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INDUSTRIAL A N D ENGINEERING CHEMISTRY DISCUSSION OF PROCEDURE
Since the color-developing reaction is very sensitive to oxidizing impurities, all glassware should be thoroughly cleaned with acid-dichromate cleaning solution or hot nitric acid, and rinsed several times with tap water and once with distilled water. As a final precautionary measure the glassware may be rinsed with a little of the 1% ascorbic acid solution, which will reduce any impurities present, such as dichromate from the cleaning solution. The most common interfering substance in this procedure is ferric iron. Sproull and Gettler have shown, however, that It must be prcscnt in excess of 20 mg. per 100 cc. to liberate iodine. The method can be carried out directly in the presence of considerable quantities of the alkali metals, magnesium, manganese, zinc, cobalt, nickel, chromium, and aluminum (6). As Wiegand, Lann, and Kalich have pointed out, using hypophosphorous acid instead of sulfite reagent, the sequence of reagent addition given above must be adhered to for satisfactory results. While it would be expected that ascorbic acid alone would exert sufficient reducing action, actual trials have proved that a little sulfite reagent must be present. Although the aqueous solution of potassium iodobismuthite is stable for scveral days if kept tightly stoppered, the extracted complex ha.; been found to remain stable for no longer than 30
Vol. 17, No. 7
minutes. The readings, therefore, must bc made without undue delay if accurate results are to be obtained. From 1 to 10 micrograms of added bismuth have been recovered from 10 to 20 cc. of normal dog serum and beef plasma with an average accuracy of 92 * 6%. These data, presented in Table I, demonstrate that the error ‘tends to become less as the amount of bismuth increases. If quantities of bismuth greater than 10 micrograms are present, it is necessary to use two or even three extractions with amyl alcohol-ethyl acetate; the extraction must be repeated until all the yellow complex has been removed from the aqueous solution LITERATURE CITED
(1) Haddock, Analyst, 59, 163-i (1934). (2) Hubbard, IND. Exo. CHEM., ANAL.ED.,1 1 , 343 (1939). (3) Leonard, J . Pharmacal., 28,81-7 (1926). (4) Oettingen, von, Physiol. Reu., 10,221-81 (1930). (5) Sandell, “Colorimetric Determination of Traces of Metals”. Vol. 111, p. 162,New York, Interscience Publishers, 1944. (6) Sproull and Gettler, IND.ENO. CHEM.,ANAL.ED., 13, 162-5 (1941). (7) Thresh, quoted by Scott in “Standard Methods of Chemical Analysis”, 4th ed., p. 79, New York, D.Van Nostrand Co., 1925. (8) Wiegand, Lann, and Kalich, IND.ENG.CHEM., ANAL.ED.,13, 912-15 (1941).
Separating and Detecting Cupric and Cadmium Ions in the Copper Subgroup of Group GERALD F. GRILLOT
AND
II
JERRY B. KELLEY
University of Kentucky, Lexington, Ky.
E
VANS, Garrett, and Quill (1) describe a method of separating cadmium from cupric ions by virtue of the fact that copper forms a complex tartrate which is soluble in an alkaline solution, whereas cadmium precipitates as cadmium hydroxide, (’lasses m elementary qualitative analysis at the University of Kentucky obtained unreliable results in using this test, as evidenced by the number of reports in which cadmium was missed. The students were cautioned to use more sodium hydroxide and a longer period of boiling, but little improvement in the detection of cadmium was noted. The authors have reasoned that it is. difficult to remove the ammonia completely by boiling; thus the cadmium remains in solution aa the ammonia complex ion. I t was decided to acidify with concentrated nitric acid the ammoniacal solution obtained in the separation of cupric and cadmium ions from bismuth. This was followed by evaporation to dryness in order to remove any ammonia or ammonium salts that were present. The residue was put into solution and was treated with sodium hydroxide and Rochelle salts solution, whereupon the soluble blue copper tartrate complex and a heavy white gelatinous precipitate of cadmium hydroxide formed. The authors recognize the fact that the cyanide method is as reliable as this method and much shorter. They, like many other instructors of qualitative analysis, object to the use of solutions of the poisonous alkali cyanides in large freshman classes of qualitative analysis. Therefore the goal in qualitative analysis seems to be the development of an alternative method which will bc as reliable and as rapid as the cyanide separation of copper nnd cadmium. This modification of the separation has given dependable rehults in a small class of pupils doing qualitative analysis, as well as in analysis carried out by the authors. Although this article dcscribes the method of separating cupric and cadmium ions
using the semimicrotechnique, it should be easily adaptable t o the macrotechnique. PROCEDURE
I t is suggested that the analysis as described by Evans, Garrett, arid Quill (f) be modified as follows: The decantate from the preci itation of bismuth hydroxide may contain Cu(SHa),++ and CJNHs),++. Copper is present if this decantate has a deep blue to purple color. If there is any doubt about the presence of copper, a small sample of the decantate can be acidified with dilute acetic acid and then 2 drops of potassium ferrocyanide can be added. A red precipitate of copper ferrocyanide confirms the presence of copper. If copper is absent, make the decantate just acid with dilute sulfuric acid and saturate the solution while cold with hydrogen sulfide gas. A yellowish precipitate of cadmium sulfide confirms the presence of cadmium. If copper is present, place the decantate in a casserole and acidify with concentrated nitric acid, then add 5 drops in excess. Evaporate this solution to dryness and heat until all ammonium salts are decomposed. Cool the casserole and add 1 to 2 drops of 6 N nitric acid and 1 ml. of water, Add 10 drops of 6 ili sodium hydroxide and l ml. of 0.5 iM potassium sodium tartrate (Rochelle salts) solution. Centrifuge. A white gelatinous precipitate of cadmium hydroxide confirms the presence of cadmium. The precipitate of cadmium hydroxide may appear to be blue. Centrifuge, decant, and wash twice with water. The precipitate should be white. A further confirmation can be carried out by just dissolving this white precipitate in dilute sulfuric acid and then saturating the cold solution with hydrogen sulfide gas. .4 yellow precipitate of cadmium sulfide confirms the presence of cadmium. LITERATURE CITED
(1) Evans, Garrett, and Quill, “Semimicro Qualitative Analysis", p. 54,Boston,Ginn & Co., 1942.
