Reference

NBS

PUBLICATIONS

NBSIR 85-3237

Review of Materials for pH Sensing

for Nuclear Waste Containment

AlllDb Q37DSS

Timothy Dietz and Kenneth G. Kreider

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Chemical Engineering

Gaithersburg, MD 20899

September 1985

—QC—

100

• 1156

Prepared for *ice of Research

5lear Regulatory Commission

shington, DC 20555

85-3257

, 1985

NBSIR 85-3237

REVIEW OF MATERIALS FOR pH SENSING

FOR NUCLEAR WASTE CONTAINMENT

Timothy Dietz and Kenneth G. Kreider

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Chemical Engineering

Gaithersburg, MD 20899

September 1985

Prepared for: Office of Research

Nuclear Regulatory Commission

Washington, DC 20555

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

national bureau

OF STANDARDS

LIBRARY

UJ VIEW OF MATERIALS FOR pH SENSING FOR NUCLEAR WASTE CONTAINMENT

Timothy Dietz*

Chemical Process Metrology Division

Center for Chemical Engineering

National Bureau of Standards

Gaithersburg, MD 20899

*presently with Integrated Ionics

300 Wal 1 Street

Princeton, N.J. 08540

TABLE OF CONTENTS

Page

I. INTRODUCTION 1

II. CRITERIA FOR pH ELECTRODE EVALUATION ..... 2

III. GLASS ELECTRODE 5

1. pH Response Mechanism 5 2. Internal Reference Electrode and Electrolyte 6

3. Extended Operation . ........

4. Expected Performance . 9

IV. YTTRIA STABLIZED ZIRCONIA 10

1. pH Electrode Performance 11

2. Mechanism of pH Response ........... 11

3. Internal Electrode Schemes ..... 13

4. Calibration and Associated Reference Electrode Questions ... 15

5. Long Term Stability . 17

6. Applicability of the 100°-200°C Regime ..... 19

V. PALLADIUM HYDRIDE 20

VI. METAL OXIDES 22

, 1. General Comments 22

2. Advantages and Disadvantages of Metal Oxides 23

3. Electrode Mechanisms .............. 24

4. Metal Oxide Suitability for pH Sensing 26

5. Transition Metal Oxides 26

A. Antimony ..... 26

B. Molybdenum . 27

C. Manganese . . ................. 27

D. Tungsten 28

E. Titanium ..... 28

F. Tantalum 29

G. Zirconium 29

6. Platinum Metal Oxides 30

A. Osmium 30

B. Palladium 31

C. Platinum 31

D. Rhodium 33

E. Ruthenium ........ 34

F. Iridium 34
VII. REACTIVELY SPUTTER-DEPOSITED IRIDIUM OXIDE

Page

40

1. Present Status of Sputtered IrOx pH Sensing .... 41

2. Reactive Deposition of Iridium Oxide ..... 43

3. Film Characteri zation ........... 44

REFERENCES ....... 50

i i
Review of Materials for pH sensing for Nuclear Waste Containment

I. INTRODUCTION

The continuing investigation by the Nuclear Regulatory Commision into the

encapsulation and disposal of high level radioactive waste requires an availability of sensors for long term, insitu, high temperature, corrosive environment pH measurements. pH sensors are also needed for 1 aboratory measurement

applications and for monitoring deep geologic aqueous fluids. In both cases,

the most immediate need is to model accurately and predict the long term viability of the various proposed waste encapsulation schemes. Laboratory measurements of encapsulation corrosion processes and rates and the subsequent modeling

efforts require accurate in-situ pH measurements. The other application is the

eventual monitoring of the pH in deep geologic nuclear waste repository sites.

These measurements are intended to indicate the effectiveness of the waste containment procedures. Prediction of metal container viability as a function of

the accelerated pit corrosion processes is one of the key issues in the evaluation of proposed waste disposal technologies.

This report defines the performance criteria of the needed pH sensors and

reviews the performance of a number of elevated temperature pH sensing technologies with respect to these criteria. The criteria of electrode performance were

developed to predict the utility of various pH electrodes in these simulated

environments. The classes of pH electrodes reviewed are the glass electrode,

yttria stabilized zirconia, palladium hydride and a variety of metal oxides.

The latter sections of the report focus on a relatively new solid state

electrode material, reactively sputter deposited iridium oxide. The performance

of this thin film material is of particular interest because its low electrical

resistivity and high corrosion resistance eliminate some of the shortcomings of
the glass and ceramic materials. The reactive sputtering technology permits

these films to be deposited and pattern defined on a wide variety of substrate

materials. Low electrical resistivity, which simplifies electrical contacts,

and a flexible deposition technology make this material a prime candidate for

micro pH sensors. These pH sensors would be small enough to measure the corrosion chemistry in high aspect ratio corrosion pits.

II. CRITERIA FOR pH ELECTRODE EVALUATION

This report evaluates high temperature pH electrodes according to the following criteria:

1. Nernstian response (where the voltage is proportional to log of the

activity) to [H+] and temperature from pH 1-14.

2. Ionic and redox interference.

3. Hysteresis and long term stability.

4. Corrosion resistance and mechanical robustness.

Four classes of electrodes will be considered: glass, ceramic, palladium

hydride and metal oxides. Of particular interest is the predicted performance

of these electrodes in aqueous solutions simulating expected repository site

conditions.

The two types of repository sites receiving primary interest at present are

the salt bed and basalt flow geologic formations. These repositories are expected to have slightly elevated pH, pH 9-10, and temperatures primarily between

90° and 160°C. More extreme conditions are possible, including temperatures up

to 260°C and pH 11. However, for laboratory investigation of corrosion rates,

satisfactory electrode performance in conditions less stringent and more representative of the majority of expected storage environments, was assumed to be

useful

.

2
Information on ionic concentrations that can be expected is available, but

information on redox levels is not definitive for basalt flows and apparently

unavailable for salt beds. As indicated by Table 1, salt beds are characterized

by high dissolved solid concentrations and the basalt flows by much reduced concentrations. The salt bed data was taken from compositional analysis of fluid

inclusions in the Salina Salt Bed. The data on basalt ground water was obtained

from samples taken out of the Grande Ronde Formation, Columbia River Basalt

Group. The two sets of data are included to indicate to what extent ionic

interference effects will impinge on electrode pH selectivity.

Hysteresis and long term drift are indications of an electrode's usefulness

in extended studies of corrosion and corrosion rates. Hysteresis indicates

irreversible electrode processes which will almost certainly result in a gradual

decrease in electrode performance or steady corrosion. These processes and

others result in electrode drift, which necessitates frequent recalibration.

Recalibration may not always be convenient.

The corrosion resistance and mechanical robustness of an electrode are

particularly relevant if the device is to be used in the field or for corrosion

studies over extended periods of time. Often, even fragile or awkward mechanical designs can be implemented in a laboratory, if sufficient care and ingenuity

are employed. However, such designs are often inconvenient and unreliable. The

design of effective electrode seals for high pressure autoclave systems is made

more difficult by fragile electrode materials. For the rugged demands of field

testing and implementation, the robustness and corrosion resistance of electrode

systems are particularly desirable.

3
Table 1

Approximate Molar Concentrations of the Aqueous Media

Existing in the Proposed Repository Sites

Species Salt(l) (M) Basalt( 2

) (10“ 3 M)

Na+ 3.0 13.0

ci- 3.1 7.9

K+ 0.09 0.1

F- 3.05xl0' 3 1.8

Si - 1.14

Ca+ 2 0.34 0.07

Mg+ 2 0.07 0.01

Fe - <2xl0‘ 4

As - <3xl0' 3

S04

" 2 0.47 1.8

C0 3

- 1.2

pH - 9.7

(1) Physical Properties Data for Rock Salt, L.H. Gevantman, ed . National Bureau

of Standards, Dept, of Commerce, NBS Monograph 167 issued January 1981.

(2) D.L. Lane, T.E. Jones, M.H. West, "Prel iminary Assessment of Oxygen Consumption and Redox Conditions in a Nuclear Waste Repository in Basalt", Geochemical Behavior of Disposal Radioactive Waste, ACS Symposium Series 246

(1984).

4
III. GLASS ELECTRODE

The combination glass electrode has, for many years, been the preferred

method for measurement of pH in aqueous solutions. The glass electrode responds

in a Nernstian manner to pH and is insensitive to the redox potential of the

solution. It is not poisoned by, nor does it respond to, common electrode poisons such as cyanides, sulfides, chloroform, phenols, alkaloids, and salts of

the noble metals. Dissolved gases such as O 2 , H

2 and N 2 also do not

affect the electrode response!. For most aqueous pH measurement environments

at ambient and slightly elevated temperatures, the glass electrode is the electrode of choice, because of its hydrogen ion selectivity.

In extreme or harsh environments, the glass electrode technology has sometimes proved inadequate. The glass electrode is known to exhibit sodium ion

error in high pH solutions, especially at higher temperatures2. Elevated temperatures above 150°C and particularly above 200°C have been reported to degrade

electrode performance due to leaching and glass corrosion phenomena^.

Response Mechanism

The mechanism of the glass electrode pH response is rapid hydrogen ion

exchange at the hydrated silicate glass surface. The glass electrode phase

boundary potential between the surface of the glass and the electrolyte solution

is dominated by the hydrogen ion concentration or activity in the solution.

Presumably, the high mobility and lability of the hydrogen ion is responsible

for hydrogen ion exchange processes dominating all other exchange equilibria!.

For solutions of high pH, cations other than hydrogen, affect the phase

boundary potential and the potential measured is no longer Nernstian with

respect to pH. As the concentration ratio of hydrogen ions to cations such as

Na+

, K+ and Li + decreases as the pH is increased, cation exchange begins

5
to compete favorably with hydrogen ion exchange processes. The result is that

above pH 10, glass electrodes begin to exhibit cation interference.

