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NIST

PUBLICATIONS

NBSIR 88-3790

Electrochemical Evaluation of

Solid State pH Sensors for

Nuclear Waste Containment

Peter H. Huang

Kenneth G. Kreider

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Chemical Engineering

Chemical Process Metrology Division

Gaithersburg, MD 20899

May 1988

CnAnrArOir| by:

nr

Regulatory Commission

100 Research

U 56 ton, DC 20555

# 88-3790

1988

c.2

75 Years Stimulating America's Progress 1913-1988
NATIONAL INSTITUTE OF STANDARDS &

TECHNOLOGY

Research Information Center

Gaithersburg, MD 20899
NBSIR 88-3790 no.

m2

C. 7- ELECTROCHEMICAL EVALUATION OF

SOLID STATE pH SENSORS FOR

NUCLEAR WASTE CONTAINMENT

Peter H. Huang

Kenneth G. Kreider

US. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Chemical Engineering

Chemical Process Metrology Division

Gaithersburg, MD 20899

May 1988

Sponsored by:

Nuclear Regulatory Commission

Office of Research

Washington, DC 20555

U.S. DEPARTMENT OF COMMERCE, C. William Verity, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Review of

Electrochemical Evaluation of Solid State pH Sensors

for Nuclear Waste Containment

Contents

Page

I . Introduction . ............ 1

II. Theoretical Models of the Oxide-Liquid Interfaces 4

A. The Electrical Double Layer 4

B. Modification of Double Layer Model 8

III. Electrochemistry of Ir02 Films 11

A. Formation and Properties of Anodic Ir02 Films 11

B. Electrochemical Behavior 14

IV. pH Testing of Sputtered Iridium Oxide Films (SIROF) 16

A. Ionic Sensor Principles 16

B. pH and its Measurement 18

C. Testing to Insure Suitability for Geochemical Metrology 21

V . Summary . 23

References 27
np«>: I

'
I . Introduction

The National Bureau of Standards (NBS) has reviewed technical reports on

the high level waste package for nuclear waste storage. The Department of

Energy (DOE) waste package test data have been evaluated and compiled by the

NBS for the Nuclear Regulatory Commission (NRC) [1]. It is clear from these

reports that pH is one of the most important parameters affecting the

degradation of the waste packages. Accurate and reliable pH measurement is

critical to the satisfactory use of a geochemical code to model the path of

reaction between nuclear waste materials, alteration products, and various

aqueous solutions. The information obtained for the reaction of nuclear waste

materials with aqueous solutions is essential for understanding and predicting

the behavior of these materials under repository conditions. The NBS has also

reviewed the performance of a number of elevated temperature pH sensing

technologies for the NRC [2]. Measurement of pH in a geologic repository

requires the use of pH sensor in high pressure and elevated temperature

corrosive brines. Recent work of Hodges and Pederson [3] at Pacific Northwest

Laboratory (PNL) indicated the chemical composition of brines interacting with

a high-level nuclear waste package in salt can affect brine pH produced at

high temperatures. However, the hydrogen ion activity of the brine is

difficult to measure directly, especially at high temperatures and pressures.

The hydrogen ion activity is related to the hydrogen ion concentration through

the activity coefficient, the value of which depends on the temperature, the

pressure and the composition of the solution of interest. The value of the

hydrogen ion activity, therefore the pH, can be determined by the direct

measurement of the hydrogen ion concentration in brines, provided the activity

coefficient is known.

The instability of the conventional reference electrodes such as calomel

or silver/silver chloride electrode is also a problem when measuring pH at

temperatures above 100°C. The contamination of the internal reference

solution is also a problem if the back pressure becomes greater than the

internal pressure. Danielson reported [4] a flowing type of reference

electrode, which may be used at high temperatures and pressures.

1
In a recent report to the Office of Research, Nuclear Regulatory

Commission the National Bureau of Standards presented a "Review of Materials

for pH Sensing for Nuclear Waste Containment" [2] (NBSIR 85-3237). The review

was based on the assumption that higher temperature pH sensors would be needed

to evaluate the corrosion potential of containment vessels. The authors

described the criteria for pH electrode evaluation, the performance of glass

pH electrodes, the developmental program on cubic zirconia electrodes and a

summary of research studies of fourteen other exploratory electrode materials.

A special emphasis was placed on the most promising of the high temperature

electrode materials - Ir02

.

The criteria for selection of a pH electrode for use at temperatures up

to 250 °C were: linear response from pH 1-14, low ionic and redox

interference, long term stability, corrosion resistance, and mechanical

robustness. The review of the applicability of the glass electrode concluded

that although the recent work at the Pacific Northwest Laboratories (PNL) has

indicated useful corrosion resistance in the 150-200°C range, the glass

electrode has serious stability problems above 150°C. Other problems with the

glass electrode relate to its high impedance, Si02 hydrolysis at high pH

levels, and lack of mechanical robustness.

Yttria stabilized zirconia pH sensors have been studied by Niedrach [5],

at General Electric, Tsuruta and Macdonald [6] at SRI, and Danielson at PNL.

Generally these Zr02 (Y) sensors are linear between pH 3-9 and at temperatures

above 150°C to 300°C, and exhibit few interference problems if any. The

inconvenience of poor room temperature performance, however, complicates the

calibration and flexibility of the sensor. The results of research on

palladium hydride and numerous metal oxide films for use as pH electrodes are

also discussed in the NBS/NRC review including Sb2 0 3 , Mo02 , Mn02 , Ti02 , Ta2 0 5 , Zr02 , 0s02 , PdO, Pt02 , Rh02 , Ru02 , and Ir02

. Of these, the Ir02 emerges as

the most promising primarily because of its wide range of stability (both pH

and T) , its low impedance in thin films and its consistent Nernstian- linear

response

.

