Determination of 1 in Haloid Salts. L97 



The sodium chloride aud potassium bromide used were 

 shown to be free from iodine, and the hydrogen potassium ar 

 oiate contained qo arsenions acid. 



The mode of proceeding was. in general, like thai of the pre- 

 vious experiments. The solution of potassium iodide was put 

 in an Enenmeyer beaker of 300 cm capacity, two grams of 

 hydrogen potassium arseniate were added in solution and fol- 

 lowed by l j <> cm' of sulphuric acid [1:1], the liquid was diluted 

 witli Mater to a little more than LOO em', a platinum spiral was 

 placed in the rlask to secure quiet ebullition, a trap made by 

 cutting ofi a two-bulb drying tube about an inch from the in- 

 side bulb was hung. Large end downward, in the mouth of the 

 flask io prevent mechanical loss, and the Liquid was boiled until 

 the level reached the 35 enr mark put upon the flask. At this 

 point the flask was cooled, the acid nearly neutralized with so- 

 dium hydrate in solution, the neutralization completed by acid 

 potassium carbonate, 20 cm" of the last being added in excess, 

 a definite portion of starch indicator added, and the contents 

 in arsenions acid determined by titration with a decinormal 

 solution of iodine for the larger amounts, and a centinormal 

 solution for the smaller. Due correction was made for the 

 amount of iodine necessary to develop the test-color in a solu- 

 tion prepared and treated similarly in all respects to the experi- 

 mental solutions excepting the introduction of the iodide — the 

 correction amounting to a single drop more of the decinormal 

 solution than was required to produce the end reaction in the 

 same volume of pure water containing only the starch indicator. 



The decinormal solution of iodine was made by dissolving 

 12*65 grms. of carefully resublimed iodine in potassium iodide 

 (proved free from iodate) and diluting to a liter. The centi- 

 normal solution was made by diluting the stronger solution. 

 The standardizing was effected by comparison with a solution 

 of arsenions oxide containing -t*95 grin, to the liter. This 

 mode of fixing the value of the solution should, and, as the 

 sequel proved, did indicate the correct standard, but for the 

 >ake of confirmation the value of the arsenic solution was re- 

 determined by titration against iodine specially purified by 

 Bnbliming off potassium iodide, re-subliming between watch- 

 glasses, and exposing during forty-eight hours over sulphuric 

 acid. Portions of the iodine thus prepared were weighed in a 

 glass-Stoppered weighing bottle, dissolved in the same without 

 danger of volatilization by introducing pure solid potassium 

 iodide and a little water, and diluted and treated with the solu- 

 tion of arsenions acid measured from a burette until the color 

 of iodine vanished. The excess of arsenious oxide was deter- 

 mined by titrating against the standard solution of iodine whose 

 value in terms of measured portions of the solution of arseni- 

 ons oxide was perfectly known. 