Lithia containing glasses have replaced sodium based glass compositions for

elevated temperature pH measurements. Lithia glasses exhibit both reduced sodium ion error and increased response linearily in high pH solutions. The

improved corrosion resistance of the lithia glasses is achieved by decreasing

the lithia to silica ratio. The lower limit to the lithium: silica ratio is

determined by the desired electrode impedance. Decreasing lithia increases the

impedance of the electrode. The impedance of the lithia based glass formulations is greater than 10^ cm at 100°C3. This impedance is comparable to

that of yttria stabilized zirconia electrodes at 100°C. These glasses also

display a sub-Nernsti an pH response which becomes more severe after exposure to

high temperature, high pressure measurement environments.

2 . Internal Reference Electrode and Electrolyte

The internal reference electrode and electrolyte have been chosen to establish stable, well characterized potentials across the inner glass membrane surface/electrolyte-fi 11 interface and the electrolyte-fi 11/sol id state reference

electrode interface. Ag/AgCl electrodes have been a common choice for the inner

reference. AgCl has been reported stable at elevated temperatures except in

alkaline solutions or solutions containing dissolved H

2 S and H^. The

electrolyte fill can be selected such that it is low pH and contains no dissolved, interfering gases.

Some speculation has been made concerning the long-term stability of the

Ag/AgCl electrode. Hydrolysis of AgCl to AgO and gradual dissolution of AgCl

into solution are possible electrode degradation mechanisms^. Elevated

chloride concentration in the filling solution can inhibit these degradation

processes

.

6
The choice of electrolyte fill is also critical in minimizing asymmetry

potentials across the glass membrane. An asymmetric potential between the

internal and external membrane interfaces can occur due to 1

a. Unequal leaching or corrosion of the glass surfaces.

b. Mechanical or chemical damage to the external surface.

c. Disturbance of exchange capacity for hydrogen ions with the adsorption

of foreign ions, grease, proteins and surface active agents. Leaching, corrosion and other damaqe to the internal surface of the membrane can be reduced by

choosing a low pH, high concentration fill. The buffers often used for ambient

temperature pH electrodes, in particular, low pH buffers generally exhibit a

temperature dependent pH variation and are poorly characteri zed at elevated temperatures. Low pH fills are desirable because the reduced pH results in reduced

glass corrosion^. High salt concentration fills are thought to reduce leaching of the internal glass surface^, which results in a stable potential across

the inner membrane interface.

The buffers employed as filling solutions for ambient temperature pH measurements are unsuitable for elevated temperature measurements. Low pH buffers,

which minimize corrosion, are too dilute, possess insufficient buffer capacity

and exhibit temperature dependent pH variations that are poorly characteri zed at

elevated temperatures

.

3. Extended Operation

The glass electrode has performed poorly during extended exposure to aqueous environments above 150°C and at pH higher than neutral. Corrosion and

leaching degrade the pH response and cause long-term drift. From 1978-1980, the

Department of Energy funded the development of corrosion resistant glass dH

electrodes for in-situ measurement of geothermal fluids. The Division of Geothermal Energy independently funded both Owens-Illinois and Leeds and Northroi ,

7
through Pacific Northwest Laboratory- Both efforts experimented with the durability of various glass compositions containing silica, 10-30% lithia and 5-10%

other oxides.

The DOE sponsored efforts produced several glass compositions that did not

suffer substantial corrosion below 200°C, but between 150-200°C exhibited a

gradual degradation of pH response. Most commercial pH glass compositions evidenced substantial corrosion even below 150°C. Of the few newly synthesized

glass compositions that survived 200°C autoclaving for 24 hours, autoclaving for

four hours at 250°C resulted in a 30% degradation of pH response.

Degradation is attributed to two separate processes, leaching and hydration. Leaching of the silica glass modifiers, L^O, CS 2 O, lanthanum oxide,

and alkaline earth oxides, occurs in high temperature water and results in an

increased electrical impedance and reduced pH response. High temperature water,

particularly above neutral pH, also hydrolyzes the S i

O

2 glass matrix. According to Filbert et al.5, silica is readily attacked by alkali but rarely, if at

all, by acids. Below pH 7, alkali or alkaline earth ions are replaced by H+

,

but the original glass matrix of Si-O-Si bonds is unaffected. Alkali solutions

differ in their mode of attack in that Si-O-Si bonds are broken to form Si-O-Na

and Si-OH, whereas for acids only SiOH forms at the sites of leached alkali or

alkaline earth ions. Since the impedance of the glass electrode is comparable

to that of the yttria stabilized zirconia electrode at 100°C, the zirconia technology must be given strong consideration because, as explained in a subsequent

section, it performs well in alkaline brines.

Due to these corrosion processes, the glass electrode which is not useful

even for short periods above 200°C, is useful for several hours at temperatures

between from 150 and 200°C.

8
It is of potential utility at 100-150°C for measurement of pH over extended periods. Above 250°C, hydrolysis of the Si -0-Si matrix occurs at substantial

rates. Below 200°C, the primary mode of degrdation is leaching. Hydrolysis and

leaching can both be roughly modeled by first order kinetic dependencies on temperature and pH. 2*3,5 Neither Owens-Illinois nor Leeds and Northrop report

extended in-situ measurement of pH between 100-150°C. However, the results of

the higher temperature survival tests and reported first order kinetic degradation processes are indications that glass electrodes are viable at 100-150°C and

should be capable of accurate, stable pH measurement for extended time periods.

4. Expected Performance

The glass electrode technology can be expected to provide reliable measurement of pH from 100-150°C. However, the presence of large concentrations of

alkali metal ions, such as Na+

, will interfere with the electrode response in

alkaline solutions. Since the corrosion resistance also decreases with increasing pH, the glass electrode is not expected to be very useful in elevated temperature, alkaline brines. The high impedance of lithia glasses below 150°C

makes calibration at ambient temperatures difficult, particularly since these

glasses exhibit subNernstian responses. Calibration is only accurate in this

case if done at the operating temperature. Attempts to reduce impedance by fabricating thinner glass membranes are constrained by durability and mechanical

strength considerations.

9
IV. YTTRIA STABILIZED ZIRCONI

A

Yttria stabilized zirconia sensors are the most developed and presently

viable technique for pH measurement at 150°-300°C. Early prototypes designed by

Niedrach at General Electric? resembled the mechanical construction of glass

electrodes, a ceramic tube with an internal electrolyte and reference electrode.

Later models developed by Niedrach^, and MacDonald and Tsurata^, used solid

internal fills such as Cu/Cu 20 as an electronic-ionic contact to the internal

surface of the insulating, ionic conducting zirconia membrane [see Table 2].

Early experiments established that these membrane electrodes exhibited nearly

Nernstian response at elevated temperatures and appeared to exhibit little sensitivity to interfering ionic, redox and other easily reduced aqueous species.

Recent published efforts by D.D. MacDonald, S. Hettiarachi and P. Kedzierzawski at SRI International 10»11 and separate work done by M. Danielson, H.

Kosk and J. Myers at Pacific Northwest Laboratory6,12,13 have focused on

improving the accuracy and temperature range of these sensors. They have investigated the suitability of ceramics from different vendors with varying yttria

concentrations, the efficiency of various internal electrode schemes and sealing

procedures, the effects of extended exposure to interfering species, and the

mechanisms of the long term degradation of sensor performance. This section on

zirconia pH sensors will discuss the state-of-the-art, briefly mention some

intimately related questions regarding internal reference electrodes and indicate the limitations experienced by these electrodes at present. This discussion is intended to indicate the present utility of these electrodes and which

aspects of electrode design might merit further development.

10
1 . pH Electrode Performance

The electrode performance reported by the groups at GE, SRI, and PNL is summarized in Table 2. All three groups report a linear open circuit potential

response as a function of log [H+

], with little, if any, observed interference

from O2 , Hg, Na+

, Cl" and H 2 S. The slope of the response is generally

observed to be approximately 10% less at 100°C than the theoretical Nernst figure of 2.303 RT/F, but the disparity generally decreases as the temperature

approaches 300°c6>H. The improved performance of the zirconia membrane

above 250°C has been attributed to ion conduction which increases with increasing temperature? »11 >13

2. Mechanism of pH Response

The mechanism for the pH response of ceramic zirconia membranes is oxide

ion conduction. The indirect relationship of oxygen concentration in the aqueous phase and pH, establish the pH dependent potential drop across the solid ion

conductor, zirconia, and the aqueous solution interface. Calculation of activation energies from membrane impedance vs. temperature plots show the activation

energies to be very close to those of high temperature oxygen

sensorsl2,13,14 e The correspondence of activation energies implies an

oxide ion conduction mechanism for the zirconia pH sensor.

The low temperature (<200°C) pH response has been correlated with the electronic impedance. Various stabilizing oxides have been added to zirconia to

reduce the impedance, but 8-10% yttria is apparently close to the optimum composition?^ Impedances around 10^ at 100°C cause several problems. Noise

and spurious charging are serious problems; Faraday cages are often required to

provide electromagnetic shielding. Electrode response time and the observed

open circuit potential are affected by resistive shunts intrinsic to the ceramic, the hermetic sealing and/or the input impedance of the el ectrometer . The

11
Table 2

Comparison of Yttria Stabilized Zirconia pH

General Electric SRI International

8% Yttria, Corning Glass

Works

9% Yttria, Coors Ceramics

CU/CU 2 O

internal reference

CU/CU 2 O, Ag/HgO

internal reference

Linear response to pH

95-285°C, pH 3-9

1:1 pH response relative

to the standard hydrogen

electrode, 125°-275°C,

pH 2. 5-9.0; >90% theoretical Nernst slope

An initial calibration

spread of 0.5 pH units

amongst the three sensors tested

Insensitivity to 100 ppm

sulfide, free from

sodium ion error

Insensitivity to O 2 , H

2 , C 1 - , 780 ppm S*2; insensitive to pressure variation of 200-1000 psi

Sensors responded poorly

at 95°C after operation

at 285°C

Report sluggish performance

below 150°C; results of

thermal cycling not

reported

Stress cracking of the

ceramic at the sealing

collar; corrosion resistance to 20% NaCl and

100 ppm H

2

S

Report sealing

difficulties

Electrodes

Pacific Northwest Lab

8% Yttria, Corning

Glass Works

(C 11 /C 112 O), Pt

internal reference

Linear response to pH

85°-300°C, pH 3-9.5;

>90% theoretical

Nernst slope

E 0 for a number of

tubes falls within

+0.5% of a pH unit

Insensitive to the

redox Dotential

Thermal cycling

(T>250°C) results in

sub-Nernsti an response

at lower temp.

