ANALYST FULL PAPER THE www.rsc.org/analyst Determination of molybdenum in steel by adsorptive stripping voltammetry in a homogeneous ternary solvent system João Carlos de Andrade,* Alex M. de Almeida, Aline Renée Coscione and Luiz M. Aleixo Universidade Estadual de Campinas, Instituto de Química, CP 6154, 13083-970, Campinas SP, Brazil. E-mail: dandrade@iqm.unicamp.br Received 31st January 2001, Accepted 12th April 2001 First published as an Advance Article on the web 22nd May 2001 A new alternative approach for the determination of molybdenum in steel is proposed, using adsorptive stripping voltammetry (AdSV). The determinations are performed in a homogeneous ternary solvent system (HTSS) composed of N,N-dimethylformamide, ethanol and water, with a-benzoinoxime (aBO) as the complexing agent and a sodium acetate–acetic acid buffer as the support electrolyte. The HTSS composition was optimized by mixture design modelling. The AdSV measurements were performed in the differential pulse mode using an accumulation potential of 21050 mV. Under these optimized experimental conditions, the Mo(VI)–aBO reduction current peak potential is observed at potentials near 21250 mV, much lower than those usually reported, and the calibration plot follows the polynomial equation I = 0.359 + 0.265 [C Mo(VI) ] 2 0.015 [C Mo(IV) ] 2 (r 2 = 0.997), for Mo concentrations up to 10.0 mg L 21 . There is a linear range in this calibration plot for Mo(VI) concentrations up to 0.20 mg L 21 , defined by the equation I = 0.353 + 0.385 [C Mo(VI) ] (r 2 = 0.980). In both cases, I is the absolute value for the current in mA and C Mo(VI) is the concentration of Mo in mg L 21 . The detection limit for this linear concentration range was estimated as 20 pg L 21 . A RSD of 0.43% is associated with the signals at a Mo(VI) level of 0.72 mg L 21 . From the common method-interfering species tested, only iron at Fe/Mo(VI) ratios above 500 and vanadium and tungsten at M/Mo(VI) ratios above 100 appear to affect the analytical response significantly. Phosphorous may also reduce the analytical signal at P/Mo(VI) ratios above 100, due to the formation of the competitive P–Mo complex. The suggested routine procedure was tested by analyzing four stainless steel samples and the results compared well with the ICP-AES measurements. The higher sensitivity of this method permits direct determination of Mo(VI) in steels, eliminating the need of analyte concentration or separation steps in the sample processing procedure. Introduction Molybdenum plays an important role in many industrial processes, mostly those related to metallurgical applications for the production of alloy steels and non-ferrous alloys. Its addition to steels results in increased strength, reduced corrosion and improved resistance to thermal breakdown. Consequently, many analytical procedures have been pro- posed for Mo determination, 1–21 but only a few, such as graphite furnace atomic absorption (GFAAS) and plasma emission (ICP- AES and ICP-MS) techniques, 1–7 present good sensitivity and lower detectability associated with lower levels of chemical interference. Iron appears to be one of the most severe interfering elements. In fact, many of the common methods used for molybdenum determinations are usually performed in association with masking, separation or concentration steps, 8–14 which can make the analytical procedures time consuming and exposed to an undesirable level of operating errors, losses and contamination. One of the most common separation procedures used in Mo determinations is liquid–liquid extraction with a-benzoinoxime (aBO), 11–14 which is often employed as a prior separation step in spectrophotometric methods for molybdenum determination. Several electroanalytical techniques have also been considered for this purpose, 15–21 due to their relatively lower complexity and lower detectability. These methods are usually run in aqueous solution or in a solution containing low concentrations of an organic solvent, added to dissolve non-soluble complex- ing agents whose metal complex will be the electroactive species to be detected. Since preliminary experiments carried out in our laboratory using adsorptive stripping voltammetry (AdSV) have indicated that AdSV method sensitivity could be greatly improved by adding organic solvents to the aqueous solution and also considering that, up to now, we know of no voltammetric methods for the determination of metals having been proposed using a homogeneous ternary solvent system (HTSS), we report in this paper our determination of molybdenum in steels by AdSV using a N,N-dimethylformamide (DMF)–ethanol–water homogeneous ternary solvent mixture. Experimental Solutions and reagents All chemicals were of analytical grade and distilled deionized water was used throughout. The solutions were stored in high- density polyethylene bottles. A 1000 mg L 21 molybdenum standard stock solution was prepared by dilution of the content of a standardized Merck– Tritisol ampoule to 1000 mL with water. The working solutions were prepared from the stock as needed. Commercial standard stock solutions of 1000 mg L 21 of Al(III), Co(II), Cu(II), Cr(VI), Fe(III), P (as phosphate), Mn(II), Ni(II), S (as sulfate), W(VI), V(V) and Zn(II) were employed for the interference studies, after appropriate dilutions. A stock aBO solution (4.0 3 10 24 mol L 21 ) was obtained by dissolution of an appropriate amount of the compound in DMF This journal is © The Royal Society of Chemistry 2001 892 Analyst, 2001, 126, 892–896 DOI: 10.1039/b101068a Published on 22 May 2001. Downloaded by UNIVERSIDAD ESTADUAL DE CAMPINAS on 04/08/2014 16:11:34. 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