Heath — Determination of Arsenic and Antimony. 515 



In the course of preliminary work the fact developed that 

 tetrathioiiic acid, which results from titration of free iodine by 

 sodium thiosulphate in the copper determination, makes trouble 

 in the subsequent operation. For, when the solution is boiled, 

 after addition of sulphuric acid, the tetrathioiiic acid decomposes 

 to give hydrogen sulphide and free sulphur, and sulphides of 

 antimony and arsenic may be precipitated. Various means 

 were tried to oxidize the tetrathioiiic acid and its decomposition 

 products before reducing the arsenic and antimony. Solid 

 iodine and potassium nitrite were tried for this purpose but 

 not very successfully. Bromine water and, better, liquid bro- 

 mine were used with success. 



It was found that if liquid bromine was added to the cold 

 solution in sufficient quantity to decompose all the excess of 

 potassium iodide present, and the solution then boiled, there 

 was very little subsequent trouble on account of tetrathioiiic 

 acid. To decompose the remainder of the potassium iodide 

 used in the precipitation of the copper about one cubic centi- 

 meter of bromine was added. If, after boiling for a short 

 time, the solution did not become clear it was cooled, a little 

 more bromine added, and the boiling repeated to ensure com- 

 plete oxidation and to expel excess of bromine. It is well to 

 concentrate the solution somewhat at this point so as to remove 

 most of the bromine ; otherwise, when potassium iodide is 

 added to bring about reduction of the arsenic or antimonic salt 

 so much of it may be decomposed by the bromine that the 

 reducing action may not be complete. Some free iodine 

 remains in the solution after the reduction process and there 

 was found to be some difficulty in bleaching it with exactness 

 in the hot solution. If the solution was cooled starch indica- 

 tor could be used. Accordingly in the cooled solution the free 

 iodine, in presence of starch indicator, was bleached by adding 

 an excess of dilute sulphurous acid. The solution was then 

 diluted to 100 cm3 or more and iodine solution added to a faint 

 color. This pale blue color was then just bleached by careful 

 addition of dilute sulphurous acid from a pipette. The solu- 

 tion was neutralized by use of sodium or potassium bicarbonate 

 and the arsenic or antimony determined by titration with stand- 

 ard iodine solution in the usual way. Following are tables 

 showing results obtained by this method. 



The procedure for the determination of copper and arsenic 

 or of copper and antimony may be outlined as follows : — To 

 the solution containing the copper and also the arsenic or anti- 

 mony in the higher condition of oxidation, add 1 grin, to 2 grm. 

 of citric acid. To precipitate amounts of copper not exceed- 

 ing -3 grm. in a volume of 50 cmS add 3 grm. of potassium iodide ; 

 in a volume of 100 cm3 add 5 grm. of potassium iodide. Titrate 



