Amperometric titration of arsenous salts by lead tetraacetate solution

MICROCHEMICAL

JOURKAL

8, 1-j

Amperometric Lead PETR Department

AD~EK,

of .4nalyticaZ

(1964)

Titration of Arsenous Tetraacetate Solution JAN Chemistry, Received

DOLE~AL, Charles’ Sovember

AND JAROSLAV University,

Prague,

Salts

by

ZPKA Cserhoslovakia

l-7, 196.3

INTRODUCTIOS

The volumetric solution of lead tetraacetate in glacial acetic acid is a strong oxidation agent that has proved its value in the determination of a number of inorganic as well as organic substances (1-4). One of the reliable determinations of this kind is the potentiometric titration of arsenous salts, suitable for amounts of Q 2 mg As. With respect to the rapid and quantitative course of the reaction, we have attempted to increase the sensitivity of the method by using amperometric indication of the point of equivalence, and a method has been worked out that is suitable for microdeterminations of arsenic. MATERIALS

AND

METHODS

-4 mercury drop electrode was used as the indicating electrode. It was connected to a saturated calomel reference electrode by means of a salt bridge filled with 50% acetic acid and saturated with ammonium acetate and potassium nitrate. The necessary applied voltage was branched off on a Heyrovsky polarograph of the V-301 type. The required polarograms were also registered on this instrument. Oxygen was removed from the solution by passing a stream of nitrogen through the solution for 1S-20 minutes before titrating. -4 0.1 S lead tetraacetate solution was prepared and standardized according to a well-proved procedure (4) and stored in dark bottles. Solutions of 0.01 X and lower were prepared by precise dilution with glacial acetic acid. All other reagents used were denoted throughout as “reagent-grade.” RESULTS

In studying the course of the amperometric titration it was found more convenient to follow changes of the limiting current of arsenous salts,

2

PETR

ADhEK,

JAN

DOLEiAL,

AND

JAROSLAV

Zi.KA

whose polarographic behavior is well known; however, it was first attempted to find out whether it would be possible to utilize in this case similarly the limiting current of Pb(IV). The polarographic behavior of lead tetraacetate was therefore studied in an orientative manner: the following was found: In a medium of acetic acid and 0.5 il4 ammonium acetate a discontinuous maximum appears on the polarographic curves of the reduction of lead tetraacetate. The maximum may be suppressed by a higher fuchsin concentration. However, when 1 M ammonium acetate is used as electrolyte in acetic acid medium, the fuchsin concentration required to suppress the maximum is greatly decreased. In a saturated ammonium acetate solution the maximum practically does not appear, or rather it may already be suppressed by a few drops of 0.01 M fuchsin solution in acetic

T

6

IO n

FIG. 1. Polarogram of lead tetraacetate. Solution: 50 ml saturated ammonium acetate in acetic acid; 0.1 ml 0.01 M fuchsin solution in acetic acid; and 0.05 M Pb (CHaCOO), in amounts of: (1) 2.0m1, (2) 1.8m1, (3) 1.6m1, (4) 1.4m1, (5) 1.2m1, (6) l.Oml, (7) OAml, (8) 0.6m1, (9) 0.4m1, (10) 0.2 ml, (11) 0. Sensitivity 1.40, starting at 0.4 V (SCE), 200 mV/abscissa.

acid. With increasing ammonium acetate concentration the height of the maximum decreases, and at the same time its discontinuous character decreases also. The dependence of the limiting current on lead tetraacetate concentration is linear, passing through the origin of the coordinate system (Fig. 1). The dependence of the limiting current on the square root of the

AMPEROMETRIC

TITRATIOS

OF

AS”’

BY

PB (IV)

SALT

3

reservoir height is linear in the graphic plot but does not pass through the origin. The suppression of the maximum by increasing the supporting electrolyte concentration is mainly due to increased viscosity of the solution. Experiments designed to study the polarographic behavior of lead (IV) salts in media of some mineral acids, the only media in which the amperometric titration of arsenous salts takes place, were unsuccessful since solutions of lead (I\‘) salts hydrolyze in such media relatively rapidly, and polarographic curves are reproducible only with difficulty. For this reason, the limiting current of arsenous compounds was utilizetl for the amperometric titration, this current being suitable for the amperometric purpose in a medium of 3 S H-SO4 [this is most suitable for the course of the reaction between As (III) and Pb(I\,‘) acetate (4) ] with the addition of gelatine. The procedure for the titration of trivalent arsenic is very simple if solutions of concentrations higher than lo-” M are used. Nitrogen is first passed through the solution to be analyzed for a period of 1.5 minutes! and after each addition of the volumetric agent, i.e., 0.1-0.01 A’ Pb(CH3 COO)r, the nitrogen stream is again passed through for 2 minutes. Current values are read after 30 seconds. During the titration, a precipitate is formed in the solution due to the reaction of the Pbf+ ions formed with sulfuric acid. The range of the trivalent arsenic limiting current is broad (5) and practically does not change with the sulfuric acid concentration. For the measurement, it is suitable to apply a voltage of -0.8 to -0.9 V Saturated Calomel Electrode (SCE). After the point of equivalence, a small increase of the current is observed due to polarographic reduction of lead (IV) ions (see Fig. 2 ). (see Fig. 2). The slope of this second part of the titration curve is greatly dependent on the titration conditions! particularly on the period of passing nitrogen through the solution. It was proven with polarographic means that this current corresponds to lead (IV) ions. When the arsenic concentration is decreased by one or two orders of magnitude, the range of the limitin, u current decreases. In this case it is necessary to work with an applied voltage of -0.7 Y (SCE). In the titrations the character of the graphic plot of titration curves, in the case of very dilute solutions, changes a little. and no perfect straight lines are obtained when connecting points giving current intensity values; however, the point of equivalence can be determined precisely even in the case of these very dilute solutions (Fig. 3a.b). With these arsenic

