1572
ANALYTICAL CHEMISTRY
ble that this was also a source of error in the determination of magnesium in the leaf samples. The titration procedure for calcium in water can be adapted easily to leaf sample solutions. It is direct, simple, and rapid for samples having a low phosphorus content. Compared with the oxalate procedure on citrus leaf samples, it had a high degree of precision and checked well within 5% of the oxalate values. The procedure for magnesium lacked precision because of the accumulation of errors from both titrations; however, fourteen of the seventeen determinations checked within 15% of the oxine values. Only the direct titration procedures described by Bet2 and Xoll were used in this work. It is probable that better agreement for magnesium could be obtained after removing calcium by an oxalate precipitation and employing the refmements in the Eriochrome black T titration which have been suggested by Connors ( 3 )and Diehl et al. (4).
LITERATURE CITED (1) -4ssoc. Offic. d g r . Chemists, “Officialand Tentative Methods of A4nalysis,”6th ed., p. 119, 1945. (2) Betz, J. D., and Xoll, C. .4., J . Am. Water Works Assoc., 42, 4956 (1950). (3) Connors, J. J., Ibid.,42, 33-9 (1950). (4) Diehl, Harvey, Goetz, C. 8., and Hach, C., Ibid., 42, 40-8 (1950). (5) Schwarzenbach, G . , Biedermann, W., and Bangerter, F., Helv. Chim. Acta, 29, 811 (1946). (6) Snedecor, G. TT., “Statistical Methods,” p. 54, Ames, Iowa, Collegiate Press, 1937. (7) Snell, F. D . , and Snell, C. T., “Colorimetric Methods of Analysis,” 2nd ed., Vol. 1, pp. 472-3, New York, D. Tan Sostrand Co., 1936. (8) Toth, S. J., Prince, A. L., Wallace, A., and Mikkelsen. D. S., S o i l S C ~ 66, . , 459-66 (1948). RECEIVED April 17, 1950.
Colorimetric Determination of Copper in Steel with Rubeanic Acid WALTER L. MILLER, ISIDORE GELD, AND MAX QUATINETZ Material Laboratory, New York Naval Shipyard, Brooklyn, N . Y .
HE determination of copper in steel usually requires a preTliminary separation from other elements, followed by various methods of measuring the copper. Several colorimetric reagents have been used for this purpose. Levine and Seaman (3) have described the use of rubeanic acid as a colorimetric reagent for copper separated from steel by electrolysis. An investigation of this reagent showed it t o be practical for use nithout the necessity of separating copper from iron and common alloying elements in steel. Interference from iron is compensated by use of a reference solution and correction is made for other elements by simple calculation.
Larger percentages require proportionately smaller aliquots, in which cases sufficient iron reference solution should be added to adjust the aliquots to 5 ml., and proper corrections made in calrulating results. Precaution. Distilled water from metal systems may contain variable interfering amounts of copper. It should therefore be tested and redistilled from glass if necessary. DISCUSSION
Ammonium acetate was used as a buffering solution ( 1 ) in order to obtain an optimum p H of about 4 ( 4 ) . Gum arabic was first
APPARATUS AND SPECIAL REAGENTS
Colorimeter. A photoelectric colorimeter, such a i the KlettSummerson, Kith a 6GO m p filter and teqt tubr cell, IS icquired. Acetate Buffer Solution. Dissolve 500 grams of ammonium acetate in a miytui e of 200 nil of glacial acetic acid and 400 nil. of distillcd water. Dilute t o 1 liter w t h distllled water. Gum Arabic Solution. Add 10 grams of guni arahic to 500 ml. of hot distilled water slo~vlywith stirring until completelv dissolved. A4110wto simmer on a hot plate foi 1 hour. Cool and keep well stoppered and refrigerated. Rubeanic Acid Solution. Diqsolve 0.5 gram of ruheanic acid (dithio-oxamide) in 500 nil. of 95% ethyl alcohol. The solution is stable for 2 to 3 months.
Table I. Copper in National Bureau of Standards Samples Sample
Identity5
NO.
Copper Present
%
Copper Found %;o
0.244 0.232 0.244 0.249
PROCEDURE
Dissolve a 1-gram sample in a 250-ml. Erlenmeyer flask with 3 nil. of concentrated nitric acid and 9 ml. of concentrated hydrochloric acid .Idd 15 ml. of perchloric acid (71%) and 5 drops of hydrofluoIic acid, evaporate to fumes of perchloric acid, and allow to reflux 15 minutes. Cocl, add 50 ml. of hot distilled water, and smirl to dissolve salts. If any residue is present, filter through a Xo. 40 Whatman filter paper. Cool the solution and adjust to 100 ml. in P volumetric flask with distilled watei. Transfer a 5-mI. aliquot to another 100-nil. volumetric flask, add 5 ml. of acetate buffer solution and 2 ml. of gum arabic solution, and dilute to 100 ml. with distilled water. Transfer the solution to a 200-ml. beaker and add 1.0 ml. of rubeanic acid solution alowly, with constant stirring. Measure colorimetrically within 30 minutes a t 660 mp, using a mild steel or iron sample solution as a zero reference. Prepare the reference solution in the same wav as the sample, preferablv with a standard sample containing a knoan low copper content. Prepare a calibration chart by adding copper sulfate solution t o aliquots of a low copper standard sample. The colorimetric reading of the sample is converted to a tentative copper percentage by reference to this chart. Correct the tentative copper percentage by adding the percentage of copper in the iron reference sample, adding 0.004% copper for each 10% of elements other than iron or nickel present in the sample, and deducting 0.002% copper for each 10% of nickel in the sample. This procedure may he used for copper contents up to 0.50%.
