A111D2 137SA5

NBSIR 82-2581

Simulated Precipitation Reference

Materials, IV

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Measurement Laboratory

Inorganic Analytical Research Division

Washington, DC 20234

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June 1982

Issued October 1 982

Prepared for U.S. Environmental Protection Agency

»Q£^MM/jronmental Research Center

search Triangle Park, NC 27711

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62-2561

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NATIONAL BUREAU OF STANDARDS

LIBRARY

NOV 16 1982

NBSIR 82-2581

i<« SIMULATED PRECIPITATION REFERENCE

W. F. Koch, G. Marinenko, and J. W. Stolz

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Measurement Laboratory

Inorganic Analytical Research Division

Washington, DC 20234

June 1982

Issued October 1982

Prepared for U.S. Environmental Protection Agency

Environmental Research Center

Research Triangle Park, NC 27711

MATERIALS, IV

a

£ *

tt

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

TABLE OF CONTENTS

Page

1. Introduction 1

2. Instructions for use 2 2.1 Levels 1 and 2 2 2.2 Levels 3 and 4 2 2.2.1 Acidity Measurement 2 2.2.2 Conductance and pH Measurements 2

3. Analysis of SPRM IV 3 3.1 Measurement of pH 4

3.2 Measurement of Specific Conductance 4

3.3 Measurement of Acidity 4

3.4 Determination of Anions 4

3.5 Determination of Ammonium Ion 5 3.6 Determination of Metals 5 3.6.1 Polarography 5 3.6.2 Atomic Absorption and Flame Emission Spectrometry 5 3.6.3 Inductively Coupled Plasma Spectrometry 5 3.7 Discussion of Analytical Results 5

4. Stability of Simulated Precipitation Reference Materials 6

4.1 SPRM I through SPRM III 6

4.2 SPRM IV 6

4.3 Long-term Stability in Polyethylene Bottles . . 6

5. Evaluation of Current Methodologies for pH and Acidity

Measurements ’ 11

5.1 Definition of pH 11 5.2 pH Experiments 12

5.3 Discussion of pH Experiments 14

5.4 Recommended Procedure for Measurement of pH 14

5.5 Measurement of Acidity 15

6. Summary and Recommendations 16

7. Acknowledgment 16

8. References 17
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SIMULATED PRECIPITATION REFERENCE MATERIALS, IV

W. F. Koch, G. Marinenko, and J. W. Stolz

Inorganic Analytical Research Division

National Bureau of Standards

Washington, D. C. 20234

This report describes work performed at the National Bureau of

Standards under the sponsorship of the United States Environmental

Protection Agency to establish the composition of a fourth series of

reference materials intended to be used for the intercalibration of

precipitation measurement stations, to evaluate the stability of the

first three series of reference materials, to evaluate current methodologies for pH and acidity measurements, and to make recommendations

to improve future reference materials and measurement protocols.

Key words: acidity, acid rain; chemical analysis; conductance; pH;

precipitation; rain; reference materials; trace elements.

1. Introduction

The analysis of the composition of rainfall and other forms of precipitation is an excellent means to monitor global atmospheric pollution. Such measurements must

necessarily be made at widely spaced geographical locations, over extended periods of

time, and by many independent investigators. It is imperative that the measuring

stations be intercal ibrated if the data obtained are to be correlated and combined

for environmental interpretation. The measurment of a series of substantially

identical samples by stations in a measurement system is an accepted method of

evaluating laboratory performance for intercalibration purposes.

This report describes work done at the National Bureau of Standards under the

sponsorship of EPA 1 to establish the composition of a fourth series of reference

materials intended to be used for the intercalibration of precipitation measurement

stations, to evaluate the stability of the first three series of reference materials,

to evaluate current methodologies for pH and acidity measurements, and to make

recommendations to improve future reference materials and measurement protocols.

This is the fourth series of solutions prepared for EPA and is designated

Simulated Precipitation Reference Materials IV (SPRM IV). The details of the first

three series are contained in: NBSIR 75-958, "Simulated Precipitation Reference

Materials," October 1975 [1]; NBSIR 77-1315, "Simulated Precipitation Reference

Materials II," September 1979 [2]; NBSIR 79-1953, "Simulated Precipitation Reference

Materials III," April 1980 [3].

SPRM IV is similar in concept to those prepared previously at the National

Bureau of Standards. It was prepared in May of 1981 by a contract laboratory under

the direct supervision of the Environmental Protection Agency and the guidance of the

National Bureau of Standards. Two reference solutions, low concentrations (Level 1)

x This work was performed at the National Bureau of Standards for the United States

Environmental Protection Agency under Interagency Agreement, EPA IAG#: AD-13-F-2-

535-0, Subagreement 5.

1
and high concentrations (Level 2), were prepared of specified components at levels

similar to those expected in natural samples. In addition, two reference solutions

(Levels 3 and 4) containing approximately 2 microequivalents (yeq) and 5 yeq of

sulfuric acid per ampoule, respectively, were prepared specifically for the measurement of acidity, conductance, and pH. The four sets of solutions were prepared in

bulk and then transferred to 20 milliliter (mL) glass ampoules using an automatic

dispensing and sealing machine.