3

PETR ADAMEK,

FIG. 2. Amperometric 1 ml 5.10-*M KaAsO,, S.lO-‘M Pb (CHsCOO),;

JAN

DOLEiAL,

AND

JAROSLAV

Z+KA

titration of As(II1) by lead tetraacetate. Titrated 50ml 3 N H&SO,, 0.6ml 0.5% gelatin. Volumetric -0.9 V (SCE).

solution: solution:

FIG. 3. Titration of As(III) by lead tetraacetate. Titrated solution: 50 ml 3 N H,SO,, 1 ml 5.10-aM K3As03, 0.7 ml 0.5% gelatin. O-l.4 ml 5.10-s&’ Pb (CH, COO), solution added in volumes of 0.1 ml. Sensitivity 1:7, start at 0.2 V (SCE), 200 mV/abscissa. (a) Polarographic titration; (b) amperometric titration.

concentrations there is also no distinct turbidity of precipitating lead sulphate to be observed. A small amount of lead (II) salt was therefore added (1 ml lead acetate trihydrate for 50 ml 10m4A4 As3+) in order to shift the reaction equilibrium in the direction of the quantitative course of the reaction. It has been found that this improves the course

AMPEROMETRIC

TITKATIOS

OF AS”’

BY PB (IV)

SALT

5

of the titration and, at the same time, the precision of graphic determination of the point of equivalence (linear course of the titration curve). When determining arsenic in concentrations lower than lop3 M, excess lead (IV) salts often hydrolyze totally, and the second straight line is then often parallel with the abscissa. It has been found experimentally that it is possible to determine amounts as low as 3.10PZM As. The presence of lead (II) ions is then necessary. In the absence of these ions, the reaction does not proceed at all. Oxygen must be removed from the solution very carefully, its limiting current being far higher than the limiting current of arsenous ions at this dilution. After every reagent addition it is also necessary to pass nitrogen through for 3-4 minutes. When arsenic is determined in concentrations lower than 3.10-5 M, the range of the limiting current becomes so narrow that it no longer can be utilized for the amperometric titration. The amperometric titration of arsenic by lead tetraacetate is relatively very precise, the mean relative error being rt 0.70% for lo-‘-lo-” iI As”+ solutions, and i- 2.1y0 for the most dilute solutions. SUMMARY The polarographic behavior of lead tetraacetate has been studied in an orientative way, and conditions have been found for the amperometric determination of arsenic (III) salts by means oi a volumetric solution of this reagent in concentrations down to 3.10-S M. REFERENCES 1.

2.

3. 4.

5.

J., AND BERKA, A., Use of lead tetraacetate as volumetric oxidimetric reagent Journal Symposium Series” for microanalytical purposes. In “Microchemical (N.D. Cheronis, ed.), (“Microchemical Techniques”) pp. 789-796. Wiley (Interscience) New York 1962. Z~A, J., AND BERKA, A., In “The Proceedings of the International Symposium held at Birmingham University (U.K.), 1962, in Honour of F. Feigl,” p. 383. Elsevier, Amsterdam, 1962. BERKA, A., DVORAK, V., NEMEC, I., AND ZGKA: J., Oxydation de Quelques Systemes Inorganiques par le Tetracetate de Plomb. Anal. Chim. Acta 23, 380-384 (1960). BERKA, A., VULTERIN, J., AND Z+KA, J., “Vybrane OxydafG-RedukEni Odm&G Metody” (in Czech) (“Selected Oxidation-Reduction Volumetric Methods”), Statni Nakladatelstvi Technicke Literatury, Prague, 1961. Lingane, J. J., Systematic polarographic metal analysis. Characteristics of arsenic, antimony, bismuth, tin, lead, cadmium, zinc, and copper in various supporting electrolytes. Ind. Eng. Chem. Anal. Ed. 15, 583 (1943). ZPXA,