Coulometric titration of acids in non-aqueous solvents

Talaata.

1968.

Vol. 15. pp. 939 to 948.

Pergamon

Press.

Printed

in Northern

Ireland

COULOMETRIC TITRATION OF ACIDS IN NONAQUEOUS SOLVENTS JAMESS. FRITZ and FRANK E. GAINER* Department of Chemistry and Institute for Atomic Research Iowa State University, Ames, Iowa 50010, U.S.A. (Received 25 January 1968. Accepted 26 February 1968) Summary-Coulometric titrations of mineral acids, sulphonic acids, carboxylic acids, enols, imides and phenols have been carried out in t-butanol or in acetone with electrically generated tetrabutylammonium hydroxide. Either a potentiometric titration or a visual indicator end-point may be used. The amount of acid titrated ranges from 10 to 60 pequiv, and the precision and accuracy of the method are excellent.

IN RECENTyears methods have been developed for the titration of acidic organic compounds in non-aqueous solvents such as pyridine,l acetone2 and t-butanoLs A quaternary ammonium hydroxide or alkoxide titrant is better than a lithium, sodium or potassium salt because a quaternary ammonium salt formed in the titration is more likely to be soluble. In addition, an alkali metal salt may prevent the use of the glass electrode as a dependable indicator electrode in some solvents. A disadvantage of quaternary ammonium hydroxide titrant is the difficulty of preparing it free from impurities. Carbonate and tertiary amines from the Hofmann degradation of the quaternary ammonium hydroxide are difficult to avoid. Marple and Fritz? showed how to remove these impurities from solutions of tetrabutylammonium hydroxide, but the process is rather long and involved. In principle, coulometric titration of acids should avoid these difficulties. Coulometric titration is especially useful for titration of very small amounts of acids. For weak acids, a coulometric titration should be performed in a non-aqueous solvent containing very little water. Crisler and Conlons titrated acids coulometrically in a benzene-methanol solution with lithium chloride as electrolyte. Johansson6 generated bases electrolytically and successfully titrated acids in isopropanol and in a mixture of isopropanol and ethyl methyl ketone. Streuli et a/.’ showed that acids may be titrated coulometrically in acetone with a quaternary ammonium halide as the electrolyte for generation of the base. Williams and his students8 generated quaternary ammonium hydroxide titrant coulometrically for the titration of acids in a benzenemethanol mixture and in t-butanol. Although successful to a degree, the methods cited have been limited in scope. The weakest type of acid titrated successfully has been carboxylic, and the accuracy has Work was performed in the Ames Laboratory of the U.S. Atomic Energy Commission. tribution No. 2250. * Present Address: Eli Lilly and Company, Indianapolis, Indiana. 939

Con-

940

J. S. FRITZand

F. E. GAINER

been limited to approximately f l-4 %. In the present work a wide variety of acidic compounds has been titrated coulometrically in t-butanol and acetone. Substances determined include phenol and other very weak acids. The precision and accuracy for the titration of 10-60 pequiv is excellent. The end-point of the titration may be detected either by use of a visual indicator or potentiometrically. DEVELOPMENT

OF

THE

METHOD

The conditions for the successful titration of a weak acid in non-aqueous media may be summarized as follows. The solvent used should not have acidic properties and should be nearly anhydrous, as the presence of water causes less sharp end-points for weak acids. A quaternary ammonium hydroxide titrant, rather than an alkali metal titrant, should be used and must be essentially free from basic impurities such as carbonate and aliphatic amines. Potentiometric titrations may be performed by using a glass and a calomel electrode. For routine titrations the use of avisual indicator is advantageous. In the coulometric titration technique developed the following system is used. Cathode compartment. A platinum electrode of large area is used. The cathode compartment is filled with 0.1M tetrabutylammonium bromide in t-butanol. Since the t-butanol contains approximately 0.2 % water (Karl Fischer titration), the probable electrode reaction is I-I,0 + e- + OH- + !$-I,(g) Anode compartment. A platinum electrode is used because difficulty was encountered with the silver halide coating that forms on a silver electrode. The compartment is filled with O*lM tetramethylammonium bromide in methanol, which has a lower electrical resistance than t-butanol. The electrode reaction is Br -+ fBr, + eCompartment divider. An anion-exchange membrane is used to separate the anode and cathode compartments. (In some cases a cation-exchange membrane was placed behind the anion-exchange membrane on the anode side, but this appears to be unnecessary.) The membrane used offers lower electrical resistance than does a glass frit. Also, hydrostatic pressure does not force the anode liquid through the anionexchange membrane as it does with a glass frit. Indicator titrations. The titration cell is a plastic box consisting of two square compartments side by side, separated by an anion-exchange membrane (Fig. 1). Each compartment contains approximately 15 ml of electrolyte solution. A suitable indicator for the titration of a particular acid is selected from the published transition ranges in t-butanoP after a potentiometric titration has been done to ascertain the end-point potential for the acid being titrated. Potentiometric titrations. The titration cell used is shown in Fig. 2. The nonaqueous salt bridge to the calomel reference electrode is of a type used by Marple and Fritz4 and gives a reproducible reference potential. A general-purpose glass indicator electrode is used. A combination glass-calomel electrode is convenient when the absolute values of the potentials are not important.