2.

Instructions for Use

2.1

Levels 1 and 2

The sample solutions for Levels 1 and 2 are contained in 20 mL glass ampoules

which have been pre-scored to facilitate opening. When these samples are accurately

diluted 1 to 50 with high-quality distilled water (such as ASTM Type II or better),

the resultant solutions simulate natural rainwater with regard to the concentrations

of major components and trace elements. The recommended procedure for preparation

of a sample for analysis is as follows:

1. Rinse the outside of the ampoule with distilled water to remove dust, and

dry in clean air or with a clean, lint-free cloth.

2. Hold ampoule in a vertical position, tip over and back, and gently tap to

transfer any liquid from the top to the body of the ampoule.

3. Hold ampoule with a paper towel (a precaution to prevent injury to hand),

and snap off the top at the scored constriction.

4. Accurately pipet 10.0 mL of the concentrate from the ampoule into a clean

500 mL volumetric flask. Use Class A volumetric glassware.

5. Add distilled water to the flask to dilute to the calibrated volume, and

mix contents thoroughly.

6. Treat the resulting solution with the same care and in the same manner as

a sample of natural rainwater.

2.2

Levels 3 and 4

The sample solutions for Levels 3 and 4 are contained in 20 mL glass ampoules

which have been pre-scored to facilitate opening. These solutions are intended for use without additional dilution.

2.2.1 Acidity Measurement

Transfer the contents of two ampoules into a suitable titration vessel. Open

the ampoules according to the recommended method outlined in Section 2.1 (1-3). Hold

the inverted ampoules over the titration vessel until the solution has drained as much as possible. Check for visible inclusions of solution in the ampoule and tap

gently if required. Do not rinse the ampoules. Any film of solution remaining in

the ampoules will not cause significant error.

2.2.2 Conductance and pH Measurements

Transfer the contents of one ampoule into a clean beaker of the proper size to

facilitate the measurement of conductivity and pH. Open the ampoule according to the

2
method outlined in Section 2.1 (1-3). Remove substantially all the contents by

shaking. However, these measurements being intensive by nature are not influenced by

the quantity of sample used and hence quantitative delivery is not required. It is

recommended to measure conductivity of the sample first, followed by measurement of

pH. This will prevent leakage from the reference electrode from affecting the

measured value of the conductivity of the solution.

3. Analysis of SPRM IV

In most cases, solutions from six ampoules at each level were analyzed for each

specific component. From a practical standpoint, all determinations were not made

from a single set of ampoules but rather, each technique analyzed a separate set.

There is no reason to believe that at the time of filling of the ampoules, the solutions were anything other than homogeneous. The recommended values of the components

of SPRM IV are given in Tables 1 and 2. The methods used for the analytical

measurements are summarized in the following sub-sections.

Table 1. Simulated Precipitation Reference Materials IV, Levels 1 and 2

(diluted 10 mL to 500 mL)

Level 1 Level 2

pH 4.36(0.10) a 4.03(0. 10) a Specific Conductance, yS/cm 20.4(1.0) 49.0(1.0)

Acidity, yeq/500 mL 27.8(0.2) 73.4(0.5)

Fluoride, yg/mL 0.17(0.1) 0.23(0.1)

Chloride, yg/mL 0.80(0.1) 3.08(0.04)

Nitrate, yg/mL 0.91(0.1) 2.26(0.02)

Sulfate, yg/mL 1.49(0.1) 3.12(0.04)

Ammonium, yg/mL 0.220(0.002) 0.562(0.006)

Sodium, yg/mL 0.314(0.005) 0.596(0.006)

Potassium, yg/mL 0.140(0.001) 0.538(0.006)

Magnesium, yg/mL 0.061(0.001) 0.172(0.002)

Calcium, yg/mL 0.197(0.004) 0.63(0.02)

Manganese, yg/mL.

Iron, yg/mL

0.034(0.002) 0.178(0.001)

0.032(0.001) 0.056(0.001

)

Copper, yg/mL 0.086(0.003) 0.302(0.012)

Lead, yg/mL 0.044(0.015) 0.12(0.05)

Cadmium, yg/mL 0.044(0.001

)

0.260(0.012)

Nickel, yg/mL 0.072(0.006) 0.16(0.02)

Zinc, yg/mL 0.081(0.005) 0.421(0.015)

a The values in parentheses represent uncertainties based on an evaluation of the

combined effects of method imprecision, possible systematic errors, and material

variabil ity.