The remainder of the review by Dietz and Kreider outlines the research

2
findings in Ir02 pH sensor studies, the key questions to be answered and a

suggested approach for developing the reactively sputtered Ir02 film as a high

temperature, harsh environment sensor.

In order to develop the pH electrode for use at high temperatures and

pressures and to understand the interaction between the electrode and the

solution, quantitative information about the electrochemical behavior at the

oxide-solution interface is essential. One of the fundamental factors

determining all electrochemical phenomena occurring at the solid- liquid

interfaces is the electric charge and potential distribution in these regions.

The simplest model reflecting the real electrical structure of the interface

region is the electrical double layer (e.d.l) model. An electrical double

layer occurs at all interfaces and its structure depends on the properties of

the contacting phases. Knowledge of the structure of the electrical double

layer and the charge and potential distribution in the layer is very important

because it will greatly influence or determine such pH electrode phenomena as

hydrogen ion sensitivity and selectivity, open circuit potentials, electrode

stability and others.

Therefore, improved understanding of the electrochemical behavior at the

Ir02

- solution interface is needed to optimize the performance of Ir02 electrodes for pH sensing at high temperatures and pressures.

Several workers have attempted to quantitatively describe the charge and

potential distribution at the oxide- liquid interface using various theoretical

models. These models are described in Section II.

Hydrous oxides are of major interest in the areas of technology such as

aqueous battery systems, colloid chemistry, electrochromic display systems and

pH sensing. Hydrous iridium oxide films are of special interest since iridium

oxide has been shown to have good chemical stability in aqueous solutions at

temperatures up to 200°C [7]. In Section III of this report, the

electrochemical preparation, redox behavior, and some important properties of

hydrous iridium oxide films are described.

3
Section IV describes pH testing of sputtered iridium oxide films (SIROF)

.

The general principles of ionic sensing are briefly described, followed by a

discussion of pH measurement methods. The scope of testing for geochemical

metrology is then described for geochemical measurements.

A summary of this report is presented in Section V.

II . Theoretical Models of the Oxide-Liquid Interfaces

This section reviews theoretical models used to describe the e.d.l for

colloidal metal oxide systems. The surface charge and potential of an

insoluble metal oxide is determined in large part by the pH of the solution in

which it is immersed. For such systems the OH' and H

+ ions are potentialdetermining ions as a result of the following reactions

H

+ OH' — M -OhJ< M—OH ^ M—O' + H2 0 ( 1

)

where M represents metal. The presence of hydroxyl groups on metal oxide

surfaces has been amply demonstrated by infrared spectroscopy. Theoretical

models useful for describing the electrochemical behavior of a colloidal

system may not be strictly applicable to iridium oxide, which has a metal- like

conductivity. Based on the more recent studies of metal- liquid and

semiconductor- liquid interfaces, however, useful information may be obtained

to characterize electrochemical properties of iridium oxide- liquid systems

within the framework of the theoretical models described in this section.

A. The Electrical Double Laver

In this section, no attempt is made to cover all studies of the

oxide- liquid interface. Recent work is reviewed to obtain the important facts

that are relevant to the nature of the interface. A considerable amount of

work in this area has also been reviewed by Lyklema [8,9], Ahmed [10], Hunter

[11], and Parsons [12].

The double layer model is the classical approach to describe the

electrical structure of the solid- liquid interface and is usually referred as

4
the Gouy- Chapman- S tern- Grahame (GCSG) model. In this model, the electrical

double layer consists of two regions: an outer diffuse region and an inner

Helmholtz region. In the outer diffuse region, the ions undergo thermal

Brownian motion and are distributed according to the Boltzmann statistics.

The inner Helmholtz region has two planes. The inner Helmholtz plane (IHP) is

the plane of the centers of adsorbed ions near the solid surface. The outer

Helmholtz plan (OHP) is the plane of the centers of hydrated ions near the

diffuse region.

The mean electrical potential at any point in the diffuse layer

satisfies the well-known Poisson-Boltzmann equation

V

2 ^ = - (e/ee Q ) £ z^exp (-z^/kT) (2)

where <f> is the mean potential, nd is the number of ions of type i per unit

volume in the liquid phase, z i is their valency with sign, e is the unit of

electric charge, e is the dielectric constant of the medium, e Q is the

permittivity of free space (8.854 x 10

“ 12 Fm" 1

, k is the Boltzmann's constant

and T is the absolute temperature. For a flat interface, Eq . (2) is solved

with the boundary conditions <f>=<f> d at x=0 , where </> d and x are the potential and

the coordinate, respectively, at the OHP and 0=d0/dx=O at x=c°. The solution

is given by

$ = (2kT/ze) In [1+ 7 exp ( -ax) ] / [

-7 exp(-ax)] (3)

where 7 = exp [ (ze0d /kT) - 1 ] /exp [ (ze<^ d /kT)+l ] , and a = 2z 2 e

2 ni /e e Q kT ) 12 = 3.29

x 10 9 z(c ) 1/2 rtf

1 at 298K, and c is the bulk electrolyte concentration in mol

dm” 3

.

The diffuse layer charge density, ad , contained in a cylinder of

unit cross-sectional area extending from x=0 to x=® is given by

od = -

(

8 nd e

Q kT )

1 1

sinh (ze0d /2kT)

5
( 4 ) = -11.74 (c) 1/2 sinh (19.46 z<f> d ) /iC cm 2

at 298 K, where <f> d is in volts.

