Thiosulphate with Iodic Acid. 237 



bring out the starch blue was uniformly applied in the ana- 

 lytical work. 



To determine whether or not the purity of the ordinary iodic 

 acid is sufficient to admit of its direct application in standard 

 solutions, a series of experiments was made. Two different 

 samples of "chemically pure" iodic acid were used. The 

 first was in coarse granular crystals, and the second was in the 

 form of fine powder. Quantities of both of these were dried 

 in a dessicator over sulphuric acid to constant weight. Neither 

 sample lost weight appreciably when left for a considerable 

 time on the scale pan. A third sample of iodic acid was pre- 

 pared by dissolving a quantity of the purest obtainable iodic 

 anhydride in water, and evaporating at ordinary temperature. 

 The resulting crystalline mass was dried over sulphuric acid in 

 a dessicator for one week, until it ceased to lose weight, when 

 it was presumed to consist of the pure normal acid. Two pre- 

 sumably decinormal solutions of each of the first two samples, 

 and one such solution of the third sample of iodic acid were 

 made by weighing out 17*585 grms. and dissolving in exactly 

 one liter of water at 15° C. Convenient portions of each of 

 these solutions were analyzed in the manner described, with 

 results shown in the following table, averaged from many 

 determinations. 







Table I. 







Analyses 



of Approximately 



— Iodic Acid. 



Solution 



Sample 



HI0 3 taken. HI0 3 found. Error. 



analyzed. 



used. 



grm. 



grm. grm. 



I 



A 



0*1055 



0-1066 0-0011 + 



II 



A 



0*1055 



0-1062 0-0007 + 



III 



B 



0-1055 



0-1065 0-0010 + 



IV 



B 



0-1055 



0-1073 0-0018 + 



V 



C 



0-1055 



0-1053 0-0002 — 



These results show that while the deviation from the theo- 

 retical strength of the solution in the case of the acid prepared 

 from the anhydride is hardly appreciable, and will not affect 

 the accuracy of any work in which the solution may be applied 

 as a means of standardization, the solutions made from the 

 purchased product, on the other hand, contain a very appre- 

 ciable amount of iodine in excess of the theoretical. That 

 iodic acid is somewhat unstable at 30-40° C, gradually losing 

 water with the formation of the anhydride,* is well known, and 

 it is quite possible that to some such gradual change as this 

 must be attributed the fact that the ordinary iodic acid cannot 



* Dammer, Anorganische Chemie, i, 564. 





