Ashing Procedures for the Determination of Copper in Plant Material.

Three mixtures were used for wet digestions: nitric and sulphuric acids, 30 per cent. hydrogen peroxide and sulphuric acid, and selenium and sulphuric acid. In the nitric acid digestion 3 ml. of conceintrated sulphuric acid was used for each gram of dry plant material, a total of 50 inl. of concentrated nitric acid was added in four portions, the mixture being evaporated to white fumes and cooled between successive additions. A description of this well-known procedure is given by PIPER (7). LINDNER and HARLEY (5) recently described a procedure usilng 30 per cent. hydrogen peroxide. The digestion is rapid and easily miianipulated. Sulphuric acid is used with successive additions of small amiounts of the peroxide. A mixture of three ml. of coneentrated sulphuric acid with one gram of dry plant material is heated to partially decompose the organic matter and the digest is then cooled. In the present study three ml. of peroxide was added and a small flame used for digestion to white fumes after which the mixture was cooled. This was repeated with one ml. of hydrogen peroxide and with four to six additions of five drops of the peroxide. Selenium is frequently used to catalyze the oxidation of organic miiatter with sulphuric acid prior to nitrogen determinations. This method is also suitable when copper is to be determined. Six ml. of concentrated sulphuric acid was used for a gram of plant material. To this was added one gram of potassium sulphate and 0.5 gram of powdered selenium. The selenium need 1 The investigation reported in this paper is in conniection with a project of the Kentucky Agricultural Experiment Station and is published hy permission of the Director. 79