The surface charge density of adsorbed ions at IHP is given by the

Stern adsorption isotherm

z + eNa

1 + (A0 /n+ M)exp[ (z+e<f>

p

+ $+)/kT

1 + (A0 /n_M)exp[ (z_ e<f> 0

+ ^_)/kT

where z± is the valency of cations or anions with sign, Na is the number of

available adsorption sites, A0 is Avogadro's number, is the bulk

concentration of the ionic species in ions/cm3

, and M is the relative

molecular mass of the solvent. <£± are specific adsorption potentials that

account for non-Coulombic interactions, such as changes in the state of

solvation of the adsorbed ions, van der Waals interactions, or hydrogen

bonding between the adsorbate and the surface. When these non-Coulombic

interactions contribute significantly to the free energy of adsorption of

ions, the adsorption is said to be specific [13]. A specifically adsorbed ion

can therefore adsorb against electrostatic repulsion such that a

d in Eq. (5)

is greater than a0 , the surface charge density at the solid surface. This

leads to a reversal of the signs of the diffuse layer charge and potential.

This possibility is of great importance for the case of adsorption of H

+ or

OH" ions which are chemically bound.

Because of electroneutrality, the charge densities at the surface,

the IHP and the OHP are related by

+ °0 + ad = 0 ( 6 >

If the potential drop between each plane is _ssumed to be linear, the integral

capacity per unit area, C : , of the zone between the surface and the IHP is

given by

6
Cl = a0 /(<f> 0

-4> p

) = e

1 e 0 /f3 (7)

where e, and are the dielectric constant and separation between the surface

and the IHP, respectively. The integral capacity per unit area between the

IHP and the OHP, C2 , is given by

c 2 = -°r d/(^“^d) = € 2

e o/d W

where e 2 and d are the dielectric constant and separation between the IHP and

the OHP, respectively. Hence the five equations (4) to (8) relate to six

variables a0 , a

^

, 4> Q , <f>p , and <f> d of the GCSG model of the electrical double

layer. Thus, if one of these six variables is known then the others may be

computed provided that the values for the parameters Na , <3? ± , C 1 , and C2 are

also known.

The total double layer differential capacity, d<7 0 /d^0 , can be

obtained by combining Eqs . (7) and (8) to give <j> Q ,

4*

= <7 0 /Ci - ad /C2 + 4> d (9)

and then differentiating with respect to oQ

d^0 /doQ = (1/C, ) - ( 1/C2 ) (dcrd /da0 ) + (d<j> d /dad ) (dad /da0 ) (10)

where C, and C2 are assumed to be independent of aQ . The diffuse layer

capacitance, Cd = -(dad /d^d ), is obtained from Eq . (4) and (dcr d /dcrQ ) may be

expressed in terms of a

0 by using Eq . (6). Therefore the total differential

capacitance, C, is given by

C’ 1 = d40 /da0 = (1/C,) + (1+ d&p/da0 ) (1/C2 + 1/Cd ) (11)

Calculated capacitance may then be compared with the experimental differential

capacitance

7
In the simple GCSG model described above, the surface charge

densities are assumed to be uniformly distributed in each plane. The effect

of discreteness of charge has been introduced by Levine, Bell and co-workers

[14,15] to account for various phenomena, e.g. adsorption at the mercury-water

interface

.

Blok and de Bruyn [16] were the first to describe a GCSG-based model

for an oxide- liquid interface. They assumed ideal Nernstian behavior for the

surface potential and compared calculated aQ -<f> Q curves for different values of

the parameters with their experimental oQ -pH curves for ZnO electrolyte

solutions. Using the following parameter values: C 1 =C2 =100 /iF cm" 2 for Eqs

.

(7) and (8); $+ = -2kT, = -4kT, and Na = 10 15 cm" 2 for Eq. (5), good

agreement between the theoretical and experimental results was obtained.

Later, Levine and Smith extended this basic model to allow for nonNernstian behavior of the surface potential. They obtained calculated a0 -pH,

C-pH, and <f> d -pH curves in reasonable agreement with published experimental

data. The double layer properties Sn02 , Si02 , and quartz were investigated

by Ahmed et al . [17], Tadros et al. [18], and Li et al . [19] through the use

of cr-pH curves. The curves were convex for these oxides. In fact, for all

oxides studied so far [20-26] including Ti02 , 7-Al2 03 , a-Fe2 03 , and Mn02 , the

slope 5cr 0 /3pH increases with pH without any indication of reaching a plateau.

This phenomenon has been explained by assuming that oxide surfaces are

somewhat penetrable for H

+

, OH", and counterions [27]. The consequence is

that aQ is not a surface charge but rather a volume charge. Healy et al. [22]

have noted that oxides generally exhibit very high charge density values

attributed by the charge-determining H

+ or OH" adsorbed ions.

The GCSG model in general can account for observed interfacial properties

provided certain values are assigned for various parameters (e.g., specific

adsorption potential, inner layer surface charge density). However, it has

been pointed out that it is not at all certain that the parameter values

assigned are reasonable [22]

.

B

.

Modification of Double Layer Model

8
In order to explain the phenomena associated with the very high surface

charge densities observed at oxide-solution interfaces as mentioned before,

various nonclassical models have been proposed. These models include the

porous double layer model [27-29], the gel layer [30-32], the transition layer

model [33,34], and the site-binding model [35-37].

(a) Porous double layer model [ 27-29 ]- -This model assumes that the oxide

surface is porous to charge -determining ions and counterions. This approach

accounts at least qualitatively for the general trends observed in interfacial

properties. The surface charge is substantially higher in solutions of

certain ions that can penetrate the solid than in solutions of other ions.

