D. L. Randall — Titration of Mercurous Salts. 137 



Art. XIY. — The Titration of Mercurous Salts vnth Potas- 

 sium Permanganate / by D. L. Randall. 



[Contributions from the Kent Chemical Laboratory of Yale University — cliii.] 



If a solution of a mercurous salt, such, as mercurous sulphate 

 in dilute sulphuric acid, is titrated with potassium permanga- 

 nate in the usual manner the bleaching of the color is rapid 

 at first, but long before the oxidation is complete the solution 

 assumes a golden yellow color and on standing the brown 

 oxides of manganese are precipitated. For this reason no defi- 

 nite end reaction is obtainable. This difficulty is, however, 

 avoided if the permanganate solution is added in excess, the 

 color then bleached with a standard ferrous sulphate solution, 

 and the end point finally reached by a few drops of permanga- 

 nate. Under these conditions the end reaction is perfectly 

 sharp and the oxidation of the mercurous salt complete, as 

 may be proved by adding hydrochloric acid to the solution. 



In the preliminary experiments this process was tested upon 

 weighed portions of commercial chemically pure mercurous 

 sulphate, which was dissolved in dilute sulphuric acid. So 

 much difficulty was experienced in dissolving the salt in quan- 

 tities suitable for experiment that recourse was taken to the 

 very doubtful procedure of heating the liquid (boiling or pro- 

 longed heating on the steam bath) to hasten the solution. 

 Although the end reaction was in every case as sharp as could 

 be desired, and the oxidation complete, the results were low 

 and variable, indicating from 90-93 per cent of mercurous sul- 

 phate in the sample used. This was probably due to the 

 partial oxidation of the mercurous salt during the process of 

 solution. 



A few experiments in which the salt was dissolved in hot 

 concentrated phosphoric acid, and the solution then diluted 

 and titrated without further addition of acid, gave results 

 much nearer the theory but still very variable. 



For the final experiments a large quantity of a mercurous 

 sulphate solution was prepared by shaking up an excess of the 

 salt in water acidified with sulphuric acid, allowing the whole 

 to stand for twenty-four hours, and finally filtering through 

 asbestos. Portions of this solution for standardization or titra- 

 tion were measured out in a 100 cm3 burette or weighed out in 

 a dry counterposed beaker. The standard of this solution 

 was obtained by precipitating the measured or weighed por- 

 tions in the cold with a slight excess of sodium chloride. 

 After twelve hours standing, the precipitated mercurous chlor- 

 ide was collected upon asbestos in a platinum crucible and 

 dried to a constant weight at the ordinary temperature in a 



