156 Gooch and Morris — lodometric Estimation of 









H 3 3 AsO 





H a KAs0 4 



H 3 3 AsO 





found by 





taken, in 



found 





titration of 





terms of 



by the 





H 3 3 As with 





HgOsAsO. 



thiosulphate. 



Error. 



iodine. 



Error. 



gram. 



gram. 



gram. 



gram. 



gram. 



0-1767 



0-1798 



+ 0-0031 



0-1776 



+ 0-0009 



0-1767 



0-1798 



+ 0-0031 



0-1777 



+ 0-0010 



0-1767 



0-1795 



+ 0-0028 



0-1785 



+ 0-0018 



0-1767 



0-1793 



+ 0-0026 



0-1785 



+ 0-0018 



0-1767 



0-1794 



+ 0-0027 



0-1780 



+ 0-0013 



0-1767 



0-1798 



+ 0-0031 



0-1785 



+ 0-0018 



The average error of the first operation is 0*0029 gram, not 



far from that of the previous series ; the error of the second 



operation, the titration of the arsenious acid, amounts on the 



average to 0-0014 gram. In the second operation the error 



due to over-use of the thiosulphate by iodine set free outside 



the main reaction is obviously eliminated. The tetrathionate 



present after neutralization with acid potassium carbonate is 



n 

 unaffected by iodine, as we have found by titrating 25 cm3 — 



iodine mixed with 25 cm3 hydrochloric acid, sp. gr. 1*16, by the 

 thiosulphate, neutralizing with acid potassium carbonate,* add- 

 ing starch and getting the starch blue with a single drop of 



?z 



— iodine. The average error of this process, therefore, 0-0014, 

 10 ° . 



is probably due to the products of decomposition of the thio- 

 sulphate in the first operation. 



From the foregoing experiments it is clear that an arbitrary 

 correction of about 0*0030 gram must be deducted from the 

 indications of Williamson's process of direct titration by thio- 

 sulphate, made with the greatest care under the conditions 

 mentioned ; and that a correction varying from one-half that 

 amount (0*0015 gram) to nothing (according to the amount of 

 arsenious acid present) when the determination is made by 

 iodine after neutralization with acid potassium carbonate. 

 After making these arbitrary corrections in the results of the 

 preceding table, the individual variations fall within reasonable 

 limits. 



On the other hand, the vaporization process, in which the 

 arseniate is reduced by boiling with sulphuric acid and potas- 

 sium iodide in the manner described,! gives indications reason- 



*It is worthy of note, that, as we have found by experience, it is not possible 

 to substitute an alkaline hydroxide for the carbonate in the early stages of the 

 process of neutralization, on account of the decomposing effect of the former 

 reagent upon the tetrathionate. This effect is in proportion to the heating of the 

 solution, but is never wholly absent even when ice is intermixed with the liquid 

 and the greatest care taken to prevent a rise of temperature. 



•f Loc. cit. 