For example, in 10“ 1 M LiCl at pH 10, on hematite, a0 is almost twice as high

as in 10“ 1 M KC1, apparently because Li+ can penetrate into the solid, thereby

occupying Fe+3 -sites. Penetration distances of less than 1 nm are usually

needed to account for the observed high charges [27]. The theory indicates

that high valence charges require much counterion compensation, therefore the

charge remaining in the diffuse double layer part is comparatively small.

(b) Gel layer model [ 30- 32 ] - -This model assumes that there is a

homogeneous gel- like layer of definite thickness (2-5 nm) at the interface

containing metal hydroxy groups M(0H)n into which various ions can adsorb.

This model accounts for the high surface charge in terms of ion penetration

into subsurface regions of the oxide.

(c) Transition layer model [ 33 , 34 ] - -This model combines the basic ideas

of porous and hydrolyzed gel- like layer theories for the oxide-solution

interface. The essential feature of this model is the existence of a transition layer of atomic dimensions in which an equilibrium between oxide

and aqueous ionic species is established.

(d) Site-binding model [ 35 - 37 ] - -This model assumes that the adsorbed

electrolyte cations and anions are distributed in two ways: first, as

interfacial ion pairs formed with oppositely charged surface groups, and

second, in a diffuse layer to balance the remaining unneutralized surface

9
charges. Potential-determining ions and counterions are considered to react

or bind with the acid-base sites on the oxide surface. This model is

generally adequate for accounting for the shapes of oQ -pH curves and specific

ion effects [38]. In this model the high surface charge is rationalized in

terms of ion-pair formation between surface ionized groups and counterions.

Yates et al. [36] assumed that a porous hydrated oxide surface layer at the

interface is penetrated by supporting electrolyte ions.

The site-binding model was recently applied to study the temperature

coefficient of the metal oxide-liquid interfacial potential as a function of

pH for ion-sensitive field-effect transitors (ISFET's) [39], At

electrochemical equilibrium, the electrochemical potentials for a metal oxideliquid system are given by

Mm 0 H 2

+ k-^ni'/ M0H2 +

=

Mm oh Mh *" kTln i^moh kTlna^+ and (12)

Mm oh + kTlni/M0H + q<l> = Mmo

- + Mh + + kTlni/M0 - + kTlnaH (13)

where M represents metal, is the standard chemical potential for the x's

species, ux is the activity of the x's species at the surface, k is the

Boltzmann constant, q is the electronic charge, and <i> is the potential

difference developed at the interface and is given by

^ = 1 ( ^mo h 2

~

1/ mo-)/(-'h (14-)

where CH is the capacitance per unit area across the electrical double layer.

Eq. (14) is valid if we neglect space charges in the diffuse layer.

It can be shown that the surface potential, the number of active surface

sit-'.s, the pH, and the temperature are related by the following equation

aH + = {Kj rj + [iqV + 4(l-r, 2 )K1 K2 ] 1/2

} exp(q<^/KT)/2 ( 1-r;) (15)

10
where aH

+ is the activity of H

+

, Kj. and K2 are the dissociation constants

given by

Kx = exp [/^moh 2

” ^moh * 4h + ) /^T ] (16)

K2 = exp [ Mmo h

~ 4m o

" 4h + ) /l- l ] (17)

and rj is given by

V = 0CH /qNs (18)

in which Ns is the density of available active sites per unit area at the

metal oxide surface. Eq. (15) was used to determine the temperature

characteristics of silicon nitride pH-ISFETs for various pH values. Good

agreement was obtained between theory and experiment [39],

The site-binding model has been extended to a two-site theory [40] for

explaining the features of the potential-pH response of both silicon nitride

and borosilicate glass ISFETs. The theory was used to interpret the behavior

of chemically treated gate materials for Si02 and Ta2 0 5 ISFETs as pH sensing

devices [41 ]

III. Electrochemistry of Ir0 2 Films

A. Formation and Properties of Anodic Ir02 Films

Burke and Lyons [42] have recently surveyed both the theoretical and

the experimental work that have been done on hydrous oxide films on several

noble metals including platinum, palladium, gold, iridium, rhodium, and

ruthenium. This section is concerned only with the electrochemical formation

and redox properties of the hydrous Ir02 films.

One of the most convenient techniques used to generate hydrous

oxides in a form suitable for studying their redox behavior is that of

potential cycling [43-45]. In this case the potential of the parent metal

electrode, which may be noble or non-noble, is cycled repetitively between

suitable limits in an aqueous solution of appropriate pH. A sinusoidal,

11
square, or triangular potential wave form may be used for hydrous oxide film

growth. The triangular wave is most commonly implemented because cyclic

voltammetry can then be used to monitor the growth and redox behavior of the

iridium oxide film.

The mechanism of hydrous oxide growth on repetitive cycling is now

reasonably well understood, at least at a qualitative level. For most metals,

but especially gold, platinum, iridium, and rhodium, extension of oxide growth

beyond the monolayer level under conventional galvanostatic or potentiostatic

conditions is usually quite slow. Under potential cycling conditions the

anodic limit plays quite a crucial role for anodic oxide film formation. The

early work on the initial stages of anodic oxide formation on Ir was described

by Belanger et al . [46]. Thick oxide films were readily generated on cycling

the potential over the normal potential range of 0-1.5 V. It is now generally

accepted that cycling the potential of Ir in a wide variety of electrolytes

between certain limits induces irreversible formation of a battery- type

,

porous, hydrous oxide layer [37] whose thickness increases with increasing

number of cycles.

