hy the Direct Action of A7'seniotcs Acid. 287 



An examination of these results discloses the fact that for 

 conditions varying within a rather wide range the oxidation of 

 the arsenious oxide reaches a fairly definite limit. The 

 bromate effects practically the same proportionate oxidation of 

 arsenious oxide in a volume of 200^°^^ or less, whether the sul- 

 phuric acid of half-strength present amounts to 3*5^°^^ or lO^'^^ 

 and whether the time of digestion is 15 minutes or 30 minutes 

 at the boiling temperature, one and one-half or four hours on 

 the steam-bath, or two days at the ordinary temperature. 

 Setting aside those experiments in which the addition of acid 

 did not exceed the equivalent of alkali carbonate present by 

 more than 1*^'''^, the average absolute variation from theory of 

 the entire list of forty-two experiments amounts to 0-OOOT— 

 grm. in terms of bromate, individual variations departing from 

 the average by about the same figure. The obvious meaning 

 of this fact seems to be that the slight error is due to impurity 

 in the potassium bromate employed and not to incomplete 

 oxidizing action on the part of the bromate. 



In the following series of experiments another preparation 

 of bromate was employed, and its value was fixed by reduction 

 with acidified potassium iodide and titration of the iodine set 

 free according to the method formerly proposed by Kratchmer.^ 

 The rate at which the action proceeds, according to the 

 equation 



6HI + HBr03 = HBr + 3H,0 -f I,, 



has been investigated by Ostwald,f by ISToyes,:}: and by Judson 

 and Walker.§ Time of action, proportion of iodide to bromate, 

 excess of acid, and dilution are all, within limits, determining 

 factors in the reaction ; but in the analytical process it is 

 usually assumed that the reaction goes soon to completion if 

 free acid and a moderate excess of potassium iodide are 

 present. In the following table are recorded the results of 

 experiments in which measured amounts of standard solutions 

 of potassium bromate (approximately 2*8 grms. to the liter) 

 were treated with potassium iodide and hydrochloric or 

 sulphuric acid for definite times in glass-stoppered bottles, the 

 iodine liberated being determined by titration with sodium 

 thiosulphate standardized against nearly decinormal iodine of 

 value fixed by comparison with decinormal arsenious oxide 

 dissolved in acid potassium carbonate. 



*Zeit. Anal. Chem., xxiv, 546 (1885). 

 fZeit. Pliys. Chem., ii, 127 (1888). 

 iZeit. Phys. Chem., xix, 599 (1896). 

 § Jour. Chem. Soc, Ixxiii, 410 (1898). 



