DETERMINATION OF CHLORIDE AND BROMIDE IONS IN A MIXTURE OF POTASSIUM CHLORIDE, BROMIDE, CHLORATE, AND BROMATE

Yu. L. LELCHUK, N. L. BATURINA

(Presented by the Department of Analytical Chemistry)

Quantitative determinations of chloride, bromide, chlorate, and bromate ions in their joint presence are difficult analytical problems. At the same time, in production control of various compounds of these halogens, such determinations often have great practical significance.

We have studied the possibility of separate determination of chloride and bromide ions in production solutions containing simultaneously potassium chloride, bromide, chlorate, and bromate, based on argentometric titration of the sum of chloride and bromide and the total weight of the precipitate of silver chloride and silver bromide formed in this process. We propose to use the same method for subsequent separate determination of chlorate and bromate ions in this mixture after their reduction to chloride and bromide ions by a suitable reducing agent.

The first experiments were carried out with mixtures of decinormal solutions of potassium chloride and potassium bromide: 1:1, 10:1, 100:1, 1:10, 1:100; then the mixture was prepared from equal volumes of decinormal solutions of potassium chloride, potassium bromide, potassium chlorate, and potassium bromate. In subsequent experiments, work was continued with solutions in which the concentration of potassium chloride varied from 1 to 10 g/l, potassium bromide from 10 to 50 g/l, potassium chlorate from 1 to 13 g/l, and potassium bromate from 1 to 17 g/l. The selected concentration range of the salts corresponds to the composition of real production solutions, in the analytical control of which certain enterprises are interested.

Course of analysis. An aliquot portion of the solution being analyzed is titrated with a 0.2 N solution of silver nitrate by Mohr’s method. The concentration of the indicator is selected so that the appearance of the silver chromate precipitate precisely marks the end of the titration of potassium chloride and bromide and excludes the possibility of any partial precipitation of silver bromate. In accordance with this, in our experiments 5–6 drops of a 10% solution of $K_2CrO_4$ were used for each titration of 10 ml of the solution being analyzed.

Into a new portion of the same volume of the solution being analyzed (without adding indicator), a volume of 0.2 N silver nitrate solution is introduced equal to the volume used for titrating the sum of chloride and bromide. The precipitate of silver chloride and silver bromide is filtered through a glass filter dried to constant weight with a filtering ...

bottom: No. 3, washed with water, and dried at 100–120°C to constant weight.

The calculation formulas for computing the content of KCl and KBr in the analyzed mixture from the volume of silver nitrate solution consumed in the titration and the weight of the precipitate are derived from the mathematical expression of the law of equivalents for the given reactions, where the left-hand side of the equations expresses the number of milligram-equivalents of silver nitrate, and the right-hand side the milligram-equivalents of silver chloride and silver bromide participating in the reaction:

\[ V_{\mathrm{AgNO_3}}N_{\mathrm{AgNO_3}} = \frac{1000x}{E_{\mathrm{AgCl}}} + \frac{(H-x)1000}{E_{\mathrm{AgBr}}}, \]

where
\(V_{\mathrm{AgNO_3}}\) is the volume of silver nitrate solution used for titrating the mixture, ml;
\(N_{\mathrm{AgNO_3}}\) is the normality of the silver nitrate solution, g-eq/l;
\(x\) is the weight of silver chloride in the total precipitate, g;
\(H\) is the weight of the mixture precipitate;
\(E_{\mathrm{AgCl}}\) is the gram-equivalent of silver chloride, g;
\(E_{\mathrm{AgBr}}\) is the gram-equivalent of silver bromide, g.

After the corresponding transformations, the following expression for \(x\) is obtained:

\[ x=\frac{3{,}221}{1000}\left(E_{\mathrm{AgBr}}V_{\mathrm{AgNO_3}}N_{\mathrm{AgNO_3}}-1000H\right). \]

For a concentration of the working titrated silver nitrate solution of 0.2, this expression, after substituting the numerical value of the equivalent of silver bromide, is further simplified:

\[ x=0{,}121V_{\mathrm{AgNO_3}}-3{,}221H \]

Taking into account the conversion factor of AgCl to KCl and AgBr to KBr \((F_{\mathrm{KCl/AgCl}}=0{,}5201,\ F_{\mathrm{KBr/AgBr}}=0{,}6337)\), the final formulas for calculating the contents of KCl and KBr in the analyzed mixture take the form:

\[ B_{\mathrm{KCl}}=0{,}0629V_{\mathrm{AgNO_3}}-1{,}675H, \]

\[ B_{\mathrm{KBr}}=2{,}675H-0{,}0767V_{\mathrm{AgNO_3}}, \]

where \(B_{\mathrm{KCl}}\) is the content of potassium chloride in the analyzed mixture, g;
\(B_{\mathrm{KBr}}\) is the content of potassium bromide in the analyzed mixture, g.

Table 1 gives the results of our experiments.

Table 1

Verification of the accuracy of the method on artificial mixtures

From these data it is evident that the method we have developed for the separate determination of KCl and KBr in a mixture of KCl, KBr, KClO$_3$, and KBrO$_3$ can be successfully used in production control.

Conclusion

A volumetric-gravimetric method has been developed for the separate determination of chloride and bromide ions in a mixture of KCl, KBr, KClO$_3$, and KBrO$_3$. The contents of KCl and KBr are calculated from the data of argentometric titration of the sum of chloride and bromide by Mohr’s method and from the weight of the total precipitate of AgCl and AgBr formed in this process.