3
Table 2. Simulated Precipitation Reference Materials IV, Levels 3 and 4

(undiluted)

pH

Specific Conductance, yS/cm

Acidity, yeq/2 ampoules

Level 3

4.12(0.10) a

36.0(1.0)

4.01(0.08)

Level 4

3.71(0.10)a

87.0(2.0)

9.40(0.33)

The values in parentheses represent uncertainties based on an evaluation of the

combined effects of method imprecision, possible systematic errors, >and material

variabil ity

3.1 Measurement of pH

Six samples of each of the four levels were analyzed for pH. Levels 1 and 2 were diluted 10 mL to 500 mL; Levels 3 and 4 were analyzed without dilution. The pH was measured with a combination glass electrode and digital pH meter which had been

calibrated with NBS Standard Reference Material (SRM) pH buffers (pH 4.01, SRM 185e

and pH 6.86, SRM 186-Ic/186-IIc) . A detailed recommended procedure for measuring pH1 in low ionic strength solutions is given in a later section of this report. The

limitations of the measurement are also discussed. Because of these limitations, the

accuracy of the pH measurement in these samples can be no better than 0.1 pH number.

3.2 Measurement of Specific Conductance

Six samples of each of the four levels were analyzed for specific conductance.

Levels 1 and 2 were diluted 10 mL to 500 mL; Levels 3 and 4 were analyzed without

dilution. Conductance was measured with a platinum dip-cell having a cell constant

of 0.09975 cm

-1 determined through use of a 0.001 denial KC1 solution at a frequency

of 1000 Hz. The values reported were determined at 23 ± 1 °C.

3 . 3 Measurement of Acidity

The acidity content of each level was determined by coulometric titration of

hydrogen ion to an endpoint as indicated by the inflection-point in the sigmoidal

titration plot. Titration to a fixed-pH-endpoint can seriously bias the results.

This matter is discussed in detail in a later section of this report.

3.4 Determination of Anions

Six samples from each of Levels 1 and 2 were analyzed by ion chromatography for

fluoride, chloride, nitrate, and sulfate. For convenience. Level 1 was diluted 1:3

with the same carbonate buffer solution as used as the eluent for the chromatography.

Similarly, Level 2 was diluted 1:13. All four anions in each sample were determined

sequentially with a single injection. The ion chromatograph was calibrated before

every sample with three multi-component standards which bracketed the levels in the

sample. A linear plot, peak height versus concentration, was used for quantitation.

Each sample was analyzed twice. The results summarized in Table 1 have been adjusted

to reflect a dilution of 10 mL to 500 mL.

4
3.5

Determination of Ammonium Ion

The chosen technique consisted of chemical measurement according to the procedure

given in the following reference: C. 0. Willits, and C. L. Ogg, "1949 Report of

Standardization of Microchemical Methods, Micro Kjeldahl Nitrogen Determinations,"

USDA Eastern Regional Research Laboratory, Philadelphia, PA. A micro-Kjeldahl steam

distillation apparatus was used to separate the ammonia (released by sodium hydroxide)

which was trapped in boric acid. The ammonia was titrated with standardized 0.01

mol/L hydrochloric acid using methyl purple indicator.

3.6

Determination of Metals

Several techniques were employed for the determination of the metals in Levels 1 and 2. These are described in the following sub-sections. When more than one technique was used for the determination of any single element, an average value was calculated and reported in Table 1.

3.6.1 Polarography

Six samples at each level were analyzed for copper, lead, cadmium, nickel, and

zinc by cathode-ray polarography. Nickel and zinc were determined in a supporting

electrolyte of dilute sulfuric acid and pyridine. Copper, lead, and cadmium were measured in an ammonium acetate buffer solution. A multi-point calibration curve was used for quantitation.

3.6.2 Atomic Absorption and Flame Emission Spectrometry

Sodium, potassium, and calcium were determined by flame emission spectrometry.

Flame atomic absorption spectrometry was used to determine magnesium, manganese,

iron, copper, cadmium, zinc, nickel, and lead. Calibration standards were prepared

by dissolution of pure salts or pure metals.

3.6.3 Inductively Coupled Plasma Spectrometry

The inductively coupled plasma technique was used to determine zinc, lead,

cadmium, iron, manganese, nickel, magnesium, and copper. Three multi-element

standards were prepared to establish calibration curves for these elements. Two

separate calibration runs were used for each sample.

3.7

Discussion of Analytical Results

The results of the analyses are summarized in Tables 1 and 2. The values in

parentheses represent uncertainties based on an evaluation of the combined effects of

method imprecision, possible systematic errors, and material variability. The rather

high degree of uncertainty in some of the values, notably copper, lead, nickel, and

zinc is a matter of great concern. It is believed that the major cause of the

problem is adsorption of the metals on the glass, with hydrolysis of the metals being

a further complication. The addition of citrate ion to counter these effects has

proved to be unsatisfactory and has created the further problem of the growth of

algae in the long term. Recommendations to produce more stable solutions will be

discussed in a later section of this report.

