Spectrophotometric determination as a molybdenum V thiocyanate complex

In a mineral-acid solution and in the presence of a reducing agent molybdenum (V) reacts with thiocyanate to form a yellowish-orange Mo(V)SCN complex. The reducing agent used is tin (II) chloride. Extraction of the colour complex, presumably as the oxonium salt of the acid H2MoO(SCN)5 (Wiinsch), allows interfering substances to be separated off. 

Assuming an initial quantity of 10 litres of the water sample, which is then evaporated to less than 1 litre and treated in accordance with the directions given, concentrations of molybdenum down to approximately 0.02 mg/1 can be determined. 

Potential sources of interference are largely eliminated by precipitation with hydrogen sulphide, extraction as cupferronate with methyl isobutyl ketone and final extraction of the Mo(V)SCN complex with diisopropyl ether. All reagents must be of the highest grade of purity. A blank test must be carried out in parallel. 

Glass dish, 1 litre Measuring flask, 30 ml Quartz or porcelain crucible Separating funnel, 50-100 ml 

Sodium hydrogen carbonate solution, saturated in the cold state Ammonium chloride, reagent purity Potassium hydrogen sulphate Hydrochloric acid, 25 % 

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