Palmer — Application of Potassium Fei*ricyanide. 399 



Art. XXXI V. — The Application of Potassium Ferricyanide 

 in Alkaline Solution to the Estimation of Arsenic, Anti- 

 mony, and Tin; by Howard E. Palmer. 



[Contributions from the Kent Chemical Laboratory of Yale Univ. — ccx.] 



In 1892, Quincke* published a method for estimating arsenic 

 and antimony gasometrically, consisting essentially in oxidizing 

 the arsenic or the antimony by a known excess of potassium 

 ferricyanide in the presence of alkali, and determining the 

 excess by measuring in a gasometer the oxygen evolved by the 

 action of hydrogen peroxide on it according to the following 

 equation: 



2K,FeC 6 N e + H 2 2 + 2KOH = 2K 4 FeC 6 N 6 + 2H.0 + 2 . 



In previous papers from this laboratory, methods for the esti- 

 mation of cerium in the presence of the other rare earths,f 

 and for the estimation of thallium, % have been described, based 

 on oxidation by potassium ferricyanide in alkaline solution and 

 titration with permanganate of the ferrocyanide formed. 



The work to be described is the result of an attempt to 

 apply this reaction to the estimation of arsenic, antimony, and 

 tin, which are oxidized according to the following equations: 



As a O, + 4K,FeC 6 N 5 + 4KOH = As 2 5 + 4K 4 FeC 6 N 6 + 2H Q 0. 

 Sb 2 3 + 4K„FeC 6 N 6 -h4KOH = Sb 2 5 + 4K,FeC B N 6 + 2H 2 0. 

 SnO + 2K,FeC a N e + 2KOH = Sn0 2 + 2K 4 FeC 6 N 6 + H 2 G. 



The ferrocyanide formed is then oxidized by permanganate 

 according to the equation 



10K 4 FeC 6 N 6 + 2KMnO 4 + 8H 2 SO 4 = 



1 0K,FeC a N 6 + 6K 3 S0 4 + 2MnS0 4 + 8H 2 0. 



Estimation of Arsenic. 



The essential procedure was to oxidize the arsenic by 

 potassium ferricyanide in alkaline solution, and after making 

 ammoniacal by the addition of ammonium sulphate, to precipi- 

 tate the arsenic by magnesia mixture, filter off the ammonium 

 magnesium arseniate, and titrate the filtrate with permanganate, 

 after acidification with sulphuric acid. 



For the work a solution of arsenious acid was prepared in 

 the usual way by dissolving pure sublimed arsenious oxide in 

 potassium hydroxide, neutralizing with sulphuric acid and 

 adding sodium bicarbonate. The magnesia mixture used 



*Zeitschr. f. Anal. Chem. , xxxi, 1. 



f This Journal, xxvi, 83. % This Journal, xxvii, 879. 



