Reference

BUM

NBS

PUBLICATIONS

NBSIR 85-3114

NATL INST. OF STAND & TEC

AlllOb D3M71M

Evaluation of Methods Used for

the Determination of Acidity in

"Acid Rain" Samples

G. Marinenko and W. F. Koch

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Analytical Chemistry

Gaithersburg, Maryland 20899

March 1985

Prepared for U.S. Environmental Protection Agency

Environmental Research Center

tese

-QC—

'i QQ tesearch Triangle Park, NC 2771

.1156

85-3114

1985

NATIONAL BUREAU OF STANDARDS

LIBRARY NBSIR 85-3114

EVALUATION OF METHODS USED FOR

THE DETERMINATION OF ACIDITY IN

"ACID RAIN" SAMPLES

G. Marinenko and W. F. Koch

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Analytical Chemistry

Gaithersburg, Maryland 20899

March 1985

Prepared for U.S. Environmental Protection Agency

Environmental Research Center

Research Triangle Park, NC 27711

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

NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Director

Introduction

The existing methodologies for the determination of acidity of

atmospheric precipitation samples were reviewed by us earlier [1]. The

purpose of this work is to establish the performance characteristics of a

number of these methods under conditions which exist in a single laboratory.

Such a test should provide a valid method intercomparison, free from

laboratory-to-laboratory variations. Furthermore, in this test, the same

solutions were used in all intercomparison measurements, and therefore, any

difference in the results could not be attributed to the compositional

differences in samples.

Experimental Part

Several different titration procedures were evaluated with respect to

their applicability to the determination of acid concentration (acidity) of

dilute solutions of a strong acid (HC1). The concentrations of these samples

were in the range of interest for acid rain studies. The following is the

list of methodology variations employed:

(1) Titration with 10

-

^ mol/kg NaOH, using phenolphthalein as indicator,

(2) Titration with 10 ^ mol/kg NaOH, using a pH meter and pre-determined

end-point pH's,

(3) Titration with 10 ^ mol/kg NaOH, using the Gran plot technique for

end-point detection [2],

(4) Coulometric titration using the Gran plot technique,

(5) Coulometric closed loop titration to pH 8.1 in the electrolyte

containing 0.1 mol/kg KC1 and 10 mg sodium citrate per 100 mL [ 3 J

2

Methodologies (1) and (2) were tested using two HC1 solutions prepared

gravimetrically from the coulometrically standardized 1 mol/kg stock solution

containing 0.0201 and 0.2001 yeq of H

+ /g of solution, respectively. The

results of these analyses are summarized in Table I.

In Table I, column 1 lists three titration end-point pH values, measured

using a combination electrode and a pH meter, and a fourth value (pH=9.2)

determined with phenolphthalein indicator. The raw titration data for

solutions 1 and 2 are listed in columns 2 and 3, respectively. Column 4

gives the apparent acidity of distilled water, determined to the corresponding end-point pH’s. By taking the data for distilled H2O as the ’’blanks" and

applying these respective corrections to the data in columns 2 and 3, the

data in columns 5 and 6 are obtained.

It is quite apparent from these data (columns 2 and 3) that titrations

with sodium hydroxide solution will yield different results for acidities

depending on the pH selected for the end-point if no "blank" correction is

applied, with recovery ranging from 1 00% for pH 6.M to the maximum of 25^$

for pH 9.2. This is due primarily to the dissolved C0

2 in the sample. If

distilled water saturated with air is used as the "blank", the C0 2 titer is

factored out of the final result. Nevertheless, even after the "blank"

correction, some residual error still remains for samples titrated to pH 8.0

and 9.2. The magnitude of this error is 3x1 yeq/g and 5x1

-

^ yeq/g for

the two end-point pH’s, respectively. The overall root-mean-square error of

the results for this titration procedure is 2x10

-

^ yeq/g.