5
4. Stability of Simulated Precipitation Reference Materials

The stability of these reference materials is an integral factor in the evaluation of laboratory performance for intercalibration purposes. To this end, periodic

checks have been made on SPRM I through III with respect to pH and conductance. This

stability monitoring program is continuing with SPRM IV. In addition, two projects

have been instituted to evaluate the use of polyethylene bottles rather than glass

ampoules as storage containers in an effort to improve the long-term stability. As

previously stated (Section 3.7), glass is not an appropriate storage container for

the transition elements or the heavy metals. There is mounting evidence that glass

is equally inappropriate for the alkali metals and hydrogen ion due to exchange

reactions, which adversely affect the concentration of the alkalis, the pH, and the

conductance of the solutions.

4.1 SPRM I through SPRM III

The results of the long-term stability check on SPRM I through SPRM III are summarized in Tables 3-5. In general, there is a decrease in conductance with time,

most markedly in those solutions containing only dilute sulfuric acid (Series 740000,

930000, and 940000). The pH of these samples also indicate considerable instability

and variability. Algae were apparent in some of the ampoules which contained

citrate.

4.2 SPRM IV

The stability of SPRM IV in glass ampoules with respect to pH, conductance, and

acidity over a nine-month period is summarized in Table 6. Again, in those solutions

composed of dilute sulfuric acid (Levels 3 and 4) there is a sharp drop in conduct- ance with a concomitant increase in pH. This is most likely due to an exchange of

sodium ion in the glass with hydrogen ion in the solution.

In an experiment to confirm this suspected "glass effect" a second set of SPRM

IV samples was placed in 60 mL polyethylene (CPE) bottles from the same bulk solutions and at the same time as the samples in glass ampoules. The improvement in

stability in all four levels over the same period of time is shown in Table 7.

However, the growth of algae in Levels 1 and 2 (which contain citrate) contained in

plastic bottles was quite apparent. There has been no evidence of algae growth in

any of the sample solutions which do not contain citrate.

4.3

Long-term Stability in Polyethylene Bottles

To evalute the stability of dilute acid solutions with respect to pH and

conductance over a long period of time, a solution of sulfuric acid was prepared on July 24, 1980 and subsequently placed in several 60 mL polyethylene (CPE) bottles.

On a periodic basis, individual bottles are opened and the solutions analyzed for pH

and conductance. The results to date of this ongoing experiment are shown in Table

8. The first date, 7/24/80, represents measurements made on the bulk solution before

bottling. There appears to be a slight upward trend in the conductance of the

solution. However, relative to storage in glass ampoules, this small change is

insignificant. No detectable change in the pH of the solution is apparent.

6
Table 3. Stability of SPRM I

Series 1975A

pH Specific Conductance (uS*cm -1

)

1975 6.18 5.8

1979 — 5.1

June 1980 5.94 5.3

July 1980 5.74 5.2

August 1980 5.66 5.3

Series 1975B

pH Specific Conductance (pS^cm -1

)

1975 6.15 18.3

1979 -- 17.1

June 1980 5.70 17.2

July 1980 5.76 17.1

August 1980 5.66 17.0

Series 1975C

pH Specific Conductance (pS’cnr 1

)

1975 6.20 59.8

1979 5.75 57.0

June 1980 5.67 53.0

July 1980 5.63 56.2

August 1980 5.62 55.9

Series 1975D

pH Specific Conductance ( pS'cm -1

)

1975 6.07 117.0

1979 5.78 112.0

June 1980 5.59 113.4

July 1980 5.59 111.2

August 1980 5.64 111.4

7
Table 4. Stability of SPRM II

I

Series 710000

pH Specific Conductance (yS'crrT 1 )

1977 4.48 28.8

1979 4.25 30.8

June 1980 4.24 29.5

July 1980 4.17 29.7

August 1980 4.14 33.4

October 1980 4.04 30.0

Series 720000

pH Specific Conductance (yS‘cm _1

} i

1 1977 4.08 77.9

1979 4.07 84.4

June 1980 4.06 80.3

July 1980 3.97 82.3

August 1980 3.97 82.0

October 1980 3.90 82.6

Series 730000

pH Specific Conductance (pS*cm -1

)

1977 5.98 190.5

1979 5.86 205.2

June 1980 5.61 200.2

July 1980 5.60 199.2 1 August 1980 5.66 198.7

' October 1980 5.55 200.8

Series 740000

pH Specific Conductance (uS*cm _1

)

i 1977 3.81 80.0

1979 4.02 —

June 1980 4.00 60.4

July 1980 3.95 60.0

August 1980 3.91 59.0

October 1980 3.83 59.1

8
Table 5. Stability of SPRM III

Series 910000

pH Specific Conductance ( pS'cm- 1 )

1979 5.03 13.2

June 1980 4.87 12.8

July 1980 4.78 12.6

August 1980 4.83 11.8

October 1980 5.18 8.9

June 1981 5.46 8.7

Series 920000

pH Specific Conductance (pS'cm -1

)

1979 4.35 43.4

June 1980 4.27 41.3

July 1980 4.25 41.8

August 1980 4.22 40.9

October 1980 4.15 38.3

June 1981 4.35 36.5

Series 930000

'

pH Specific Conductance (yS*cm-1

)

