lower figure. This value does not appear to be excessive for plant use. The error produced by temperature changes in the polarographic cell, approximately 391, per degree, can be eliminated by maintaining adequate temperature control of the diluent and the cell contents. The diluent reservoir is easily kept in a large mater bath equipped with a thermostat. On the other hand, if the cell is immersed in another water bath, a siphon arrangement is required because of the drain line. For a plant instrument a better arrangement for maintaining the cell a t constant temperature would be to use a water-jacketed cell. Water from the reservoir water bath then could be circulated to the polarographic cell. The precision of the method has been s h o m above to be within 0.5y0 (relative standard deviation). The precision (and accuracy) is dependent upon the proper functioning of the proportioning unit. Some minor difficulties with this unit have been experienced. The Teflon plug of the rotary valve has a diameter of approximately 0.63 inch, a length of 1 inch, and a 1 to 10 taper. I n operation it was necessary to use stopcock grease on the plug to ensure a tight seal. If the diameter of the plug is made larger (from 1 to 1.5 inches) and the power of
the drive is increased, a tight seal can probably be made without the use of lubricant, which has to be applied regularly. A single selector valve could be used for changing from one feed solution to another in plant operation. The valve would open a given pair of feed and return lines to the circulating pump and proportioning system within the instrument. With proper design, such a valve might acconimodate four or five pairs of lines to permit sampling of several points in the plant. For plant operation, it would be desirable to include a standard uranium solution in the operational sequence by means of a standard uranium line as one of the multiple lines mentioned above. Solutions of different representative feed samples of known uranium content could be used to obtain specific calibration curves for each type of feed. ACKNOWLEDGMENT
The authors wish to thank A . R. Eberle and W. A. Peavy for the chemical analyses of the feed solutions. LITERATURE CITED
(1) Bisby, H., Brown, L. H., Chapman, D. R., J. Sci. Imtr. 33, 467 (1966).
(2) Eberle, A. R., Lerner, M. \V., ANAL.CHEM.29, 1134 (1957). (3) D. J.. Kellev. 11. T.. U. S. . , Fisher. Atomic Energy“ ‘ Commission, ORNL-1233, 1 (1951). (4)Ibid., ORNL-1361, 1 (1952). (5) Kelley, M. T., Fisher, D. J., U. S. Atomic Enernv Commission. ORNL-1276, 7 71952). ( 6 ) Kelley, > T., ‘I. Fisher, D. J., Meeks, L. A, Palmer, J. P., U. S. Atomic Energy Commission, ORNL-1423, 1 (1952). (7) Kolthoff, I. AI., Lingane, J. J., “Polarography,” 2nd ed., Vol. I, p. 331, Interscience, Xew York, 1952. (8) Koyama, IC., Michelson, C. E., Alkire, G. J., U. S. Atomic Energy Commission, HW-30148 (1953). (9) . , Leveaue, Lf. P , Roth, F , J . c h m . phis. 46, 480 (1949). ‘ (10) Overton. K. C.. U.K. ~, Lewis. J. 8.. Atomic Energy Authority,’ CRL,/ AE-79 (1951). (11) Michelson, C. E., General Electric communiHanford Works,. private cation. (12) Rodden, C. J., “Analytical Chemistrjof the Manhattan Proiect.” 1st ed , p. 68, McGraw-Hill,’ SenYork, 1950. (13) Rodden, C. J., U. S. Atomic Energy Commission, TID-5295, 215 (1956).
RECEIVED for revieiv May 15, 1957. Accepted Kovember 26. 1957. Division of Analytical Chemistry. Beckman ilward Symposium, 131st Meeting, ACS, Miami, Fla., .kpril 1957
Photometric Determination of Chromium in Electronic Nickel C. L. LUKE Bell Telephone luborutories, Inc., Murruy
b A
method for the determination of
0.001 to 0.0270 of chromium in nickel has been developed in which the chromium i s oxidized to the sexivalent state, nickel i s removed b y precipitation as nickel ammonium perchlorate, and the chromate in the filtrate is determined b y the photometric diphenylcarbazide method.