Coulometric

titration of acids in non-aqueous

941

solvents

ET

ANODE

ION’ EXCHANGE MEMBRANE(s)

FIG. l.-Coulometric

titration assembly for indicator end-points

in t-butanol.

CATHODE.CATHODE ANION EXCHANGE MEMBRANE

TO REFERENCE ELECTRODE

FIG. 2.-Coulometric

ILASS ELECTRODE

titration assembly for potentiometric

end-points

in t-butanol.

EXPERIMENTAL Apparatus Current supply. Constant current for the generation of basic titrant was supplied by a Sargent Model IV Coulometric Current Source. Generating electrodes. Two sheet platinum electrodes served as generating electrodes (anode and cathode) for all titrations. For potentiometric end-points the cathode was 19.3 cm* in area, and the anode 16.6 cm*. Contact with the current source was made through platinum wires spot-welded to the platinum sheets. When indicator end-points were used both generating electrodes were about 4 cm* in sire.

942

J. S. FRITZ and F. E. GAINER

Measuring electrode system. Solution potentials were measured with a Corning Model 12 pH Meter and the glass-modified calomel electrode system developed by Marple and Fritz4 Titration cells. The potentiometric titration cell for the generation of base is shown in Fig. 2, and is just a small scale replica of the titration cell recommended by Marple and Fritz.* Only half of the assembly is shown in the figure. The cathode compartment was made of glass tubing 8 cm long and 3 cm in internal diameter. The anode compartment was made of Plexiglas tubing 10 cm long and 9-S mm in internal diameter, with a disc of anion-exchange membrane fixed to one end with epoxy glue or Eastman No. 910 adhesive. The Eastman adhesive had the advantage of being readily usable and quick-drying. Figure 1 shows the cell design that was used for coulometric titrations with indicator end-points. The cell was made of &in. thick Plexiglas having all sides cemented by use of ethylene dichloride. The cell consisted of two halves of a rectangular box which were clamped together by threaded rods and wing nuts. The membr~cs and plastic, if used, were placed between these halves, thus forming the anode and cathode compartments with internal dimensions 1 x 1 x 2 in. The cell was made leakproof by placing rubber cement in the cracks between the two halves (sides and bottom) after clamping them together. A Plexiglas cover was made and used on the box to reduce atmospheric effects on the test solutions. Zen-exchange membranes. The cation-exchange membrane Nepton 61AZL065 and the anionexchange membrane Nepton I1 lBZL065, were obtained from Ionics, Inc., Cambridge, Mass. The selective cation~xchange membrane was comprised of sulphoMted co-polymers of vinyl compounds. The selective anion-exchange membrane was comprised of co-polymers of vinyl compounds containing quaternary ammonium groups and tertiary amine groups. Both types of membrane were homogeneous films cast in sheet form on a synthetic cloth backing. Reagents Reagent grade t-butanol was puritied by fractional distillation. Reagent grade tetramethyl~monium bromide and polarographic grade tetrabutylammonium bromide were used. Solutions of acid samples in t-butanol were prepared such that a 230 ml aliquot could be used for titration. Dilute solutions of the various indicators were prepared. A O.lM solution of tetrabutylammonium bromide in t-butanol was used as the supporting electrolyte. Procedure A (visual indicator) Add IO-15 ml of O-L@ tetrabutyl~monium bromide in t-butanol and several drops of the appropriate indicator to the cathode compartment. Add an equal volume of O*lM tetrame~ylammonium bromide in methanol to the anode compartment. Set the cell cover and generating electrodes in place and bubble a slow stream of nitrogen through the cathode solution to purge it of dissolved carbon dioxide. With a slow stream of nitrogen passing over the cathode solution and with the magnetic stirrer on, generate titrant at the 0.005 setting (~5 mA), 0.01 setting or, 0.02 setting until the indicator changes colour. Then add the sample and titrate it to the same colour change using the same setting to generate the titrant base. Calculate the amount of acid present from the number of coulombs passed, after subtracting the blank. Procedure Z?(potentiometric titration) Use the titration cell shown in Fig. 2, with a glass indicator electrode and a calomel reference electrode with a salt-bridge of the type previously described. 4 If reproducible potentials are not important, a combination glass-calomel electrode may be used instead. Place in the cathode compartment approximately IO-15 ml of O.lM tetrabutylammonium bromide in t-butanol and purge with nitrogen. Add approximately 5 ml of @l&f tetramethylammonium bromide in methanol to the anode compartment. With a slow stream of nitrogen passing over the cell solution and with magnetic stirring, generate the base in several increments at the O-005 setting (~5 mA). After each increment, measure and record the potential of the glass-calomel pair. Continue this process somewhat beyond the blank end point so that a blank titration curve may be constructed. Add the sample, switch to the 0.02 setting (~20 mA), and repeat the incremental generation of base until the acid is titrated and data for construction of a potentiometric titration curve are available. Determine the difference between the acid and blank titrations. RESULTS To test the current efficiency a large number of samples of primary standard benzoic acid were titrated in t-butanol, 2,4-dinitro di’phenylamine being used as visual Results for 20-60 pequiv samples showed a current efficiency of indicator.

