Abstract.A spectrophotometric method for the determination of gold has been developed, based on the reaction of gold(III) with molybdate and nile blue (NB) to form an ion-association complex in the presence of poly(vinyl alcohol). The molar absorptivity at 595 nm is 2.71 x 105 1 mo1-1 cm -1. Beer's law is obeyed over the range 0-16 #g of gold per 25 ml. The relative standard deviation evaluated from seven independent determinations of 0.4 #g/ml of gold is 2.6%. The limit of detection is 0.011 #g/ml. The molar ratio of Au to NB in the complex is established to be 1 : 2. Over 30 foreign ions were tested for interference; Pt(IV), Sb(III), W(VI) and SiO 2-interfered and had to be separated from gold on polyurethane foam. The method can be applied to the spectrophotometric determination of trace amounts of gold in powdered carbon and some ores.Key words: gold, ion-association, spectrophotometry, nile blue.Gold can be determined with various chromogenic agents [-1-18], such as di-2-thienylketoxime Eli, anthranilic acid [-2] and diethyldithiocarbamate [3]. In most of the methods, the sensitivity is very poor and the colour fades after 10 min. In some instances the complex is formed only after heating for a long time and requires extraction with organic solvent, whereas others suffer from interferences from other metal ions. The spectrophotometric determination of niobium [-19], tantalum [-20], tellurium [21] and zirconium [22] as ion-association complexes of the metal ion with molybdate and nile blue in aqueous solution in the presence of PVA has been reported. However, the possibility of the determination of gold using an ionassociation complex of this type has not been studied to date. This paper presents a detailed investigation of various factors for the spectrophotometric determination of gold in aqueous solution, e.g., the acidity, reagent concentrations, the stability and the effects of foreign ions. The method has been applied to the determination of' gold in powdered carbon and some ores, with satisfactory results.

A sensitive method for the determination of trace amounts of scandium by spectrophotometry is described, based on the formation of the ion associate between scandium(III), molybdate and Nile Blue in the presence of poly(vinyl alcohol). The molar absorptivity at 585 nm is 3.59X105 dm3 mol -' cm -'. Beer's law is obeyed over the range 0 -2.0 µg scandium per 25 ml. Optimal conditions for the determination and the interferences from foreign ions are reported.Ten-fold amounts of rare earth elements do not interfere with the determination of scandium, except cerium. The proposed method has been applied successfully for the spectrophotometric determination of scandium in some geological samples.

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