Spectrophotometric determination of small amounts of mercury in copper

Abstract

A spectrophotometric method for the determination of small amounts of mercury in pure copper has been presented. A small amount of mercury is selectively extracted as mercury-diethyldithiocarbamate complex from an alkaline aqueous solution containing large amounts of copper and EDTA into toluene and is backextracted from the organic layer into a 0.7 N nitric acid aqueous solution containing 0.0075 M potassium iodide, 0.005 M potassium bromate and 0.025 M potassium bromide (backextraction solution). Then Crystal Violet colorimetric method is carried out for the determination of mercury in the backextraction solution. The recommended procedure is as follows: Two grams of sample was gently dissolved in nitric acid (1+1) solution. The pH was ajdusted to 8.0~9.0 with ammonia after an addition of 40ml of 1 M EDTA solution. It was transferred into a separatory funnel, and 5.0 ml of 0.1% sodium-diethyldithiocarbamate solution was added. The solution was shaken vigorously with 5 ml of toluene for 5 min and the aqueous layer was discarded. Ten ml of the backextraction solution was added into the separatory funnel and the mixture was shaken vigorously for 5 min and the aqueous layer was transferred into another separatory funnel. Five drops of 20% sodium metabisulphite solution was added in order to reduce the free iodine, then 0.4 ml of 1.0% Crystal Violet solution was added and the solution was immediately shaken with 5 ml of toluene for 10 sec. The amount of mercury was determined from the absorbance of the toluene layer at 605 nm within 3 min after the extraction. In case of the presence of copper and EDTA, many elements such as Pb, Zn, Bi, Ni, Co, Sn and Sb are not extracted as DDC complexes into the organic layer. Not more than 0.5 mg of silver, 0.1 mg of thallium and 5 µg of gold do not interfere. Silver at high concentration interferes the first separation by extraction of mercury as DDC complex from the solution containing copper and EDTA. More than 10 µg of gold interferes seriously but gold are selectively removed from the toluene layer containing DDC complexes by washing with 0.5 N nitric acid solution. In this case, mercury and copper are not removed into the aqueous solution. The mercury content in some vacuum melt copper and oxygen free highpurity copper was determined to be less than 0.25ppm. © 1972, The Japan Society for Analytical Chemistry. All rights reserved.