Coulometric

titration of acids in non-aqueous

solvents

943

100~0 f 0.5%. Therefore, it was assumed that 100-O% current efficiency could be attained in all subsequent titrations. Results for some 354 titrations of different acids, with visual indicators, are summarized in Table I. The precision and accuracy are extremely good, especially for l-10 mg samples. A gas chromatogram showed that the sample of 2,4-pentanedione contained some impurities. It will be noted that the coulometric method is applicable to the titration of many types of acids including weakly acidic phenols. To obtain results of the precision and accuracy reported, attention must be paid to careful measurement of the titration blank due to impurities in the solvent and reagents. Potentiometric titration curves for some typical weak acids are shown in Fig. 3. It was found not to be possible to record titration curves while the titrant is being generated, so titration curves have to be plotted manually by generating an increment of titrant and then shutting off the coulometer while the potential is measured. In Fig. 4 curves for the titration of toluenesulphonic and benzoic acids are shown, and in Fig. 5 titration curves for some diprotic acids are illustrated. Results for quantitative potentiometric titrations in t-butanol are given in Table II. For the accurate titration of strong acids such as those in Fig. 5, it is necessary to use very pure tetrabutylammonium bromide, to purify the solvent used and to subtract the blank which remains despite these precautions. The curves in Fig. 6 show that the solvent blank may be reduced appreciably by double distillation of the t-butanol. Purification also largely avoids the buffering effect of weak acid impurities on the titration of strong acids such as sulphuric or aromatic sulphonic acids. The coulometric method outlined above gives excellent results for the titration of a variety of acidic compounds either potentiometrically or with visual indicators. The titrant generated electrically is of excellent purity, and the tedious preparation of pure quaternary ammonium hydroxide titrant by chemical means is obviated. However, a limitation of the coulometric method is that low current densities must be employed. With the titration cells described, the maximum current for precise results is approximately 20 mA. With a current much higher, the cell resistance increases and the current efficiency is lowered. The crux of the problem seems to be the difficulty in transporting ions at a sufficiently fast rate through an ion-exchange membrane or glass frit. COULOMETRIC

TITRATIONS

IN

ACETONE

Apparatus

The cell for potentiometric titrations in acetone solution differed somewhat from the cell already described. The glass cathode compartment was 8 cm high and 3 cm in internal diameter, and the anode compartment was a piece of glass tubing 10 cm long and 10.5 mm in internal diameter with an ultratine-porosity glass fritted disc on one end. The measuring electrode system was a slenderbodied combination glasscalomel electrode with the aqueous saturated potassium chloride solution replaced by a saturated solution of potassium chloride in methanol. The generating electrode system remained the same. Reagents

A spectrographic grade of acetone was obtained from Eastman Organic Chemicals and used in this studv, without further drvine or vuriflcation. The acetone contained 0.35% water WV). The electrolyte solution for the cathode compartment was O.lMpolarographic grade tktrabutylankonium bromide in spectrographic grade acetone. Tetramethylammonium bromide in methanol served as electrolyte solution in the anode compartment. ,Y

I

Procedure

As previously described for potentiometric

titrations

in t-butanol.

