T . H. Whitehead and Georgia K. Hatcher The University of Georgia, Athens
The Separation and Identification of Cadmium and Copper Ions
Many methods for the separation and identification of cadmium and copper ions have been proposed, avoiding the use of potassium cyanide because cyanides are considered too toxic for use in elementary laboratories. These methods fall into several categories: (1) the cupric ion is tied up in a complex with tartrates (1, 2 ) , or with citrates (4), or with cyanate (5) after which cadmium is precipitated as its hydroxide, sulfide, or cyanate; (8) copper(I1) ion is reduced to copper(1) ion and precipitated as copper(1) thiocyanate (6, 6),or reduced to metallic copper with iron filings or sodium dithionate (7) followed by precipitation of cadmium ion with H2S or thioacetamide; (3) the CdS and CnS are reduced to their metals with Na2C03and carbon in a glass tube by heating, more intense heat is applied until metallic cadmium vaporizes and condenses on the upper part of the glass tube (8); (4) copper is precipitated as pyridiie-thiocyanate complex and cadmium as its sulfide (9); (5) other methods were suggested by Chandra (10). While all of these methods no doubt have merit, they also have disadvantages. Some are long, some do not provide complete separations, and some do not produce distinctive colors for the confimnation of the ions.
The method proposed here is rapid, gives complete separation of Cd+2 and Cu+%,is very sensitive, and provides distinct colors for confirmation of the ions. The method assumes that a traditional scheme of analysis is used in which cadmium, copper, bismuth, and lead have been precipitated as their sulfides from acid solution; lead is then precipitated as its sulfate, the remaining solution is made alkaline with ammonia, and bismuth precipitated as its hydroxide. This leaves cadmium and copper in solution as Cd(NHa)a+' and CU(NH&+~ ions. When a solution containing Cd(NH3)4+2and Cu(NH&+Z ions is treated with aqueous hydroxylamine hydrochloride, the copper(I1) ion is reduced to copper (I) ion but the cadmium ion is unaffected. The addition of acetylene (gas or solution) to the above solution precipitates a very insoluble red compound of copper. This is readily removed by filtration?'or centrifugation, leaving a clear colorless solution containing the cadmium ions. This precipitate will form in solutions containing so little copper that the blue color of the ammonia complex is not distinct, so it is very sensitive. Cadmium can then be confirmed in the colorless solution by bubbling HzS through the solution and
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observing the formation of yellow CdS. Ammonium sulfide or thioacetamide may be used instead of H2S, if desired. Semimicro Procedure To 2 ml of the solution containing Cd(NH&+' and Cu(NHs),+B after precipitation of bismuth with ammonia, add by drops a. solution of hydroxylamine hydrochloride in water (120 g/l) until the blue color is no longer visible. (If no blue color was visible, add 3 drops.) Now add, by drops, a ~olutionof acetylene in aeetone until the formation of the red precipitate seems complete. Filter or centrifuge. Test the supernatant solution or filtrate with more acetylene in acetone until precipitation is complete. The red precipitate definitely identifies copper. To the clear colorless solution from ahove, add 2 drops of conc. ammonia and seturrtte with H.S. The formation of yellow CdS confirms cadmium. If copper is absent, no red precipitate will form and if cadmium is absent, no yellow precipitate will form.
A saturated solution of acetylene in acetone is easily prepared by placing a small lump of calcium carbide in a 30-ml test tube fitted with a one-hole rubber stopper and delivery tube. Add 10 ml of water to the carbide, insert the stopper, and allow the gas to bubble through
400 / Journal of Chemical Education
acetone for several minutes. This solution will keep for days if kept tightly corked. I t has been stated that copper(1) acetylide is explosive and this may be true, but the precipitate formed in this procedure is quite stable. I t has been dried a t 1 0 5 T for 24 hours without any decomposition. Perhaps it is not copper(1) acetylide; efforts are presently being directed toward determining the structure of this compound. Literature Cited (1) MCREYNOLDS, J. P., J. CHEM.EDUC.,17, 532 (1940). (2) GRILLO,G. F., AND KELLEY,J. B., Anal. Chem., 17, 458 (1945). , , (3) DAVIS,M. V., AND HEATH,F. H., J. CHEM.EDUC.,26, 277 (1949). (4) HUSTED,HELENG., J. CHEM.EDUC.,35,403 (1958). D., J. CHEM.EDUC.,18,434 (1941). (5) BRAWLEY, (6) WAGGONER, W. H., J. CHEM.EDUC.,37.411 (1960). (7) GAINEE,P. C., AND WOODRIFF, R., J. CHEM.EDUC.,26, 166 (1949). (8) WASLEY, W.L., J. CHEM.EDUC.,18,31 (1941). J. T.,A N D GILREATH, E. S., J. CHEM.EDUC.,22, (9) DOBBINS, 119 ilU451. ~ - ~ - ~ , ~ (10) C ~ ~ ~ DR.,R J. ACHEM. , EDUC.,38,409 (1961)