T
of the present investigation has been to develop a suitable method for the determination of chromium in nickel used for the fabrication of electronic devices ( I ) . As the chromium content of such nickel is usually very low, a photometric method was indicated (I). Unsuccessful attempts were made to determine the chromium directly in acid solution without prior separation of the nickel or any of the impurities therein. except HE PURPOSE
Hill, N. J. silicon or tungsten. The photometric dichromate method proved to be too insensitive. The diphenylcarbazide method was found to be sufficiently sensitive, but copper caused low results and no suitable method was found for destroying the permanganate formed during the oxidation of the chromium. Some reduction of the dichromate invariably occurred. ilfethods for isolating the chromium were next considered. An attempt was made to separate it from the bulk of the nickel, cobalt, copper, manganese, and magnesium by precipitating it from ammoniacal solution as chromic hydroxide, using 10 mg. of ferric iron as a coprecipitant. Recovery of the chromium was incomplete. Eventually a successful method of separation was evolved in which copper is removed by dithizone-chloroform extraction, chromium is oxidized to the sexivalent state,
and nickel, cobalt, iron, titanium, and aluminum are removed by an ammonium hydroxide separation-the nickel and cobalt being separated in the form of their difficultly soluble ammonium perchlorate salts. The chromate in the filtrate from this separation is then determined by the diphenylcarbazide method. REAGENTS
Standard Chromium Solution (10 y of chromium per ml.). Dissolve 0.283 gram of Bureau of Standards sample No. 136a of potassium dichromate in water and dilute to 1 liter in a volumetric flask. Dilute 100.0 ml. of this solution t o 1 liter in a volumetric flask. Dithizone Solution. Dissolve 0.5 gram of dithizone-diphenylthiocarbazone-in 500 ml. of chloroform. Keep the solution in a refrigerator when not being used. VOL. 30, NO. 3, M A R C H 1958
a
359
Diphenylcarbazide Solution. Dissolve 0.2 gram of diphenylcarbazide in 20 ml. of methanol. Prepare fresh just before use. PREPARATION OF CALIBRATION CURVE
Working individually, transfer 0, 4.0, 8.0, and 12.0 ml. of standard chromium solution (10 y of chromium per ml.) to a 125-ml. conical flask. Add 3 drops of sulfurous acid (6Yc) from a dropping bottle and swirl to reduce the chromium. Add 5 ml. of colorless nitric acid, dilute to 55 ml., and transfer to a 125-m1. Squibb-type separatory funnel. Add 35 ml. of dithizone solution, stopper, and shake vigorously for 30 seconds. Allom a minute for the layers to separate, and then drain off and discard the lower layer. Wash down the inner walls of the funnel with about 10 ml. of chloroform, and then drain off and discard the lower layer. Add about 15 ml. of chloroform, stopper, and shake for about 10 seconds. Allow to settle, and then drain off and discard as much of the lower layer as possible. Transfer the aqueous solution to a 250-ml. conical flask and add 0.5 nil. of sulfuric acid (I 1) plus 10 ml. of perchloric acid (70%). Boil vigorously, without a cover, until most of the water arid nitric acid have been expelled. Finally, swirl over a flame, heating the upper malls of the flask as m-ell as the bottom, until copious fumes of perchloric acid are being evolved. Do not, howevrr, expel any more perchloric acid than is necessary. Cool somewhat. Add 50 ml. of water, 1 drop of 1% potassium permanganate solution, 1 drop of 1% manganese sulfate solution, and then heat to gentle boiling. Boil for 3 minutes, cool to about io" C., and add 25 ml. of ammonium hydroxide. Swirl, and then cool to 20" C. in an ice bath. Filter through an 11-em. KO 42 Whatman filter paper into a 100-ml. volumetric flask, m-ashing the conical flask and paper well. Discard the paper. Warm the solution in the flask to room temperature, dilute to the mark, and mix. Transfer 25.0 ml. of the solution to a 150-1111. beaker, and add 50 ml. of water and a small piece of Congo red paper (about 0.5 em. square). Khile stirring, neutralize with sulfuric acid (1 1) until the paper just turns blue. Remove the Congo paper immediately to minimize reduction of the chromium. Without delay, transfer the solution to a 100-ml. volumetric flask, cool to room temperature under cold running .sater. add 5 ml. of sulfuric acid (1 9) followed by 1 ml. of diphenylcarbaside solution, dilute to the mark, and mix well. Allom to stand 1 minute to attain full color development, and then immediately measure the solution photometrically in a 5-cm. absorption cell a t 540 mp using distilled water as the reference solution. Do not delay making the photometric measurement, as the color tends to fade slightly on standing. Prepare R calibration curve. This
+
+
+
360 *
ANALYTICAL CHEMISTRY
curve will be the usual Beer's law calibration curve. ANALYSIS OF SAMPLE
Transfer 0.500 gram of the nickel sample to a 125-ml. conical flask. Add I) plus, if 10 ml. of nitric acid (1 the sample contains no tungsten, 1 drop of hydrofluoric acid (4@5). Cover and heat gently until solution of the sample is complete. Remove the cover and boil just long enough to expel all brown fumes. Cool to room temperature, add 45 ml. of water, remove a n y precipitated tungstic acid by filtration, and transfer to a 125-ml. Squibb-type separatory funnel. Proceed to the dithizone extraction and steps following, as directed above. If the dithizone extract is mine colored, indicating the presence of considerable amounts of copper, repeat the extraction with one or more 10-ml. portions of dithizone solution until the extract remains fairly green. Then proceed to the chloroform wash. After precipitation of the nickel, cool to 20" C., and then filter with suction through two sheets of 5.5-cm. Whatman No. 42 filter paper on a small Buchner funnel into a 100-ml. volunletiic flask. Allom the precipitate on the paper t o be sucked as dry as possible but do not wash. Carry a reagent blank through the entire analysis. With the aid of the calibration curve, determine the weight of chromium present in the blank and sample.
