ON THE QUESTION OF A NEW TITRIMETRIC METHOD FOR THE DETERMINATION OF IODIDE, BROMIDE, AND CHLORIDE IONS

Yu. L. LEL’CHUK, O. V. CHASHCHINA, T. V. SONINA, F. V. DITRIKH

(Presented by Prof. A. G. Stromberg, Doctor of Chemical Sciences)

Until now there have been no sufficiently satisfactory chemical methods for determining large amounts of chlorine, bromine, and iodine ions when they are present together. Berg’s method [1], adopted by the State Standard, is not only cumbersome, but also often leads to substantial errors [2, 3].

In this connection, a new variant of the titrimetric method for determining halogens, proposed by Vorob’ev [4], is of definite interest. According to this method, all three halogens are determined from one weighed portion of the substance under investigation. The iodide ion is titrated with 0.1 N potassium iodate solution in a sulfurous-acid medium at a sulfuric acid concentration from 0.8 to 1.3 gram-equivalents per liter, in the presence of acetone; bromine is titrated in the same solution, after the determination of iodine, with 0.4 N potassium iodate solution at a sulfuric acid concentration from 6.7 to 7.3 gram-equivalents per liter; chlorine at concentrations of 0.0017 N and higher is determined by the Folgard method [5], and at lower concentrations—nephelometrically.

According to the author, the method is simple, precise, and the duration of a single determination does not exceed 12–15 minutes.

From Vorob’ev’s report it is evident that he bases his conclusions on two series of experiments. In one series, the working solutions used were 0.01 N potassium iodate solution for titrating the iodide ion and 0.4 N iodate solution for titrating the bromide ion. The halogen mixtures were prepared from equal volumes of 0.01 N potassium iodide, 0.2 N potassium bromide, and 0.02 N potassium chloride solutions. In the second series of experiments, the working solutions used were 0.1 N potassium iodate solution for titrating the iodide ion and 0.4 N iodate solution for titrating the bromide ion. The mixtures were prepared from equal volumes of 0.1 N potassium iodide, 0.2 N potassium bromide, and 0.1 N potassium chloride solutions.

On the basis of these experiments it is impossible to say within what limits of the ratios of halogen concentrations the new method gives satisfactory results.

There are no indications in the literature concerning the mechanism of interaction of iodide and bromide ions with iodate in an aqueous-acetone medium; there are no data on the influence of chloride and bromide ions on the accuracy of iodine determination, of chloride ion and iodacetone on the accuracy of bromine determination, and, finally, of bromine and iodacetone on the accuracy of chlorine determination. In order to clarify these questions, the present study was undertaken.

The first series of experiments was carried out by us in order to check the accuracy and reproducibility of the titrimetric determination of the iodide ion with potassium iodate in an aqueous-acetone medium in the absence of other halogens. In these experiments we used 0.1 M solutions of potassium iodide and potassium hydrogen iodate, prepared from chemically pure reagents, chemically pure sulfuric acid (1:3), and acetone purified by double distillation. Preparation of the solutions and dilution of the sulfuric acid were carried out with bidistillate.

Into a conical flask of 250 ml capacity, using a pipette, 10 ml of 0.1 M potassium iodide solution, 20 ml of water, 15 ml of acetone, and 5 ml of sulfuric acid (1:3) were introduced. The titration was carried out with 0.1 M potassium iodate solution. Until the end of the titration, with each drop of iodate the solution being titrated acquired a yellow color, which disappeared on shaking owing to the formation of iodoacetone. At the equivalence point the yellow color does not appear. The titration proceeds quite clearly; the end of the reaction is observed sharply. Reproducibility is good. In ten experiments carried out by us, for 10 ml of 1 M potassium iodide, 4.96 ml of 0.1 M potassium iodate were consumed each time. Changing the volume of potassium iodide from 10 to 30 ml, with a corresponding decrease in the volume of water, does not lead to any noticeable changes in the ratio between the potassium iodide and potassium iodate solutions.

The results of these experiments confirm the correctness of the overall equation of the process proposed by Mitchell and Ward [2]. However, the error in these determinations of iodine reaches −0.8%. Alypina [6], having obtained increased results in the determination of chlorine in the presence of bromo- and iodoacetone, explains this fact by the hydrolysis of bromo- and iodoacetone. In order to check the validity of such an explanation, the titrated iodine solution was left by us for 6–8 hours and then checked with potassium iodate. However, the solution did not require further titration, even after considerable dilution with water and heating to 60–80°. This refutes Alypina’s opinion.

