154 Gooch and Morris — Iodometric Estimation of 



an equal volume of water, were titrated with nearly — iodine. 



The condition of acidity when the volume of 50 cm3 contains 

 25 cmS of hydrochloric acid, sp. gr. 1*16, is that of the beginning 

 of titration of Williamson's process. In order that the effect 

 of error due to such action upon the determination of arsenic 

 acid may appear immediately, the thiosulphate and iodine used 

 are expressed in terms of that acid. 



These two sources of error, the one due to a liberation of 

 iodine and the other due to decomposition of the thiosulphate, 

 would naturally tend to overcome one another, but the com- 

 pleteness of such neutralization would naturally be largely a 

 matter of chance in the varying conditions of actual analysis. 

 The experiments of the following table, however, in which 



— thiosulphate, to the amount of 1, 2, and 5 cmS , was added to 



the liquid, 50 cm3 and 75 cm3 , containing 25 cm3 acid, and titrated 

 with iodine at once, and after five minutes, were made to test 

 the matter for the conditions of dilution at the beginning and 

 at the end of a titration. 



HCl 



Sp. gr. 



1-16. 



KI. 



Volume. 



i 



Na 2 S 2 3 



nearly — 



in terms of 

 H 3 3 AsO. 



Iodine in 

 terms of 

 H 3 3 AsO, 

 at once. 



Iodine in 



terms of 



H3O3ASO, 



after 



5 min. 



Error in 



terms of 



H 3 3 AsO. 



cm 3 . 



gm. 



cm 3 . 



cm 3 



gram. 



gram. 



gram. 



gram. 



25 



2 



50 



1 



0-0071 



0-0057 





— 0-0014 



25 



2 



75 



1 



0-0071 



0-0071 





0-0000 



25 



2 



50 



2 



0-0141 



0-0131 





—o-ooio 



25 



2 



75 



2 



0-0141 



0-0143 





+ 0-0002 



25 



2 



50 



5 



0-0353 



0-0322 





— 0-0021 



25 



2 



75 



5 



0-0353 



0-0357 





+ 0-0004 



25 



2 



50 



1 



0-0071 





0-0028 



— 0-0043 



25 



2 



75 



1 



0-0071 





0-0067 



— 0-0004 



25 



2 



50 



2 



0-0141 





0-0116 



— 0-0025 



25 



2 



75 



2 



0-0141 





0-0139 



— 0-0002 



25 



2 



50 



5 



0-0353 





0-0314 



— 0-0041 



25 



2 



75 



5 



0'0353 





0-0361 



+ 0-0008 



It is clear that under the conditions covered by the experi- 

 ments of the two preceding tables the decomposition of the 

 thiosulphate is likely to occur in greater or less degree, and 

 that when the acid of sp. gr. 1*16 is not much diluted, the 

 products of decomposition are not oxidized by the iodine com- 

 pletely. The latter observation is quite in harmony with the 

 fact that sulphur dioxide bleaches iodine in strong hydrochloric 

 acid only slowly and incompletely. In such cases dilution 