1979 3.91 69.1

June 1980 4.05 48.2

July 1980 3.97 46.0

August 1980 3.95 47.9

October 1980 3.89 46.6

February 1981 4.18 44.2

June 1981 4.04 41.8

Series 940000

pH Specific Conductance (pS‘cm-1

)

1979 3.62 121.4

June 1980 3.72 91.6

July 1980 3.62 99.8

August 1980 3.59 97.3

October 1980 3.53 94.3

February 1981 3.78 91.4

June 1981 3.66 92.6

9
Table 6. Stability of SPRM IV in Glass Ampoules

Level pH

Specific Conductance

(wS/cm)

Acidity_

(peq)

6/81 3/82 6/81 3/82 6/81 3/82

1 4.36 4.50 20.4 18.6 27.8 27.3

2 4.03 4.13 49.0 46.4 73.4 73.2

3 (per 2 ampoules) 4.12 4.30 36.0 29.3 4.01 3.97

4 (per 2 ampoules) 3.71 3.80 87.0 76.1 9.40 9.91

Table 7. Stability of SPRM IV in Polyethylene Bottles

Level- pH

Specific Conductance

(m S/cm) —Acidity___

(neq)

6/81 3/82 6/81 3/82 6/81 3/82

1 4.34 4.41 22.4 21.0 — 32.8

2 4.03 4.08 50.9 48.4 — 74. &

3 (per 2 ampoules) 3.87 3.90 52.4 53.0 — 6.19

4 (per 2 ampoules) 3.59 3.59 105.9 104.1 — 11.58

Table 8. Stability of Dilute Sulfuric Acid in Polyethylene Bottles

Date pH

Specific Conductance

(pS/cm)

7/24/80 3.90 45.0

7/25/80 3.90 45.3

8/5/80 3.87 45.0

8/6/80 3.90 46.0

8/14/80 3.90 45.5

10/27/80 3.79 44.4

1/30/81 3.86 47.0

6/24/81 3.95 48.0

3/5/82 3.93 48.1

1 0
5. Evaluation of Current Methodologies for pH and Acidity Measurements

Research is being conducted at the National Bureau of Standards to improve the

measurement of pH and acidity in low-conductivity, unbuffered solutions such as encountered in natural precipitation. A survey of the literature and experiments

performed at NBS has shown that serious errors may affect the reliability and

comparability of acid rain analyses in the important parameters of pH and acidity.

Preliminary results of this research have been presented at the Symposium and Workshop on Sampling and Analysis of Rain, sponsored by ASTM Committee D-22 and held in

Philadelphia, PA on October 7, 1981. The following sub-sections are excerpted from

that presentation.

5.1 Definition of pH

In 1909, the Danish chemist, S. P. L. Sorensen, published two papers in which he

distinguished between the degree of acidity (the intensity factor) and the total

acidity (the capacity factor) of a solution [4]. Both the concept and the notation

of pH grew from these publications. In pure terms, pH is intended to represent the

negative logarithm of the hydrogen ion activity of a solution. The realization of

this pure concept of pH is not attainable with present theory and technology. Hence,

reliance on standards and conventions (i.e., an operational definition) is mandatory.

The history of and rationale behind the various pH scales used in the last 70

years is a story in itself. Roger G. Bates summarizes the major developments in his

book "Determination of pH: Theory and Practice" [5] and updates them in a recent

monograph [6]. Today, the most widely used convention is the NBS practical pH scale,

the physical realization of which is embodied in the NBS pH buffers issued as Standard Reference Materials [7]. These seven primary and three secondary standard

buffer materials cover the pH range from 1.68 to 12.45 and define the pH(S) scale.

It must be stressed that these buffers form the basis of an operational

definition of pH as given by the equation [7]

pH(X) pH(S)

(E

X -EJF

RT In 10

where: E<. is the electromotive force of the cell —

reference electrode | KC1 ( >_3 . 5 mol /L) | | sol ution S|H 2 (g); Pt;

E^ is the electromotive force of the same cell substituting sample solution X for the

standard buffer solution S; F is the Faraday constant; R is the gas constant; and T

is the thermodynamic temperature. This definition incorporates several assumptions,

conventions, and compromises which have been discussed by Bates [5,6] and Durst [7].

As such, the NBS scale (and any other conventional scale) has imposed limitations as to its use and more importantly as to the interpretation of results obtained through

it. According to Bates, quantitative interpretation of measured pH values should

not be attempted unless the medium closely resembles the standard of reference, that

is, aqueous solution of buffers and simple salts within the pH range of 3 to 11 and

with ionic strengths between 0.01 and 0.1. Acid rain samples generally do not meet

these constraints. Hence, pH measurements of such samples have. limited accuracy

even under favorable laboratory and field conditions. Furthermore, fundamental

interpretation in terms of hydrogen ion activity is rendered impossible.