Report sealing

difficulties

12
resistance of the ceramic to the formation of small cracks, the effectiveness of

the high pressure seal around the brittle zirconia tube, and the ohmic/ionic

intimacy of the internal electrode determine the accuracy of low temperature pH

measurements.

3. Internal Electrode Schemes

The first internal electrode used by Niedrach was a Ag/AgCl type inserted

into the zirconia tube filled with a potassium chloride solution'7 . Difficulties with long-term drift due to changes in the concentration of the internal

electrolyte, deterioration of the Ag/AgCl electrode and sealing difficulties

posed by the vapor pressure of the hot internal electrolyte solution led to the

consideration of several solid fill internal electrodes. Solid fill electrodes

were hoped to possess long-term stability, thermodynamic reversibi 1 ity and physical characteristics permitting simple, effective sealing at high temperature

and pressures. If the internal electrode is not reversible, space charge or

other polarization phenomena will cause unreproducible open circuit potential

readings. An electrochemical equilibrium must exist to connect reversibly the

pure electronic conduction of the internal metal electrode with the ionic conduction of the zirconia ceramic^.

Cu/Cu 2C)7’8»9, Hg/HgC)ll, Ag/Ag20,H, Agl3 and Pt 1

3

have been tested as internal electrodes. Hettiarachi; et al.H found the

Cu/Cu20 filled tubes to exhibit unreproducible responses. X-ray diffraction

indicates an irreversible decomposition of CU 2 O; this decomposition results in

the expansion of the internal electrode material, placing the ceramic tubes

under mechanical tensionll*^. The Ag/Ag20 fill was also shown by x-ray

analysis to degrade^ as the Ag20 thermally decomposes to Ag.

Hettiarachi, et al . demonstrated stable electrode performance with a Hg/HgO

internal electrode^. They assert the stability of the Hg/HgO internal

13
reference electrode*! at their operating temperature (<275°C) was "beyond

any doubt". As communicated by M. Danielson, operating stability is not necessarily evidence of reversibility***. Further evaluation of the reversibi 1 ity

of the Hg/HgO half cell appears to be indicated if it is to be used in experiments involving extended periods of time. The Hg/HgO must also be used with

discretion, because mercury has a high vapor pressure and boils at 356°C. Mercury is also very toxic.

M. Danielson*^, et al . reports the application of metallic paint to the

internal surface of the zirconia tube. The authors suggest that the loss of pH

response after the sensor was at ambient temperature for several days was due to

the adsorption of water at the metal-zirconia interface. Water at the interface

may interfere with the electrochemical conversion of oxygen into the oxide ion,

resulting in a net polarization.

Recent work at PNL has resulted in a platinum internal electrode which,

from thermodynamic evidence, appears to be reversible^. The bottom two cm of

the zirconia tube is coated with a platinum ink and fired at 400°C for 18 hours.

Electrical contact to the platinum film is made with a metal spring contact.

The advantages of the platinum internal electrode are:

a. It appears to be reversible, which should result in long term

stab i 1 ity.

b. Contact of metal to ceramic is intimate, resulting in an order of magnitude reduction in total impedence of the electrode over the powder

fills.

c. The electrode is poised after the 400°C firing and does not need to be

"seasoned" at hiqh temperature in the autoclave.

d. The E 0 (the pH=0., intercept) appears to be within a range of 10 mV

for all the sensors tested, consequently simplifying calibration

procedures

.

14
4. Calibration and Associated Reference Electrode Questions

Some of the most difficult and important problems associated with high temperature pH sensing are calibrating the pH electrodes and implementing well

characteri zed buffer solutions and reference electrodes. The performance of

reference electrodes and buffer solutions with increasing temperature, particularly above 200°C, has been addressed, but there is certainly a need for further

investigation. The development of the pH zirconia sensor is hoped to provide a

tool for making direct measurements of hydrogen ion concentrations. "The lack

of a simple and reliable pH electrode for environments above 100°C has been a

great impediment to assembling the thermodynamic data base to make aqueous chemistry at elevated temperatures a quantitative science^". Since the zirconia electrodes operate best at 250-300°C, it is important to understand their

performance and the aqueous solution chemistry at these temperatures in order to

characterize their behavior at lower temperatures. At this temperature, electrode impedence is substantially increased and slopes drop further from the theoretical Nernst response. With most other pH electrode systems, high temperature measurements are an extension of low temperature measurements; the zirconia

membrane is different in this respect.

Calibration of a pH electrode requires establishing the E 0 and the slope

of the open circuit potential (OCP). E 0

is generally taken to be the y-intercept of the OCP. Its temperature dependence can be calculatedll>13 using

theoretical hydrogen-oxygen electrode potentials under standard conditions. The

actual value of E 0 varies amongst zirconia tubes from the same supplier, from

different internal electrode systems and different preconditioning histories.

Differences amongst internal electrode systems emphasizes the necessity of a

reversible system. An irreversible system is far more likely to accentuate

preconditioning history differences.

15
The PNL group investigated the effect of preconditioning on electrode performance and report reduced preconditioning sensitivity and improved low temperature performance for a platinum internal electrode. They report that sensors

with Cu/Cu20, Ag/Ag20, Ag and graphite filled electrodes require "seasoning"

in the autoclave at temperatures exceeding 250°C. Between 250° and 300°C, newly

made Cu/Cu 20 or silver fill sensors suddenly jump several hundred millivolts

(several pH units) in the oxidizing direction and become poised by oxygen. Once

the electrodes are poised (preconditioned), returning them to 100°C does not

reverse the poising. However, if the sensor is allowed to sit at ambient temperatures for more than a few days, it is no longer poised to oxygen and suffers

significant loss of pH response. Returning the electrodes to 250°C results

again in oxygen po

i

s ed electrodes^. Not only is the necessity for "seasoning" inconvenient, and an indication of non-equilibrium processes, but the high

temperature seasoning degrades the long-term stability of the sensor for low

temperature measurements. Some apparently irreversible phase or structural

changell>1.3 occurs at 250300°C which does not affect the high temperature

performance of the sensor, but does result in reduced sensor longevity for low

temperature pH sensing. Changes in the ceramic during extended autoclaving periods is discussed in more detail in the subsequent section on long-term

stabi 1 ity.

The platinum fill electrodes developed by PNL^ give immediate thermodynamic potentials without the need for a 300°C autoclaving and have a greater

than 90% Nernstian pH response at 100°C. After 30 days at 100°C, this sensor

retained a greater than 90% response. Presumably sensor performance improves as

the temperature increases. The authors claim that if the pH accuracy required

is only a few tenths of a pH unit, then calibration is not necessary^. This

implies that both E 0 and the slope of the response are reprodicible from electrode to electrode to within a few percent.

16
Recent work at SRI which compared the pH response of 9% yttria tubes to

that of a standard hydrogen electrode placed in the same autoclave establishes a

pure Nernstian response for the zirconia electrodes. Earlier measurements were

not as definitive, because of uncertainties in buffer behavior of elevated temperatures, particularly at low pHsll, and uncertainties in the accuracy of

several reference electrode schemes^, 16, 17 Hetti arachchi and coworkers

measured the OCP of a Hg/HgO zirconia sensor and an internal reference hydrogen

electrode against an external Ag/AgCl reference electrode. They report a 1:1

correlation of the potentials of each electrode against the common Ag/AgCl reference from 125°-275°C. The experimental slopes of the potential (E sensor/

pH) plots were calculated by linear regression analysis; the linearity was shown

by the regression coefficients obtained near unity. The experimental slopes are

less than the theoretical Nernst values by approximatel y 11% at 125°C and gradually decrease to a less than a 5% deficit as the temperature is increased to

225°C. These results unequi vocably establish that yttria stabilized zirconia

can be an accurate, linear pH sensor. However, the less than theoretical slope

value raises interesting questions about high temperature solution chemistry and

the standard pi atinum-hydrogen electrode.

5 . Long Term Stability

Experimental work has shown the zirconia sensor can provide reasonably

accurate and stable pH measurement for periods greater than 30 days at both low

(100°C) and high (250°C) temperatures6>7>9 However, problems with mechanical pressure seals, fine cracking of the sensor ceramic, and corrosion of the

ceramic at the grain boundaries results in either premature sensor failure or

degradation. Further effort directed at optimizing the ceramic for both its

mechanical and electrochemical properties seems appropriate.

17
The composition and density of the ceramic have been related to the electrochemical properties at elevated temperatures* Scanning electron microscope

(SEM) micrographs of the surface of a sensor after extended exposure to 300°C

autoclaving, revealed a significant widening of grain boundaries. Apparently, a

second phase is present which contains segregated Ca, A1 , and Si. These elements may have been added to aid the sintering process during manufacturing.

SEM studies of higher purity materials showed no significant widening of grain

boundaries^. It has been suggested that second phase materials may have been

responsible in part for the very high impedance at low temperatures which result

in a sluggish pH response^-.

A.C. impedence studies done at PNL.6 indicate a shunt resistance is

responsible for both the less than theoretical Nernst slope at low temperatures

and for further reduction of the slope at low temperatures after extended autoclaving at higher temperatures. As temperature decreases, ion-conduction also

decreases, increasing the electrical impedence of the membrane. Thus, any

shunting conductance due to whatever mechanism would result in a resistive loading of the open circuit potential. This would be observed as a lower millivolt

output and an apparent degradation of pH sensitivity. At higher temperatures,

membrane impedence is low enough that the effect of resistive shunting is negligible and even sensors with seriously degraded performance at low temperature,

perform acceptably at high temperature.