Variation (3) in apparent H

+ concentration as a result of titrating the

acid with 10 ^ mol/kg NaOH and employing Gran’s method for establishing friend-point of titration was studied on a limited basis. The Gran's plot

method, which requires incremental delivery of reagent and measur- rr- r.t :

3
after each delivery, is very tedious and difficult to implement, unless it is

automated. The primary difficulty lies in the ability to control precisely

the size of small reagent increments. When the increments are not of exactly

the same size, the graphed function of AV/ApH as a function of V is not very

smooth (where AV refers to the standard NaOH solution increment, ApH is the

pH change resulting from the addition of the reagent increment, and V is the

amount of reagent added which corresponds to the particular AV/ApH). On the

basis of three titrations, the recovery of acid was 104$ with a standard

deviation of 11$.

A far more productive procedure is the fourth method listed above, which

entails use of a Gran’s plot technique for the end-point determination

combined with coulometric in-situ generation of reagent (base) at constant

current. By selecting an appropriate value of the current, one can

'fine-tune' the system so that the time increments for reagent generation are

precisely controlled, e.g., 5 seconds. In fact, the procedure is so easily

adaptable to point-by-point incremental measurements that one can easily

discern two different segments of the titration curve: one for the titration

of a strong acid and the second for the titration of the weak carbonic acid,

present in the sample due to the absorbed atmospheric C02

. This is shown in

Figure 1, which illustrates a Gran's plot of the titration of a 24 g sample

of 2*1 0~-> mol/kg HC1 . Two distinct inflections are apparent.

For the purposes of intercomparison, two other sets of the same titration

data are plotted on a nearly identical scale. Figure 2 contains a conventional acidimetric titration graph of pH as a function of coulometrically

added titrant, expressed in seconds. Also, Figure 3 shows a differential

titration graph of ApH/s as a function of added titrant. Both the

4
differential plot and the Gran's plot reveal quite vividly the two

inflections. The conventional plot of pH vs titration time in Figure 2, on

the other hand, is far less sensitive. It was of interest to establish if it

were feasible to separate graphically, and determine quantitatively, the two

dissimilar acids (HC1 and H^O^) in a mixture by Gran's technique. The

results of this phase of the study are summarized in Figure 4. Three samples

[deaerated 2*10”^ mol/kg HC1 (A), air-saturated distilled H

2 0 used for

preparation of the above HC1 solutions (B) and air saturated 2»10 ^ mol/kg

HC1 (C)] were coulometrically titrated at 0.9649 mA. At this titration

current, the rate of reagent delivery is 0.01 yeq/s. The calculated value of

the end-point for the 24 g sample of HC1 corresponds to 49.2 s, and the titer

obtained for deaerated HC1 sample (A) is nearly the same (50 s). The

end-point for air-saturated H

2

0 sample (B) is 28 s. Thus, on the basis of

the additivity of titers, one would expect to find the titer for the airsaturated sample of HC1 to be 78 s. Unfortunately, as seen on graph C, this

is not the case. The titer is 92 s. Therefore, a positive bias of about 18$

is encountered for this particular end-point evaluation of the sum of the two

acids. It is also quite apparent that the quantitative graphic separation of

the two acids in a mixture is quite impossible. Hence — some indication of

the presence of the two acids in the mixture is there but quantitation is not

possible. This situation may be of some concern to those who determine

acidity of rain samples.

The fifth method, coulometric closed loop titration, consists of the

following: 20 mL of electrolyte containing 0.1 mol/L KC1 and 1 mg/L potassium citrate is delivered into a coulometric titration cell, deaerated by

purging with argon and pretitrated by coulometric generation of 0H~ to pH

8.1. Subsequently, a sample of unknown solution (e.g., 2g) is weight,:! into

5
the coulometric cell with the pretitrated electrolyte and purged with argon.

The generation of titrant (0H~) is resumed until the pretitration pH value is

restored. The number of coulombs required to return the system to the pH of

the pretitrated electrolyte corresponds to the titer of the sample.