On the other hand, the monolayer oxide growth on Ir has been investigated

by several researchers. Rand and Woods (44) used a solution of 5 mol- dm" 3

H2 SOa , in which the hydrous material is soluble, and observed hydrogen peaks

on the anodic sweep at 0.09 and 0.25 V, with oxygen adsorption commencing

above 0.4 V. Two very broad peaks were observed in the oxygen adsorption

region at 0.7 and 1.0 V, respectively. Similar behavior was also reported by

Mozota and Conway [47]. They assumed that a monolayer of OH species was

formed in the region of 1.1 -1.2 V, conversion to an IrO monolayer being

completed at 1.4 V.

The conditions required to produce a thick hydrous oxide layer on iridium

under potential cycling conditions were investigated by Buckley et al . [48]

and Conway and Mozota [49]. The optimum upper and lower limits in acidic

media were found to be 1.60 and 0.01 V, respectively. Oxide growth on cycling

was observed with the upper limit just above 1.4 V. Significant penetration

of oxygen into the second, or even the third, layer of metal atoms of the bulk

12
lattice is probable. Increasing the upper limits above 1.6 V causes

dissolution of oxide species at the interface. Critical values for both upper

and lower limits required for oxide growth on cycling are influenced by a

range of kinetically and thermodynamically important factors, e.g., sweep

rate, solution composition, temperature, degree of oxygen penetration, and

oxide stability. Among other factors which affect the hydrous oxide growth

reaction are sulfuric acid concentration, pH, the presence of chloride ions

and temperature [43,49-51]. The presence of chloride ion in solution is

assumed to inhibit the formation of an anodic oxide film on a clean iridium

surface [43]. This effect can be explained in terms of specific adsorption of

chloride ions onto the iridium surface in aqueous solution. However, the

oxide layer region of the cyclic voltammogram of iridium was unaffected by the

addition of chloride [43]. The rate of growth of iridium oxide films depends

on the type of and concentration of supporting electrolyte. It decreases in

the order 0 . 5M H2 S04 > 1M HC10A » 0 . 5M Na2 C03 » 0 . 5M NaOH [49]. This order

follows qualitatively the order of decreasing specific adsorption of the

anions and presumably arises from the effects of these ions on the continuing

monolayer formation and reduction that takes place on the iridium substrate

beneath the growing hydrous oxide film. Cruz et al. has discussed the effect

of temperature on the potentiodynamic behavior of iridium in 0.5 M H2 S0 A

. It

may be assumed that increasing the temperature modifies the structure and

composition (degree of hydration) of the highly hydrated oxide layer formed

under continuous potential cycling [51]. This modification can then change

the physical and electrochemical characteristics of the surface oxide layers

on iridium.

Rand and Woods [44] postulated that cycling an iridium electrode in acid

resulted in irreversible oxide formation at the metal surface. However, they

assumed the material to be a porous form of the anhydrous oxide, Ir02 , attributing the density value of 11.68 g/cm3 of the latter to the surface

layer. Buckley and Burke [43,48] claimed that the layer formed was a hydrated

oxide or oxyhydroxide , such as Ir02 -2H2 0. They used a redox transition

between the Ir(III) and Ir(IV) states in the surface layer to explain the main

charge storage reaction:

13
Ir02 + H (19) + + e~ = IrOOH

Mitchell et al. [52] showed that the film formed on cycling in acid was quite

rough and not very adherent based on preliminary electron microscopy work.

The grain size was quite small (< 50 nm) . It was suggested that the film was

amorphous having a hexagonal structure. Gottesfeld and Srinivasan [53] showed

that potential cycling resulted in the formation of relatively thick film (250

nm after 15 hours of cycling), which they suggested was a hydrated hydroxide.

By combining coulometric with optical data, McIntyre et al . [54] found that

the apparent density of the hydrous material was 2.0 g cm' 3

, i.e., one-sixth

of that of an hydrous Ir02 . They showed, using electron microscopy, that the

film was porous, with microvoids on the order of 2.5 nm diameter.

Hackwood et al . [55] carried out a combined thermal and gas evolution

analysis of sputtered iridium oxide films and suggested these were probably

not as dispersed or hydrated as films produced by cycling. Some water was

lost in heating in vacuum at 120° C and it was assumed that an amorphous to

crystalline transition occurred at 300°C.

Augustynski et al. [56] studied films produced by potential cycling and

it was found that the initial layer was not Ir02 but the hydrated hydroxide.

The water was assumed to be chemisorbed on the surface beneath the Ir(0H) 4 species

.

B . Electrochemical Behavior

The electrochemical behavior of acid-grown hydrous iridium oxide films

are affected by: (i) the acidity of the hydrated material and the variation of

this property with change in the iridium oxidation state [57]; and (ii) the

difference in electrical conductivity between the reduced and oxidized forms

of the surface film [58,59]. According to Burke et al . [60], both propeities

are strongly dependent on the dispersed hydrated nature of the iridium oxide

layer. Both oxidized and reduced forms were assumed to be polymeric network

structures. There has been considerable controversy in the literature as to

14
whether cations [61-63] or anions [64-67] act as charge balancing ions for

maintaining electroneutrality in the film in the course of electron insertion

or withdrawal reactions

The ac impedance measurements by Glarum and Marshall [58], and faradaic

experiments by Gottesfeld [68], have shown that the hydrous film in the

reduced state, Ir(III) , has a low electronic conductivity while in the

oxidized state, Ir(IV), it is highly conductive. Gottesfeld has interpreted

this behavior in terms of conventional band theory. However, Burke and Whelan

[60] have proposed a chemical approach involving electron hopping between

neighboring Ir centers for interpretation of this behavior. The high

conductivity of the oxidized state was attributed to the presence of Ir(VI)

sites that behave as acceptor states in the predominantly Ir(IV) film. The

direct evidence for these Ir(VI) sites was obtained by ESCA investigation of

the oxidized state [56].

