Arsenic Acid. 



155 



favors further action of the iodine, but results obtained by 

 titration with iodine in the acid solution diluted with an equal 

 amount of water are unmodified by further dilution. 



In the following tables are recorded actual determinations of 

 arsenic according to Williamson's process. To each 25 cmS of 

 the arseniate were added 1, 2, or 3 grams of potassium iodide 

 and 25 cmS hydrochloric acid, sp. gr. . 1*16. The iodine was 

 bleached by nearly decinormal thiosulphate without addition of 

 the starch indicator, which loses all delicacy in the presence of 

 strong acid. The time occupied by each titration was about 

 five minutes. The standards of the arseniate were determined 

 by the vaporization process,* the purity of reagents employed 

 in that process having been proved by trying the process in the 

 estimation of a solution of arsenic acid made by oxidizing 

 pure decinormal arsenious acid by iodine. 







Volume at 



Volume 



H 2 KAs0 4 











beginning 



at 



in 











of 



end of 



terms of 



H 3 3 AsO 





HCl 



KI. 



titration. 



titration. 



H 3 3 AsO. 



found. 



Error. 



cm 3 . 



gram. 



cm 3 . 



cm 3 . 



gram. 



gram. 



gram. 



25 



2 



50 



51 



0-0062 



0-0085 



+ 0-0023 



25 



2 



50 



52 



0-0125 



0-0156 



+ 0-0031 



25 



2 



50 



55 



0-0312 



0-0350 



+ 0-0038 



25 



2 



50 



55 



0-0624 



0-0666 



+ 0-0042 



25 



2 



50 



73 



0-1559 



0-1588 



+ 0-0029 



25 



2 



50 



73 



0-1559 



0-1587 



+ 0-0028 



25 



2 



50 



73 



0-1559 



0-1591 



+ 0-0032 



25 



2 



50 



73 



0-1559 



0-1595 



+ 0-0036 



25 



3 



50 



73 



0-1559 



0-1595 



+ 0-0036 



25 



1 



•50 



73 



0-1559 



0-1581 



+ 0-0022 



25 



2 



50 



73 



0-1559 



0-1581 



+ 0-0022 



25 



2 



50 



73 



0-1559 



0-1588 



+ 0-0029 



The range of error in these results is from +0*0023 gram to 

 + 0*0042 gram with a mean of +0*0031 gram — not very dif- 

 ferent from what might be expected from the effect of the 

 interaction of the strong hydrochloric acid and the iodide 

 alone. The counter-effect due to the decomposition of the 

 thiosulphate is not large, yet it is probably real, as will appear 

 in the sequel. In the following series of determinations, made 

 with new solutions and new standards throughout, the arsenic 

 acid was determined, first, by titrating the iodine set free by 

 25 cm3 of hydrochloric acid, sp. gr. 1*16 and 3 grams potassium 

 iodide, the solution having a total volume of 50 cmS at beginning 

 and of 75 cm3 at the end of titration and, secondly, the arsenious 

 acid produced in the first reaction was titrated, after being 

 neutralized with acid potassium carbonate by iodine in the 

 presence of the starch indicator. 



* Gooch and Browning, loc. cit. 



Am. Jour. Sci. — Fourth Series, Vol. X, No. 56. — August, 1900. 

 11 