+
DISCUSSION OF
METHOD
The oxidation of chromium to the sexivalent state is a very critical step in the proposed method. Thus, if, in the procedure described for the preparation of the calibration curve, the addition of 0.5 ml. of (1 1) sulfuric acid and 1 ml. of potassium permanganate solution is omitted, the transmittancy readings obtained correspond to only 50 to 90% of the chromium added-depending on how much of the perchloric acid has been fumed off. The more acid expelled, the less chromium recovered. By arranging to oxidize subsequently with permanganate, somewhat better recovery of the chromium can be obtained, but the results are still too low to be acceptable. Eventually, i t was found that the low recoveries can be virtually eliminated by fuming for a limited time, in the presence of a little sulfuric acid, followed by oxidation with permanganate. The cause of the loss of chromium described above is not apparent. The fact that subsequent oxidation with permanganate does not eliminate the low recoveries suggests that most of the loss may be due to volatilization or hydrolysis of the chromium rather than to reduction to the chromic state. To compensate for any slight negative errors that may result from incomplete oxidation or from occlusion losses in the ammo-
+
nium hydroxide separation, it, is best to include these steps in the preparation of the calibration curve. The amount of permanganate used in the oxidation of the chromium must be kept to the minimum because, when the solution is subsequently made ammoniacal, the permanganate tends to oxidize cobalt and prevent it from precipitating as cobalt ammonium perchlorate. If appreciable amounts of cobalt accompany the chromium, the resulting pink color will cause slight,ly high values for chromium. I n the proposed method, a little manganous ion is added just before the neutralization to ensure complete removal of any excess permanganate. The removal of nickel and cobalt is satisfactory, if t,he solution is cooled to 20" C. before filtration. Xoreover, as much as 1 gram of nickel can be precipitated in this manner 11-ithout incurring any loss of chromium. On the other hand, if appreciable nniounts of impuritiese.g., cobalt, iron, titanium, aluminum, manganese, etc.-are present, in the nickel, slightly low recoveries of chromium are encount,ered. For this reason, in the proposed method, the sample size has been limited to 0.5 gram a s a precaution. The diphenylcarbazide nietliod was chosen in preference to the chromate method for the photometric deterniinat,ion of the chromium. The latter method tends to give high and variable results due to the small amounts of nickel, cobalt, and, in all probability, traces of organic matter and colloidally suspended oxides or hydroxides that escape removal in the ammonium hydroxide separation. The diphen>.lcarbazide method has one disadvantagenamely, that copper causes the results for chromium t,o be appreciably loa. This phenomenon does not seem to have been previously reported. The copper acts as if it were reacting with sexivalent chromium to form a nonreactive compound of some kind. Once this postulated compound has been formed, it is not readily destroyed by removing copper by complexing with cyanide or (ethylenedinitri1o)tetraacetic acid, by solvent extraction, or by electrolytic deposition. The only practical solution to the problem appears to be removal of the copper while the chromium is still in the trivalent state. This can be easily accomplished by means of a dithizone extraction frcm strong acid solution. The deleterious effect of copper can be demonstrated as follows: Transfer i5 ml. of water, 5 ml. of sulfuric acid (1 9), 3 ml. of standard pot#assium dichromate solution (10 y of chromium per ml.), and various amounts of copper in the form of aliquots of a copper sulfate solution to 100ml. ~-01umetricflasks in the order men-
+
tioned, swirling after each addition. Add 1 ml. of diphenylcarbazide solution, mix, dilute to volume, and measure photometrically at 540 mp as directed in the proposed method. Typical results obtained by such an experiment are shown in Table I. Some types of nickel used for the fabrication of electronic devices contain about 4y0of tungsten. Loss of chromium by occlusion in precipitated tungstic acid is negligible, but when hydrofluoric acid is employed in the dissolution of the nickel sample some tungstic acid appears during the fuming with perchloric acid. EXPERIMENTAL
To test the accuracy of the proposed method, synthetic samples of known chromium content were prepared and analyzed for chromium. The synthetic samples were made by dissolving 0.5gram portions of pure nickel (Kivac) in 10 nil. of nitric acid (1 l), and then adding aliquots of a standard solution of trivalent chromium plus aliquots of solutions of salts of other metal impurities normally present in electronic nickel. I n all cases the maximum expected amount of impurity was added (1). The results obtained are shotvn in Table 11. The values listed in the last column were obtained by subtracting the weight of chromium present in the nickel plus reagents, from the total weight of chromium found in the synthetic samples. The blank on the reagents plus 0.5 gram of Sivac amounted.
