EFFECT OF ORGANIC SOLVENTS ON THE PROPERTIES OF SILVER BROMATE PRECIPITATE

Yu. L. Lel’chuk, E. I. Kapkova

(Presented by the scientific seminar of the Faculty of Chemical Technology)

Precipitates of silver chloride and silver bromide are widely used for quantitative determinations of chloride, bromide, and silver ions. These same precipitates are often used for gravimetric determinations of chlorate and bromate ions after their reduction by suitable reducing agents to chloride and bromide. However, the use of these precipitates in gravimetric analysis is associated with overcoming many difficulties. Chief among them are the high sensitivity of silver chloride and bromide to light and the tendency of these precipitates to form colloidal solutions.

It is now generally recognized that the decomposition of silver chloride and bromide is caused mainly by rays in the blue-violet region of the light spectrum [1]. Moreover, the free halide and silver released as a result of this photochemical reaction, depending on the precipitation conditions, affect the final results of the corresponding gravimetric determinations in different ways.

In all determinations of chlorine and bromine, analytical precipitation of silver chloride and silver bromide is carried out with a certain excess of silver nitrate. For the most complete separation of the precipitate, N. A. Tanaev [2] recommends using one and a half times the amount of silver nitrate. However, under these conditions the halogen liberated under the action of light reacts with the excess silver ions according to the equation

\[ 3\mathrm{Cl}_2 + 3\mathrm{H}_2\mathrm{O} + 5\mathrm{Ag}^{\cdot} = 5\mathrm{AgCl} + \mathrm{ClO}_3' + 6\mathrm{H}^{\cdot}\ [3]. \]

As a result, additional amounts of silver chloride are obtained, which in turn leads to erroneously high results.

In all determinations of silver, analytical precipitation of silver chloride is carried out with a certain excess of hydrochloric acid. Under these conditions the final results of gravimetric determinations are low, since the chlorine liberated under the action of light volatilizes. In connection with this, Lendell and Hoffman [4] recommend, in accurate determinations, completely excluding daylight and carrying out all operations with precipitates of silver chloride and silver bromide only under the light of a red lamp.

The tendency of these precipitates to form colloidal solutions also causes great inconvenience. In order to prevent forma-

of a colloidal solution of silver chloride, precipitation is carried out from a hot solution acidified with nitric acid. A nitric-acid medium is also necessary in all determinations of chlorine and bromine in the presence of carbonate, phosphate, arsenate, and oxalate ions. However, nitric acid, on heating, may cause additional losses of halogens through volatilization of the hydrogen halide formed as a result of the exchange process of halide salts with nitric acid.

The precipitate of silver bromate has a number of advantages over precipitates of silver chloride and silver bromide. As our investigations have shown [5, 6, 7, 8], silver bromate is stable in solutions with a wide range of concentrations of nitrates of the alkaline-earth metals, nitrates and sulfates of aluminum, zinc, manganese, cadmium, copper, lithium, potassium, and sodium. The high specific gravity of AgBrO$_3$ (5.206) and its crystalline structure [9] account for the rapid settling of the precipitate. Complete clarification of the solution usually occurs as early as 10–15 minutes after the required amount of precipitant has been added. The precipitate does not change appreciably even under comparatively prolonged exposure to daylight and, under ordinary precipitation conditions, is not prone to the formation of colloidal solutions. It is readily filtered through glass filters with a No. 3 filtering bottom, and to obtain the weighable form it is sufficient to dry it at 120–130°.

The principal drawback of the AgBrO$_3$ precipitate is its comparatively high solubility (8.25·10$^{-3}$ mol/l). In this connection, use of the precipitate for gravimetric determinations of bromate and silver ions is possible only when the solubility of the precipitate is considerably reduced by employing appropriate media. In our opinion, such dispersion media could probably be the corresponding aqueous solutions of certain organic solvents. However, the literature contains no necessary data on the solubility of silver bromate in the most important organic solvents and their aqueous solutions. In this connection, we have carried out a physicochemical study of the solubility of the AgBrO$_3$ precipitate at 25°C in the systems: silver bromate—acetone—water, silver bromate—methyl alcohol—water, and silver bromate—ethyl alcohol—water.