A second series of experiments was carried out in order to check the influence of potassium bromide on the titration of iodide iodine with potassium iodate. Into a conical flask, by pipette, were introduced 10 ml of 0.1 M potassium iodide solution, 10 ml of 0.1 M potassium bromide, 10 ml of water, 15 ml of acetone, and 5 ml of sulfuric acid (1:3).

In ten determinations of iodine under these conditions, the consumption of 0.1 M iodate ranged from 5.10 to 5.15 ml. Thus, the presence of potassium bromide leads to an increased consumption of potassium iodate by approximately 2–3%. After each hour of standing, the titrated solution required, for further titration, 0.40–0.45 ml of potassium iodate (Table 1). On the basis of these experiments we come to the conclusion that the increased consumption of potassium iodate in the titration of iodine in the presence of potassium bromide is associated with a reversible exchange process between iodoacetone and potassium bromide. This conclusion is also confirmed by the fact that the greater the relative content of bromine in the analyzed solution, the more potassium iodate solution is consumed for the titration of the same amount of iodine (Table 2).

Table 1

Additional consumption of potassium iodate as a function of the standing time of the titrated solution

Table 2

Effect of the bromide ion on the titration of potassium iodide

The third series of experiments was carried out in order to check the effect of potassium chloride on the titration of the iodide ion with potassium iodate. Into a conical flask were introduced 10 ml of 0.1 M potassium iodide, 10 ml of 0.1 M potassium chloride, 10 ml of water, 15 ml of acetone, and 5 ml of sulfuric acid (1:3). In ten determinations of iodine under these conditions, the consumption of iodate did not differ from its consumption in the titration of iodine in the absence of other halogens. Thus the chloride ion does not interfere with the determination of iodine.

The fourth series of experiments was carried out in order to check the accuracy and reproducibility of the results of titrating the bromide ion with potassium iodate under the conditions described in Vorob’ev’s article. Into a conical flask were introduced 10 ml of 0.1 M potassium bromide, 20 ml of water, 15 ml of acetone, and 15 ml of sulfuric acid (2:1). The titration was performed with a 0.2 M solution of potassium iodate. Under these conditions it was not possible to titrate bromine in any way, since the solution acquired any noticeable coloration only upon the addition of large portions of iodate, and it is impossible to observe the equivalence point in this titration. Exactly the same result was obtained when an attempt was made to titrate the bromide ion in a mixture with potassium chloride and potassium iodide after determination of iodine.

The determination of bromine by adding an excess of potassium iodate and titrating the remainder with potassium iodide also does not give satisfactory results.

Our observations of the process of the oxidimetric determination of iodine with potassium iodate in an aqueous-acetone medium allow us to suggest that the reaction proceeds in two stages. In the first stage, potassium iodide is oxidized and potassium iodate is reduced to electroneutral iodine. In the second stage, acetone is oxidized by electroneutral iodine to iodoacetone.

Conclusions

1. An experimental verification was carried out of the titrimetric method for determining chlorine, bromine, and iodine ions in their mixture according to the procedure proposed by A. S. Vorob’ev. It was shown that the error in determining the iodide ion with potassium iodate in an aqueous-acetone medium, in the absence of chlorine and bromine ions, reaches 0.8% in the direction of a lower consumption of potassium iodate. Chlorine ions have no noticeable effect on the course of the titration of iodine. Bromine ions lead to an increased consumption of potassium iodate. Bromine cannot be titrated by the proposed procedure.

2. Considerations are presented on the mechanism of the reaction of interaction of the iodide ion with potassium iodate in an aqueous-acetone medium in the presence of sul-

acid. The increased consumption of iodate in the presence of the bromide ion is apparently associated with partial replacement of iodine in iodoacetone by bromine.

REFERENCES

1. Berg R., Z. Anal. Chem. 69, 342, 368, 1926.
2. Mitchell A. D. and Ward A. M., Methods of Quantitative Chemical Analysis. Goskhimizdat, ONTI, p. 42, 1933.
3. Vorob’ev A. S., Zh. anal. khimii 2, 96, 1947.
4. Vorob’ev A. S., Zh. anal. khimii 12, 395, 1957.
5. Kolthoff I. M. and Sandell E. B., Quantitative Analysis, Goskhimizdat, p. 492, 1948.
6. Alpine M., J. Am. Chem. Soc. 51, 4, 1929.