Peters and Moody — Determination of Per sulphates. 371 



The average of the corrected figures gives a value to 12 - 5 cm 3 

 of the persulphate solution of 0-1219 grm. of the salt. 



Method of Mondolfo. 



In certain experiments conducted as described by Mondolfo* 

 a mixture, 25 cm3 in volume, containing 12*5 cm3 of the persul- 

 phate solution, 1*0 grm. of potassium iodide, was heated in a 

 stoppered bottle 10 minutes on the steam bath, and after cool- 

 ing, the liberated iodine was titrated by thiosulphate. The 

 readings were made without starch, which, however, was added 

 afterwards to verify the observation. In other experiments, 

 similarly conducted, the volume of the liquid containing the 

 persulphate and the potassium iodide was increased to 100 cm3 

 and the mixture heated 10 minutes on the steam bath, the 

 temperature being 60°-S0°. The repeated return of the iodine 

 color when the volume of the liquid was 100 cm3 showed plainly 

 that only a partial reduction of the persulphate had taken place. 



When, however, the mixture at a volume of 100 cm3 was 

 heated on the steam bath for 30 minutes, the results agreed 

 with those obtained by the treatment at a volume of 25 cm3 . The 

 results of these two sets of experiments are given in sections 

 A and B of Table IY. After the iodine in the experiments 

 which are recorded in section A of Table IY had been titrated 

 by thiosulphate, sulphuric acid was added to the mixture and 

 a blue color appeared immediately, showing, even in these ex- 

 periments conducted at a volume of 25 cm3 , that the reduction 

 of the persulphate had been incomplete. This fact suggested 

 a study of the effect of the presence of sulphuric acid on 

 the process conducted otherwise as described by Mondolfo. 

 Accordingly experiments were made similar to those already 

 described excepting that a small amount of sulphuric acid was 

 present. The results of these experiments are given in section 

 C of Table IY. 



In a blank test made with l cm3 of sulphuric acid present, a 

 small amount of iodine was liberated ; and when, in a similar 

 experiment the air over the liquid was replaced by carbon 

 dioxide the amount of iodine liberated was less. The results 

 of these experiments in blank are given in section D of Table 

 IY. It is seen from the results of experiments given in sec- 

 tions A and B of the Table that the method as described by 

 Mondolfo gives constant results provided the volume is small 

 and the heating prolonged. The instantaneous appearance of 

 the iodine color upon acidifying with sulphuric acid showed, 

 however, that a very small amount of the persulphate had 

 escaped reduction. Still, the presence of sulphuric acid in the 



* Loc. cit. 



