Enrichment and determination of molybdenum in geological samples and seawater by ICP-AES using calmagite and activated carbon

The present paper proposes a procedure for separation, preconcentration and determination of trace amounts of molybdenum in geological samples by using atomic emission spectrometry with inductively coupled plasma (ICP-AES). It is based on the complexation of molybdenum(VI) ions by calmagite reagent and its sorption onto activated carbon. Parameters, such as calmagite amount, pH effect on the complexation and sorption, shaking time, mass of activated carbon, desorption of molybdenum from activated carbon, sample volume, effect of other ions and analytical features like precision, accuracy and others were studied. Results demonstrated that the molybdenum(VI) ions, within the pH range from 1.0 to 3.0, as calmagite complex, had been quantitatively sorbed (>90%) onto activated carbon. The shaking time required for sorption of the complex was 15 min by using 50 mg of activated carbon. A successfully desorption of molybdenum for its determination by ICP-AES was achieved by heating the loaded activated carbon with concentrated nitric acid at 120 degreesC and treating the residue with 20% (v/v) nitric acid solution. The effect of some ions in the separation procedure was assessed and iron(III) should be reduced to iron(II) before the complexation, if present in fairly high amounts. This procedure was also efficient for elimination of spectral interference of aluminium in molybdenum determination by ICP-AES. The precision of the preconcentration method, calculated as the relative standard deviation by using 50.0 mug l(-1) molybdenum solutions, was 2.21 and 2.89%, regarding repeatability and reproducibility, respectively. The limit of quantification and detection were 2.50 and 0.75 mug l(-1), respectively, for a sample volume of 1000 mi. The achieved enrichment factor was 100. Achieved analytical results for (JR-I) Rhyolite, (RGM-1) Rhyolite, (SGR-1) Green river shale and (IPT-73) iron pure standard reference materials were in good agreement, between 92 and 103% of the certified values. The preconcentration procedure proposed was applied also to determine this element in seawater collected from Salvador City, Bahia, Brazil. (C) 2001 Elsevier Science B.V. All rights reserved.