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V O L U M E 2 7 , NO. 1 2 , D E C E M B E R 1 9 5 5 ( 2 ) Bickford, C. F., Jones, W. S., and Keene, J. S., J . Am. Pharm. Aasoc., Sci. Ed., 37, 255-61 (1948). (3) Clark, L. J., Ph.D. thesis, University of Maryland, 1954. (4) Clark, R. E. D., Analyst, 61, 242-5 (1936). (5) Grim, J. L.. Ibid., 78, 470-3 (1953). Hamince, J. H., Ibid., 65, 152-4 (1940).
Hillebrand, W. F., Lundell, G. E. F., Bright, H. .4, and Hoffman, J. I., “Applied Inorganic Analysis,” 2nd ed., p. 385, Wiley, Kew York, 1953. Ibid., p. 673. Hoffman, J. I., J . Research Natl. Bur. Standards, 25, 379-83 (1940).
Miller, C . C., Analyst, 69, 109-12 (1944). Miller, C. C., J . Chem. SOC.,151, 792 (1941). Perrin, D. D., New Zealand J. Sci. Technol., A28, 183-7 (1946). Piper, C. S., and Beckwith, R. S..J . Soc. Chem. Ind., 67, 374-9 (1 948).
Rankama, K.,and Sahama, T. G., “Geochemistry,” p. 655, University of Chicago Press, Chicago, 1950. Robinson, W. O., and Alexander, L. T., Soil Sci., 74, 287-91 (1953).
Robinson, W. O., Edgington, G., Armiger, W. H., and Breen, A. V., Ibid., 72, 267-74 (1951). Short, H. G., Analyst, 76, 710-14 (1951). Wells, J. E., and Pemberton, R., Ibid., 72, 185-8 (1947). RECEIVED for review June 1, 1955. Accepted August 31, 1955. Abstracted from a dissertation submitted by Lewis J. Clark to the graduate faculty of the University of Maryland in partial fulfillment of the requirements for the degree of doctor of philosophy, June 1954. Subsequent confirmatory intercomparisons on referee soil samples and applications to diverse fertilizer materials were made in the laboratories of the Soil and Water Conservation Research Branch, Agricultural Research Service, U. 5. Department of Agriculture, Beltsville, Md. Scientific paper A505, contribution 2624 from Maryland Agricultural Experiment Station, Department of Agronomy, University of Maryland.
Photometric Determination of Copper in Molybdenum Products with Alpha-Benzoinoxime JOSEPH MADERA Climax Molybdenum Co. o f Pennsylvania, Langeloth, Pa.
The a-benzoinoxime method of Dunleavy and others has been applied to a wide range of molybdenum compounds. Procedures for the solution of a variety of samples are described. Ratios of calcium to copper greater than 2 to 1 caused appreciable interference. The procedure is suitable for samples containing from 0.001 to 1.00% of copper. The technique is simple and results are reproducible under routine conditions.
REAGENTS AND EQUIPMENT
a-Benzoinoxime (Eastman Kodak), 0.5Q/, solut.ion in 0.25N sodium hydroxide. Sodium hydroxide, 5 5 . Sodium hydroxide, 2.5iV (use volumetric flask). Rochelle salt,, 300 grams in 500 ml. of water. Chloroform, reagent grade. Spectrophotometer (Beckman, Model DU was used). Phenolphthalein indicator. PREPARATION O F CALIBRATION CURVE
A
SPECTROPHOTOMETRIC method utilizing a-benzoinoxime has been found suitable for rapid and accurate determinations of copper on a aide range of molybdenum compounds. Feigl ( 4 ) introduced a-benzoinoxinie as a specific reagent for copper in ammoniacal solution. Kar (6) and Silverman (8) reported its use for gravimetric determinations of copper in steels. Although Feigl had stated previously that a-benzoinoxime was specific for copper, Jennings ( 5 ) reported that nickel, cobalt, platinum, and palladium also formed complexes with the reagent. Dunleavy, Wiberley, and Harley ( 3 )reported a direct colorimetric method for the determination of copper in plain and low-alloy steels and cast irons. The effect of the variables-pH, amount of reagent (a-benzoinoxime), amount of Rochelle salt, stability of complex, interference of other ions (specifically nickel and c o b a l t t a r e discussed thoroughly. EXPERIMENTAL
An attempt was made to apply the a-benzoinoxime method in this laboratory. The general recommendations found in the literature (a) were followed and early efforts to obtain reproducible values on standard copper solutions were successful. The pH range of 11.3 to 12.3 was obtained by using phenolphthalein indicator and a measured volume of a standard sodium hydroxide solution. The sample solution was neutralized with 5A7sodium hydroxide until 1 drop imparted a pink color to the solution. A measured volume of 2.5N sodium hydroxide was added in excess to give the desired p H range, as read on a pH meter. The 2.5N sodium hydroxide solution was used as a standard reagent in routine analysis, eliminating the necessity for using a pH meter.
