Use of the molybdenum–thiocyanate–rhodamine 6G ternary complex for spectrophotometric molybdenum determination without extraction

A modified thiocyanate method without extraction by using rhodamine 6G as a secondary ligand was developed. Molybdenum in 1.0×10^-2 M HCl, after the addition of ascorbic acid, was heated for 10 min in a 90 °C water bath for reduction. Suitable amounts of glycerine, Triton X-100, rhodamine 6G solutions and 2+1 (v/v) 9 M H2SO4+3 M KHSO4 were added in this order. This solution was allowed to cool to room temperature and the absorbance at 570 nm was measured against a reagent blank 45 min after the addition of thiocyanate solution and the second aliquot of Triton X-100 solution. The complex was stable for at least 4 h, the order of reagent addition was important, and thiocyanate should be in large excess. Beer’s law was obeyed over the range 0.9×10^-6 to 1.1×10^-5 M Mo with the molar absorptivity being 1.1×10^5 l mol^-1 cm^-1. The R.S.D. for the determination of 0.7 mg Mo l^-1 was 1.83% (n=8). Possible interferences of various cations and anions on molybdenum determination were studied. The proposed method was applied to the determination of molybdenum in a dental alloy, Wiron 99.