72 



J. Bbaduri — Transformation of 



[No. 2, 



Action of acids on potassium iodide. 



Having had frequent occasion to notice the slight colouration of 

 potassium iodide, both in the solid state and in solution, the action of 

 dilute acids such as had been used in previous estimations was tried. 

 Potassium iodide as ordinarily sold for medicinal purposes is decom- 

 posed by most acids — pure dilute phosphoric, hydrochloric, sulphuric, 

 acetic acids, &c. It has been asserted (Fresenius's Quantitative Analysis, 

 seventh edition, p. 104) that the pure iodide should not liberate iodine 

 by the action of dilute sulphuric acid. This statement appears to be 

 incorrect. Various samples, purified according to different methods, to 

 remove the iodate (reduction by sodium amalgam, fusion with and 

 without charcoal, and crystallization from alcohol) gave invariably the 

 blue colouration with starch paste and dilute sulphuric acid. Water 

 boiled and cooled in absence of air was used to prevent the action of 

 dissolved oxygen. The amount of iodine which was thus liberated, 

 when the titrations were made in dilute solutions, was however too 

 small to affect the accuracy of results, and hence no blank experiments 

 were necessary. 



The distillation (with hydrochloric acid) method is equally applic- 

 able, but greater precautions are necessary owing to the ready decom- 

 position of hypochlorites. The hypochlorite which I had to use being 

 mixed with excess of free alkali, previous acidification was necessary. 

 When dilute hydrochloric acid was used, free chlorine was liberated, a 

 small portion of which escaped before potassium iodide was added. 

 Hence the operation was performed in a closed vessel. A mixture of a 

 solution of potassium iodide and dilute phosphoric acid was found to act 

 equally well, and gave results identical with the first. It also simplified 

 the operation. When however the iodide was added first, and then the 

 acid, invariably a greater quantity of iodine was liberated. Mixtures 

 containing different quantities of chloride, chlorate, hypochlorite and 

 free alkali were analysed, and the following results obtained: — 



Volume of 

 solution used. 



Volume of thiosulphate used 



(first KI and then acid 



being added.) 



Volume of thiosulphate used 



(a mixture of KI and 



acid being added). 



I. 



II. 

 III. 



IV. 



10 c.c. 

 10 c.c. 

 10 c.c. 

 10 c.c. 



331 c.c. (mean of three) ... 

 28'88 c.c. (mean of five) ... 

 2900 c.c. (mean of two; ... 

 37'50c.c. (mean of three)... 



32 8 c.c. (mean of three). 

 28'6 c.c. (mean of three). 

 288 c.c. (mean of two). 

 37"2 c.c. (mean of three). 



