1920] Determination of Arsenic in Arsenates 91 



A better method of eliminating free iodine was sought. The fol- 

 lowing procedure was finally employed: To 25 c.e. of a solution con- 

 taining about 0.1 gram of AS2O5 were added one to two grams of 

 potassium iodide and after solution 5 c.c. of cone, hydrocloric acid. 

 This mixture is then boiled and as free iodine appears a dilute so- 

 lution of sodium sulphite is added from a burette to destroy the yel- 

 low color of the solution. Sodium thiosulphate cannot be used, as it is 

 decomposed by hot acid solutions with the liberation of free sulphur. 

 Boiling is continued for a few minutes after the last appearance of 

 iodine in order to remove any excess of sulphite, as SO2, which may 

 have been used. The flask is then cooled in cold water and caustic 

 soda solution is dropped in from a pipette until litmus paper turns 

 blue. Dilute hydrochloric acid is added drop by drop until the litmus 

 paper just turns red. Crj'stals of disodium phosphate, 3 to 5 grams, 

 are now added, turning the litmus blue, and titration against N/10 

 iodine solution follows immediately. 



As a test of the accuracy of the above method, a quantity of AS2O3 

 was dissolved in dilute hydrochloric acid solution; the solution was 

 made alkaline and then faintly acid ; an excess of disodium phosphate 

 was added and titration against iodine solution followed immediately. 

 After this titration, the solution containing arsenic only as AsoOs 

 was reduced by the method above described and the titration repeated. 

 The results will be seen to agree within tlie experimental error : 



Xo. 1 Xo. 2 Xo. .T Xo. 4 



First titration 17..j5cc 17.80cc 17.79cc 17.76cc 



Second titration 17.58ec 17.79cc 17.76cc 17.74cc 



The quantities of arsenic taken in each of the trials was not the same, 

 so tliat no comparison should be made of the different samples witli 

 each other. The agreement between the two titrations for the same 

 quantity of arsenic shows that the ]iroposed method is as accurate as 

 the Mohr method for arsenious oxide. The time re<|uired. after the 

 sample is in solution, is from 15 to 20 minutes for reduction to the 

 arsenious condition and subsequent neutralization and titration 

 against the standard iodine solution. 



CiiAPEL Hill, X. C. 



