Peters and Moody— Determination of Per sulphates. 367 



Art. XL. — The Determination of Persulphates ; by 

 Charles A. Peters and Seth E. Moody. 



[Contributions from the Kent Chemical Laboratory of Yale University — CIIL] 



In a recent article by Namias, of which, however, only the 

 abstract is at our disposal,* it is proposed to substitute for the 

 method of reducing the persulphate by a ferrous salt and esti- 

 mating the excess of the latter — a method which the author 

 characterizes as inaccurate : — the treatment of the persulphate 

 by potassium iodide for a period of ten or twelve hours, and 

 titration by thiosulphate of the iodine set free ; and Griitznerf 

 has recently stated that a solution of arsenious oxide is well 

 suited for the determination of the oxidizing power of persul- 

 phates, the reaction being hastened by heat and the presence 

 of alkaline hydroxide. A test of the processes of Namias and 

 Griitzner led us to consider also the method of LeBlanc and 

 Eckardt,^ as well as the previously proposed method of 

 Mondolfo,§ in order that the indications of all these methods 

 might be brought into comparison. 



LeBlanc and Eckardtjj state and bring experimental results 

 to show that when a mixture containing a persulphate, a suffi- 

 cient excess of ferrous salt, and sulphuric acid, is heated at 

 60°-80°, or allowed to stand ten or twelve hours, the persul- 

 phate is reduced, and the amount of ferrous salt oxidized is the 

 measure of the amount of persulphate originally present in 

 solution. This is the method to which Namias takes exception. 

 Marshall^ is authority for the statement that persulphates 

 liberate iodine from potassium iodide, and that the action is 

 hastened by heat and affected little. by the addition of dilute 

 sulphuric acid. Upon this reaction Mondolfo** has based a 

 method for the estimation of persulphates, which consists in 

 heating a mixture containing the persulphate and potassium 

 iodide in a stoppered bottle for ten minutes at 60°-80°, and 

 titrating by thiosulphate the iodine set free. The process 

 which JSTamiasff proposes for the estimation of persulphates — 

 without knowledge of Mondolfo's method — differs from the 

 iodometric method of Mondolfo in the point that the interac- 

 tion between the iodine and the persulphate is carried out at 

 the ordinary atmospheric temperature and prolonged ten or 

 twelve hours, when the estimation is made hy thiosulphate of 

 the iodine set free in the action. 



* Chem. Centralblatt. 1900, ii, p. 806. f Ibid., 1900. i, p. 835. 



i Zeit. f. Electrochemie, 5, 355-7. § Chem. Zeitung, 23, p. 699. 



|| Loc. cit. «jf Jour. Chem. Soc, 59, 171. 



** Loc. cit. f f Loc. cit. 