50b
4Cr-lV-18W
0.110
0.123 0.123 0.128 0 113
0 120 0 128 106
1Cr-1A1
126
36Ki
0.096
0.092 0,092 0.094 0.094 0 103 0 103 153 4Cr-8Co-2W-8RI0-2V 0 099 0 111 0 117 0 120 0 111 0 115 0 123 0 Samples except 6e are steel. Numerals denote approximate percentages of major alloying elements.
V O L U M E 2 2 , NO. 12, D E C E M B E R 1 9 5 0 used by Hoar ( 3 ) to stabilize the copper rubeanate color. I n the present work it was noted that the intensity of copper color varied with the condition of the gum arabic solution. Freshly prepared solution without heat treatment gave somewhat high color intensities, which became progressively lower with the age of the gum arabic solution until a stabilization point was reached. When the gum arabic solution was stabilized by heating near boiling for 1 hour, the results remained constant and permitted the use of a permanent calibration chart. No investigation was made of this characteristic in relation to different grades of gum arabic. Although iron does not react with rubeanic acid in the above procedure, it does produce some color with ammonium acetate, with an interference equivalent to 0.004% copper for each 10% of iron in the sample. This is rompensated by use of the iron reference sample, which also includes the reagent blank. Xirkel gives an intense color with ruheanic acid (4);but only in the presence of a relatively high concentration of rubeanic acid. When the reagent n as added sIow1~-with stirring, to prevent local contentration, no reaction occurred with nickel. Honever, the greenish color of nickel salts causes an interference equivalent to 0.006% copper for each 10% nickel in the sample. Because this is 0.002% more than the iron interference, the net correction is 0.002% for each 10% nickel. Manganese, molybdenum, titanium, aluminum, vanadium, cobalt, and hexavalent chromium do not interfere in the quantities usually present in steels. Certain other elements such as tungsten and columbium give insoluble residues, which are filtered off. However, the presence of these alloying elements reduces the amount of iron present as rompared nith the reference solution, and a positive correction is required for the iron replaced. Representative results tor copper are shown in Table I. The first four samples show arcuracy obtained for single determinations of copper in stainless steels. The remainder show accuracy and precision for samples of various types. Kith two eucep-
1573 tiohs, good agreement is shown with Sational Bureau of Gtandards values. Results for samples 50b and 153 n-ere someJrhat higher than the certificate values, although separate tests of the alloying elements shoned no interference in the method. Because the nature of the elements present in these samples causes some question of the exact copper values, it would he difficult to make N close evaluation of accuracy at this time. Any doubt as to the arrurary of the method for a complex steel could be removed, however, t)y using a similar sample with knoirn copper content i n preparing the reference solution. Although large amounts of aluminum :ause interference with ropper rubeanate color (j), the small amounts usudly present ill steel have no measurable effect. Slightly high rrsults ma>- br obtained lvith high-aluminum steel, such as siimple 106 in Table I. Correction for nivkel intcrferenc,e is acxwatr. as sho\rrr by results for sample 126 containing 36% nirkel. Except for high-nickel alloys, the method has t w r l n used ewexitially as n-ritten Cor the past 3 years for all types of steels arid various other ferrous alloys. As a result of the above investigation, the method has I ) w n extended to high-nirkrl stwls arid has proved rPliah1r and convc,nient for routine usc~'. LITERiTURE CITEI)
(1) Center, E. J., and Macintosh, H. JZ., 1x1). EN(:. CHEM.,A w r . .
ED., 17,239 (1945). (2) Hoar, T . P., .Innlyst, 62,Ci57 (1937). (3) Levine, W. S.,arid Seaman. H., IND. ESG. CHKX., A X I L . ED., 16 80-2 (1944). (4) Willard, H. H., and Dirhl, H . , "Advanced Quantitative Analysis," %-en.York, D. Van Sostrand ('o., 1948. ( 5 ) Willard, H . H . , RIoshrr. E:. SI., and Boylr, -4.,J., .\s.\r,. ('HEM.,21, 598 (1949). RECEIVED Jiinr 6, 1950. The opinions euprrh-ed iri t h i r arti