Gaining further insight into the degradation processes, as indicated above,

will enable an improvement of the low temperature, long term pH sensing capability of these sensors. Components of an electrical equivalent circuit model have

not yet been correlated with the physical and electrochemical degradation mechanisms of the ceramics. Once these mechanisms are understood, modifications for

optimization may be made intelligently. Changes in density, composition and

18
membrane thickness are suggestions that have been made to improve the low temperature pH response and long term stability.

6 . Applicability to the 100°-200°C Regime

The insensitivity to ion and redox interference makes this sensor very

attractive for corrosion studies in hot brines. However, there is no question

that the sensor performs best between 250°-300°C. Implementation at 100°-200°C

will require careful selection of the ceramic vendor and internal electrode.

GE, SRI, and PNL all report variability from vendor to vendor. Elimination of

resistive shunting due to microcracking is very important. Other considerations

concerning membrane thickness, composition and density apparently require further research. The internal electrode is directly related to the total impedance of the electrode. The two lowest impedance, reproducibl e, internal electrodes to appear in the literature are the Hg/HgO filial and Pt ink^. The

Pt ink electrode has been used successfully down to 85°C, the Hg/HgO to 125°C.

At these low temperatures , as mentioned previously, the very high impedence

(10^ ) of the electrodes results in long response times and necessitates Faraday shielding from electromagnetic interference.

19
V. PALLADIUM HYDRIDE

Palladium hydride may be useful for high temperature reference electrode

applications, but is generally considered of limited utility for high temperature pH measurement. The electrode potential is usual ly measured in the cd-6

region of the palladium hydride phase diagram. Difficulty in maintaining the

a+6 phase region long enough to record potentials, potential dependence on H£

and O2 partial pressures and evidence of irreversible electrode processes

limit the potential usefulness of this material for pH measurement.

Palladium hydride exists in two forms, a and 6. The a phase contains the

smaller percentage of hydrogen and the phase contains the greatest. The a+6

mixture is of particular interest because its el ectrochemical potential in aqueous solution is insensitive to the actual Pd : H ratio. As long as the palladium

hydride electrode remains in the a+6 region of the phase diagram, its potential

with respect to the solution will not change.

The open circuit electrode potential can be used to monitor the progress of

hydroqen diffusion out of the electrode. If the electrode begins in the hydrogen rich phase and hydrogen is permitted to diffuse out of the metal into the

solution, then the potential will increase as the palladium is depleted. As the

electrode enters the + phase, the potential remains nearly constant until

enough hydrogen has escaped to place the electrode into the a region of the

phase diagram. Once into the a region, the potential continues to increase as

the depletion of hydrogen continues. The potential of the a+6 plateau has been

reported to exhibit a Nernstian relation to pH up to 195 °Cl 8 - 21 .

In a more recent study, D.D. MacDonald and coworkers reported their investigation into pH measurement with palladium hydride from 25°-275°C.22,23

The mode of electrode operation was to discharge hydrogen cathodically onto the

20
surface of the metal, Dushing the palladium into the 3 region of the phase diagram. A brief anodization pulse was applied to oxidize and remove any metal

impurities resident due to cathodi zation; the current was interrupted and the

electrode desorbed hydrogen into the solution (as long as the fugacity of H 2

in the solution was less than that of the metal). As the metal entered the ct+s

region, hydrogen continued to desorb, but the potential reached a plateau. D.D.

MacDonald and coworkers found that plateau potentials were reproducible to better than +10mV up to 275°C or to better than +0.1 pH units.

Unfortunately, the electrodes exhibited a non-Nernsti an temperature dependence, irreversibi 1 i ty, and very short residence times at the olateau potential.

The non-Nernsti an temperature dependence was clearly observed above 200°C.

Irreversibi 1 i ty was indicated by the lack of Nernstian temperature dependence

and hysteresis between the absorption and desorption isotherms of the oc+s mixture. Deviation from the Nernst response of a one electron exchange process

(H+ + e~ = H(p(j)) is also an indication of either a mixed potential

electrode process or simply that the adsorption-desorption of hydrogen from the

palladium lattice is an irreversible or at best a quasi-reversible process.

Above 200°C, the plateau residence times are too short to discern plateau potentials. Even at 150°C, the plateaus are short enough that accurate determination

of plateau potentials is difficult. Apparently, above 200°C, the hydrogen

escapes so rapidly from the surface that the a+B phase cannot be maintained at

the interface.

The plateau potential is strongly dependent upon the partial pressure of

both H 2 and O 2 in solution. The O 2 dependence appears to be almost linear

for certain regions of O 2 partial pressure and temperature. The electrodes

were insensitive to large amounts of carbonate ions and carbon dioxide.

21
VI. METAL OXIDES

1. General Comments

The literature indicates extensive use of metal oxide electrodes to measure

pH. The mechanism of pH response for these electrodes is generally quite different from the simple ion exchange processes thought to dominate the response

of glass and ceramic electrodes. The increased electronic conductivity of metal

oxides relative to glasses and ceramics is responsible for the substantially

different electrode characteri sties . Most of the studies reported in the literature were done at temperatures below 100°C. The literature is fragmented in

its discussion of metal oxide materials and electrode characteristics above

100°C. Problems with electrode stability and strong redox interferences have

probably discouraged systematic investigation into using metal oxides for high

temperature pH sensing.

Recent work at SR 1^4 ancj the University of Pennsylvania^ are indications of a continuing interest in metal oxides for high temperature pH sensing. SRI, in a recent study of suitable pH measurement techniques for power

water reactors supported by the Electric Power Research Institute (EPRI), considered oxides of Zr, Pt, Ti, Ir and Ti-Nb. The results of the University of

Pennsylvania investigation into iridium oxide prepared by reactive ion sputtering are particularly encouraging. They report a Nernstian pH response at both

ambient and elevated temperature and their measurements indicate redox in sensitivity at ambient temperatures. This redox insensitivity is the primary characteristic that distinguishes the sputtered iridium oxide from all other reported

metal oxides and identifies this material as a candidate for pH sensing in aqueous environments with uncontrolled redox potentials. Redox sensitive electrodes

are of limited value for high temperature pH sensing, because of the difficulty

22
of controlling the redox potential or dissolved gas concentrations in high temperature, aqueous solutions.

M.J. Madou et al . at SR 1 26 have suggested the use of flatband potential

measurements as an alternative to the standard open circuit potential measurements. This capacitance measurement technique is not expected to be sensitive,

for appropri ately doped semiconducting metal oxides, to the redox potential of

the solution, because the Helmolz potential drop at the el ectrode-el ectrol yte

interface is dominated by adsorption/desorption phenomena of protons rather than

electron exchange. Unfortunate! y, this technique requires low impedance solutions and substantial ly more sophisticatd measurement instrumentation than open

circuit potential measurements.

2. Advantages and Disadvantages of Metal Oxides

The metal oxide electrodes have a number of advantages with respect to the

glass and ceramic electrode materials. The relatively low electrical impedance

of some of them is at least partially responsible for the primary advantages.

Reduced resistance reduces thermally generated electrical noise and shielding

requirements . Open circuit measurement techniques require inexpensive electrometer instrumentation and place no special requirements on the conductivity of

the solution being measured.

The incorporation of these materials into electrode structures is often

greatly simplified due to convenient thin film deposition techniques and the

elimination of the need for including internal electrode structures. The more

highly conductive oxides have been implemented as electrically free standing

thin films. The film can be deposited on any convenient substrate such as

Al 2 O 3 and electrical contact can be made remote from the region of the film

exposed to the solution. Deposition technlogies such as evaporation, reactive

sputtering, and chemical vapor deposition are compatible with most

23
microelectronic planar fabrication techniques and facilitate the development of

microsensor structures. The availability of micro pH sensors would be of particular utility to present efforts to understand and model pit corrosion processes. Sufficiently small pH sensors could probe the aqueous chemistry inside pit

corrosion structures.

The two primary problems generally associated with metal oxide pH probes

are an almost universal response to solution redox potential and instability in

the oxide-hydroxide stoichiometry. The problem of redox interference appears to

be inseparably linked to the property of high electrical conductivity which is

also directly responsible for the advantages these electrodes possess over the

insulating glass and ceramic materials. High conductivity is generally thought

to dictate a dominance of electron exchange over proton exchange processes.

1

The paradox of sputtered iridium oxide, which appears to possess both relatively

high conductivity and redox insensitivity, suggests that a high temperature,

metal oxide, pH electrode does not necessarily respond to redox potentials. The

possible origin of the paradox and suggestions for further research are discussed in some detail in a subsequent section on iridium oxide. Many metal

oxide electrodes have been observed to "age" and drift in potential over periods

of days at ambient temperatures. This "aging" phenomena has been associated

with hydration. Katsube, et al .27 report acceleration of hydration times by

autoclaving. Madou suggests28 that anomolies in the temperature dependence

of Pt electrode at 25-300°C may be due to the poorly understood formation of

mixed oxides and hydrates.

3. Electrode Mechanisms

A. Fog and R.P. Buck in a recent paper formulated five different explanations of oxide electrode mechanisms [p. 142, A. Fog, R.P. Buck, Sensors and

Actuators, 1984)].

24
i. Simple ion exchange processes occurring in a hydrated surface layer at

electrode-electrolyte interface. This ion exchange phenomena is responsible for

the glass electrode pH response.

ii. A redox equilibrium between two different solid phases, e.g., a lower

and higher valence oxide, or an oxide and pure metal phase, such as the

antimony electrode:

Sb 2 0 3 + 6H+ + 6e“ 2Sb + 3H 2 0

iii. A redox equilibrium involving only one solid phase, whose hydrogen

content can be varied continuously by passing current through the

electrode:

H

x MOn + yH+

+ ye" H x _y + yH 2 0

iv. Or similarly a single phase oxygen intercalation:

M0X + 2yH+ + 2ye" MOx _y + yH 2 0

v. A steady-state corrosion of the electrode material may, in some cases,

cause a pH dependent potential. No verified example of such pH electrodes has been reported.

