374 Peters and Moody — Determination of Per sulphates. 



stand 20-25 hours, the air about the liquid in some experi- 

 ments being replaced by carbon dioxide. Though carbon di- 

 oxide shows no tendency to liberate iodine in the blank test, 

 the persulphate appears to liberate in its presence a little more 

 iodine than in its absence. This experience naturally suggests 

 an effect of acidity. The effect of the presence of sulphuric 

 acid in the process otherwise conducted as described by JSTamias 

 was therefore tried, the air over the liquid being replaced by 

 carbon dioxide. The results of these experiments are recorded 

 in section C of Table Y. 



It is seen from the experiments conducted as described by 

 Kaunas that the reduction of the persulphates in the time 

 stated by .Namias, 10-12 hours, is plainly incomplete unless 

 the volume of the liquid is small. If the experiments are con- 

 ducted at the greater dilution the time of standing must be 

 increased in order that subsequent standing may not result 

 in the return of color. In the presence of sulphuric acid 

 more iodine is liberated than in its absence, and the amount is 

 greater when the air above the liquid is not replaced by carbon 

 dioxide, and when the volume is small. The amounts of iodine 

 liberated under similar conditions in blank experiments prove 

 to be appreciable and naturally greater when the atmosphere 

 above the liquid is air. When these amounts, averaging the 

 equivalent of 0*0007 grm. of the persulphate when the atmos- 

 phere is carbon dioxide, and about 0*0010 gram when the 

 atmosphere is air, are deducted from the actual indication, the 

 figures agree well with those found for the same amount when 

 sulphuric acid is not present. Plainly, the addition of the acid 

 adds nothing to the regularity and value of the process. The 

 value of 12*5 cm 3 of the persulphate solution obtained by the 

 method of Namias is 0*1208 grm. 



Arseniate — Iodide Method. 



The estimation of chlorates is accomplished according to 

 Gooch and Smith* by allowing a mixture containing the 

 chlorate, a definite amount of standard potassium iodide solu- 

 tion, an arseniate, 20 cm of 1:1 sulphuric acid, to boil from a 

 volume of about 100 cm3 to 35 cm3 ; the difference between the 

 amount of iodine required to oxidize the arsenious acid pro- 

 duced and the amount of iodine in the potassium iodide origi- 

 nally present being the measure of the chlorate taken. Jt is 

 interesting to note that persulphates can be estimated in a 

 similar manner. Mixtures containing 12*5 cm3 of the persulphate 

 solution, 0*5 grm. of potassium iodide, 2*3 grm. of hydrogen 



* This Journal xlii, 220. 



