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Based on the formation of a Keggin-type [PMo12O40]3- complex, a sensitive capillary electrophoresis (CE) method was developed for the determination of P(V) with direct UV detection at 220 nm. A mixture of alpha- and beta-Keggin-type [PMo12O40]3- complexes was readily formed in a sample solution consisting of a trace amount of P(V), 2.5 mM Mo(VI), 0.050 M p-C6H3(CH3)-2-SO3H (XSA), and 60% v/v CH3CN. When a 0.05 M HCl and 60% v/v CH3CN solution was used as a migration electrolyte, the Keggin complexes exhibited a sharp and well-defined peak in the electropherogram. The peak area was linearly dependent on the P(V) concentration in the range of 5 x 10(-7)-5 x 10(-5) M; a detection limit of 1 x 10(-7) M was achieved. In comparison with indirect UV detection, the direct UV detection is about ten times more sensitive, because the Keggin complexes possess high molar absorptivities. The developed CE method was applied to the determination of P(V) in river water, and the results were in good agreement with those obtained by ion chromatography (IC) and colorimetry (COL) based on the formation of mixed-valence heteropoly blue species.

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The formation of heteropoly complexes has been the basis for a widely utilized determination of oxoanions as central hetero-ions. 1 We have demonstrated the formation of new heteropolymolybdate complexes in aqueous-organic media. For example, PHO 3 2-and P 2 O 7 4-react directly with an Mo(VI)-Mo(V) reagent to form the corresponding heteropoly-blue species in CH 3 COCH 3 -containing media, and the color-forming reaction was applied to a direct spectrophotometric determination of both oxoanions. 2 The formation of a Dawson-type 18-molybdobis(sulfate)(VI) complex in aqueous CH 3 CN media was utilized for the voltammetric and spectrophotometric determination of SO 4 2-. 3,4 Among the heteropolymolybdate complexes, the Keggin complexes are of particular interest because the yellow complexes undergo reversible multi-step reductions to the corresponding heteropoly-blue species. However, most of the works have been limited to the Keggin complexes based on Si(IV), Ge(IV), P(V) and As(V). Recently, we synthesized a novel Keggin complex with S(VI) as a central hetero-ion. 5,6 In an attempt to develop new analytical methods, we have continued to prepare Keggin complexes incorporating metal ions as the central hetero-ions. Based on the formation of a 12-molybdovanadate(V) complex 7 , we developed a simple voltammetric method for the determination of V(V) and V(IV). 8 In continuation of such preparative studies, we have found that Ga(III) also reacts directly with Mo(VI) to form a 12-molybdogallate(III) complex in acidic solutions containing CH 3 CN. Raman spectroscopic and voltammetric studies have demonstrated that the yellow complex possesses the Keggin structure. The present study was undertaken to elucidate the formation conditions of the 12-molybdogallate(III) complex and to establish a voltammetric method for the direct determination of Ga(III). The voltammetric reduction current for the Keggin complex was directly proportional to the Ga (III) concentration, and the presence of many metal ions including Al(III) and In(III) did not interfere with the determination.In general, Ga(III) has been determined spectrophotometrically as a complex with chelating reagents such as tetraethylrhodamine (Rhodamine B) 9-11 , 4-(2-pyridylazo)resorcinol (PAR) 12 , 1-(2-pyridylazo)-2-naphthol (PAN) 13 and 8-hydroxyquinoline (oxine). 14 Lumogallion and Eriochrome Red B have also been used as reagents for the fluorophotometric determination of Ga(III). 15,16 In contrast, few methods have been developed for the voltammetric determination of Ga(III), although Kammori et al. described an ac polarographic method with the use of HClO 4 +NH 4 SCN as a supporting electrolyte. 17 Experimental Apparatus and materialsRaman spectra were obtained on a Jobin Yvon Model Ramanor U-1000 spectrophotometer. In order to make quantitative measurements, 0.1 M (M=mol/dm 3 ) LiNO 3 was added to each solution and used as an internal standard; the NO 3 -line was observed at 1048 cm -1 . When the Raman intensities were measured as a function of the Ga(NO 3 ) 3 con...