SEPARATION O F COPPER FROM CADMIUM WITH SALICY LALDOXIME IN QUALITATIVE ANALYSIS L. P.BIEFELD Purdue University, Lafayette, Indiana
DETECTION of cadmium in solutions containing copper is a common test in qualitative chemical analysis. The usual procedure involves the use of potassium or sodium cyanide as a masking reagent for copper. Some instructors object to the use of cyanide solution on the grounds that it is a hazardous reagent, especially in the hands of beginners. Various other schemes have been tried as substitutes for the cyanide separation but none of them seems to be entirely reliable for use by inexperienced students. We have tried salicylaldoxime and found it very dependable for the copper-cadmium separation. It is now being used by some of our elementary classes in semimicro qualitative analysis. Salicylaldoxime is practically a specific reagent for copper in slightly acidic solutions ( 4 ) . The only other substances forming precipitates with the reagent in acid solution are Au+++, Os04, Pdff, and V03(6) ; Fef + forms a soluble red complex (4). As shown by hydrogen sulfide saturation tests, copper is completely precipitated in acetic acid solutions. The weak yellow-green copper salicylaldoximate is practically insoluble in acetic acid but soluble in mineral acids. The limit of detection for Cu++ in slightly acid solution is one part of metal in 2,000,000 of solution (6). Cadmium is not precipitated by salicylaldoxime unless the solution is neutral or basic. White cadmium salicylaldoximate is easily soluble in acids or excess ammqia (6). Copper can be guantitatively separated from cadmium with salicylaldoxirne by control of acidity (8). If one desires to measure the amount of copper by weighing the precipitate as CU(C,H~O~N)~, the pH value of the acetic acid solution must be 2.6 or greater (2). If the reagent is used only to separate out the copper from solution, then the amount of excess acetic acid is not critical. A one per cent solution of salicylaldoxime is used as the precipitant for copper. One liter of the solution can be prepared, according to Ephraim (a), by dissolvf
ing 10 grams of salicylaldoxirne in 50 ml. ethyl alcohol and pouring the solution into 950 ml. of water a t 80°C. This mixture is thoroughly stirred, cooled, and filtered. Another good method for preparation of the solution is given by Astin and Riley ( I ) . Dissolve 8.88 grams of salicylaldehyde in 32 ml. of ethyl alcohol and add the solution to 5.08 grams of hydroxylamine hydrochloride dissolved in 8 ml. of water. Dilute the resulting mixture with 80 ml. of ethyl alcohol. Pour the solution into 900 ml. of water at 80°C. Stir well, cool, and filter. Solutions prepared by either of these methods are dependable for u& and are fairly stable. They do darken, but solutions several months old still give reliable results. The separation of copper and the identification of cadmium is simple and reliable. In the usual scheme of qualitative analysis the solution that might contain copper and cadmium has an excess of immonia, due to the previous separation of bismuth. A deep blue coloration indicates the presence of copper. Add dilute acetic acid dropwise until the blue color turns to green, and then add a few drops more. If the deep blue color is absent but copper is detected by a more sensitive test, add acetic acid until the solution is acid to litmus, and then a few drops more. Add a one per cent solution of salicylaldoxime dropwise until precipitation is complete. Separate the precipitate from the solution and test for complete precipitation. After complete separation of the copper, saturate the clear colorless solution with hydrogen sulfide. A yellow precipitate confirms the presence of cadmium. Various mixtures of copper and cadmium were satisfactorily analyzed according to these directions. The reagent was used with excellent results in a small summer class in elementary semimicro analysis. The only troubles encountered were due to failure to make the solution acid with acetic acid, which resulted in coprecipitatiou of the cadmium with the copper, and the darkening of the cadmium sulfide due to incomplete removal of lead or bismuth earlier in the analysis.
From the viewpoint of safety in handling, specificity for copper, and ease in use the reagent is excellent. It is also economical. According to current list prices the cost of one liter of reagent solution prepared from purchased salicylaldoxime is about $1.50, hut if the solution is prepared by the method of Astiwand Riley, one liter costs only about 18 cents. For semimicro qualitative analysis a liter of solution should suffice for about 1000 separations. Some instructors will object to the use of salicylaldoxime in qualitative inorganic analysis because the reagent is an organic compound. There is a definite trend toward the use of the more specific oganic reagents for inorganic analysis; salicylaldoxime is one of these. At least 30 published papers deal with its use in analytical chemistry and seven books on inoreanic analvsis eive methods utilizine the reagent (7y. Feigl 15) and Engelder, ~un!&ber~er, and Schiller (3) describe the use of salicylaldoxime for the identification of copper in qualitative analysis. If
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the objectives of a course in qualitative inorganic analysis include proficiency and accuracy in the analysis of unknown mixtures and acquaintance with reactions that are very useful in analytical chemistry, then salicylaldoxime should be included along with dimethylglyoxime and aluminon as standard reagents for class use. LITERATURE CITED
(1) ASTIN AND RILEY, 1.Chem. Soc., 1933, 314. Id. Eng. Chem., Anal. Ed., 11, 251 (2) BIEFELD AND HOWB, (191(9\ -- - - ,. (3) ENGELDER, DWI~LBERGER, AND SCH~LLER. "Semi-micro aualitative analvsis." 2nd ed.. Tohn Wilev and Sons.. Inc... ~ e York w City; 1940, p. 162: (4) E P F I R Ber., ~ , 63, 1928 (1930); ibid., 64, 1210 (1931). (5) EEIGL, "Qualitative analysis by spat tests," Nordemsn Publishing Co.,New York City, 1937, p. 42. AND FURMAN, Id. Eng. Chem., Anal. Ed.. 12, 520 (6) FLAGG \
,*"*",.
(196113
(7)
LIGETT,Unpublished
Thesis, Graduate School, Purdue University, Lafayette, Indiana. (8) Isnrsas~rAND Krsnr, J. CChem. Soc. Japan, 55, 1060 (1934); C. A., 29, 702 (1935).