An intercomparison of the results of closed loop titrations with Gran’s

plot procedure is presented in Table II. The data reported in columns 2 and

4 were obtained with an argon atmosphere maintained over the electrolyte

during the titration. This procedure precludes atmospheric C02 from

redissolving in the electrolyte in the course of titration. However, any C0 2

in the sample solution is not excluded by this procedure, and thus it becomes

part of the titer. The data in columns 3 and 5 were obtained with solutions

subjected to continuous argon purge. Thus virtually all of the C0 2 should

have been expelled in this procedure, and only nonvolatile acids would

contribute to the titer.

In Table II, solution A was 0.0500 mol/kg potassium acid phthalate, pH 4

buffer, prepared from SRM I85f. Its concentration corresponds to 49.50

yeq H

+ per gram of solution (yeg/g). The determined titer for samples which

were not deaerated, both in the case of closed loop and Gran’s plot, was

significantly higher than the calculated value (by 0.25 yeq/g and 0.3 yeq/g,

respectively). The closed loop titration of deaerated solution A yielded a

value of 49.57 yeq/g, which is only 0.07 yeq/g higher than the computed

value. This difference is well within the uncertainty of the measurements on

this solution. The Gran plot procedure yielded a value of 49.3 yeq/g. Again

a significant reduction in the titer resulted from deaeration with argon.

Similar behavior was observed for strong acid samples B, C, D, and E in Table

III. As a final step of this study, it was of interest to establish the

contribution of C0 2 to the determined acidity of rain-like samples. For this

6
purpose, three solution concentrates of acid-precipitation research material

prepared by the U.S. Environmental Protection Agency were used (1000, 2000,

and 3000 series). The samples of the investigated solutions were weighed

into a coulometric cell containing a deaerated solution of KC1 (1 mol/L),

which was initially pretitrated to pH = 7.0. The samples were either taken

as the concentrates directly from the original bottles, or were diluted 1:50

by weight with distilled, deionized H

20. After sample delivery, the cell

contents were either titrated immediately or were deaerated by purging the

solution with argon gas for ten minutes. An argon atmosphere was maintained

over all solutions while in the titration cell to prevent reabsorption of

CO 2

The titrations were performed ’'cyclically” , i.e., by measuring the time

necessary to return the system to the starting pH (7.0) while electrogenerating titrant in the solution. The data obtained are summarized in Table

III. The following conclusions may be drawn. First, samples deaerated with

argon (first three entries in Table III) show no significant difference

(Column 5) in the determined acidity using either concentrated or (1:50)

diluted samples. This clearly shows that after deaeration the distilled H

2 0

introduces no additional acid/base modifications to the samples. Secondly,

materials which were not deaerated exhibit significant differences between

diluted and undiluted samples. If one attributes this difference to the

dissolved C0 2

, then the extrapolated value for carbonic acid is 0.69 ueq H

+

per 49 g of distilled H

2

0. Finally, direct determination of carbonic acid in

distilled H

2

0 yields the value 0.70 peq H

+ per 50 g of distilled H

2 0,

confirming the extrapolated value. In view of the fact that difficulties are

encountered in maintaining the C0 2 concentration invariant, deaeration of

solutions for acidity determinations is recommended.

7
Conclusions

Most accurate and precise determination of acidity by any method requires

deaeration of samples prior to titration to remove the dissolved C0 2

. Five

methods for the determination of acidity of acid solutions of low concentration (down to 1(T 5 mol/kg) were investigated. Four of the methods provide

satisfactory results. Method (3), Gran's plot end-point detection using 10~^

mol/kg NaOH, in a conventional titration suffers from the inability to reproduce the size of reagent increments, which is essential for this method.

Automated titration systems could remedy this deficiency. "Closed loop"

titration procedures are most convenient, where applicable.

Resolution of the two segments of a titration curve, in a reliable and

quantitative manner, into strong acid and carbonic acid components does not

appear feasible, even when using Gran's plot method.

