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A novel spectrophotometric method is described. The procedure involves the use of Cu (II) as vitamin C oxidant. After completion of oxidation reaction, excess of Cu (II) is determined by complexation with alizarin red s (ARS). Thiocyanate ion is used as stabilising agent for Cu (I) which is the product of oxidation reaction. This method is used for determination of vitamin C in fruits and pharmaceutical products. Linearity was found to be perfect in the range 3.5 · 10 )6 -4.8 · 10 )5 m (0.6-8.2 ppm). The detection limit and relative standard deviation of method are 2.0 · 10 )6 m (0.35 ppm) and 1.03%, respectively. The proposed method is simple, safe, inexpensive and rapid.

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AB ST R ACT : L-cysteine (Cys) was adsorbed by bentonite (Ben) from aqueous solutions of variable concentrations and pHs. The raw materials and the adsorbed molecules (Ben-Cys) were characterized using X-ray diffraction (XRD), infrared spectroscopy (IR), thermal analysis methods (TG, DTG and DSC). The results indicated that the adsorption process depended strongly on pH, temperature and concentration of amino acid. Cysteine is specifically adsorbed on bentonite surface sites, probably through its NH 3 + moiety. The pH dependence suggests that these sites may be silanolate groups (:Si-O À ). It is presumed that ion exchange interaction governs the adsorption process. The experimental data of adsorption isotherms obtained and analysed with the Langmuir model showed that this model fitted the adsorption data. Calculated thermodynamic parameters (DHº, DSº, and DGº) indicate that the adsorption processes were exothermic.