Determination of molybdenum in steel by adsorptive stripping voltammetry in a homogeneous ternary solvent system

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 alpha-benzoinoxime (alpha BO) 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 -1050 mV. Under these optimized experimental conditions, the Mo(VI)-alpha BO reduction current peak potential is observed at potentials near -1250 mV, much lower than those usually reported, and the calibration plot follows the polynomial equation I = 0.359 + 0.265 [CMo(VI)] - 0.015 [CMo(IV)]2 (r2 = 0.997), for Mo concentrations up to 10.0 micrograms L-1. There is a linear range in this calibration plot for Mo(VI) concentrations up to 0.20 microgram L-1, defined by the equation I = 0.353 + 0.385 [CMo(VI)] (r2 = 0.980). In both cases, I is the absolute value for the current in microA and CMo(VI) is the concentration of Mo in microgram L-1. The detection limit for this linear concentration range was estimated as 20 pg L-1. A RSD of 0.43% is associated with the signals at a Mo(VI) level of 0.72 microgram L-1. 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.