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ANALYTICAL CHEMISTRY
posed primarily as a method for the separation of iron, preparatory to the determination of other metals in the filtrate. LITERATURE CITED
Blum, UT.J., J . Am.’Chem. SOC.,38, 1291 (1916). Bodansky, M., Ind. Eng. Chem., 13, 096 (1921). Chalk, L. F., Analyst, 55, 187 (1930). Fairhall, L. T., and Prodon, L., J . Am. Chem. SOC.,53, 1321 (1931).
Gordon, L., and Caley, E. R.. ANAL.CHEM.,20, 560 (1948). Gordon, L., Vanselow, C. H., and Willard, H. H., Ibid., 21, 1323
(10) Willard, H. H.. and Fogg. H. C., J. Am. Chem. SOC.,59, 1197 (1 937). ( 1 1 ) Willard. H. H., and Freund, H., IND. ENG.CHEM., A N ~ LED., . 18, 195 (1946). (12) Willard, H. H., and Gordon, L., ANAL. CHEH.,20, 165 (1948). (13) Willard. H. H., and Greathouse, L. H., J. Am. Chem. Soc., 39. 2366 (1917). (14) Willard, H. IT., and Haiin, R. B., ANAL. CHEM.,21, 293 (1949). (15) Willard, H. H., and Tang, N. K., IND.ENG.CHEM.,ANAL.ED., 9, 357 (1937).
(1 949).
Hazel, Fred, and Ayres, G. H., J . P h y s . Chem., 35, 2933 (193i). Hosfetter, J. C., and Roberts, H. S.,J . Am. Chem. SOC.,41, 1348 (1919).
Scott, W. W., “Standard Methods of Chemical Analysis,” 5th ed., p. 1068, Sew York, D. Van Nostrand Co., 1939.
R E C E I V EFebruary D 1 1 , 1950. Presented before the Division of Analytical Chemistry at the 117th Meeting of the . ~ M E R I C A NCHEMICAL SOCIETY, Houston, Tex. From a dissertation submitted b y John L. Sheldon to the Graduate School of the University of Michigan i n partial fulfillment of the requirements for the degree of doctor of philosophy.
Colorimetric Determination of Copper with Pyridine and Salicylic Acid V. A. GORDIEYEFF‘ Clarkson College of Technology, Potsdam, N . Y . If an’aqueous solution of pyridine containing a large amount of salicylic acid is added to a solution of copper salt, a complex precipitate is formed. This precipitate dissolves readily in most organic solvents. The intensely blue color of the extract may be used for the colorimetric determination of copper. The method may be applied to a variety of metallurgical samples, and is suitable for routine determinations. A microprocedure may be adopted for samples where copper is present in extremely low concentrations.
I
T WAS found during a study of some organic copper com-
plexes that copper could be readily removed from an aqueous solution of its salts if both an organic acid, such as benzoic or salicylic acid, and pyridine were present. The reaction was similar to that between copper and pyridine in the presence of potassium thiocyanate, applied analytically by Spacu (9) and Biazzo (Z), and improved by Elvehjem and Lindow (3) and Goethals (4,10). The purpose of this investigation was to study the combined effect of salicylic acid and pyridine upon aqueous solutions of copper salts and its analytical possibilities. When an excess of pyridine and salicylic acid is added to a solution containing even minute amounts of copper ions, a voluminous greenish white precipitate is formed. This precipitate dissolves readily in chloroform, carbon tetrachloride, ether, and other organic solvents with the formation of *a brilliant and intense blue color. The composition of this precipitate was investigated by Ley and Erler (1, 6), who defined it as a cupric salicylate coordinated with two pyridine rings [Cu(Py),(CsHr OHCOO),]. The author is inclined t o believe that when the precipitate is formed in the presence of a very large excess of pyridine this formula should be corrected by addition of two more pyridine rings. The color of the solvent extract is specific for copper, its intensity is proportional to the concentration of copper, and any interference due to the presence of other colored ions may be easily prevented or limited. APPARATUS AND REAGENTS
A Beckman quartz spectrophotometer (Model DU) with its standard equipment (10-mm. absorption cells) was used for all measurements. No appreciable difference was recorded when cells of a different type were used. Changing the slit width or 1
Present address, Columbia University. New York, N. Y .