1

1
5.2 pH Experiments

The customary way of measuring pH is el ectrometrical ly with a glass electrode

and a reference electrode, often combined into a single unit for convenience. The

glass elctrode replaces the hydrogen electrode in the aforementioned electrochemical

cell. Various configurations are commercially available tailored for specific

applications as well as for general laboratory use. It is evident that such an arrangement measures not a single quantity (i.e., hydrogen ion activity), but rather

a sum of potentials. These "unwanted" potentials include asymmetry potentials,

1 iquid-junction potentials, streaming potentials, and several others related to the

non-ideal behavior of the solution and of the electrodes [8]. Insofar as these

potentials are constant between standard S and sample X, they can be electronically

offset and the equation defining pH holds true. However, in solutions such as acid

rain, which are unbuffered and low in ionic strength, there can be substantial

changes in these potentials, particularly the liquid-junction potential. These

residual potentials can easily bias the pH reading by 0.1 pH number. Several

experiments were conducted to reveal and confirm these conclusions.

1. Several combination pH electrodes of various configurations, sizes,

and manufacturers were calibrated simultaneously in a single

solution of NBS buffer potassium hydrogen phthalate, SRM 185e [pH ( S

)

at 23.0 °C, 4.002]. After copious rinsing with distilled water, the

electrodes were all dipped into a solution of 10~ 4 mol/L hydrochloric

acid. The solution was stirred initially to ensure homogeneity, then

allowed to settle to a quiescent state. Stable pH numbers were recorded after five minutes. Readings were then taken with the

solution stirred. These data are summarized in Table 9.

Table 9. Response of Combination pH Electrodes in 10“" mol/L HC1

Recorded pH Number

Electrode Quiescent Stirred A(Q-S)

1 3.944 3.816 0.128

2 3.864 3.661 0.203

3 3.908 3.741 0.167

4 3.899 3.841 0.058

5 3.875 3.755 0.120

Range 0.080 0.180

2. The above experiment was repeated using the glass sensors of the

combination electrodes but using a single silver/silver chloride

electrode as the reference for all. These data are summarized in

Table 10.
Table 10. Response of pH Electrodes with a Common

Electrode in 10" 4 mol/L HC1

Reference

Recorded pH Number

Electrode Quiescent Stirred a(Q-S)

1 3.942 3.889 0.053

2 3.943 3.883 0.060

3 3.944 3.877 0.067

4 3.942 3.888 0.054

5 3.956 3.878 0.078

Range 0.014 0.012

3. Several reference electrodes (both saturated calomel and silver/silver

chloride types) with various kinds of junctions (fiber, ceramic,

annular, frit, sleeve, and double) were tested in three solutions:

1 mol/L potassium chloride; 0.05 molal potassium hydrogen phthalate;

and 10" 4 mol/L hydrochloric acid. Potentials were measured versus

a saturated calomel electrode with a ceramic junction in quiescent

and stirred solutions. These data are tabulated in Table 11.

Table 11. Response of Reference Electrodes Versus a Common Reference

Electrode in Various Aqueous Media

Potential (mV)

Electrode 1 mol/L KC1 KHP Buffer 10‘ 4 mol/L HC1

A(fiber) Q -48.7 -46.7 -43.6

s -48.7 -47.4 -47.2

B(frit) Q -3.3 -0.4 +1.6

S -3.3 +0.7 +6.7

C(double) Q -3.8 -1.4 +7.9

S -3.8 -0.6 +10.5

D(sleeve) Q -2.9 +0.7 +4.7

S -2.9 +2.0 +10.0

E(micro) Q -35.7 -33.8 -31.9

S -35.6 -32.5 -26.8

F( ceramic) Q -49.8 -48.5 -47.6

S -49.7 -47.6 -45.5

G(annular, double) Q -32.1 -19.2 -6.3

s -31.9 -17.9 -3.5

Q = quiescent S = stirred

13
5.3 Discussion of pH Experiments

The data in Table 9 clearly demonstrate that five randomly selected electrodes

calibrated in identical fashion yield five different pH numbers with a range of 0.08

when used to test a solution that approximates the pH and ionic strength of acid

rain. Even larger deviations have been noticed with other electrodes. Also illustrated is the dramatic effect (sometimes referred to as streaming potentials) that

stirring the solution has on the pH number. It should be noted that stirring has

negligible effect (less than 0.01 pH number) in the standard buffer solutions.

The fact that the liquid junction of the reference electrode is the major source

of the problem is indicated by comparison of Table 10 with Table 9. Using a single

reference electrode, all glass electrodes read virtually identically. Again the

effect of stirring is obvious.

