Microdiffusion Analysis and Volumetric Error

Abstract

655 cause coma at levels above 0.30%. Therefore the special tests are not essential in the hospital emergency room. A number of procedures for the colorimetric determination of 1 copper in soil and in plant material have been published. Lagerwerff (8) surveyed the best known of these and recommended the diethanolamine method. Beeson and Gregory (1) preferred the carbamate method for determining copper in plants. Lundblad el al. (9) determined copper in soil by a carbamate method in which interferences from iron, cobalt, or nickel were prevented by a series of separations. Steenbjerg and Boken (16) precipitated copper on a cathode by electrolysis, dissolved it in acid, and determined it colorimetricaily by adding an excess of concentrated ammonia. Dithizone is also commonly used for determining copper in soil and in plant material (IS). However, the methods mentioned are subject to interference and should be carried out under carefully controlled conditions. The object of this paper is to propose two simple and specific methods for determining copper in soil and in plant material. These are an improved carbamate method and the biquinoline method. IMPROVED CARBAMATE METHOD Sodium diethyldithiocarbamate reacts with copper to give copper diethyldithiocarbamate. This salt has a golden brown color. This reaction has been considered one of the most sensitive for copper that has been developed. The compound formed in this reaction has been assigned the following formula (15): S / \ (C2H5)2 = X-C Cu2 In very dilute copper solutions a colloidal suspension suitable for colorimetric comparison is obtained (3), especially if it is stabilized by gum arabic or similar substances. Since the copper carbamate is soluble in many organic solvents such as isoamy] alcohol, isoamyl acetate, bromobenzene, and carbon tetrachloride, an extraction procedure is often used to decrease the effect of interfering ions and to increase the sensitivity of the reaction. Most metals, other than calcium a.nd magnesium, may interfere if present in sufficient amounts. The chief interfering metals are iron, manganese, and nickel, and the method has been criticized (2) because of the many interferences. Many complexing reagents such as pyrophosphate, citrate, and ammonium hydrox-ide have been recommended for eliminating iron interference, provided the amount of iron is not too large (12). Interference by nickel and cobalt can be prevented by adding dimethyl-glyoxime to the sample solution before adding the ammonium hydroxide. The precipitate is separated by filtration or centrif-ugation (9). Cobalt remains in the aqueous layer, to which it 1 Present address, Commercial Solvents Corp., Terre Haute, Ind. imparts an orange color not extracted by carbon tetrachloride. Manganese interferes to a considerable extent in the extraction procedure by imparting a pinkish color to the organic layer. This color is more or less unstable. The solution becomes virtually colorless if only small amounts of manganese are present, or if the solution is allowed to settle for at least 1 to 2 hours. , When appreciable amounts of iron, nickel, or manganese are present, copper cannot be determined by the carbamate method without some chemical separation (17). The principle of employing sequestering reagents to complex metals, as used by Schwarzenbach and coworkers (14), has long been known and has been applied many times in the field of analytical chemistry (7,10-12,15). If a mixture of Versenate and citrate solution is used in the carbamate procedure for copper, all interference can be eliminated except that from bismuth, which is ordinarily present in negligible amounts in both soils and plants. This principle is utilized in the following procedure. Reagents. Carbamate solution. Dissolve 1 gram of sodium diethyldithiocarbamate in 100 ml. of redistilled water, and filter. Versenate and citrate mixture. Dissolve 20 grams of am-monium citrate and 5 grams of the disodium salt of (ethylene dinitrilo) tetraacetic acid (Versenate) in 100 ml. of redistilled water. Ammonium hydroxide, concentrated. Carbon tetrachloride. Procedure. Place 25 to 50 ml. of the solution to be analyzed, containing not more than 50 micrograms of copper, in a suitable separatory funnel. Add 10 ml. of the Versenate and citrate mixture and 1 to 2 ml. of concentrated ammonium hydroxide to obtain a pH of 7 to 10. Mix, and add 1 ml. of 1% carbamate solution. Then add exactly 10 ml. of carbon tetrachloride, stopper, and shake vigorously for more than 2 minutes. Allow the carbon tetrachloride layer to settle, and, when it is free from water droplets, run it directly into a dry absorption cell. Then pour another 5 ml. of carbon tetrachloride into the separatory funnel, and shake for 1 minute to extract the remainder of the copper carbamate from the solution. Add this second extract to the cell which contains the first extract. Usually the second extract is almost colorless. Measure the color in the combined extracts with a spectrophotometer at 500 m/i. with a 1-cm. light path length. Make a calibration curve in exactly the same manner with standards containing from 0 to 50 micrograms of copper. If the carbon tetrachloride extract is cloudy, clear it by centrifu-gation, or by adding 0.5 ml. of methanol. Elimination of Interference by I'ersenate. Versenate forms a soluble chelate complex with many bi-and trivalent metals in the following way: Na2H2Y4 + Me + +-> Xa2MeY4 + 2H + where Me represents bi-or trivalent metals. The stability of the metal Versenate complex varies with different metals. Since, in genera], Versenate requires an alkaline reaction in order to