The mechanism of oxygen intercalation can be rewritten to include hydration

effects:

M(0H) x + yH+

+ ye" M(0H) x _y + yH 2 0

M0 n (0H) x + yH+

+ ye" -— M0n (0H) x

. y

+ yH 2 0

or some combination of these reactions to account for the hydrate nature of some

of the oxide materials in aqueous solution.

Each of these equations can be used to calculate an electrode potential

from standard reference data. Comparison of calculated and experimental values

is sometimes instructive in determining possible reaction mechanisms and evaluating likely electrode performance at elevated temperatures . Poor performance

at low temperatures, as evidenced by mixed or unstable electrode potentials.

25
oxide solubility or vulnerabi 1 i ty to common complexing species, or a strong temperature dependence for E 0 , are not necessarily proof of poor elevated temperature performance, but do indicate possible problems.

4. Metal Oxide Suitability for pH Sensing

In the sections that follow, some of the most promising oxides are discussed individually. Some results from the literature on their performance as

pH electrode materials are indicated. Standard reference data and other published literature sources are used to discuss metal and oxide stability, and

probable electrode reaction mechanisms. Minor emphasis is placed on the method

of oxide preparation, although this generally has a pronounced effect on the

composition morphology and electrochemistry of the material. Emphasis was

placed on determining whether any form of the oxide might be suitable for further investigation as to its suitability for high temperature pH sensing. A

significant portion of such an investigation would indeed focus on developing

and optimizing preparative techniques. Suggestions as to how such an investigation might be done are indicated in the discussion of iridium oxide.

5 . Transition Metal Oxides

A. Antimony

The antimony metal -metal oxide electrode has been used extensively to measure pH.29 7h e equilibrium controlling the response of this electrode

i s29 ;

Sb 2 0 3 + 6H+ + 6e“ = 2Sb + 3H2 0

The antimony electrode is not a good choice for high temperature pH sensing

because of drift, non-Nernsti an temperature dependence and significant oxide

solubility even at room temperature. Instability observed as drift is likely to

be due to the formation of intermediate valence oxides. 30 Like most other

metal oxides, this electrode is sensitive to redox interference.

26
8. Molybdenum

Molybdenum is a reactive metal that readily passivates forming non-stoichiometric, intermediate oxide phases^. The E 0 reported by I.M. Issa, et

a 1 - ^ 1 is indicative of the redox couple

Mo02 + H 2 0 M0O3 + 2H+ 2e"

rather than a metal-oxide redox couple. They also report a slope of 51 mV/pH at

35°C, pH2- 11, which is substantially lower than the theoretical Nernst value of

61.6 mV/pH. The color of the electrode used was reported to be black which cannot be the white, anhydrous, Mo0 2 , but rather must be a hydroxide.32 The

Mo02 anhydrous and hydrous form is not stable at pH 9 and above. MnC>3 is

relatively soluble even at room temperatures.32 jhe poor stability of the

oxide and non-Nernsti an pH response are augmented by a third unfavorable feature, sensitivity to solution redox potential .31

C. Manganese

Mn02 is reported to respond in a Nernstian manner between pH 1-5, 25°C,

for both the stoichiometric 6 form and the MnOOH form. 33 jhe potential

determining reaction is thought to be

Mn0 2 + H+ + e* MnOOH

J. Caudle, et al.33 also report the electrode process to be reversible and

thus they observed no hysteresis. The potential-pH diagram32 indicates that

both Mn02 and MnOOH are stable oxides, particularly for pH 4 and above. The

paper by J. Caudle, et al . did not discuss the hydrogen ion response above pH 5,

because their interest was in acid batteries.

Unfortunately, the real problem with this material is its known cation

exchange properties. It favors exchange with cations in the order:

K+> Rb+> Cs+> Na+

> Li +

27
Interference potentials are of the order of 50-100 mV for 0.001 M concentrations

of these cations. 33 This level of interference would not permit accurate pH

determination in even weak electrolyte solutions.

D. Tungsten

Tungsten does not have a stable oxide form for pH values higher than four

according to Pourbaix's pH-potential diagram. 32

E . T i t an i urn

According to thermodynamic data on titanium, the Ti

O

2 oxide should be a

stable oxide for pH sensing in most severe environments. The tetravalent native

T i

O

2 oxide is stable with respect to corrosion in nearly all non-complexing

aqueous solutions up to 150°C. Above 150°C, the T i

O

2 phase begins to experience corrosion in strongly alkaline solutions. The two other reduced oxide

forms, TiO and T i 2^3 are n °t thermodynamical 1 y stable in water. 34

T i

O

2 electrodes have been reported with nearly Nernstian OCP response, to

pH. A. Fog30 reports 55 mV/decade at ambient from pH 2-12, with increasing

electrode error from pH 10-1435. «. Kinoshita, et al . 34 report similar

results, but observe better responses for thermally oxidized electrodes. However, K. Kinoshita and M.J. Madou35 also report the probable existence of a

mixed potential for the anodically prepared film according to E 0 data. They

also express uncertainty as to whether the high temperature pH response

observed, was due to access to the titanium substrate via pinholes or due to a

membrane potential established by proton conduction in the oxide^^.

The observed mixed potenti al -1 i ke behavior and alkaline pH error may be due

to instability in the hydration character of the oxide. 29 The 30 mV of

hysteresis observed by A. Fog30 and the superior performance of thermal

oxides over anodically formed oxides are also indications that the nature of the

oxide hydration is correlated with OCP pH response.

28
For HO2 to become a useful pH sensing material for high temoerature measurements further development is necessary. OCP measurement techniques remain

attractive for this oxide, because ionic interference is reported lower than the

platinum group oxides, except of course in reducing and oxidizing

solutions . 30 However, work done by M.J. Madou, shows that voltage flatband

techniques on Nb-doped T i O2 may hold more promise^.

F. Tantalum

Tantalum pentoxide, Ta205, is an insulating oxide exhibiting a Nernstian potential response to pH. It does not possess the benefits of electrical

conductivity, but is noteworthy for its Nernstian response and exceptional chemical stability. The mechanism of pH response is thought to be hydroxyl ion

exchange; other anions do not interfere with the observed potential^.

Potential-pH diagrams for tantalum indicate a single phase passive layer of

Ta205 that is stable in ambient, aqueous solutions^, it is not

attacked by HC 1 , HNO3, aqua regia, H2SO4, or caustic alkali solutions. HF

etchants require a platinum catalyst for substantial etch rates to be achieved.

250 °C autoclave testing in water showed Ta205 dissolution rates an order of

magnitude below those of yttria stabilized zirconia and 1/3 that of

M2O328. a doped form of Ta205 might be useful for OCP or flatband

pH measurements.

G. Zirconium

Zr02 stabilized with yttria has already been discussed in this report as

a material for measuring pH between 150 - 300 °C. It is an insulator and possesses

none of the advantages of the electrically conductive metal oxides.

In a systematic study of high temperature pH monitoring techniques done for

EPRI, K. Kinoshita and M.J. Madou of SRI-37 found that thick films of Zr02

were insensitive to pH and thin anodic and thermal oxide films on zirconium

29
metal exhibited nearly Nernstian behavior. The low impedance and long pH

response time constants for the thin films were attributed to electron exchange

reactions occurring at pinholes in the films. They suggest that at pinholes,

electronic conductivity is high enough to sustain an electrochemical reaction

involving protons and oxide species. They conclude that OCP measurement of pH

with Zr-ZrC>2 electrodes does not show much promise, because the pH response is

dependent upon defects in the oxide film.

Experiments on flatband potential response to pH were equally discouraging.

They suggest doping of single crystal ZrC>2 as an alternative. This technique

differs from OCP yttria-zirconia electrode, in that it does not have an internal

reference electrode structure.

6 . Platinum Metal Oxides

A. Osmium

Osmium is the most chemically reactive of the platinum metals. There are

only two stable forms of osmium oxide, OSO4 and 0s02- The existence of any

other forms is dubious even in the gaseous phased except for gaseous OsO

and OS3O4. The 0s02 OSO4 redox couple is of little utility because

OSO4 is very soluble, reactive and volatile (mp. 40 . 6 °C, bp 131 . 2 °C). Furthermore, 0s02 is not stable in strongly alkaline solutions^ or in the

presence of complexing species such as Cl“, I", Br

-

, NO2 -, NO,

etc . 32 The susceptibility of the binary oxide to corrosion

and complexing make osmium a poor candidate for high temperature pH sensing.

B. Pal 1 adi urn

Palladium is a noble metal whose PdO oxide has been used by W.T.

Grubb39 a t General Electric for ambient temperature pH sensing, pH 3-11.

The electrode reaction from x-ray diffraction data and the E 0 value is assumed

to be:

Pd + H 2 0 PdO + 2 H+ + 2e

-

30
Grubb reports very stable electrode slopes even after aging the electrodes for

several years in neutral H

2 O. The potential-pH diagram predicts the stable

formation of several hydroxides which probably is a good indication of electrode

drift due to mixed oxide-hydroxide equilibria. PdO and all of the oxides are

reported to be reasonably soluble at ambient temperatures^; presumably at

elevated temperature increased solubilities would greatly reduce the utility of

the PdO electrode.

The PdO electrode responds strongly to interference from 0.05M potassium

ferri and ferrocyanide , and oxygen39.

C. PI atinum

Platinum has been extensively used in high temperature studies in aqueous

solution studies. The nobility and stability of platinum in aqueous solution up

to at least 300°C, as well as its catalytic properties have resulted in the

extensive use of this metal as a counter electrode in electrochemical studies of

other corroding metals. Potential-pH diagrams show platinum and its oxides to

be stable in most aqueous solutions with little variation in the diagram with

increasing temperature. Platinum corrodes only in strongly oxidizing solutions

at pH less than three. 34

PtO, Pt02 and mixed Pt0-Pt02 oxide have been utilized for pH sensing.

