152 Gooch and Jlorris — lodometric Estimation of 



1*16. The precaution is recommended that the strength of 

 the solution of the arseniate shall not exceed the decinormal 

 value, in order that the dilution consequent upon titration bj 

 the thiosulphate may not be too great — the reducing action 

 brought about by the action of the strong acid upon the arseni- 

 ate and iodide being reversible upon the dilution of liquid 

 with water. This procedure thus limits the process to the 

 determination of about 018 gram of arsenic acid in 25 cm3 of 

 the solution to be treated with an equal volume of hydrochloric 

 acid of sp. gr. 1*16. Obviously, however, the process should, 

 so far as the reduction is concerned, be applicable to larger 

 amounts of arsenic provided the strength of the acid is kept up 

 proportionately. It is essential that the liquid at the end of the 

 titration should contain approximately ten per cent of its mass 

 of absolute hydrochloric acid or about one-third of its volume 

 of the aqueous acid of sp. gr. 1*16. 



The arsenic acid is measured either by the amount of stand- 

 ard thiosulphate required to bleach the iodine or by the 

 amount of iodine required to reoxidize the reduced arsenious 

 acid, after neutralizing with acid potassium carbonate. If the 

 former alternative is followed, the end-reaction must be the dis- 

 appearance of the yellow color of the iodine, since in solutions 

 so strongly acid it is impossible to place dependence upon the 

 starch indicator; in using the latter alternative, the starch 

 indicator is, of course, permissible and preferable. 



In the direct titration of the iodine by thiosulphate two 

 sources of error present themselves as possibilities ; first, the 

 excessive liberation of iodine by the action of air upon the 

 strongly acidulated iodide ; and second, the liability of the 

 thiosulphate,* if present even in momentary or local excess 

 during the process of titration, to break down under the action 

 of strong acid, thus changing its capacity to convert iodine to 

 hydriodic acid. The latter contingency should be remote in 

 proportion to the caution used in adding the thiosulphate and 

 in keeping the liquid well stirred ; the former must of neces- 

 sity vary with the acidity of the solution containing the iodide, 

 the time of exposure to atmospheric action, and the degree of 

 contact with the air incidental to stirring. We have thought 

 it desirable, therefore, to see how far each of these possibilities 

 is likely to interfere in the practical conduct of an ordinary 

 analysis. 



The effects likely to result simply from the strong acidifica- 

 tion of the solution containing potassium iodide and their 

 variation for conditions of dilution representing the beginning 

 and the end of a titration on the lines laid down are shown in 



* Norton, this Journal, vii, 287. 



