The estimation in sea-water solutions of micro-quantities of mercury in the presence of copper by means of dithizone.

With the development of leaching methods (Ketchum, Ferry, Redfield & Burns, 1945; Harris, 1945) for the study of anti-fouling compositions and the in~reased interest in the toxicity of copper and mercury to marine organisms, it has become desirable to have a moderately rapid method for the estimation of mercury in the presence of copper ions. Frequently, in these solutions, the copper is in large excess; commonly, leachates contain from 300 to 900 (Lg. of copper and up to 5° (Lg.of mercury per litre of sea water. Mercury in the absence of copper may be readily estimated in these concentrations by the use of dithizone, and the tentative curves of Wichmann (1939) suggest that it is completely extracted even at pH 2'0. However, according to these curves copper is also partially extracted at this acidity. The two most recent methods for the determination. of mercury by d~thizone in the presence of copper are those of Laug & Nelson (1942) and Reith & van Dijk (see Sandell, 1944). The former workers separate the mercury from the copper by shaking an acid potassium bromide solution with the mixed 'dithizonates. The mercury is thereby transferred to the aqueous 'phase and is determined by a mixed colour method after bringing the pH of solution to 6'0. The method of Reith & van Dijk is much longer and involves regulated. extraction of the mixed dithizonates. In addition, Hubbard (1940) has separated the two metals with a dithizone technique by shaking the combined dithizonates with sodium thiosulphate solution; the mercury is transferred to the aqueous phase and after destruction of the thiosulphate with permanganate and of the latter with hydroxylamine, the mercury may be estimated in the aqueous layer. The method outlined below is simpler; it depends upon the slow rate of reaction of a chloroform solution of dithizone with copper and the complex forming action of potassium cobalticyanide with copper salts.