using different phototubes (red or blue light-sensitive) did not affect the accuracy of measurements. Basic Reagent. It was found that optimum sensitivity is obtained with the reagent prepared by dissolving 22 to 25 grams of C.P. salicylic acid in 32 to 35 ml. of copper-free pyridine. This solution should be diluted to a volume of 100 ml. with distilled water. The reagent is stable for many weeks if protected from bright light and excessive heat. One milliliter of the reagent provides enough excess to precipitate up to 20 mg. of copper ions. Solvent. C.P. chloroform is the best extraction solvent. Standard Copper Solution. A solution containing 1 to 2 mg. of copper per ml. (preferably as sulfate) is the best standard, either for the preparation of a calibration curve or as a comparative solution. This solution may be prepared by dissolving 1 to 2 grams of pure (hydrogen-reduced) copper shot in 10 to 15 ml. of concentrated nitric acid. It is diluted with distilled water to about 30 ml. and evaporated almost to dryness. Then 20 to 30 ml. of water and 4 to 6 ml. of concentrated sulfuric acid are added to the cooled residue. The solution is evaporated t o dense white fumes of sulfuric acid, cooled, diluted with about 200 ml. of water, transferred quantitatively to a volumetric flask, and diluted to 1000 ml. The copper content of the solution may be checked, if necessary, either electrolytically or iodometrically. GENERAL PROCEDURE
Weigh a sample containing 0.100 to 5.0 mg. of copper and, depending on the nature of the sample, char or digest with a mixture of oxidizing acids. The amount of acids may be adjusted according to the weight of the sample and its copper content. I n most caaes 3 to 5 ml. of concentrated nitric acid mixed with an equal volume of concentrated perchloric or sulfuric acid provide a satisfactory excess for digestion. After the digestion is completed, cool the solution, dilute it carefully with a few milliliters of distilled water, and filter off any residue, washing it repeatedly with small portions (1 to 2 ml.) of a hot 2% solution of nitric acid. The final volume should not be larger than 40 to 50 ml. Evaporate if a larger volume has been obtained. Remove the excess of acids either by evaporation or by adding 8 M ammonium hydroxide to p H 4.5 to 7.0. Transfer the solution to a separatory funnel (about 60-ml. capacity), and add 3 to 5 ml. of chloroform, followed by 1 ml. of the pyridine-salicylic acid reagent. Shake vigorously, allow the chloroform phase to settle, and withdraw
V O L U M E 2 2 , NO. 9, S E P T E M B E R 1 9 5 0
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i t into a volumetric flask (10- to 25-ml. capacity). Repeat the extraction with small portions of the solvent ( 2 to 3 ml.) until no more blue color is visible in the solvent phase. Test for completeness of extraction by adding 1 t o 2 droplets of the reagent before each extraction. Dilute the extract to the mark. If the extract is cloudy, transfer it quantitatively to a larger volumetric flask and add more solvent. Measure the transmittantc in a suitable spectrophotometer or colorimeter, using the range of 590 to 620 mp. If the sample contains less than 0.150 to 0.100 mg. of copper, proceed as above, using smaller volumes of acids, reagent, and solvent (about 50% of the amount given above). Collect the extract in a microcrucible and evaporate i t t o dryness over a hot plate. Dissolve the residue in the smallest carefully measured volume of the solvent to which one droplet of the reagent has been added. Transfer 0.05 to 0.1 ml. to a microabsorption cell [described for the Beckman spectrophotorncter by Lowry and Bessey ( 7 ) and recently by Kinsey ( 5 ) ]and measure the transmittance in the usual way. Using this technique it was possible to iiiialyze as little as 50 to 100 cu. mm. of the extract without decreasing the length of the light path ( 1 cm. for Beckman spectrophotometer). The calibration curve may be prepared by recording readings obtained with chloroform extracts containing a known amount of copper. Most common ions do not interfere with the color of the complex. There is no interference from. silver, aluminum, arsenic (as arsenite and arsenate), bismuth, chromium (ic), mercury (ous) and (ic), magnesium, manganese (ous), lead, antimony, tin (ic) (stannous tin should be oxidized), vanadium (as vanadate); wolfram, zinc, alkali and alkaline earth metals, and ammonium compounds (if the solution is acid or neutral), even if these ions are present in concentrations many times exceeding that of copper.
Table I.
Determination of Copper in Samples of Medium and High Concentrations
--
Weight of Sample
Present
MU.
Mo.
CopperFound Mu. 8.20 10.35 12.35 20.05 35.60 65.50
454.6: 8 2 456.1 10.4 449.Ba 12.3 459.2b 20.1 455.Ob 35.6 448.3b 65.6 461.3C 95:5 95.55 494.5= 108.1 108.10 465,3c 120.4 120.30 460.8c 145.6 145.70 456.7d 196.7 196.70 461.4e 426.7 426.50 458.gC 432.1 432.00 a Copper furnace slag (commercial). b Copper ore, low grade (commercial). Copper ore, higher grade. d Nonferrous alloy. e Brass a n d bronze samples.
Error
% 0.0 0.5 0.4 0.2 0.0 0.2 40.1 0.0 40.1