+
Table 1.
Deleterious Effect of Copper
Copper Added, Mg.
Chromium, y Added Found 30.0 30.0 0 0.03 30.0 28.8 30.0 28.0 0.3 27.5 3 30.0 26.9 15 30.0 26.7 15a 30.0 30.0 25.7 36 15 ml. of H2S04 (1 9) added instead of 5 mi. * 5 ml. of diphenylcarbazide solution added instead of 1 ml.
+
Table II. Determination of Chromium in Synthetic Nickel Samples
Chromium, y Impurities Added Added Found ... 60 60 ... 120 119 1 . 5 mg. Cu 60 60 30 mg. Co 60 61 2 mg. A1 5 mg. Fe 120 120 2 . 5 mg. T i 2 . 5 mg. Mn 60 59 20 19 120 119 120 119 a 1 5 mg. Cu 1 . 5 mg. Fe 2 mg. A1 + 2 . 5 mg. Ti 2 . 5 mg. Mn 5 mg. co.
++
+ +
++
in the present investigation, to 1 y of chromium-Le., 0.25 y for the 25-nil. aliquot of the filtrate. Three typical samples of electronic nickel and two Bureau of Standards samples of nickel oxide were analyzed in duplicate, as directed in the proposed
Table 111. Analysis o f Nickel and Nickel Oxide Samples
Sample A-3 1a
Chromium Found, %
220
0.002 0.002 0.002 0.002 0.025
0.0002 0.0002
225 Oxide 6715
0.026 0.003 0.003 ‘Sample contains about 453 of tung-
Oxide 6725
sten. * Bureau of Standards recommended values for chromium are 0.02570 for 671 and 0.003y0 for 672.
method, except that 0.3-gram samples were used in the analysis of nickel oxide No. 671. Also, in the analysis of the two nickel oxide samples, it was necessary to add a few drops of hydrochloric acid in order to obtain complete dissolution of the samples. The results obtained are recorded in Table 111. LITERATURE CITED
(1) Am. SOC. Testing hlaterials, Philadelphia, Pa., “BSThI Methods of Chemical Analvsis of Metals.” v. 240, 1956. (2) Sandell, E. B., “Colorimetric Determination of Traces of Metals,’ p. 257, Interscience, Sen. Yo1 k, 1950.
RECEIVEDfor review May 17, 1957. Bccepted December 2, 1957.
Volumetric Determination of Magnesium in Titanium M. J. MILES, W. J. MESIMER, and MAE ATKIN Titanium Metals Corp. of America, Henderson, Nev.
b The volumetric determination of magnesium b y titration with disodium (ethylenedinitri1o)tetraacetate (EDTA) has been adapted for use with sponge and other samples having appreciable concentrations of titanium. Titanium and other elements that interfere with the EDTA titration are eliminated b y an ammonium acetate separation. Samples containing manganese in amounts considerably greater than normally found in titanium sponge require additional treatment with sodium sulfide. The magnesium is titrated with a standard solution of disodium EDTA using Eriochrome Black T mixed with methyl red as the indicator. This method is suitable for application to titanium samples containing magnesium in amounts greater than 0.02c7,.
T
of magnesium in the production of titanium has created an urgent need for a convenient method for determining this element in sponge and other products. This analytical problem has been studied intensively by the laboratories belonging to the Task Force of Magnesium and others interested in the problem. Several methods h a w been proposed (S, 8 ) and tested. The D u Pont method (8) and the Frankford Arsenal method (5’) have been applied to the task force samples, but the time required has been rather long. I n experienced hands, both methods have yielded satisfactory results. A revised and shortened adaptation of the Frankford Arsenal method Kas formerly used in sponge analysis, but was still too long to be entirely satisHE EXTEKSIVE USE
factory. A flame photometric method was also tried; however, the high background emission restricted its application to fluoboric acid solutions, and even these had too much background emission to be entirely satisfactory. The numerous recent uses of disodium (ethylenedinitri1o)tetraacetate (EDTA) for the titration of magnesium in various kinds of materials (1, 2 ) prompted this investigation to find a fast, accurate method for determining magnesium in titanium. Direct titration of magnesium lyith E D T A was not possible because titanium, iron, and other elements used in the alloys form chelates with the EDTA reagent in the p H range required for the titration of magnesium. An effective VOL. 30, NO. 3, MARCH 1958
361
