Solid Phase Spectrophotometry for the Determination of Cobalt in Pharmaceutical Preparations Leonardo Sena Gomes Teixeira 1;2; , Anto Ãnio Celso Spõ Ânola Costa 2 , Ju Âlio Ce Âsar Rosa Assis 2 , Se Ârgio Luis Costa Ferreira 2 , and Mauro Korn 3 1 UNIFACS ± Universidade Salvador, Departamento de Engenharia, Av. Cardeal da Silva 747, Federac Ëa Äo, 40220-141, Salvador ± BA, Brasil 2 Instituto de Quõ Âmica, Universidade Federal da Bahia, 40170-290, Salvador ± BA, Brasil 3 Departamento de Cie Ãncias Exatas e da Terra, Universidade do Estado da Bahia, Campus I, 41195-001, Salvador ± BA, Brasil Abstract. A new sensitive method exploiting solid- phase spectrophotometry is proposed for the determina- tion of cobalt in pharmaceutical preparations. The chromogenic reagent 1-(2-thiazolylazo)-2-naphthol (TAN) was immobilized on C18 bonded silica loaded into a home-made cell with 1.5 mm of optical path for cobalt determination. Cobalt(II) reacts with TAN on C18 material, at pH 6.0±7.5, to give a coloured complex which has maximum absorption at 572 nm. In this way, the sample was passed through the cell and Co(II) ions were quantitatively retained on the solid-phase. After the direct measurement of light-absorption in the solid phase, only the cobalt was eluted with 0.1 mol L 1 hydrochloric acid. The cell was washed with water and then another sample solution could be passed through the cell. The procedure allowed the determina- tion of cobalt in the range of 10±160 mgL 1 with coef®cient of variation of 4.7% n  10 and apparent molar absorptivity of 2:62  10 6 L mol 1 cm 1 using sample volume of 3-mL. Key words: Cobalt determination; solid-phase spectrophotometry; 1-(2-thiazolylazo)-2-naphthol; pharmaceutical preparations. Thiazole azo compounds, such as 1-(2-thiazolylazo)- 2-naphthol (TAN), have attracted much attention, since they are sensitive chromogenic reagents and important complexing agents. TAN has been used for spectrophotometric and extraction-photometric deter- minations of many metal ions. These dyes have been applied for spectrophotometric determinations due to its good selectivity and sensitivity over a wide range of pH and because they are relatively easy to synthesize and purify. Nevertheless for conventional spectrophotometric analysis in aqueous solution, the low solubility of these azo compounds and their complexes is a signi®cant drawback, that can be overcome by adding organic solvents or surfactants. However, the low solubility of these compounds can become an advantage when the solid phase spectro- photometry (SPS) technique is applied because the reagent can be immobilized through physical sorption on a proper support [1]. In recent years, a growing popularity of SPS has been observed due to its simplicity, rapidity, ease auto- mation and low detection limits achieved [2]. SPS is a technique that uses a solid matrix for preconcentration of the species of interest with the aid of a chromogenic reagent and the absorbance of the color complex of the analyte sorbed on a solid support is measured directly [3, 4]. The cell containing the sorbent is placed in a spectrophotometer and the light transmitted is mea- sured. These measurements may be carried out with the help of conventional spectrophotometers, resulting in low cost and good availability. SPS offers the advan- tage of in situ preconcentration of the analyte due to accumulation of the analyte in a small volume. Furthermore, an increase in selectivity is observed due to different retention of the analyte and the inter- fering substances on the solid state support [2, 5±7]. Several solid supports have been used for SPS: cation and anion exchanger resins, Sephadex QAE Mikrochim. Acta 137, 29±33 (2001)  To whom correspondence should be addressed