Peters and Moody — Determination of Per sulphates. 375 



potassium arseniate, and 20 cm3 of 1:1 sulphuric acid, and water 

 enough to make the total volume about 100 cm3 , were boiled in 

 a trapped Erlenmeyer beaker until the volume decreased to 

 35 cm3 , and the arsenite present after the solution was made 

 alkaline with potassium bicarbonate was estimated with iodine. 

 The results of these experiments are given in Table YII. 







. Table VII. 







Ammonium 



persulphate 



solution. 



cm 3 . 



KI 



present. 



grm. 



Iodine 

 required 



for 



oxidation 



of arsenite. 



grm. 



Iodine 

 liberated 



by 



persulphate. 

 grm. 



(NH 4 ) 2 S a 8 



equivalent 



to iodine 



liberated. 



grm. 



12*5 



0-1875 



0*0514 



0-1361 



0*1225 



12-5 



0-1875 



0-0522 



0-1352 



0-1217 



12-5 



0-1875 



0-0514 



0-1361 



0-1225 



12-5 



0-1875 



0-0516 



0-1359 



0-1222 



These results agree closely with one another and in the aver- 

 age, 0*1222 grm., accord well with the results of the process of 

 Griitzner and of LeBlanc and Eckhardt. 



To compare the values obtained for the persulphate solution 

 the averages of the results obtained by the different methods, 

 together with the average of all the experiments, are given 

 in Table VI. 



Table VI. 



Number of 

 Process. experiments. 



Mondolf o 6 



Namias 8 



LeBlanc and Eckhardt 12 



Griitzner (corrected) ._ 4 



Arseniate-iodide method 4 



Average of whole 



Average of results, 

 grm. 



0-1207 

 0-1208 

 0-1217 

 0-1219 

 0-1222 



0-1213 



The results obtained by the process of Mondolfo and the 

 process of E"amias, both of which involve the liberation of 

 iodine from potassium iodide and the titration of that iodine 

 by thiosulphate, are practically identical and lower than the 

 results obtained by the other three methods. The results 

 obtained by the process of LeBlanc and Eckardt in which the 

 persulphate is reduced by a ferrous salt, by the process of 

 Griitzner in which an arsenite solution is the reducing agent, 

 and the arseniate iodide method in which the persulphate is 

 determined by the difference between the amount of iodine in 



Am. Jour. Sci. — Fourth Series, Vol. XII, No. 71. — November, 1901. 

 26 