Prepare a calibration curve by adding aliquots of the standard copper solution containing 0.1 to 0.9 mg. of copper to 0.25-gram samples of pure molybdenum trioxide and follow the procedure described. Carry a reagent blank containing no added copper along with each set of samples and correct the spectrophotometric readings for all samples for the reagent blank. Plot the corrected readings against the concentration in the conventional manner. The readings are made a t 440 mp and 1-cm. Corex cells are used. PROCEDURE FOR DETERMINATION OF COPPER
I n all cases aeigh a 0.25-gram sample into a 250-ml. borosi1icat.e glass beaker. A. For molybdenite concentrates and purified molybdenum disulfide, add 5 ml. of sulfuric acid and 10 ml. of nitric acid. Heat to fumes of sulfur trioxide, and continue to add nitric acid in 10-ml. portions until sample is in solution. On final addition, heat to strong fumes of sulfur trioxide. Cool, add 10 ml. of water and 3 ml. of nitric acid, and heat to dissolve soluble salts. Cont’inueas for sample F. B. For molybdic oxide (technical), add 5 ml. of hydrochloric acid and 5 ml. of nit’ric acid. Heat to decompose and evaporate to dryness. Bake, cool, and add 10 ml. of water and 10 ml. of 5h’ sodium hydroxide. Heat t,o boiling, and add 10 ml. of water and 10 ml. of nitric acid. Heat t.o dissolve soluble salts. Continue as for sample F. C. For molybdic oxide (pure), weigh a &gram sample into a 250-ml. borosilicate glass beaker. Add 10 ml. of sulfuric acid and heat to strong fumes of sulfur trioxide. Cool, add 5 ml. of nitric acid cautiously, and heat to fumes of sulfur trioxide. Cool, add 10 ml. of water, and heat to dissolve soluble salts. Continue as for sample F. D. For ferromolybdenum and molybdenum metal, add 50 ml. of water, 5 ml. of hydrochloric acid, and 5 ml. of nitric acid. Heat carefully to boiling until reaction ceases and the volume of the solution is reduced one half. (Digestion period is approximately 0.5 hour.) Continue as for sample F. E. For molybdenum silicide, add 10 ml. of nitric acid and hydrofluoric acid a few drops a t a time, until decomposition is complete. The solution may be heated gently between additions
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ANALYTICAL CHEMISTRY
Table I.
Comparative Data from Thiocyanate and Photometric Methods of Analysis
Copper Found, G r a m ThiocyanateThiocyanateCopper Present, electrolytic photometric Photometric Gram method method method 0.0050 0.0048 0.0049 0,0045 0.0047 0.0048 0.0050 0,0049 0.0050 0.0047 0.0047 0,0050 Av. 0.0050 Av. 0.0046 0.0048 0.0049 Standard copper nitrate solution-0.05 gram copper foil per liter of (1 t o nitric acid solution. 1 ml. = 0.00005 gram of copper. 9gkverage copper found b y thiocyanate method 92.0%. Average copper found b y thiocyanate-photo&etric method, 96.0%. Average copper found b y photometric method, 98.0%.
.
Table 11. Effect of Calcium on Standard Copper Solutions Calcium Added, Copper Present, Copper Found, ME. Mg. Mg. 0.480 0.00 0.500 0.475 0.60 0.500 0.468 1.30 0.500 0,463 1.90 0.500 0.458 2.50 0.500 0.445 3.10 0,500 0.443 3.80 0.500 Standard copper solution, 1 ml. = 0.00005 gram of copper. Standard calcium solution, 1 ml. = 0.000025 gram of calcium. Teat
purposes. I n addition, a series of production samples of roaster concentrates, ferromolybdenuni, and molybdenite concentrates were analyzed for copper. The samples were dissolved by standard acid treatment as indicated in the pievious procedure. Comparative data are found in Tables I11 and IV. RESULTS AND DISCUSSIOS
Copper values on standards and production samples compare favorably with the a-benzoinolirne and thiocyanate methods (Tablea I11 and IV). Reproducibility of results was satisfactory on all samples analyzed. Copper values on synthetic standards indicate that the abenzoinovime method of analysis is more sensitive in detecting minute amounts of copper than is the method of weighing an equivalent amount as is done in the thiocyanate-electrolytic method. Loss of copper by a-benzoinoxime is st a minimum in actual procedure. The calcium ion interfered in the determindtion of copper in calcium molybdate. Ratios of calcium to copper greater than 2 to 1 caused definite interference in the recoverv of copper (Table 11). Separation of calcium is possible, but would lengthen the method. I n other molybdenum compounds the calcium ion concentration is negligible and results are found to be favorahle. REFERENCES