In sulfuric acid media [69] a shoulder is usually observed at 0.6 V on

the cathodic side of the main anodic peak. According to Burke and Whelan [57]

it is probably an Ir (III/IV) transition in species containing anions other

than OH" within the hydrated material. Conway and Mozota [49] attributed the

large currents above the main anodic peak mainly to charging of the rather

extensive oxide-solution interface. However, Burke and Whelan [57] assumed

that the dispersed, hydrated oxide material scarcely constituted a distinct

phase, and postulated that the currents in this region were due to changes in

stoichiometry in a mixed valence state, Ir ( IV) /Ir (VI ) . A peak showing a shift

of 3/2 (2.303 RT/F) V/pH can be observed [60] above the main anodic peak on

transferring an acid grown film to solutions of higher pH. Such a peak was

also observed [70] following transfer of a base-grown film to acid.

Although hydrous iridium oxide deposits grown in acid show little change

in redox activity on transfer to base, it was demonstrated by Buckley et al.

[48] that the film could not be grown on cycling a clean iridium electrode in

such a medium (1.0 mol. dm” 3 NaOH, 0.01 to 1.50 V, 30 VS” 1 ). In a detailed

investigation, it was confirmed [70] that hydrous oxide formation was possible

in dilute, though not in strong, base. It was also observed that after

15
subjecting the electrode in base to preliminary cathodic pretreatment, which

probably involved removal of any residual oxide film, no growth was observed.

This result was explained by assuming that local pH changes within the pores

of a superficial oxide layer play a crucial role in the conversion of

anhydrous to hydrous oxide.

The base-grown films were found to dissolve readily in 1.0 mol . dm" 3 mH2 S0A or NaOH and this was attributed to their more amphorous character. On the

other hand, the acid-grown films were found to be more crystalline having

larger grain sizes and this was used to explain their greater stability in

acidic media [70]

.

Yuen and Lauks [71] showed significantly different cyclic voltammo grams

for anodically grown and sputtering deposited iridium oxide films. An anodic

iridium oxide film had a pH dependent voltammogram. The results of the

voltammogram and the derived density-of -states at the Fermi level for the case

of a sputtered iridium oxide film (SIROF) , however, are independent of pH. It

was suggested that the SIROF had a more disordered structure in acidic and

alkaline electrolytes.

Reactively sputtered iridium oxide films were shown to have good chemical

stability against corrosive environments even at high pressures and

temperatures. An extensive series of open circuit potential measurements on

d.c. reactively sputtered iridium oxide films was performed for the pH range

of 3 to 10. The testing showed promising results for sputtered iridium oxide

films as pH probes [7]. An Ir02 electrode was used by Fog and Buck [72] as a

pH sensor and showed near-Nernstian behavior in the pH range of 2 to 12.

Unfortunately, Ir02 as a pH sensor material is subject to instability, ionic

interference, and hysteresis effects [7,72]. The instability was attributed

to a change in the physical and electrochemical properties of the oxide with

time [73]. Kreider et al . also observed a shift of open circuit potential

with air bak.'ng and autoclave heating at high temperature and pressure [4,74].

IV . pH Testing of Sputtered Iridium Oxide Films (SIROF)

16
A, Ionic Sensor Principles

Described here is a conventional electrochemical electrode for sensing of

ions. The other new chemically sensitive electronic devices can be found in

reference [75] . A solid membrane separating two liquids of different ionic

content can develop an electrical potential difference across it. This

phenomena can be expressed by the Nernst equation

E =

where E

R

T

F

RT

In ( 20 ) nF

= voltage

universal gas constant = 8.32 J/mol.K

temperature, K

Faraday constant = 9.65xl04 C/mol

ionic change

activity of ion to be measured in solution on one side of membrane

a2 = activity of same ion in solution of other side of membrane

n =

a, =

The ionic activity is related to ionic concentration, c, by

a = 7c ( 21 )

where 7 is the activity coefficient and has a value of one for highly dilute

electrolytes. In Eq. (20), if a2 of the reference solution is known, then a:

of the measurand can be determined provided that no other ions are present

which would interfere with the measurand.

For ionic sensors using a solid membrane, a basic problem is making

electrical contact with the solutions. If metal electrodes were used for this

purpose, then additional voltages would be generated in the electrical circuit

17
due to the chemical reactions at the electrode surfaces. In general, a stable

and reproducible ionic sensor requires half cells coupled to the electrolytes

by salt bridges or buffer solutions. A half cell consists of an electrode

immersed in an electrolyte with which it is in electrochemical equilibrium.

Only half cells with electrochemically reversible cell reactions give

reproducible results and are useful. The calomel cell (mercury in contact

with mercurous chloride/potassium chloride paste) and the silver/silver

chloride cell are the most common reference cells.

The output voltage at 25 °C is given by

E = E° +

0

log ( 7c) volts (22)

n

where E° is a constant for the particular configuration of electrode materials

and n is the number of ionic charge. A high- input impedance is required so

that the voltage may be measured at near zero current. Traditionally this

type of ionic measurement is referred to as open circuit potentiometry

.

B . pH and Its Measurement

Hydrogen ion concentration is usually defined by the logarithmic measure

of pH , i . e . ,

pH = log (1/CH +) (23)

At 25° C the concentration of H

+ and 0H“ ions in pure water is 10

“ 7 mol dm” 3

each. pH values are temperature dependent because the extent of H2 0

18
dissociation increases with temperature. At 100° C pure water has a CH + of

10" 6 mol dm-3

, i.e., the pH is 6, but the water is neutral. The product (CH +)

(C0H ~) remains constant when the concentration of either ionic species in an

aqueous solution is increased by addition of a solute.