Experiment 3 confirms the serious nature of residual liquid-junction potentials

and further illustrates the effect of stirring. The change in potential between two

reference electrodes in going from standard buffer to a solution similar to acid rain

is typically 3 mV (which will bias the pH reading by 0.05) and in the case of a doubl e- junction electrode is almost 13 mV. It must be noted that this observed

change in potential is somewhat muted by the fact that the liquid potential of the common reference electrode, against which all others are being measured, is also

shi fting

These experiments indicate that the imprecision of pH measurements of acid rainlike solutions using different glass/reference electrodes is at least 0.1 pH number

and further that the measurement can be biased by a minimum of 0.05 pH number. Much

larger deviations are sometimes observed and are not unexpected. The use of doublejunction reference electrodes does not remedy the situation and may compound the

problem. It is often suggested to add potassium chloride to a low ionic strength

solution to obviate residual liquid junction potentials. Indeed, this will reduce

the residual potential but will not eliminate it. Furthermore, the addition of salt

may cause new problems. Of course, the absence of acidic or basic contaminants in

the salt must be established. More importantly the effects of the salt on the species

present in solution must be understood. These effects are opposite in sign and

varying in magnitude between strong acids and weak acids. It is, therefore, not

recommended to add potassium chloride to acid rain solutions. Alternate pH scales

have also been proposed [9]. Although these scales can prove helpful in specific

situations, they are generally not recommended because of the confusion they may

cause. This confusion could result from the lack of universal acceptance of these

alternates, the lack of standards, the failure to properly identify these scales as alternates, and the absence of a unifying link to the national pH(S) scale.

5.4 Recommended Procedure for Measurement of pH

Therefore, until further research improves the situation, the following protocol

is recommended for the routine measurement of pH in acid rain. Obtain a suitable

glass/reference electrode pair or combination pH electrode. (Glass electrodes with

the standard-sized sensor appear to give a fast and stable response. The junction in

the reference electrode which gives the least change in potential in different solutions appears to be of the ceramic type). Standardize the electrodes and meter using

SRM 185e, potassium hydrogen phthalate [pH(S) 4.004 at 25.0 °C, 0.05 molal].

Ascertain the Nernstian response of the electrodes with a second buffer, either SRMs

186 Ic and lie CpH(S) 6.863 at 25.0 °C] or SRM 189 [pH(S) 1.679 at 25.0 °C].

Re-establish calibration of the pH measuring system at regular intervals with the

phthalate buffer. Rinse the electrodes thoroughly with distilled water and then dip

them into a clean beaker containing a portion of the acid rain. After stirring,

allow the solution to settle to a quiescent state. Most electrodes reach a

14
reasonable degree of stability after five minutes. Record the pH number and repeat

the measurement with a fresh portion of the acid rain. The two readings should agree

to within 0.1 pH number. Non-compliance generally indicates that the electrodes were

not rinsed adequately after standardization with the buffers. Report the pH of the

rain sample to the nearest 0.1 pH number. Exercise caution in deriving thermodynamic

significance from the value. Rather, view the pH measurement as a screening procedure

to indicate whether additional study of the sample in terms of acidity is warranted.

5.5 Measurement of Acidity

In a related way, the measurement of total acidity, while a simple matter in

solutions of moderate and high acid content, is beset with potentially serious errors

in solutions of extremely low acidity. The consequences of such errors are perhaps more important than errors in pH measurements in that the capacity factor rather than

the intensity factor is the more meaningful term in acid-base chemistry.

Procedures that depend on titration to a fixed pH endpoint are particularly prone

to error. To illustrate this point, two solutions of hydrochloric acid (nominally

10.4 and 0.16 yeq/g) were titrated by three procedures. All three procedures involved

control led-current coulometric titration with electrogenerated hydroxyl ion but

differed in the manner in which the endpoint was determined. In method A, the endpoint was determined by location of the inflection point in the sigmoidal plot of pH versus microequivalents of base added. Method B used a closed-loop procedure in which

sodium citrate was added to the supporting electrolyte and pre-titrated to pH 8.3 (the

neutrality point for citrate). Then the sample of acid was added, and the solution

was titrated back to pH 8.3. Method C involved addition of the acid sample to neutral

supporting electrolyte (about pH 7) and titration to pH 8.3. The data are summarized

in Table 12. Agreement between method A and method B was excellent. However, method

C gave consistently high results with the relative error increasing as the amount of

acidity decreased. Furthermore, it has been observed that method C exhibits a sample

size dependency on the acidity titer, which is not apparent in either of the other

methods

.

Table 12. Dependence of "Total Acidity" on Titration Procedure

Total Acidity (yeq/g)

Sample Method A Method B Method C

1 10.38 10.36 10.53

2 0.161 0.165 0.222

It must be concluded that procedures which require titration to a fixed-pH

endpoint should not be used for acid rain analyses. Closed loop procedures are satisfactory but with concomitant loss in sensitivity at endpoints of higher pH.

Titration to a inflection point (or multiple inflection points for mixed acid

systems) is the preferred technique. The method of Gran [10] is equally acceptable,

since it differs from inflection-point titrations only in the way the endpoint data

are manipulated.
6. Summary and Recommendations

The fourth series of Simulated Precipitation Reference Materials (SPRM IV) has

been characterized with respect to chemical composition. A program to monitor the

stability of SPRM IV, stored in glass and in polyethylene, has been initiated. The

program to monitor the stability of SPRM I through SPRM III over the long term has

been completed. Experiments are in progress to assess the long-term stability of

dilute acid solutions in polyethylene.