Am. SOC. T e s t i n g Materials, “dST;\I lIethotis for Chemical of hydrofluoric acid if necessary. After decomposition is comAnalysis of Metals,” p. 139, 1946. lete, add 10 ml. of sulfuric acid (1 to 1) and evaporate to heavy Bendix, G. H., and Grabenstelter, Doris, ISD. E x . CHEM., Fumes of sulfur trioxide. Cool, wash down sides of beaker, and ANAL. ED.,15,649-52 (1943). again evaporate to heavy fumes of sulfur trioxide to eliminate Dunleavy, R. A., Wiberley. 8 . E., and Harley .J H. - 4 s . 4 ~ . hydrofluoric acid. Cool, and add 10 ml. of water and 3 ml. of CHEM., 22, 170 (1950). nitric acid. Heat to dissolve soluble salts. Continue as for Feigl, F., Be?., 56 (b), 2083 (1923). sample F. Jennings, J., and others, J . Chem. Soc., 138,818 (1933) F. After samples A, B, C, D, and E are in solution, cool Kar, H. A, ISD.ENG.CHEM., ANAL. ED.,7, 193 (1935). slightly and add 25 ml. of Rochelle salt solution. ildjust the pH Sandell, E. B., “Colorimetric Determination of Traces of Metals,” to range between 11.3 and 12.3 with sodium hydroxide using Vol. 111, p. 305, Interscience, Sew York, 1950. phenolphthalein indicator. Use the following procedure: Add Silverman, L., IXD.ENG.CHEM.,AUAL.ED..1 2 , 3 4 3 (1940). 3 t o 5 drops of phenolphthalein indicator and 5N sodium hydroxide from a buret until 1 drop imparts a pink color to RECEIVED for review March 23, 1955. Accepted September 16, 1955. the solution, which should persist f o r at least 30 seconds. Add 1 ml. of 2.5N sodium hydroxide in excess and 2 ml. of 0.5% a-benzoinoxime solution (use Table 111. Determination of Copper on Standard Molybdenum Products pipet,). % Copper Transfer to a 250-ml. separatory Analyst 5 Analyst 5 funnel. add 50 ml. of chloroform (buret) (thiocyanate (photometrio and shake vigorously for 5 minutes. Sample Analyst 1 Analyst 2 Analyst 3 Analyst 4 method) method) Filter the chloroform through a retenXlolybdic oxide standard 0.26 0.24 0.25 0.25 0.22 0.24 tive pzper into a test cell. Ferromolybdenum standard 0.22 0.21 0,20 0.23 0.22 0.23 Notes: Average per cent deviation on standard molybdic oxide technical (results from four laboraRead the absorption of the solution, tories) is -0.01% copper (photometric method of analysis). after having previously set the instruAverage per cent deviation on standard ferromolybdenum (results from four laboratories) is + O . O l % ment a t zero with a blank on reagents copper (photometric method of analysis). Analysts. 1 . Climax Molybdenum Co., Detroit, hlich. 2. Climax 3Iolybdenum Co., Climax, carried along simultaneously with the Colo. 3. Climax Molybdenum Co Golden, Colo. 4. Ledoux and Co., Inc.. Teaneck, N. J . 5. sample. Climax Molybdenum Co., Lange1oth:’Pa. Measure the absorbance a t 410 mk. Slit nidth is 0.065 mm. I
APPLICATION OF METHOD
Synthetic Samples. The reported specificity of a-benzoinoxime for copper was investigated by adding pure molybdic oxide to synthetic solutions. Different methods of analysis Tere used in order to compare the per cent recovery of copper. The results of these tests are found in Table I. A straight-line curve was obtained for the copper- a-benzoinoxime complex when plotting “absorbance” as the ordinate and “concentration of copper” as the abscissa. The calcium ion was found to interfere with the determination of copper in calcium molybdate. This was investigated by adding calcium ion to synthetic copper solutions. Results of these tests are found in Table 11. A straight-line curve was obtained when plotting “per cent copper found” as the ordinate and “concentration of calcium” as abscissa. Standard Samples of Roasted Concentrates and Ferromolybdenum. These samples were produced and values were determined several years ago by four laboratories using various methods. They are used in the author’s laboratory for control
Table IV.
Determination of Copper on Molybdenum Products
Molybdenum Product
Molybdenite concentrates
Sample NO.
Copper, % Analyst 5 Analyst 5 (thiocyanate (photometric method) method)
0.09 0 OR 0.12 16 0.10 17 0.14 Average per cent deviation is +0.02% copper (photometric analysis). Analyst 5 , Climax Molybdenum Co., Langeloth, P a . 13
n io
14
0.04
15
0.15 i o . 13
0.17 method of
