Indirect Spectrophotometric Determination of Copper(II) by Means of Precipitation and Separation of Copper(I) Thiocyanate

Abstract

A simple method has been developed for the determination of micro amounts of copper (II). It is based on the reduction of copper (II) to copper (I) with L-ascorbic acid, and on the spectrophotometric measilrement of an excess thiocyanate for the copper (I) with iron (III), after the removal of copper (I) thiocyanate precipitate yielded on addition of thiocyanate to the copper (I). The recommended procedure is as follows. Pipette 5 ml of sample solution containing copper (II) up to 158.9 μg in a 15-ml glass-stoppered tube, 0.5 ml of 0.05 mol/l Lascorbic acid and 1 ml of a formic buffer solution (pH 3.2), and then allow it to stand for 5 min. To this mixture, add 1 ml of 0.006% (w/v) himoroc solution as a coagulating agent and 1.3 ml of 2×10^-3 mol/l standard thiocyanate solution. After standing for 10 min, add about 1 ml of carbon tetrachloride, then shake the mixture vigorously by hand, and centrifuge i t in order to collect the precipitate of copper (I) thiocyanate formed on the interface between the aqueous and organic phases. Transfer a 5-ml aliquot of the aqueous phase to the other glass-stoppered tube, and add 2 ml of 1 mol/l iron (III) nitrate in 3 mol/l perchloric acid. Finally, measure the absorbance against distilled water at 460 nm.
The absorbances for copper (II) decrease with an increa se in the concentration of copper (II), b ut the calibration plot for copper (II)formed a straight line, and coincided with that for thiocyanate (expected graph) when the molar concentrations for copper (II) and thiocyanate were scaled as in Fig.2. In 11 results for a 5-ml aliquot of 2.5 × 10^-4 mol/l copper solution, the present method gave a mean absorbance value of 0.538 with a standard deviation of 0.002and a relative standard deviation of 0.3%.