It has been found that the conventional glass pH electrode has a good

selectivity for H

+

. Its construction, operating principle, and the

composition of the glass are discussed in detail in references 73 and 74. Its

operation involves ion transport through a thin glass membrane, made from a

glass containing appropriately chosen ionic constituents. The overall

mechanism of pH sensing, however, is not simple. The glasses are formulated

for optimum behavior under specific conditions of use. Soda glasses

containing sodium ions are generally inaccurate above pH 9; lithia extends the

range to about 13. Glasses incorporating tantalum, niobium, or rare earth

oxides having lower electrical resistance can be used to make thicker glass

membranes for electrodes.

General purpose glasses can measure pH from 0 to about 12 at temperatures

from 0 to near 100°C (and sometimes beyond) . Their electrical resistances at

25 °C are in the several hundred megaohm range, the exact value depending on

glass composition and membrane thickness. Resistance of the glass varies

exponentially with temperature. A glass whose resistance is 100 megaohms at

25°C may be around 1000 megaohms at 0°C and 1 megaohm at 80°C. General

purpose glasses show an interference response to sodium ions above

approximately pH 10. Glasses formulated for use at higher pH show a greatly

reduced response to sodium and other alkali metal ions but also have a higher

19
electrical resistance, five or more times that of general purpose glass.

The measurement of hydrogen ion activity uses a galvanic cell constructed

by combining the glass electrode with a reference electrode, usually the

saturated calomel electrode or silver/silver chloride electrode.

glass | H

+ solution || sat . KC1 | Hg2 Cl2 , Hg

The emf of this cell is given by

where the liquid junction potential, EL , at the analyte and saturated KC1

interface has been included. If one uses the formal definition of pH of Eq.

(23), then the pH value of the analyte cannot be deduced from Eq. (24) unless

the liquid junction potential is known. Therefore, the formal definition of

pH has been replaced by form of Eq . (24). Consider the measurement of the

cell emf for a buffer solution of known pH. If the pH value is pHs and the

emf value Es , then Eq. (24) can be written as

For an analyte solution of unknown pH, if the pH value is PHU and the emf value

Eu , then Eq . (25) becomes

E = E(Cal)-E' (glass) + EL + (2.303 RT/F)pH (24)

Es = E(Cal)-E' (glass) + EL + (2.303 RT/F)pHs (25)

Eu = E(Cal)-E' (glass) + EL + (2.303 RT/F)pHu (26)

20
Assume that the liquid junction potential does not change in changing the

standard buffer solution for the unknown pH analyte, then from Eqs . (25) and

(26) , one obtains

pHu = PHS + (Eu -Es )/(2 . 303 RT/F) (27)

Here we have noticed that the definition of pH is based on carefully defined

buffer solutions. The National Bureau of Standards (NBS) has proposed a set

of standard buffer solutions. Their pH values have been established from the

emf of cells without a liquid junction. The operational definition of pH

accepts the pH of these buffer solutions as correct [77].

For the pH range of 2 to 12 and for low ionic strength solutions (<0.1

mol dm-3

), the measured pH using the method discussed should be within 0.02 of

the correct value. Outside these ranges, errors in excess of 0.1 are

possible

C . Testing to Insure Suitability for Geochemical Metrology

To insure the applicability of SIROF pH electrodes for geochemical

fluids, the SIROF electrodes will be evaluated for the environmental

conditions of the particular repository site of interest. The environments of

different repository sites may differ significantly in expected moisture, pH

values, redox conditions, etc.

The proposed Tuff Repository in Yucca Mountain, Nevada has many features

that make it attractive as a site for burial of nuclear waste material. The

21
repository is located approximately 400 meters below the surface in a

compacted, hardened volcanic ash referred to a welded tuff. This level is

about 100 meters above the water table. Water influx in the welded tuff is

estimated to be less than one millimeter per year and the total moisture

content is six percent by weight. Chemical analysis of groundwater [1] taken

from Well J-13 indicates that the pH is near neutral at 7.1 and the

concentrations for all ions is low. These ions are HC03 (120 ppm), Si02 (61

ppm), Na (51 ppm), S04 (22 ppm), Ca (14 ppm), Cl (7.5 ppm), N03 (5.6 ppm), K

(4.9 ppm), F (2.2 ppm), and Hg (2.1 ppm). The water also contains 5 to 6 ppm

of oxygen. The estimated temperature of a spent-fuel waste-package for a Tuff

Repository [1] is less than 60°C during the operations period. However, it is

estimated to be in the range 100 to 300 °C during the thermal period of the

first 300 years.

The nuclear waste will affect the long-term temperatures and pressures of

the containment system. The pH sensors, therefore, must be able to withstand

these elevated thermal conditions without undergoing changes in sensor

properties

.

The measurements of long-term stability and ionic interference at

elevated temperatures have not been addressed before for SIROF electrodes.

Laboratory simulation will be performed for these measurements up to 250°C and

600 psi in accordance with the environmental conditions for a Tuff Repository.

The stability testing will be performed up to 30 days at various temperatures

and pH levels. The selectivity of SIROFs will be investigated by studying the

effect of various ionic species as previously described, on the electrode

22
response. Special attention will be paid to the ions of HC03 , Si02 , Na, S04 ,

and Ca, because these ions have higher concentration than other type of ions

in the solutions in a Tuff Repository [1].