Benzoic acid Phenol 2,4,6_Trimethylphenol ZHydroxyphenol Anthranilic acid Succinic acid (1st H) (2nd H) Acetoacetanilide Dibenzcyhnethane Succinimide Hydantoin 2Cyanoacetamide Ethyl cyanoacetate 2,CPentanedione p_ToIuenesulphonic acid

Compound titrated

6 41 13 17 10 22 2.5 16 12 10 8 12

96 I

59

No. of samples

~.--cOULOhIETRIC

10-60 12-61 20 20 10-60 10-20 10-60 20 20-48 10-60 20 20 20 30 2&60

Amount taken, pequiv

TABLE

100.1, 100.1 99.7, 99.4 100.1 99.7 99.4 99.8, 99.9 99.9 99.5 99.7 99.1 97.5 99.9

Recovery, %

TITRATIONS

IN

@I7 0.21 041 0.19 0.13 0.08 0.13 0.12 0.11 0.22 0.11 0.15 0.07 0.06 @OS

Std. deviation,

t-BUTANOL

%

2&dinitroaniline 2nitroaniline Znitroaniline Azo Violet 2,k.linitroaniline Bromothymol Blue Azo Violet Azc Violet Azo Violet Azo Violet Azo Violet p-nitro-p’-aminoazobenzene Azo Violet Azo Violet Bromothymol Blue

Indicator

I

2

m

k .F

$

Y VI

E

Coulometric titration of acids in non-aqueous solvents I

I

I

I

I

I

I

945

I

I. ACETOACETANILIDE 2 SUCClNfMlDE 3 DlWNZOYLMETHAt+E 4 IrHENOL.

_

FIG. 3.-Titration of enols, imides, and phenois in t-butanol with el~rol~ically generated titrant.

-6OO-700 -800

0

’ 4

’ 8

’ ’ 12 ‘6

’

’

20

24

’

’

28

32

J

36

BASE GENERATED blICROEQUIVALENTS)

I 200

I

I

I

I

I

2

loo

I

I

I

I

I

I

I

I

l.p-TOLUENE~LFONIC 2MXTURE OF GENZOIC AN0 p-TOLUENESULFONIC 3.GENZOIC

-

o-100 7 -2OO-

2 ti

-300

$400-I ii g

-5OO-

5 +Oo-

-700 -

I 16 BASE

24

GENERATED

32

40

6

(MICROEQUIVALENTS)

Fro. 4.-Differentiating titration of henzoic acid and p-toluenesulphonic in t-butanol with electrolytically generated titrant.

acid

946

J. S. FRITZ and F. E. GAMR 300

,,,

IIll

II

11

11

11

200 I SULFURIC 2 OXALIC 3 SUCCNC

100 2

!

0 -1 -7 3 -100 3 r

~

-1

-200

ij

_rno-

-

-400

-

I

-5oo-

-6OO-

-800

0

I

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 64 32 40 40 56 a 16 24 6~s~ GENERATED (MICR~E~UIVALENW

Era.

K-Titration

of dibasic acids in t-butanol

with electrolytically

generated titrant.

Because acetone has a much higher dielectric constant than t-butanol, the electrical resistance of solutes in acetone is much lower. This makes it possible to use a glass frit, rather than an anion-exchange membrane, to separate the anode and cathode compartments. (Also, acetone tends to dissolve the binder in ion-exchange membranes.) TABLEII.--COUL%XTRIC

Compound

TITRATIONS OF ACIDS IN GBUTANOL Em-POINTS

titrated

Benzoic acid Phenol Acetoacetanilide Dibenzoylmethane p-ToluenesuIphonic

acid

Sulphuric acid (1st end-point) Sulphuric acid (2nd end-point) Oxalic acid (1st end-point) Oxalic acid (2nd end-point)

No. of samples

WITH

POTENTIOMETXXC

Taken, pequiv

Found, pequiv

20.21 20.08 20.03 20.02 20.11

20-24 20.26

20.70 50.18 19.25 40.09

20.70 50-21

20.14 20.34 20.07

19.20 40.03

Recovery, % lOO*l 100.9 1005 101.6 99.8 100.0 100.1 99.7 99.9

Coulometric 100

I

titration of acids in non-aqueous

I

I

I

solvents

947

I

I =UNDISTiLLED SOLVENT 2’SINGLE DISTILLED 3=DOUBLE DISTILLED

04

0

00

BASE

Fro. 6.-Titration

GENERATED

of t-butanol

I.6

12

2 .o

(MICROEOUIVALENTS~

solvent blanks with electrolytically

generated titrant.

