Thiocyanate Complexation of Molybdenum in Solutions of Reduced Molybdate

Abstract

A frequently used method for the determination of trace amounts of molybdenum utilizes the ability of this element in one of its oxidation states to form an intensely colored complex, or complexes, with thiocyanate. This method requires the reaction of an acid solution of molybdate with a suitable reducing agent, such as stannous tin, then the addition of alkali thiocyanate to develop a characteristic red color. Since the aqueous mixture of thiocyanate, reducing agent, and reduced molybdate is not in a state of equilibrium under optimum conditions for the development of maximum color, it is necessary, for analytical purposes, to stabilize the system by the addition of a homogeneous organic solvent, such as acetone, or to extract the thiocyanate-complexed molybdenum into an immiscible solvent, such as diethyl ether. It is the purpose of this investigation to establish, unequivocally, the oxidation state of molybdenum in the thiocyanate complex, or complexes, so formed, and to evaluate the molybdenum to thiocyanate ratio for the predominating colored specie. Suitable experimental conditions for the spectrophotometric determination of molybdenum with thiocyanate have been developed recently by Crouthamel and Johnson (2). The mechanism which was suggested for the reduction of molybdate by stannous tin is as follows: Mo(VI) + Sn(II) --+ Mo(IV) + Sn(IV) 2 Mo(IV) --+ Mo(V) + Mo(III). From a comparison of the absorption spectrum of the molybdenum thiocyanate complex formed when using copper(I), a one-electron reducing agent, with that using tin(II), it was concluded that the oxidation state of molybdenum in its red thiocyanate complex is plus five. This conclusion is in agreement with the findings of other investigators (1, 3).