V. Summary

The pH values and ionic contents of geochemical solutions are important

characteristics of a nuclear waste repository site [1]. The changes in

solution chemistry conditions with time, which are likely to exist during the

period of repository closure, must be taken into account if adequate

containment is necessary for a long period. Therefore, the pH determination

of the geochemical solution must be measured for long-term elevated thermal

conditions

Various metal oxide electrode materials have been described in a recent

report and their application for measuring pH has been reviewed [2], The

requirements of a pH electrode for geothermal fluids are rather difficult to

fulfil, e.g., the oxide must be stable at temperatures up to 250°C corrosion

resistance, and robust, and exhibit a low sensitivity to ionic and redox

interferences. Electrode materials of this type are discussed in the NBS/NRC

review, of which the Ir02 emerges as the most promising candidate because of

its consistent near Nernstian response and its property of low impedance in

thin films.

In order to optimize the performance of Ir02 electrodes for pH sensing at

23
high temperatures and pressures, improved understanding of the electrochemical

behavior at the Ir02 -solution interface is essential. Electrochemical

phenomena associated with the electrical charge and potential distribution in

the interface have not been studied for SIROF - solution systems. The

quantitative information can contribute to a better understanding of the

electrode response, e.g., open circuit potential, stability, sensitivity, and

selectivity

.

For inorganic oxides, experimental studies have revealed the important

role of H

+ and OH" ions in determining the surface charge and potential. The

classical GCSG model has been used by most colloid chemists in qualitatively

interpreting observed phenomena at the oxide-liquid interface. Several

workers have attempted to quantitatively describe the charge and potential

distribution at the oxide- liquid interface using this model. Non-classical

models have been proposed to account for the high surface charges observed for

oxides. The porous double layer or gel layer model has been postulated in

which H

+ and OH" and counter ions are thought to penetrate into the solid

oxide. However, independent measurements of oxide surface permeability cannot

support porosity as a general model. The site-binding model involving surface

complexation of protons and counter ions does appear to account for most

observed experimental results.

Progress is obviously being made in the area of oxide- liquid interfaces.

The basic phenomena are better understood, but it appears that no totally

satsifactory model for the oxide-solution interface has emerged to date.

Hydration, hydroxy lation , and acid-base properties are all factors which

24
require special attention. As far as the iridium oxide-solution interface is

concerned, two fundamental properties affecting the electrochemical behavior

the variation of this property with change in the iridium oxidation state

[57] , and (ii) the difference in electrical conductivity between the reduced

and oxidized form of the surface film [58,59]. According to Burke et al.

[78,79] both properties are strongly influenced by the hydrated nature of the

iridium oxide layer.

This report provides the general background and also some specific

information regarding a number of oxide- liquid systems. However, this does

not imply that the results from colloidal metal oxide studies can all be

applied to the oxides used as pH electrodes. This is simply due to the lack

of information presently available on the electrochemistry of the latter group

of materials. Iridium oxide has a metal- like conductivity, so that there is

no potential drop in the oxide. Its Fermi level is taken as the equilibrium

potential of the electrode:

For these types of oxides having a metal- like conductivity the band structures

are different from those of insulating oxides (band gaps greater than 3 eV)

and of n-type or p-type semiconductors. Recently, theories for the metalliquid and semiconductor- liquid systems have begun to appear, as well as

models that attempt to model the interfaces realistically [80].

No attempt has been made so far to examine the GCSG theory's

applicability to the Ir02

- solution system. Interfacial phenomena for this

of hydrous iridium oxide films are (i) the acidity of the hydrated film and

(28)

25
system are presently poorly understood. A detailed study of the capacitance

properties of Ir02 in contact with electrolyte pH solutions might give

extremely valuable information concerning the dielectric properties of the

contacting phases. The Ir02 -solution system may be characterized by its

capacitance-potential, capacitance-pH , capacitance-charge, and capacitancetime curves within the framework of the GCSG theory. This is of importance

for interpretation of adsorption phenomena for H

+

, OH" or other ions, of

double layer effects in electrode reaction kinetics and of electrode

instability in pH measurements. It is proposed that these capacitance

characteristics may be obtained using d.c. and a.c. voltammetry.

26
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31
NBS-1 14A rev. 2-ec > U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 88-3790

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

MAY 1988

4. TITLE AND SUBTITLE

Electrochemical Evaluation of Solid State pH Sensors for Nuclear Waste Containment

5. AUTHOR(S)

Peter H. Huang and Kenneth G. Kreider

6. PERFORMING ORGANIZATION (If joint or other than N BS

,

see in struction s)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

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

Nuclear REgulatory Commission

Office of Research

Washington, DC 20555

10. SUPPLEMENTARY NOTES

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

1L. ABSTRACT (A 200-word or I e s s factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

This report contains a literature review for electrochemical evaluation of solid state

pH sensors. The requirements of pH electrode for geochemical fluids in a nuclear waste

repository site are rather difficult to fulfill, that is, the electrode must have

stability at temperatures up to 250°C, low ionic and redox interferences, corrosion

resistance, and robustness. Among the potential electrode materials, the Ir02 emerges

as the most promising because of its consistent near Nernstian response and its

property of low impedance in thin films. However, improved understanding of the

electrochemical behavior at the lrC> 2 -solution interface is essential in order to

optimize the performance of Ir02 electrodes for pH sensing at high temperatures and

pressures. This report reviews theoretical models of the oxide-solution interfaces

based on the theory of the electric double layer. Electrochemistry of IrC>2 films with

emphasis on the properties of anodic films is summarized. A plan for pH testing of

sputtered iridium oxide films (SIROF) for geochemical measurements at a Tuff

Repository is described.

1 2. KEY WORDS (Six to twelve entries ; alph abeti cal order; capitalize only proper names; and separate key wtprds by semicolons)

electric double layer; electrochemistry; iridium oxide; oxide-solution interface;

pH sensors; votammetry

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