Coulometric delivery of titrant is recommended as a reference method for

the following reasons: (1) in- situ generation of OH

- at constant volume

eliminates the need for dilution corrections; (2) coulometric apparatus are

highly amenable to maintaining inert atmospheres, hence excluding atmospheric

C0

2

; (3) aliquot delivery of titrant can be easily controlled. All of the

titration procedures are biased in a positive direction by the presence of

dissolved atmospheric C0 2

. It is recommended that titrations of acidic

precipitation be performed on deaerated samples only, to eliminate this

source of variability and uncertainty.

8
REFERENCES

[1] Marinenko, G. and Koch, W. F., "A Critical Review of Measurement

Practices for the Determination of pH and Acidity of Atmospheric

Precipitation," Environment International (accepted for publication,

August 1984).

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

Titrations," Acta Chem . Scan . 4. 559-577 (1950).

[33 Koch, W. F., Marinenko, G. and Stolz, J. W. , Simulated Precipitation

Reference Materials, IV, NBSIR 82-2481, October 1982.

9
Figure 1. Gran's Plot of Coulometric Titration of 2x10”5 md/kg ^Cl

10
Figure 2. Conventional Plot of Coulometric Titration of 2x10~5 m0 j/ k g hci.

1
A

pH/5

s

Figure 3 . Differential Plot of Coulometric Titration of 2x1 0’^ mol/kg HC1.

12
Hd

V/ss

0 20 40 60 80 100 120

Titration Time at 0.9649 mA, seconds

Figure 4. Determination of Acidity by Coulometric Titration Using Gr-m':

Plot Technique.

A ~ 24g sample of 2x10^ mol/kg HC1 , deaerated with argor.;

B ~ 24g sample of air saturated distilled, HoO;

C ~ 24g sample of air saturated 2x10~5 mo l / kg HC1

13
Table

I.

Gravimetric

Titrations

of

HC1

Solutions

Using

0.9989

peq/g

NaOH

C.o

<M

CM

•o CM * =T in c 0) c O o o o '

1 4-3 0) CM CM CM CM o O <—1 • • • • CO 0) OQ

tS-o o o o o ooo

<M

T— o ^ cr> CM vO c Q> C CM «

CM CM 1—1 4-> 05 O o O O o O rH • • • • CO CD CD o o o o c C, 1—1 t- o o

CO o

bO Ncr T3

CD on CD

i—i cr> •=T in c rH O r o CM * 1—i •H O o o o + o 40 x A • • • X CO CO o o o •rH Cm Q Oco bO

••H

N 4-3 cr

03 CMQ

S- 3. CM o GO o 4-3 c i—

O 'on c rH *- O CM CM CM CM

(1) o o x • • • • o CO o O o o o c CM o • o o

bO \

cr '

d) t

GO vO

r

c *—

CM CM on LPl 1—1 >- O O O o O o O X • • • • CO CM o o o o OO / X c

•H

0)

4-3 rH c 03 •H s: o •=r o o CM 4.3 Ou x • • • • jC r a vO c— CO CTV Q.

•oc o w C

CD szCl — • CMOoo

It m

4-> 03 •H>o

TDT3L

03 •oc

oJ

4-3 co •o

CD i—

oo

a.

U
Table

II.

Acidity

Determinations

of

Five

Solutions

by

the

Coulometric

Closed

Loop

Titration

and

Gran's

Plot

Techniques

a:o. 
<

-PoI—

Ou co

*i"4 •P c

03 o 05 mo *-0 in

C- bOO •=r tb- oo

CD t. on CM o o <—

03 < • » • • • 0 O' o o o o a -=T

4-5

(1) -rH -P 3

CD I—

asoo o

CObO \

cr

a)d

<POco

•rH -P

03

4-5 c

<13 Ocoo c

03

S- o

rH LC5 o LTi M0

CD i

M0

C

05

•rH 00 C\1 o o T X • • • 0 • CO ON O o o o co

bO

SCD

4-5

tfl

4-5

•rH OE3

•rH too

00

COX

o.

