DIFFERENTIAL-PULSE POLAROGRAPHIC-DETERMINATION OF TRACE AMOUNTS OF VANADIUM AND MOLYBDENUM IN VARIOUS STANDARD ALLOYS AND ENVIRONMENTAL-SAMPLES AFTER PRECONCENTRATION OF THEIR MORPHOLINE-4-CARBODITHIOATES ON MICROCRYSTALLINE NAPHTHALENE OR MORPHOLINE-4-DITHIOCARBAMATE CETYLTRIMETHYL AMMONIUM BROMIDE NAPHTHALENE ADSORBENT

A highly selective, sensitive, and fairly rapid and economical differe ntial pulse polarographic (DPP) method has been reported for the deter mination of trace amounts of vanadium and molybdenum in standard alloy s and various environmental samples. The morpholine-4-carbodithioates of these metals were retained (> 99% recovery) quantitatively on micro crystalline naphthalene in the pH range 4.5-6.9 for vanadium and 1.5-4 .5 for molybdenum. These metals were determined by DPP after desorptio n with 10 ml of 1 M HCl. Vanadium and molybdenum may also be preconcen trated by passing their aqueous solutions under similar conditions on morpholine-4-dithiocarbamate CTMAB-naphthalene adsorbent packed in a c olumn at a flow rate of 1-5 ml min(-1) and determined similarly. The d etection limits are 0.20 ppm for vanadium and 0.04 ppm for molybdenum at minimum instrumental settings (signal to noise ratio = 2). The line arity is maintained in the following concentration ranges, vanadium 0. 50-10.0 and molybdenum 0.10-9.0 ppm, with a correlation factor of 0.99 96 (confidence interval of 95%, slopes 0.0196 and 0.01497 mu A mu g(-1 ), intercepts 3.65 x 10(-3) and - 1.92 x 10(-3) respectively) and rela tive standard deviation of 1.1% in the microcrystalline method, while in the column method, the linearity is maintained in the concentration ranges, 0.50-6.5 for vanadium and 0.10-5.5 ppm for molybdenum with co rrelation factor of 0.9994 (with confidence interval of 95%, slopes 0. 0194, 0.015 mu A mu g(-1), intercepts 3.60 x 10(-3) and - 1.90 x 10(-3 ) respectively) and relative standard deviation of 1.4%. Various param eters such as the effect of pH, reagent, naphthalene and CTMAB concent rations, volume of aqueous phase and interference of a large number of metal ions on the estimation of vanadium and molybdenum have been stu died in detail to optimize the conditions for their voltammetric deter mination at trace level in various standard alloys and environmental s amples. (C) 1998 Elsevier Science B.V. All rights reserved.