Experimental Part

In all experiments on determining solubility and solubility product of the silver bromate precipitate in the corresponding media, we used pure acetone with a specific gravity of 0.79 and a boiling point of 56.5°, methyl alcohol with a specific gravity of 0.79 and a boiling point of 64.7°, and a 94-percent aqueous solution of ethyl alcohol obtained after double distillation of commercial alcohol. The procedure for studying the solubility and for obtaining chemically pure silver bromate has been described by us previously [5].

The concentration of AgBrO$_3$ in all the solutions studied by us was determined from the content of bromate ions by a volumetric method. For most points these data were checked by gravimetric determinations of silver in the form of silver chloride. Precipitation and settling of the precipitates were carried out in beakers protected from the action of light by caps of thick black paper. Filtration and drying of the silver chloride were carried out in glass filters with a No. 3 filtering bottom. The volumetric determination of bromate ions was performed iodometrically in a sulfuric-acid medium with the addition of several drops of a 3-percent solution of ammonium molybdate, according to the procedure described

In accordance with the change in solubility in these systems, the solubility product of the precipitate also changes over wide limits. If, in accordance with the new theory of strong electrolytes, it is assumed that $\mathrm{AgBrO_3}$ is completely dissociated in all aqueous solutions, then the solubility product of the precipitate in a 94-percent aqueous solution of ethyl alcohol will be equal to $8.85 \cdot 10^{-9}$, which is 7684 times less than the solubility product of the precipitate in water.

Fig. 3. Solubility of $\mathrm{AgBrO_3}$ in the $\mathrm{AgBrO_3}$--$\mathrm{C_2H_5OH}$--$\mathrm{H_2O}$ system.
Graph labels: solubility of $\mathrm{AgBrO_3}$ in water; ordinate $C_{\mathrm{AgBrO_3}} \cdot 10^4$; abscissa, $\%\,\mathrm{C_2H_5OH}$.

Analysis of the experimental data presented shows that even at comparatively high concentrations of the above-listed organic solvents, the solubility and the solubility product of the silver bromate precipitate remain considerably higher than those of precipitates presently used in gravimetric analysis. In this connection, effective use of the $\mathrm{AgBrO_3}$ precipitate for analytical determinations of bromate and silver ions is possible only with strict allowance for losses of the precipitate due to its solubility during precipitation and washing. At present we are verifying the possibility of quantitatively taking account of these losses on the basis of data previously obtained by us on the influence of electrolytes of various types on the solubility and solubility product of the silver bromate precipitate.

Conclusions

1. The solubility of the silver bromate precipitate was studied in ternary systems: silver bromate--acetone--water, silver bromate--methyl alcohol--water, and silver bromate--ethyl alcohol--water.

2. In all three ternary systems, the solubility and the solubility product of the precipitate decrease regularly with increasing concentration of the organic solvent.

3. In their influence on the solubility and solubility product of the silver bromate precipitate, all three solvents differ little from one another. At concentrations of organic solvents above 40%, the silver bromate precipitate is least soluble in aqueous solutions of ethyl alcohol.

4. The experimental data obtained by us for the ternary system silver bromate--ethyl alcohol--water are proposed for use in developing a new gravimetric method for determining bromate ions in soluble bromates.

References

1. Nekrasov B. V. Course of General Chemistry, Goskhimizdat, 705, 1952.

2. Tananaev N. A. Gravimetric Analysis, ONTI NKTP, 110, 1936.

3. Kolthoff I. M., Sandell E. B. Quantitative Analysis, GONTI NKTP, 273, 1938.

4. Lundell G. E. F., Hoffman J. I. Bur. of Standards, J. of Research, 4, 109, 1930.

5. Lelchuk Yu. L., Sasonko S. M., Izv. TPI, 71, 52, 1952.

6. Tananaev I. V., Lelchuk Yu. L., Petrovitskaya B. Kh., ZhOKh, 19, 1208, 1949.

1. Lel’chuk Yu. L., Izv. TPI, 77, 104, 1953.

2. Lel’chuk Yu. L., Surnina L. V., Barkhatova V. I., ZhOKh, 25, 1685, 1955.

3. Lange N. A., Handbook of Chemistry, Handbook Publishers, Inc., Sandusky, Ohio, 248, 1946.

4. Kolthoff I. M., Sandell E. B., Quantitative Analysis, GONTI NKTP, 555, 1938.