I 5.5-OIMETHYL-1.3~CYCLOHEXANEOIONE 2. ACETOACETANILIOE 3 OIBENZOYLMETHANE

- 200

0

8

BASE

16

GENERATED

FIG. 7.-Titration

24

32

(MICROEQUIVALENTS)

of enols in acetone with electrolytically generated titrant.

-Boo0

4I BASE

8I

12 I

16 I 20I

24 I

28 I

32I

36

GENERATEDDMICROEQUIVALENlS)

FIG. S.-Titration of imides in acetone electrolytically generated titrant.

with

J. S. FRITZ and F. E. GAINER

948

TABLEIII.--COULOMETRIC Compound

TITRATIONS

titrated

Benzoic acid Benzoic acid Benzoic acid Benzoic acid Acetoacetanilide Acetoacetanilide Acetoacetanilide Acetoacetanilide Acetoacetanilide 5,5-Dimethyl-1,3-cyclehexanedione Dibenzoylmethane Succinimide Phenol 2,4-Dinitrophenol S-Dinhenvlthiourea p-To&tene&.tlphonic acid p-Toluenesulphonic acid p-Toluenesulphonic acid p-Toluenesulphonic acid Electrolyte:

O.lM tetrabutylammonium

OF

ACIDS,

WITH

POTFNTIOMETRIC

END-POINTS

No. of samples

Taken, pequiv

Found, pequiv

3 3 3 4 3 3 4 5 3 4

10.06 20.11 30.17 10.11 20.21 30.32 40.42 60.16 20.16

10.16 20.08 3094 60.81 IO*08 20.32 30.36 40.28 59.81 19.55

101.0 99.9 102.6 99.9 99.7 100.5 100.1 99.7 99.4 97.0

3 2 1 2 2 3 3 3 3

20.07 20.26 21.13 20.20 2011 10.37 20.73 31.10 60.66

20.17 20.53 21.15 20.20 20.13 10.34 20.76 31.39 60.59

100.5 101.3 loo*1 lOO*O 100.1 99.7 100.1 loo*9 99.9

60.89

Recovery, %

bromide in acetone.

Titration curves for some weak acids in acetone are shown in Figs. 7 and 8. Results for quantitative coulometric titrations in acetone are given in Table III. It may be concluded that coulometric titrations in acetone by the system proposed are quite successful. However, t-butanol seems better suited to the titration of mixtures of acids, and gives a lower solvent blank than acetone does. Zusannnenfassnng-Mineralsluren, Sulfonsauren, Carbonsiiuren, Enole, Imide und Phenole wurden in t-Butanol oder Aceton mit elektrisch erzeugtem Tetrabutylammoniumhydroxid coulometrisch titriert. Man kann den Endpunkt potentiometrisch oder visuell mit einem Indikator bestimmen. Die titrierte S&rremenge reicht von 10 bis 60 r&q, Genauigkeit und Richtigkeit der Methode sind ausgezeichnet. R&n&-Gn a men6 des titrages coulometriques d’acides mineraux, acides sulfoniques, acides carboxyliques, enols, imides et phenols en t-butanol ou en acetone au moyen d’hydroxyde de tetrabutylammonium produit electriquement. On peut utiliser soit un titrage potentiometrique, soit un indicateur visuel pour le point de fin de dosage. La quantite d’acide tit& se situe entre 10 et 60 ,UBquiv., et la precision et la justesse de la methode sont excellentes. REFERENCES 1. 2. 3. 4. 5. 6.

R. H. Cundiff and P. C. Markunas, Anal. Chem., 1956,28,792. J. S. Fritz and S. S. Yamamura, ibid., 1957, 29, 1079. J. S. Fritz and L. W. Marple, ibid., 1962, 34, 921. L. W. Marple and J. S. Fritz, ibid., 1962, 34,796. R. 0. Crisler and R. D. Conlon, J. Am. Oil Chemists Svc., 1962, 39, 470. G. Johansson, Talanra, 1964, 11,789. 7. C. A. Streuli, J. J. Cincotta, D. L. Maricle and K. K. Mead, Anal. Chem., 1964,36,1371. 8. C. Cotman, W. Shreiner, J. Hickey and T. Williams, Talanta, 1965, 12, 17. 9. L. W. Marple and J. S. Fritz, Anal. Chem., 1963, 35, 1305.