J.V. Dobson, et al .40 report a Nernstian response at room temperature with

the E 0 reflecting the electrode equilibrium:

Pt + 2H 2 0 = Pt0 2 + 4H+ + 4e~ E 0

= 0.837V

However, their measurements done to 250°C showed both the E 0 to be temperature

dependent and the temperature dependence of the slope to be non-Nernsti an . A.

Fog reports different OCP slopes for Pt02 electrodes above and below pH

5.30 He used a pure, Dowdered form of Pt02 on a Ruzicka electrode, unlike

other efforts mentioned in this section which oxidized base metal electrodes of

31
platinum. They also report 100 mV of hysteresis and strong interference from

0 2 , I-, Fe (CN) 6

“ 3

, and Fe (CN) 6

“ 4 .

Certain oxidation techniques coat the platinum base metal with predominant

ly PtO rather than Pt0 2 34 » 33 * 4 l. K. Kinoshita, et al . report a Nernstian

response at ambient temperature with an E 0 very close to the equation^:

Pt + H 2 0 PtO + 2H+ + 2e- E 0

= 0.980V

However, they also observe a more rapid drop in potential with increasing temperature than allowed for by a simple Pt/PtO couple. They suggest temperature

dependent hydration phenomena may be involved^. The stability of the

oxide, its hydrate and exchange equilibrium with higher oxides and hydroxides

should also be considered. Potential-pH diagrams show a narrow band of PtO sta

bility. A. and M. Hecq report reactively sputtering pure PtO4 -; an electrochemical study of the elevated temperature response and stability of this singl

oxide sputtered thin film material would be very instructive.

At present, platinum metal oxide electrodes are of limited utility at elevated temperatures because they are difficult to calibrate, possess unknown sta

bility and hydration characteri sties and exhibit strong redox interference.

However, they may merit further attention, because platinum is such a noble

metal

.

D. Rhodium

From potential-pH diagrams 3 ^ a nd literature on the anodic oxidation of

base metal electrodes, 43 * 44 ruthenium oxides appear neither to be as corrosion resistant nor as reversible as ruthenium and iridium. It is not as noble

as platinum. Bulk rhodium forms a passivating layer except in strong oxidizing

or complexing solutions. Rhodium and its passivating oxides are more stable in

alkaline than acidic solutions.

32
Some work on anodically prepared oxide is reported in the literature. Generally, the oxide films exhibit poor adherence to glass and high solubility in

acids. Unlike iridium oxide, which is formed anodically in an acid solution,

rhodium is oxidized in basic solutions and most rapidly above pH 13 44 .

Vol tammograms indicate the existence of an abnormal second anodic peak appearing

before the main anodic peak 43 . This peak is also observed for vol tammograms

of iridium oxide at 70°C 43 . Decreased el ectrochromic activity and electrical conductivity of rhodium oxide are associated with the appearance of this

peak

.

A. Fog and R.P. Buck^O report non-Nernsti an pH response for crystals of

Rh02~nH20. The slope of the potential-pH curve was not linear, possessed

super Nernstian slope at ambient temperature and did not follow the standard

Nernstian temperature dependence at 35° and 45°C. They also report severe redox

interference and also interference from 1“ and Br*. These results plus the

other negative features such as hydrated sesquioxide solubility, susceptibility

to complexing 33

, and apparent relative irreversibi 1 ity with respect to

ruthenium and iridium, make rhodium a poor choice for pH sensing. However, its

accelerated oxide growth in base, as opposed to accelerated ruthenium and iridium growth in acid, raises intriguing questions about the properties of mixed

platinum metal oxides.

E . Ruthenium

Ru02 , the most stable of the oxides of ruthenium3 ^ is stable in acid

solutions, but according to potential-pH data, unstable above pH 10 33 . This

information by itself, discourages further interest, because repository pH is

expected to exceed pH 10. However, of the platinum metal oxides tested by Fog

and Buck 3 ^, only the Ru02 electrode showed a reproducible , Nernstian pHpotential over part of the pH scale in the presence of oxidizing or reducing

species such as ferricyanide, ferrocyanide and hydrogen peroxide. Others also

33
report an easily established reversible potential for the redox couples

Fe+2/Fe+

3, Ce+ 3/Ce+

4, I-/I 2 and Cl -/Cl 2

46 -

Ruthenium may be useful in a mixed platinum metal oxide, pH sensitive thin

film. At low pH, RuO x films exhibit reversible behavior during cyclic voltammetry measurements. It is thought that like IrOx , the thick oxide film undergoes internal electrochemical changes rather than metal /metal-oxide redox at the

metal/metal -oxide interface4^. XPS results show the joint existence of

ruthenium Ru (IV) and Ru (VI), whereas Ir (IV) does not appear to go to a higher

redox state in thick oxide films during potential cycling. 47,48

F. Iridium

The tetravalent oxide of iridium has been used for a number of widely varying pH sensing applications. S. Bordi and co-workers used Ir/IrO? electrodes

for rapid pH sensing in < 0°C hydro-organic solutions'^. I. Lauks, et al

.

have used sputtered films for measurements in concentrated HF solutions and for

elevated temperature, acid titration experiments^ . R. I an i el 1 o and A.

Yacynych used urease enzyme immobilized on an iridium oxide substrate to monitor

urea concentration; changes in pH, caused by the enzyme-catalyzed hydrolysis of

urea near the electrode surface, yield a linear potentiometric response with

logarithmic variation of urea concentration^.

Iridium oxide electrodes are generally reported to respond in a linear,

Nernstian manner to pH. Super and sub-Nernsti an responses are reported, indicating that preparative technique is important51»52. Fog and Buck report a

Nernst slope at 25°C, pH 2-10 for a pressed powder electrode. They also report

2 mV accuracy, 25 mV of hysteresis (pH 2 - pH 12 - pH 2), the smallest redox

interference of all the platinum oxides and ionic interference from Cl", Brand i —30 - Lauks, Katsube, Yuen and Zemel report similar results for reactively sputtered films^?, except that they report recent improvements in

34
deposition technology have produced films with no response to bubbling 0 2 gas.

This apparent insensitivity to redox interference is discussed in more depth in

the final section on sputtered IrO x

.

The mechanism of the electrode response is unresolved in the current literature. The majority of the most recent literature on thick (>500°A) oxide films

agrees that some sort of internal processes in the oxide film and on the surface

are responsible for the characteristics observed by OCP, vol tammogram, and impedance measurements. The Ir/Ir02 couple is apparently only dominant for either

thin 1 ayers or large electrode poetential excursions. Most of the conjecture

centers around the nature of the hydration, identification of aqueous species

involved in exchange currents across the electrode solution interface, and the

importance of oxide morphology and stoich iometry.

The response mechanism is unresolved because: the electrode potential has

not been assigned to a known reaction; the nature of the hydration is poorly

characterized; and the species responsible for the exchange currents across the

electrode-electrolyte interface have not been positively identified. The measured OCP may not be due to one particular reaction, but may in fact represent a

mixed potential. The thermally prepared oxide according to K. Kinoshita,

et al .24 an d Ardizzone et al .53 has an E 0 (vs. SHE) very close to

either of the three reactions:

2Ir0 2 + 2H+ + 2e- = lr 2 0 3 + H

2 0

Ir0 2 + 4H+ + 4e~ = Ir + H 2 0

lr 2 0 3 + 6H+ = 2Ir + 3H2 0

At elevated temperatures, Kinoshita, et al report the absolute value of

the OCP to decrease faster than can be accounted for by either a Nernstian temperature dependence or calculated solution pH temperature dependence. They

tentatively suggest a change in the degree of oxide hydration with increasing

35
temperature as a possible explanation for the presumed shift in E 0

. The stability or degree of instability of the hydrated oxide will determine whether a

stable, reproducible electrode potential is possible.

The hydrous nature of iridium oxide thin films is largely responsible for

the unusual electrochemical characteristics and extraordinary corrosion stability of iridium electrodes. The unusually large charging capacity of iridium

oxide during voltage cycling is well documented^. The porous structure of

the hydrous film provides a large surface area for ionic exchange. Thick films

(> 50 nm) exhibit a charge capacity enhancement of two orders of magnitude above

bare iridium metal. The porosity of the films is confirmed by scanning electron

microscope micrographs.

Several models of the hydrous structure have been proposed to account for

the observed, intimately related, electronic and electrochemical film characteristics. Each model defines the amorphousness of the hydrated film differently.

S. Gottesfeld55 u sed electronic band diagram levels to model the five orders

of magnitude electronic conductivity shift between the el ectrochromic bleached

and colored states. The conductive, colored film is predominantly I r ( IV) and

the insulating bleached state is predominantly the reduced Ir(III) . 58 Electronic band models must be used with care in the discussion on non-crystal 1 ine

materials, but may be useful for elucidating equilibrium rate constants for

electron exchange processes across the oxide-electrolyte interface. To identify

the band structure, several optical and surface spectroscopic techniques might

be utilized. Ultraviolet photoelectron spectroscopy (UPS) is particularly useful for identifying the electronic structure close to the Fermi level involved

in exchange rates. No substantive data using UPS is available, so it is difficult to assess whether the broad density of states functions are resolvable by

36
and whether it will help to model electronic resistivity and exchange processes

at the solution interface.

Burke and Whelan^S, 57 proposed a cross-linked polymer skeleton as an

alternative to the implied extended electronic band strucutre of Gottefeld's

band model. They propose a mixture of linear and branched chains of hydrated

oxyhydroxides. Conductivity is modeled from one dimensional solids theory,

where the ability of electrons to hop from site to site in the reduced chain,

Ir(III), is much less than for I r ( IV) sites. This polymer structure has also

been proposed for metal oxides of molybdenum and iron. 67, 68

The third model assumes a polycrystalline structure with very small grain

size53,59. Hydration character is proposed to be different for grain boundary and intra-grain regions. This model is more fully developed for ruthenium

oxide films and is most plausible for thermally decomposed iridium cloride or

reactively sputtered iridium oxide films. Rispon and Gottesfeld using a.c.

impedance measurement techniques were able to resolve fast and slow charging

processes^. They hypothesize that the fast charging process occurs in the

double layer at the outer surface sites along the grain boundaries of the microcrystalline structure. The slower process is associated with the charging of

bulk grains. The rate of the slow charging can be enhanced by applied cathodic

potential and is thus likely related to a slower, more radical form of oxide

reduction.

