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Senin, 02 Agustus 2010

DETERMINATION OF COPPER AS THE DIETHYLDITHIOCARBAMATE COMPLEX

Sodium diethyldithiocarbamate (B) reacts with a weakly acidic or ammoniacal solution of copper(I1) in low concentration to produce a brown colloidal suspension of the copper(I1) diethyldithiocarbamate. The suspension may be extracted with an organic solvent (chloroform, carbon tetrachloride or butyl acetate) and the coloured extract analysed spectrophotometrically at 560 nm (butyl acetate) or 435 nm (chloroform or carbon tetrachloride). Many of the heavy metals give slightly soluble products (some white, some coloured) with the reagent, most of which are soluble in the organic solvents mentioned. The selectivity of the reagent may be improved by the use of masking agents, particularly EDTA. The reagent decomposes rapidly in solutions of low pH.

Procedure. Dissolve 0.0393 g of pure copper(I1) sulphate pentahydrate in 1 L of water in a graduated flask. Pipette 10.0 mL of this solution (containing about 100 pg Cu) into a beaker, add 5.0 mL of 25 per cent aqueous citric acid solution,render slightly alkaline with dilute ammonia solution and boil off the excess of ammonia; alternatively, adjust to pH 8.5 using a pH meter. Add 15.0mL of 4 per cent EDTA solution and cool to room temperature. Transfer to a separatory funnel, add 10mL of 0.2 per cent aqueous sodium diethyldithio- carbamate solution, and shake for 45 seconds. A yellow-brown colour develops in the solution. Pipette 20 mL of butyl acetate (ethanoate) into the funnel and shake for 30 seconds. The organic layer acquires a yellow colour. Cool, shake for 15 seconds and allow the phases to separate. Remove the lower aqueous layer; add 20 mL of 5 per cent sulphuric acid (v/v), shake for 15 seconds, cool, and separate the organic phase. Determine the absorbance at 560 nm in 1.0 cm absorption cells against a blank. Al1 the copper is removed in one extraction. Repeat the experiment in the presence of 1 mg of iron(II1); no interference can be detected.

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