THE BEHAVIOR OF BENZIDINE TOWARD SELENIC 
AND TELLURIC ACIDS. 
ARTHUR W. DOX. 
Within quite recent years benzidine ( p-diaminodiphenyt ) has 
come into use as a precipitant for the sulfate ion. It was first 
applied as a quantitative reagent for the determination of sul- 
fate by Rasehig 1 in 1903. Other investigators subsequently in- 
troduced modifications in the original method of Rasehig and 
succeeded in obtaining very satisfactory analyses with this re- 
agent. For example, in the analysis of water samples which con- 
tain iron salts, hydroxylamine hydrochloride is added to prevent 
oxidation of the benzidine. The precipitated benzidine sulfate 
is collected in the usual way and is either weighed direct or 
titrated with sodium hydroxide, using phenolphthalein as an 
indicator. Titration is rendered possible by the very weak basic 
properties of benzidine. Bruckmiller 2 states that the benzidine 
method for sulfates in water compares favorably with the time- 
honored barium chloride method in point of accuracy, and has 
the advantage of being more rapid. 
The writer undertook to determine whether the corresponding 
acids of selenium and tellurium, two elements closely analogous 
to sulfur and occurring in the same group of the periodic sys- 
tem, would react in the same manner, with benzidine. 
The benzidine reagent was prepared as follows : Two grams 
of Merck ’s benzidine were stirred to a paste with a little water, 
washed into a 250 cc. volumetric flask, 2.5 cc. concentrated hydro- 
chloric acid added, and the solution made up to the mark. A 
slight sediment was removed by filtration. 
Qualitative tests were first made with this reagent. When 
added to a solution of sodium sulfate, as was expected, a white 
granular crystalline precipitate began to form instantly. With 
a solution of Kahlbaum’s sodium selena’te the same phenomenon 
was observed, except that the precipitate was more granular and 
settled out more readily. However, a solution of Kahlbaum’s 
sodium tellurate gave no precipitate whatever with the benzi- 
dine reagent. 
1 Raschig, Z. angew. Chem., 1908, 617, 81S. 
2 BruckmilIer, J. Ind. Eng. Chem., 7, 600, 1915. 