The equilibrium of I r ( IV) and Ir(III) valence states is widely accepted, 65, 66 5 u t the mode or precise species involved in ion insertion into

the oxide film is not. Conflicting evidence for cation vs. anion insertion is

reported in the literature60»61 . [_i + ions have been inserted using nonaqueous el ectrolytes , but primary ionic interference during OCP pH measurement

is reported due to anions, Cl", Br

_ and 1-27,30. Other work by

37
Mozota and Conway62,64 indicates that a preferential adsorption of Cl"

during anodic cycling reduces oxide growth rates. The importance of understanding the insertion mechanism is to quantify a minimum expected ionic interference

and identify the dominant process of electrode pH response. If the insertion

mechanism involves cations, then observed cation interference is easily

expl icabl e.

Rispon and Gottesfeld59 propose a double injection process for ruthenium oxide which is equally valid for iridium:

IrOx

- n(H20) + ye" + yH+ = IrOx _y(OH)y - n(H20)

This particular form of the equation represents injection of a proton from solution and an electron from the ohmic contact. The equation does not represent a

redox process. The ion exchange at the electrolyte interface consists only of

either single or hydrated hydrogen ions. The double injection equation also

indicates how high concentrations of small cations would compete with hydrogen

ion injection. A similar equation can be written to model hydroxyl and anion

insertion. The equations of Fog and Buck discussed previously, indicate how

this can be done. Gilarum and Marshall^ report fast kinetics for both acid

and alkaline solutions but not for neutral pH. This observation suggests the

existence of two different injection mechanisms and perhaps a pH dependent oxide

structure as well. Further investigation is necessary to determine whether a

particular form of the oxide may prove useful for very high pH sensing. The

double injection model describes a possible injection/exchange process and also

includes the interaction of hydrous structure of the oxide with these processes

and solution pH.

Even though the complex electrode processes are not fully understood or

controllable, iridium oxide has provided accurate, stable pH measurement at room

temperature. It does have the characteri stic redox interference behavior of

38
conductive metal oxides, but the sputter deposition technology may reduce redox

interference even at elevated temperatures . A more significant limitation is

interference from anions, particularly Cl". Significant anion interference

above 0.01 molal concentrations limit the utility of this oxide. The key

unanswered question which impinges directly on the high temperature performance

is the stability of the hydration structure. Preliminary indications are that

the oxide changes with temperature, resulting in a nonNernstian temperature

dependence^, it is not known whether the sputtering technology which can

be utilized to deposit well-characterized, uniform oxide films of known

stoichiometry, valence state, morphology, and density will lead to a stable,

homogeneous, hydrated oxide in elevated temperature aqueous solutions.

39
VII. REACTIVELY SPUTTER-DEPOSITED IRIDIUM OXIDE

The method of oxide preparation strongly influences the observed electrochemical properties. The difference in reported properties of iridium oxides

obtained from different preparative techniques raises several interesting questions. The questions of prime technological importance are why does the d.c.

reactively sputtered film exhibit such apparently superior performance and what

preparative procedures produce superior films? One answer might be that anodic,

thermal and chemically deposited oxides of iridium are polycrystalline phases

whereas sputtered iridium oxide can be amorphous. Reactive sputtering is an

ideal preparation technique for an investigation to answer these two questions

because reactive sputtering offers substantial flexibility in tailoring film

characteri sties by process parameter variation^. The other techniques of

thermal and anodic preparation are not so flexible. The anodic technique is

further limited because samples so prepared are already hydrated. The electrochemical properties of reactively sputtered oxide films can be measured by traditional electrochemical techniques in solution but must be correlated with the

atomic and molecular structural information obtained in high vacuum by surface

science techniques. Information gained about the nature of the interaction of

the solution with the atomic structure of the oxide and its hydrated form will

indicate how sputtering conditions can be tailored to produce optimum thin films

for a variety of different situations. For example, the optimum film for high

pH sensing might be different than one developed for strong acid solutions.

Film optimization might entail variation in IrOx stoichiometry, film morphology or even doping with other platinum group metal oxides.

1 . Present Status of Sputtered IrO pH Sensing

Results published by I. Lauks and co-workers at the University of Pennsylvania establish reactively sputtered IrO x as a candidate material for pH

40
sensinq according to the criteria of Nernstian response, small or acceptable

ionic and redox i nterference, corrosion resistant and possible incorporation

into mechanically robust electrode structures. At room temperature to 100°C,

the following electrode characteri sties were observed .27,69

1. Nernstian response as described by the equation:

E = E 0 - [2.303 kT/q] pH

At 100°C, the measured slope is approximate! y 3% less than predicted by this

Nernst equation. Linearity was measured from pH3-12. E 0 is independent of

temperature.

2. Cloride ion interference less than +10mV for sodium concentrations

below 0.1M. The effect of dissolved Pb+

2, Fe+3, Fe+2, Cu+2, and

Ag + are less than 8 mV for concentrations less than 0.01 M.

3. Excellent reproducibil ity of the open circuit potential slope (OOP - pH

vs. E). However, the E 0 exhibits drift apparently due to hydration. The E 0

is reported stable after hydration is complete. Autoclaving the electrodes in

high temperature, aqueous solutions results in stable E 0 values; autoclaving

is thought to accelerate the hydration of anhydrous as-prepared sputter deposited films.

4. The reactive sputtering technology permits thin films to be deposited

onto a number of different substrates including metals, glasses and ceramics

such as A1 2 O 3

- This technology leads to the design of relatively simple

electrode structures that are robust, corrosion resistant and easily installed

through autoclave seals.

IrOx exhibits strong utility for <100°C aqueous pH measurements in dilute

aqueous solutions.

The performance of IrO x at higher temperatures has not been extensively

documented. Early studies^ subjected thin film samples to autoclaving in

41
deionized water at 160°C and 200°C for two hours. No degradation of room temperature pH response resulted later, from an in-situ, 156°C pH measurement.

800°A thick IrOx anodic films on ceramic alumina substrates accurately monitored the acid titration of 0.01 M KC1 from pH 9 to pH 3 with 0.01 M HC1. The

slope of OOP vs. [H+

] at 156°C agreed well with the Nernst slope at 156°C,

2.303 k(156°C/q = 85 mV/pH). The measurement of ionic and redox interference

,

long term stability and hysteresis at elevated temperatures was not addressed by

Lauks and co-workers. Madou's investigation of sputtered films^S indicated

that they responded to the solution redox potential above 150°C. He also experienced problems with film adhesion, due perhaps to the bubbling molecular oxygen. No such adhesion difficulties are reported by Lauks, et al. Either the

presence of high concentrations of bubbling oxygen is responsible for the adhesion/corrosion instability or the two groups sputtered different thin films of

iridium oxide. Film morphology and stoichiometry are sensitive to the deposition parameters.

The remainder of this report proposes a research approach to develop wellcharacteri zed, reactively sputter-deposited iridium oxide thin films for pH

sensing. The approach can be segmented into three progressive sections:

i. Establish a well controlled technology for reactively sputtering iridium oxide.

ii. Establish a relationship between sputtering conditions, film morphology

and electrochemical properties in solution,

iii. Analyze and understand the mechanisms responsible for el ecrochemical

behavior so that these films may be intelligently optimized for high

pH, elevated temperature, non-dilute, aqueous solutions.

A systematic investigation such as this is expected to provide a metal oxide

alternative to other materials such as the yttria stabilized zirconia.

42
2. Reactive Deposition of Iridium Oxide

The method of depositing the iridium oxide by reactive sputtering provides

substantial flexibility to tailor film morphology and composition. The characteristics of deposited films are very sensitive to process parameters during

deposition^* 77 . Characteristics vary with partial gas plasma pressures,

reactive to inert gas ratio, system configuration, substrate temperature, power,

target to substrate distance, target composition, and several variables of lesser importance such as power frequency in an a.c. plasma. 7 ^ The ability to

reproducibly deposit well controlled films with identical characteri sties is

very important to ensure consistency in the analysis. The analysis is a comparison of properties as measured by a number of different techniques, each often

requiring a different sample form.

Since the deposition of iridium oxide is done reactively, the chemistry of

the reactive gas, O 2 , on the substrate, the target, and in the plasma largely

determines the characteristics of the deposited film. The critical process control parameters determining the chemistry are the partial pressures of O 2 , the

temperature, and rate of deposition. The relationships between adsorption and

reaction kinetics on the surface are certainly complex and do not need to be

directly addressed (others have approached this by attempting to sample the

plasma at the substrate surface7 ^). However, the kinetics can be empirically controlled by regulating substrate temperature, substrate positioning, power,

and gas flow into the sputtering chamber.

The pH sensing iridium oxide films prepared by Lauks and coworkers at the

University of Pennsylvania were sputtered at low power densities and for periods

of time measured in hours to produce films of 100 nm thickness^. Other

workers report similar sputtering times 7 ^. They also report significant

sample variation with 10% changes in gas mixtures and power densities. Th^

43
proposed work will fabricate films in a wide range of sputtering conditions

using a planar magnetron in both d.c. and a.c. modes. Morphological, electrochemical and surface analysis measurements are expected to provide information

detailing why certain sputtering conditions produce el ectrochemical ly useful

films and other conditions do not. Such information will prove useful to others

involved in reactively sputtering other platinum group, metal oxides as well as

iridium oxide. At present, very little published work exists on reactively

sputtered platinum group metal oxides which are potentially important not only

for numerous types of solid state sensors but also in the production of catalytic films and solid state electronic structures.

