Browning and Palmer — Estimation of Thallium. 379 



Art. XXXI. — The Volumetric and Gravimetric Estimation 

 of Thallium in Alkaline Solution by Means of Potassium 

 Ferricyanide ; by Philip E. Browning and Howard E. 

 Palmer. 



[Contributions from the Kent Chemical Laboratory of Yale Univ. — cxcviii.] 



In a former paper from this laboratory* it was shown that 

 reactions involving oxidation by ferricyanide in alkaline solu- 

 tion and reoxidation by permanganate of the resulting ferro- 

 cyanide in acid solution might be applied to the determination 

 of cerium in the presence of the other rare earth compounds. 



In the work to be described the same reactions were applied 

 to the volumetric estimation of thallium. A solution of thal- 

 lium nitrate was made up by dissolving 10 grms. of pure 

 thallous nitrate in water and making up to a liter. This solu- 

 tion was standardized by precipitating definite portions with 

 potassium bichromate in alkaline solution, filtering, and weigh- 

 ing the thallous chromate,f and also by evaporating measured 

 portions of the solution with an excess of sulphuric acid and 

 weighing as the neutral sulphate ;J the mean of closely agreeing 

 results by both methods was taken as the standard. 



Volumetric. — The procedure was as follows: Measured 

 portions of the solution of thallous nitrate were drawn from a 

 burette, water was added to about 100 cm3 a sufficient quantity of 

 a solution of potassium ferricyanide to give an excess, and potas- 

 sium hydroxide to complete precipitation of the brown thallic 

 hydroxide. The precipitate was filtered off on asbestos, gener- 

 ally without settling, and washed thoroughly. The filtrate was 

 acidified with sulphuric acid and titrated with standard perman- 

 ganate. From the following equations, representing the reac- 

 tions, the amount of thallium present may be readily calculated: 



Tl 2 + 4K 3 FeC 6 N 6 + 4KOH --= Tl 2 3 + 4K 4 FeC 6 N 6 + 2H 2 

 5K 4 FeC 6 N 6 + KMn0 4 + 4H 2 S0 4 = 



5K 3 FeC 6 N 6 + 3K 2 S0 4 + MnS0 4 + 4H 2 



It was found necessary to apply a correction for the amount of 

 permanganate used to give the first tinge of pink color to the 

 amounts of ferricyanide used in the determinations. This 

 seldom exceeded '1 of l cm3 of the permanganate. 



The following table shows the results obtained with different 

 amounts of the thallium salt: 



* Browning and Palmer, this Journal (4), xxvi, 83. 



f This Journal (4), viii, 460. % This Journal (4), ix, 137. 



