A Kinetic Experment to Determine Traces of Mo(VI ) Malachite Green is reduced to a colorless product by TiCh; this is catalyzed by Mo(V1). Even.traces of tungsten (W(VI1of the order of 1 ue must he ahsent. a s this also eatalvzes the reaction. Shiokawal studied the reaction usine a
Distilled water, polyethylene storage containers, and reagent-grade chemicals are used throughout. Solutions used L Malachite Green, 0.04M Tic13 (the 15% eommereial solution2 appropriately diluted and stored wer a little zine-amaleam. the mixture freauentlv shaken to ensure the reduced state). 0.75 M HzS04 and ammonium molvbdate (1104 mg/2. fo; 600 pg metallml or 20 pg metddrop, assuming 30 drops to the milliliter using a Pasteur $pet of moderate bore). In a spot-plate cavity are placed (in drops) 1 of Malachite Green, 1of the HzS04, ldrop of MOW),and 4 of TiCb solution, starting a stop-watch on the latter and timing to the calor end-point (faint pink, due to the violet-color of excess Ti3+).The procedure is repeated, using 40,60,80,100pg M d M ) (and a blank in other cavities. Average times for the samples are (in seconds) 256,204,170, 147, 128,330(blank), respectively, a t room temperature, 20 'C. A plot of micrograms of Mo(VI)against reeiproeal time is linear, whence "unknowns" can he interpolated. Feh, Pd2+, V5+,Ni2+,and N o 3 interfere (they react with Ti3+). The limit of detection is of the order of 20 pg. were 0.5g
'~hiokawa,T. Sci R e p . Resarch. Insts. Tohoku. Univ. 1950,2,770-773. 'care: Hazardous 10 skin; close stock solution lightly and store away from water.
M. Lampard 3 Nockolds Ave. Punchbowl, NSW 2196.Austrailia
Volume 69 Number 4 April 1992
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