Detection of Cadmium F. A. VAN ATIA1 and W. L. WASLEY2 Armour Institute of Technology, Chicago, Illinois
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N THE systematic analysis of the Group I1 metals by the hydrogen sulfide method a number of schemes have been used for the separation of cadmium from copper. These, in general, can be divided into (1) systems depending upon the depression of ion concentration by the formation of complex ions and (2) those depending upon the difference in reduction potential between cadmium and copper ions. Typical of the first is the system of holding copper in solution by the addition of an alkali cyanide to the ammoniacal solution of copper and cadmium ions, followed by precipitation of CdS, thus leaving the Cu(CN)a-' complex in solution. The same system is exemplified by the method of saturating a strongly acid solution of copper and cadmium ions with NaCl and precipitating CuS from the CdClr-2 complex. The other principle is used in the precipitation of the copper ion by the addition of iron filings to an acid solution of copper and cadmium ions. From the teaching point of view any of these methods is satisfactory as each is an example of a principle of wide application. However, for the production of a clean-cut separation, none is entirely satisfactory in the hands of students. The use of tbe alkali cyanide is not liked by most instructors because of the danger of poisoning (and in this connection the possibility of a severe contact dermatitis3 as well as general systemic poisoning should not be overlooked), although i t is generally agreed that it gives the best separation of any of these methods. When it is used, the presence of even very small amounts of bismuth in the solution will darken the CdS precipitate until it cannot be recognized. If the second method is used i t is difficult to maintain the concentration of chloride ion sufficiently high to retain small amounts of cadmium ion in solution, while a t the same time precipitating copper completely enough so that i t will not interfere with the subsequent precipitation of cadmium. In the use of the third method it is difficult to prevent the dissolution of sufficient iron so that it will not be retained, and a dark precipitate again result upon the addition of HtS. The method which we have used with considerable success for two years seems to have been first suggested by R. Biewend4 in 1902 and has since been mentioned twice in the literature, by W. Geilmanns and by van Nieuwenburg and Dulfer.' While it has thus been
Present address:
Illinois Department of Labor. Division of Factory Inspection, 205 West Wacker Drive, Chicago. Illinois. a Present address: McArdle Memorial Laboratory. University of Wisconsin. Madison. Wisconsin. vO~cupation and health," International Labour 0 5 c e . Geneva, Switzerland, 1930, Vol. 11, p. 647. BTEWEND. Bew und Hiittenmdnnischc Zeilunn. 61.. 401 (1902). GEILMANN. 2.anorg. Chem.. 155, 192 (1926)7 8 VAN NIEUWENBURG AND DULFER,"A short manual of systematic qualitative analyris by means of modem drop reactions." Centen, Amsterdam. Holland, 1933.
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known, and used to some extent, in assay work and as a specific test in drop reactions, for nearly forty years it seems never to have been incorporated into a systematic scheme of analysis. It seems to us that the method has sufficient merit that it should be widely used in systematic analysis. The operation is as follows: When a blue color is obtained in the ammoniacal solution, indicating copper, the solution is evaporated to dryness. It is not necessary to drive off the ammonium salts, so the evaporation can be camed out over a steam bath with no danger of spattering. Since the test is very sensitive only a two-ml. aliquot of the solution need be evaporated in the macro method of analysis. The whole solution is used in semimicro analysis. The resulting dry salts are mixed with about their own volume of a mixture of approximately equal quantities of anhydrous sodium carbonate and powdered carbon. This mixture is transferred to a small ignition tube--conveniently made from four inches of 7-mm. soft glass tubing-and heated to a dull red. Any cadmium in the mixture will distil up into the cooler portion of the tube and deposit as a gray metallic mirror. In order to make the mirror more easily discernible the tube is cooled, a small particle of sulfur is introduced into the bottom, and the tube is again heated until the sulfur distils up over the mirror. This treatment results in the formation of the orange modification of cadmium sulfidewhich turns yellow on cooling. The color change is typical of cadmium sulfide. The quantity present may be estimated from the size of the cadmium sulfide deposit as well as from the size of a precipitate, and with equal certainty, as the distillation is essentially quantitative. The only difficulty we have observed in several hundred tests conducted by students has been caused by tubes left so covered with carbon that the mirrors could not be seen. The chemistry involved in these changes can be represented by the following equations: (1) (2) (3) (4)
+ - + + + +
CdCI, NarCOl CdCOs CdCO* CdO CO* 2Cd0 C 2Cd COS S CdS Cd
+ 2NaCI
Of course, either copper or bismuth will show the same series of reactions equally well but they are completely separated from cadmium because they will not distil a t any temperature which can be obtained with a Bunsen burner or in glass apparatus. In the opinion of the authors the fact that this procedure is a little unusual in qualitative analysis gives it a very definite advantage from the teaching point of view. It permits a separation by a method fundamentally different from any other used in the pro-
cedures. This involves a discussion of the physical properties of the metals involved, and also of these reactions in the dry way, which are of great commer-
cia1 importance and with which the student wiU not otherwise come in contact in the course of the lahoratory work.
