Determination of Small Amounts of Cobalt by Isotope Dilution Analysis WILLIAM
D. RALPH,
Jr.,l THOMAS R. SWEET, and INARA MENCIS
Department of Chemistry, The Ohio Stafe University, Columbus 10, Ohio
b Isotope dilution analysis i s combined with solvent extraction and spectrophotometry to produce an effective general method for the determination of constituents that are not extracted quantitatively, Combining isotope dilution with extraction of the cobalt-2-nitroso-1 -naphthol complex into chloroform results in an accurate and reliable method for the determination of small amounts of cobalt in alloys of high copper content and in ingot irons. C060 is used as a radioactive tracer.
I
spectrophotometric determination of cobalt in ferrous and nonferrous alloys by extraction into chloroform with 2 - nitroso - 1 - naphthol, Claassen and Daamen (S) experienced difficulty with samples containing more than about 25 mg. of copper. Recovery of cobalt was incomplete unless a preliminary electrolytic separation of copper was performed. I n the analysis of a n ingot iron sample, results that were low by over 15% indicated the possibility of nonquantitative cobalt recovery in this case, also. Results with a high degree of accuracy and precision were obtained for both types of samples by using isotope dilution analysis, with C060, in conjunction with this solvent estraction-spectrophotometric procedure. An isotope dilution method for the determination of cobalt in vitamin BIZ and biological materials was reported (6) soon after the present paper was submitted for review. It involves the use of cobalt-60 and a spectrophotometric procedure that is based on the extraction of cobalt thiocyanate with methyl isobutyl ketone (4-methy1-2pentanone). K THE
APPARATUS
All measurements of radioactivity were made with a RIDL Model 49-54 scaler. The detector employed was a Harshaw, 2 x 2 inch, thallium-activated sodium iodide crystal. ilbsorbance measurements were made on a Beckman D U spectrophotometer, 1
Present address, Vandenberg Air Force
Base, Calif.
92
ANALYTICAL CHEMSTRY
using 1-cm. matched, rectangular cells with ground-glass stoppers. Solvent extractions were performed with 125-ml. glass-stoppered borosilicate glass separatory funnels. REAGENTS
Two standard solutions containing 1.2 and 12 pg. of cobalt per ml. were prepared from a solution containing 0.239 mg. of cobalt per ml. The last solution was made by dissolving 0.8 gram of cobaltous chloride hexahydrate in 50 ml. of distilled water and 10 ml. of 0.1M hydrochloric acid, and diluting to 1000 ml. This solution was standardized by titration with a standard 0.009.11 solution of the disodium salt of (ethy1enedinitrilo)tetraacetic acid (EDTA), using murexide indicator ( 5 ) . An active solution, containing Co60 as Cofz ion, was prepared by diluting a hydrochloric acid solution of cobaltous chloride of high specific activity obtained from the Oak Ridge National Laboratory. The activity iF-as adjusted so that when 10 ml. of this solution were added to 90 ml. of an aqueous sample solution, a S-ml. portion of the resulting solution gave a n activity of about 35,000 counts per minute with the detector employed. This solution contained about 0.6 pc. and 5.6 X pg. of cobalt per ml. The 2-nitroso-1-naphthol solution was prepared (3) by dissolving 1 gram of reagent in 100 ml. of glacial acetic acid. This solution was shaken with 1 gram of powdered activated charcoal. It was filtered immediately before use. The dithizone solution was prepared by dissolving 0.7 gram of dithizone in 100 ml. of chloroform. It w m filtered immediately before use. PROCEDURE
Preparation of Working Curve. Various volumes of the standard inactive cobalt solutions were transferred t o each of a number of 100ml. volumetric flasks. After the addition of 10 ml. of a 40y0 sodium citrate solution and one drop of methyl orange indicator solution, concentrated hydrochloric acid was added dropwise until the solution was pink. A 10-ml. volume of 12% H z O solution ~ and 10 ml. of the cobalt-60 solution were added and the volumetric flask was filled t o the mark with distilled water. Fifty milliliters of this solution were transferred to a 125-m1. separatory funnel, 2 ml. of the 2-nitroso-lnaphthol reagent were added, and the
