Fast determination of aluminium reactive to 1,2-dihydroxyanthraquinone-3-sulfonic acid in sea-water

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Abstract
A modification of the method for the determination of aluminium by adsorptive cathodic stripping voltammetry of its 1,2-dihydroxyanthraquinone-3-sulfonic acid (DASA) complex is described. Rapid determination is achieved by using staircase modulation and high potential scan speeds (20 V s–1). Square wave modulation was found inefficient above 2 V s–1 because the potential pulse is distorted by the ohmic drop. The staircase modulation is also distorted, but the determination is not disturbed and high faradaic currents are obtained. The method was not sensitive to the turbulence of the solution during the scan, as a short scan time is required (several ms) and very high currents (µA) are obtained from the reduction of the DASA–Al complex. Interference from oxygen was also overcome as DASA–Al to oxygen current ratios are increased and background subtraction is feasible. The natural calcium concentration of sea-water enhances the DASA–Al peak height; calcium should therefore be added to fresh water to enhance the determination. The method is proposed in particular for the determination of Al on board oceanographic vessels. There is no need to purge samples, noise due to vibrations of motions of the ship during scanning is avoided, and the sensitivity is enhanced. The total analysis time can be decreased to 8–12 min per sample by using an adsorption time of 40 s. The technique has been tested successfully for the on-board determination of Al in sea-water. Results obtained for sea-water samples irradiated by ultraviolet light were similar to those obtained using an existing fluorimetric method.