238 . 0. F. Walker— Titration of Sodium 



be safely employed as a means of standardization unless its 

 purity be directly determined by analysis. 



To determine whether a solution of iodic acid, once pre- 

 pared and standardized, will retain its strength for a long 

 period of time, two such solutions were kept for four months 

 (in the dark) and then again analyzed. The results (averages 

 of several determinations), given in Table II, substantiate the 

 observation of Eiegler that a solution of iodic acid will remain 

 of constant strength. 





r 



fABLE 11. 





Constancy of Strength of Iodic Acid Solutions. 



Iodic acid 



First analysis. 

 HI0 3 found. 



Second analysis, 

 (after four months) 

 HI0 3 found. 



Variation 



Solution. 



grm. 



grm. 



grm. 



I 



0*1073 



0-1072 



o-oooi- 



II 



0*1049 



0-1046 



0-0003- 



An approximately one-twentieth normal solution of " chem- 

 ically pure" sodium thiosulphate was made and its exact 

 strength ascertained by titrating it with standardized iodine. 

 A series of analyses made by oxidizing the sodium thiosul- 

 phate to sulphate, and precipitating and weighing as barium 

 sulphate, gave results identical with those obtained with iodine, 

 proving that all the sulphur present in the solution was in the 

 form of thiosulphate. According to Riegler's equation, sodium 

 thiosulphate and iodic acid react molecule for molecule, and 

 solutions of these substances should therefore require for their 

 mutual saturation volumes inversely proportional to their con- 

 centration. It was found, however, that when the one-twen- 

 tieth normal solution of sodium thiosulphate that has been 

 described was titrated in the presence of starch emulsion with 

 an approximately decinormal solution of iodic acid, prepared 

 from the anhydride, a distinctly blue color was produced long 

 before the theoretical amount of iodic acid had been added. 

 It was further noticed that the end-point of the reaction was 

 far from distinct, a faint tinge of blue at first being visible, 

 then suddenly becoming deeper, and immediately reappearing 

 when bleached with sodium thiosulphate. The deficiency in 

 the amount of iodic acid actually required to produce the blue 

 color was not lessened by titrating only three-fourths of the 

 theoretical amount of iodic acid, and estimating the residual 

 thiosulphate with iodine. It was found, however, that the 

 addition of a considerable quantity of potassium iodide to the 

 solution, either before or during the titration, had the marked 

 effect of making the reaction sharp and distinct, entirely pre- 

 venting the " after separation " of iodine, at the same time 