solution was si! irled to ensure mking. After 5 minutes’ reaction, 25.0 nil. of chloroform were added to the separatory funnel and it was shaken for 1 minute. The chloroform layer lyas n nshed with 20 ml. of 2 X hydrochloric acid and then with 20 ml. of 2M sodium hydroxide. Five milliliters of the chloroform layer were placed in a n 8-ml. vial and counted for 10 minutes. Five milliliters of the aqueous solution remaining in the 100-ml. volumetric flask were placed in a n 8-ml. vial and counted for 10 minutes. These 8-ml. vials were washed after each use by allowing them t o soak in hot chromic acid cleaning solution for 5 minutes and rinsing them thoroughly with distilled mater. A portion of the chloroform layer was placed in the cell and its absorbance was measured a t 535 mp against a blank. Since the concentration of the standard cobalt solution was known, the concentration of the solution in the 100-ml. volumetric flask could be calculated. From this, the n-eight of cobalt in the 5-ml. portion that was counted was determined. From the activity measurements taken with the aqueous solution, the specific activity of cobalt was calculated as counts per minute per microgram of cobalt. Using the activity of the chloroform solution and the specific activity, the weight of cobalt in the portion of chloroform counted was calculated. Since the volume of this portion is known, the concentration of cobalt in the chloroform solution could be determined. 4 working curve was obtained by plotting absorbance values us. the concentrations of cobalt in the chloroform solutions (Table I). I t was a straight line passing through the origin. Analysis of Copper-Nickel-Zinc Alloy (NBS 157). Four 3.7-gram samples were weighed and placed in 250-ml. volumetric flasks, each of which contained 40 ml. of 1 t o 1 nitric acid. After the samples dissolved, the solutions were boiled gently until brown fumes were no longer observed ( 2 ) . After being cooled t o room temperature, the flasks were filled to the marks with distilled m t e r . Fifteen milliliters of sample solution were pipetted from the 250-ml. volumetric flask into a 100-ml. volumetric flask and 10 ml. of 40% sodium citrate solution were added. The p H of the solution was checked with indicator paper. However, no adjustment was needed, since it was always found to be between 3 and 4. Hydrogen peroxide
was not added t o the sample solution at this point, since dithizone is oxidized under weak oxidizing conditions (4). After addition of 10 ml. of C060 solution, the flask was filled to the mark with distilled water. A 25-ml. aliquot was transferred to a 125-ml. separatory funnel, 50 ml. of 0.7% dithizone in chloroform solution were added, and the mixture was shaken vigorously for 5 minutes. The chloroform layer was drawn off and discarded. Another 50nil. portion of dithizone solution was added and the extraction was repeated. Again, the chloroform phase was discarded. Two 25-ml. chloroform extractions (each shaken for 1 minute) were performed. The chloroform layers n-ere discarded. Four milliliters of 12% hydrogen peroxide solution were added to the separatory funnel, 2 ml. of filtered 2-nitroso-1-naphthol reagent solution were added, and the solution n n s snirled. After standing for 5 minutes a t room temperature, 25 ml. of chloroform were added and the separatory funnel was shaken vigorously for 1 minute. ilfter complete separation of the layers, the chloroform layer was drawn off into a clean separatory funnel which contained 20 ml. of 2 M hydrochloric arid. From this point on, the laboratory procedure was the same as that for preparation of the working curve.
The weight of cobalt in the 100-ml. volumetric flask was calculated by equation
nhere a , is the measured activity of v , nil. of the aqueous solution, a, is the measured activity of uo ml. of the organic phase, and Va is the volume of the aqueous solution (100 ml. in this case). (“o is the concentration of cobalt in the organic phase and is obtained from the rneasured absorbance and the spectro~)liotometricaorking curve. This equation assumes that the weight of cobalt introduced by adding the Co60 solution is negligible compared to the weight of cobalt present in the aqueous sample solutions.
Analysis of Ingot Iron (NBS 5 5 e ) . Four 8-gram samples were placed in 400-ml. beakers and 100 ml. of 1 to 1 hydrochloric acid were added t o each ( I ) . The covered beakers were heated gently on a hot plate for about 3 hours. -4t the end of this period the samples ere dissolved except for thin layers of insoluble material on the surface of the solutions. Ten milliliters of concrntrated nitric acid were added dropwise to each of the hot solutions. The solutions were evaporated to about 40 nil. The hot acid solutions were transferred to 100-ml. volumetric flasks and the beakers ere rinsed with hot 1 to 1 hydrochloric acid. After cooling to room temperature, the volumetric flasks \\ere filled to the marks with distilled wter. Twenty-five milliliters of sample solution were pipetted into a 100-ml. volu-
Table I.