o.oo

P3TO

cd

COoI—

o co

•rH

4-5 03t-

<D

030) a

CD

4-5 CD i—

a.Eoo x

ooco

bO <co

bO t>- b—O

LT5

LC5 =r

CO MOC~- S- LO CM O o T < • • • • •

-P

CT» ^r

OO o o

CM

C

05 •rH f-4 IT) T LC5 05 05 > >

b- CM

O o r— CD 0 • . • • • TO TO 05 O O o o

bO \cr

CD3. bO MO \ O cr o

CD • a. o

CO II o• co o

II cu

co o

4-5

CQ <

CO

(D <D i—I i—

Q. Q. E E

03 03 CO CO

t- C. O O

<4-1 <4-1 C C O o

•r4 *i-4 4-5 4-5 03 05

to

s03 o) •o TO C C

03 03 43 4-5 CO CO

CQ

O

Q CU

-o -o

CD CD <—I i—

o o o o

a. cl o

oo

15
Table

III.

Comparison

of

the

Results

for

Diluted

(1:50)

and

Undiluted

"Acid

Rain"

Reference

Solutions

<u

4-5

ctj C— CO -=T e'- 05 u O T— o CO in en 50

4-5 < • • • • • • • c o o o o o o o

CD + n t + + + + OCoO / s • t— c i—1 .. r—

CT3 r— o O C

V-4 in 05 CO •H U

bO i—1 o ^T 50 in •=T CO o bO <3. o 05 •=r -=T CO T— t- bO •H £-« •o a • • • •o • • • • s

t- CD CTJ in -=T CM 0) in in CO o + o £4-5

4-5 3i—1

CO ' 4-5

CTJ s-

*”* 2

o* o •» •H CD CD

»*

Q CTJ 3- bO N o

Cl) 1 •»

CD Q CO + o 2 nJ

0) 4-5 • 4-5 o o cr £-05

nj

<u

CTJ 2 =r

d) t- <D 05 in CO t- o 50 «— (0 3. 4-> 1—

05 o in o in o •H c Q. • • • • • • • C

U4 CD

a =r in CM in in CM o c

•H

O CTJ i

*

o C CO •rH o» o 4-5 s- O CTJ

CTJ •»-4 >

CO CD 4J C •o 1—

o 3 •f—

o o O o o O •o

CO 4-5 • o o O o o O o C.

<u 3 o o o O o o O CM CTJ cc i—

2 co C\J t

CO CM *— 2 •o o C

cn CTJ 4-5 CO

<1) •o

rH CD O. • »—

a O 1— CM CO -=T in 50 C- o

nJ 2 o

CO Ou

16
5-1 14A IBEV. 2*80 ) U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date I

BIBLIOGRAPHIC DATA

REPORT NO.

March 1985 SHEET (See instructions) NBSIR 85-3114

4. TITLE AND SUBTITLE

Evaluation of Methods Used for Determination of Acidity of "Acid Rain" Samples

5. AUTHOR(S)

G. Marinenko, W. F. Koch

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

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Gram No.

8 . Type of Report & Period Covered

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

Same as No. 6

10. SUPPLEMENTARY NOTES

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

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

bi bl iography or literature survey, mention it here)

Five methods for the determination of acidity of acid solutions of low concentration

(down to 10 5 mol/kg) were investigated. Four of the methods provide satisfactory

results. Method (3), Gran's plot end-point detection using 10”^m NaOH titrant, suff »r from the inability to control exactly the size of reagent increments, which is esaen • tial for this method. Automated titration systems could remedy this deficiency.

"Closed loop" titration procedures are most convenient.

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

acid rain; acidimetric method evaluation; acidimetry; acidity; closed loop tiinti n end-point determination

13. AVAILABILITY

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PRINTEO PAGES

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