Results of the stability projects indicate that the simulated precipitation

reference materials as made to date and as stored in glass ampoules are not stable

with respect to pH, conductance, acidity, alkali metals, and several of the transition

elements. It is recommended to make the following changes in future SPRMs:

1. Prepare solutions of two levels simulating the major components of

precipitation, composed only of dilute mineral acids, alkali salts, and

alkaline earth salts. These solutions would be analyzed for pH,

conductance, acidity, sodium, potassium, magnesium, calcium, fluoride,

chloride, phosphate, nitrate, and sulfate. Store in clean polyethylene

(CPE) bottles.

2. Prepare solutions of two levels simulating the trace element composition

of precipitation, stabilized with 0.5 mol/L nitric acid and stored in

clean polyethylene (CPE) bottles.

Serious errors may affect the reliability and comparability of acid rain analyses

in the important parameters of pH and acidity. Additional research is needed to gain

a better understanding of this complex analytical problem, to establish practical and

theoretically sound protocols for the measurement of pH and acidity in unbuffered,

low ionic strength solutions, and to develop stable reference standards.

7. Acknowledgment

The participation of the following members of the staff of the Inorganic

Analytical Research Division in the analysis of SPRM IV is gratefully acknowledged:

E. R. Deardorff, E. J. Maienthal, T. C. Rains, T. A. Rush, T. A. Butler, and

R. L. Watters, Jr.

The work was performed for the United States Environmental Protection Agency

under Interagency Agreement, EPA IAG#: AD-1 3-F-2-535-0, Subagreement 5.
8 . References

[1] Taylor, J. K. , E. R. Deardorff, R. A. Durst, E. J. Maienthal, T. C. Rains, E. P. Schiede. "Simulated Precipitation Reference Materials,"

National Bureau of Standards (U.S.), NBSIR 75-958, October 1975,

p. 21.

[2] Taylor, J. K. , E. R. Deardorff, T. C. Rains. "Simulated Precipitation

Reference Materials II," National Bureau of Standards (U.S.), NBSIR

77-1315, September 1977, p. 21.

[3] Deardorff, E. R. , Rains, T. C. , Koch, W. F. "Simulated Precipitation

Reference Materials III," National Bureau of Standards (U.S.),

NBSIR 79-1953, April 1980, p. 21.

[4] Sorensen, S. P. L. , "Enzymstudien , II Uber die Messung und die

Bedeutung der Wasserstoffionenkonzentration bei enzymatischen

Prozessen," Biochem. Z. 1909, 21_, 131, Continuation Biochem. Z. 1909, 21, 201.

[5] Bates, R. G. "Determination of pH: Theory and Practice," 2nd ed.

,

John Wiley & Sons, New York, 1973.

[6] Bates, R. G. "The Modern Meaning of pH," CRC Critical Reviews in

Analytical Chemistry 1981, 1J3 (3), 247.

[7] Durst, R. A. "Standard Reference Materials: Standardization of pH

Measurements," National Bureau of Standards (U.S.), Spec. Pub. 260-

53, December 1975, p. 39.

[8] Durst, R. A. "Sources of Error in Ion-Selective Electrode Potentiometry ,"

Chapter 5 in Ion-Selective Electrodes in Analytical Chemistry, Vol

.

1, H. Freiser, ed. , New York, NY: Plenum Publishing Corporation,

1978, p. 311-338.

[9] Galloway, J. N., B. J. Crosby, Jr. "Acid Precipitation Measurement

of pH and Acidity," Limnol. Oceanogr. 1979, 24_, 1161.

[10] Gran, G. "Determination of the Equivalence Point in Potentiometric

Titrations," Acta Chem. Scan. 1950, 4, 559.

17
NBS-114A IREV. 2-80) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

1. PUBLICATION OR

REPORT NO.

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

SHEET (See instructions) NBSIR 82-2581 November 1982

4. TITLE AND SUBTITLE

Simulated Precipitation Reference Materials, IV

5. AUTHOR(S)

W. F. Koch, G. Marinenko, and J. W. Stolz

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

national bureau of standards

7. Contract/Grant No.

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

8. Type of Report & Period Covered

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

10. SUPPLEMENTARY NOTES

Q21 Document describes a computer program; SF-185, FIPS Software Summary, is attached.

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

bibliography or literature survey, mention it here)

This report describes work performed at the National Bureau of Standards

under the sponsorship of the United States Environmental Protection Agency to

establish the composition of a fourth series of reference materials intended

to be used for the intercalibration of precipitation measurement stations,

to evaluate the stability of the first three series of reference materials, to

evaluate current methodologies for pH and acidity measurements, and to make

recommendations to improve future reference materials and measurement protocols.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

acidity; acid rain; chemical analysis; conductance; pH; precipitation; rain;

reference materials; trace elements

13. AVAILABILITY 14. NO. OF

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