3. Film Characterization

The purpose of these measurements is to establish the relationship between

materials properties and electrochemical properties, particularly as they relate

to pH sensing. The measurements proposed for the iridium oxide thin films fall

into two distinct classes. The first set of measurements is designed to characterize the materials properties of the films: density (specific gravity),

resistivity, morphology and stoichiometry. The second set of measurements is

aimed at defining the electrochemical and surface chemical properties.

An index of each as-prepared sputtered film will be obtained by measurement

of resistivity, density, degree of crystallinity, stoichiometry and morpholoqy.

Resistivity is calculated from four point probe and film thickness measurements.

Resistivity of an unhydrated, dry film indicates the electronic characteristics

.

Thickness, along with area and weight, define density; a discussion on the

expansion and porosity of hydrated films has been published^. Scanning

electron microscopy (SEM), transmission electron microscopy (TEM), and optical

microscopy are standard tools for defining morphology and will be used in this

study. X-ray diffraction results are expected to indicate both the degree of

44
crystallinity and stoichiometry. If the films are amorphous and x-ray

diffraction results are not very informative about either stoichiometry or

molecular composition, then a combination of XPS, SIMS, and SEM microprobe

measurements might yield the desired quantitative information. Implementation

of XPS and SIMS techniques is discussed further below.

Once a film has been properly characteri zed according to its density,

resistivity, morphology and stoichiometry, conclusions concerning sensing mechanisms might be drawn with more confidence from electrochemical and surface measurements. The proposed set of measurements are designed to clearly identify

the electrochemical mechanisms defining the response of the films to pH and to

other interfering redox or ionic species. The measurements fall into two traditionally separate disciplines, those of the electrochemist and those of the surface scientist. The correlation of the results of these two different approaches is expected to be far more definitive with respect to mechanism than either

measurement approach by itself. Recommendation for this sort of correlation has

been expressed in several papers^, 76 and measurements have been initiated

on the IrC>2 system by two research groups based in Switzer! and71 >78

.

The three electrochemical measurements proposed are open circuit potential

(OCP), vol tammograms and complex plane impedance plots of the electrode in solution. OCP is most likely the mode in which these films will ultimately be used

to measure pH. Measurements of electrochemical potential versus temperature

indicate the range of Nernstian operation. OCP as a function of different

solution compositions shows the extent of ionic interferences . Voltammograms

and impedance plots are intended primarily to elucidate electrode mechanisms,

kinetics at the interface and bulk transport through the film. The impedance

plots are a new technique as applied to iridium oxide. However, substantial

voltammogram studies of iridium oxide, formed primarily by anodic techniques.

45
have already been done. Results have been interpreted to define ion

insertion-extraction^! , redox state changes?!, and electronic band

structure^. it has been postulated that the superior pH performance of

sputtered films over anodically deposited films is due to valence band structure

differences associated with the amorphous nature of the sputtered

f i lms69>??.

No published work explores the pH response or electrochemical characteristics of iridium oxide films as a function of reactive sputtering conditions.

Efforts have been directed at the electrocatalytic^S an d electrochromic^S behavior of iridium oxide. Films prepared anodically have been reported to have poor characteristics for pH sensing35,52 . Anodic preparative

techniques fall short of the flexibility inherent in reactive sputtering. The

sputtered films can be prepared in an anhydrous or hydrated form depending on

whether water vapor is introduced into the sputtering chamber. Also, as

detailed in a previous section, morphology, stoichiometry, etc. are very sensitive to sputtering process parameters. If small changes in physical characteristics can be correlated to changes in surface electrochemistry and molecularelectronic structure, then the sputtering technology itself becomes an incisive

tool in deconvoluting the simultaneous processes determining iridium oxide

el ectrochemi stry.

The three high vacuum spectroscopy measurements proposed are x-ray photoemission spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), and ultraviolet photoemission spectroscopy (UPS). The nature of the high vacuum measurement environment makes these techniques static measurements of a chemical state.

Kinetic information must be obtained from already described electrochemical

methods which are carried out in solution. However, knowledge of the bonding

configurations, extent and character of hydration, complexing and diffusion of

46
interfering species such as ionic iron, and adsorption of water and molecular

oxygen are valuable in understanding the chemistry of an iridium oxide surface

in solution.

XPS and SIMS are complementary measurements . Identification of fragments

observed by SIMS provides confidence for deconvol uting XPS results. SIMS provides identification according to mass number of different species, e.g. H

2 O,

OH, IrO, etc; but strong matrix effects have made quantitative efforts of dubious value. However, knowledge of existing species makes deconvolution of XPS

data much more reliable and increases confidence in the quantitative analysis of

bonding in hydrous and anhydrous sputtered films. Also, since the methods differ both in excitation source and detected emission, measurement distortions

inherent in each technique due to sensitivity variation, matrix effects and surface vs. bulk property variations, should be quantified more confidently with a

comparison of measurement results.

The ultrahigh vacuum measurement system that possesses both SIMS and XPS

capabilities contains two chambers. One is for sample preparation at atmospheric to 10-10 torr pressures and the second chamber is reserved for ultrahigh

vacuum surface measurements. The system is also equipped for thermal desorption

experiments which are used for studying the adsorption sites and interaction

energy of gases onto surfaces. Sorption-desorption measurements of gases and

water vapor onto iridium and iridium oxide should well be very informative.

Fortunately, some clean metal surface science work on iridium has already been

reported in the literature79,80,81 . This serves to simplify and shorten

the proposed task, since these results will aid in the interpretation of measurements made on the more complex, intrinsically disordered amorphous materials. The primary thrust of the proposed research is the study of the alteration of the amorphous oxide surface by the solution. The alteration of bulk

47
properties is harder to access with these techniques because any attempt to

remove surface layers, by for instance, ion bombardment during SIMS, is almost

certain to alter bulk characteri sties

.

XPS has very recently been used to study oxidation state changes and hydration of anodically prepared iridium oxide films as a function of solution

ph|23. Samples were prepared by anodically oxidizing iridium electrodes in

different pH solutions. The authors then used XPS to study oxygen bonding to

iridium. They correlated oxygen's core level energy shifts with voltammogram

structure to propose a reaction pathway for oxide valence changes and hydration.

Their results answered only the questions of surface properties and could not

address bulk valence changes or bulk hydration. In particular, the bulk OH

insertion mechanism proposed by Beni et. al .61 could not be addressed.

Depth profiling measurements, such as those possible using SIMS, would be particularly germaine for investigating differences in surface and bulk composition.

As already mentioned, any conclusion about bulk properties using this technique

must be reached cautiously.

SIMS may also be a very useful approach for studying ion and redox interference. SIMS could be better suited than XPS for identifying trace metals that

may have diffused into the iridium oxide matrix. The types of fragments

observed by SIMS or alternately the bonding character indicated by XPS would

help to define the nature of ionic and redox interference. It may prove possible to optimize the film morphology or stoichiometry to minimize ionic and redox

interference.

Of the three UHV spectroscopic techniques, UPS will probably provide the

most definitive information towards identifying reasons for differences in sputtered oxide film electrochemical performance. XPS measurements provide information about oxidation states, molecular coordination and atomic bonding through

48
core level electronic shifts. The UPS technique directly probes the valence

electrons and electronic density of states near the Fermi level. The density of

states near the Fermi level determnes the nature of the electronic exchange currents and ionic adsorption ohenomena at electrode surfaces^. Metals, semiconductors, and insulators exhibit characteristically different behavior as a

result. The two primary questions of iridium oxide are: "What is the relationship of electronic structure to surface aqueous el ectrochemi stry" ; and "What is

the origin of surface electronic structure with respect to surface and bulk

solid state structure, e.g. surface states, oxygen vacancies, hydration,

etc...". Fortunately, recent data on rutile iridium oxide band structure using

high resolution synchrotron radiation techniques^ will provide a reference

point for the proposed UPS measurements on amorphous films.

Once the three phases of measurement - physical, electrochemical, and surface are completed, the connection between the physical characteristics of the

sputtered film and its electrochemical performance should be evident. The knowledge of how structure, composition, and electronic band structure relate to

performance will enable these films to be tailored for different pH measuring

environments. This tailoring may only involve iridium oxide, but more than

likely mixed platinum group metal oxides will be used to produce pH sensors and

pH based sensors for a wide range of measurement needs and environments.

49
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55
NBS-U4A (rev. 2 -ec) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publication Date

SHEET (See instructions) NBSIR 85-3237 SEPTEMBER 1985

4. TITLE AND SUBTITLE

Review of Materials for pH Sensing for Nuclear Waste Containment

5. AUTHOR(S)

Timothy Dietz and Kenneth G. Kreider

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7. Con tract/ Grant No. national bureau of standards

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

Nuclear Regulatory Commission

Office of Research

Washington, DC 20555

10.

SUPPLEMENTARY NOTES

{

1 Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a si gnificant

bibliography or literature survey, mention it here) This report defines the performance criteria of the needed pH sensors and reviews

the performance of a number of elevated temperature pH sensing technologies with

respect to these criteria. The criteria of electrode performance were developed

to predict the utility of various pH electrodes in these simulated environments.

The classes of pH electrodes reviewed are the glass electrode, yttria stabilized

zirconia, palladium hydride and a variety of metal oxides. The report focus on

a relatively new solid state electrode material, reactively sputter deposited

iridium oxide. The performance of this thin film material is of particular

interest because its low electrical resistivity and high corrosion resistance

eliminate some of the shortcomings of the glass and ceramic materials. The

reactive sputtering technology permits these films to be deposited and pattern

defined on a wide variety of substrate materials. Low electrical resistivity,

which simplifies electrical contacts, and a flexible deposition technology make

this material a prime candidate for micro pH sensors.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) Chemical sensors; corrosion sensors, iridium oxide; pH sensors.

13. AVAILABILITY

|y ) Unlimited

| | For Official Distribution. Do Not Release to NTIS

n Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

61

15. Price

$ 10.00

USCOMM-OC 8043-P80