Total Weight of Cobalt Added, fig. 11.97 23.92 95.63 119.61 179.48
Preparation of Spectrophotometric Working Curve
Corrected Activity, C./M. per 5 M1. Aqueous Chloroform 66090 66947 63069 71516 67910
33734 33683 34392 35233 34259
metric flask and 60 ml. of 40% sodium citrate solution were added to the flask. The solution was cooled to room temperature after swirling. After addition of 10 ml. of Corn solution, the flask was filled to the mark with distilled water. A 50-ml. aliquot was transferred to a 125-ml. separatory funnel and this was adjusted to a pH of 3 to 4 with 28% ammonium hydroxide. p H indicator paper was used to detect the pH of the solution. About 2.5 ml. of the ammonium hydroxide solution were required to reach the proper pH range. The solution was cooled t o room temperature and 10 ml. of 12% hydrogen peroxide were added. Two milliliters of filtered 2-nitroso-1-naphthol reagent solution were added and the solution was swirled to ensure mixing. After standing for 10 minutes a t room temperature, 25 ml. of chloroform were added and the separatory funnel was shaken vigorously for 1 minute. The chloroform layer was drawn off into a clean separatory funnel containing 20 nil. of 251 hydrochloric acid solution. The rest of the laboratory procedure n-as the same as that previously described. DISCUSSION
The basic method described by Claassen and Daamen (3) was modified to a considerable extent to make it adaptable to the isotope dilution technique and to make use of the special advantages of this technique. Whereas those authors formed the 2-nitroso-lnaphthol complex of cobalt in a beaker,
Table II.
Total Weight of Sample Taken, Grams 3.7435 3.7176 3.7467 3.7245
Concn. of Cobalt in Chloroform, fig./iy. at 25 C. 0.234 0 477 1,760 2.429 3.558
Absorbance 0.060 0.121 0.440 0.609 0.894
it A as necessary in this investigation to begin with the aqueous cobaltous solution in a volumetric flask, since the volume of this solution had to be known accurately. A portion of this solution was then transferred to a separatory funnel, where complex formation was carried out. From the standpoint of keeping the volume of the aqueous solution below the limits imposed by the volumetric flask used, it was found advantageous to use concentrated HC1 or 28% XH40H for pH adjustment, rather than 2N HC1 or 2N NaOH, since such large volumes of the latter were usually required. Since Claassen and Daamen needed total recovery of cobalt in their procedure, their working curve is a plot of absorbance z’s. weight of cobalt in the sample. The present method produces a working curve that is a plot of absorbance us. the concentration of cobalt in the extracted chloroform phase. Here the weight of cobalt in the chloroform phase may be less than the weight of cobalt in the sample. Instead of 30 minutes or longer, a reaction time of about 5 minutes was used for the preparation of the morking curve and sample 157 and about 10 minutes for sample 558, Instead of extracting the cobalt complex once with 25 ml. of chloroform and two additional times with 10-ml. portions of chloroform, a single 25-ml. chloroform extraction Kas made. Xational Bureau of Standards sample 157 was chosen for analysis because of
Analysis of National Bureau of Standards Samples
Corrected Activity, C./M. per 5 M1. Aqueous Chloroform Sample 157 34405 34426 34657 34573
Total Weight of Cobalt Found, Mg.
Cobalt 970 in
5.05 4.97 5.06 5.04
Av.
0.135 0.134 0.135 0.135 0.135
Av.
0.00615 0,00616 0.00623 0.00619 0.00618
28809 27902 31042 27467
Sample
Sample 55e 8.0039 8,0108 8.0094 8.0119
34325 .~ . 34446 34191 34506
~
2893 1
n 492
48633 42153 40198
0 499 0.496
. ~ _ . _ _
0 493
VOL. 34, NO. 1, JANUARY 1962
a
93
its high copper (72.147,) and low cobalt (0.136%) content. Claassen and Daamen (3) reported recovering about 10% of the cobalt when 100 mg. of copper were present in the sample analyzed, using largely increased amounts of 2-nitroso-1-naphthol. I n initial attempts to analyze this alloy, no preliminary separation of copper was made, since it was believed that the isotope dilution method would give satisfactory results under these conditions. However, in addition to competing successfully for the complexing agent, copper formed a complex which could not be completely removed by the acid and base extractions and which absorbed slightly a t 535 mp, causing high results to be obtained for cobalt. Extensive modification of the acid and base extraction steps, such as increasing acid concentration stepwise up to 12M HC1, increasing base concentration up to saturated KOH or 28% NH40H, increasing the number of acid and base extractions, increasing the time of extractions, and increasing the volumes of acid or base used, failed t o remove the source of interference. This approach vias abandoned and an attempt was made to find a complexing agent that could be used to remove most of the copper in an extraction separation. Dithizone appeared to be ideally suited for this purpose (4, since it was a very high extraction constant for copper and
a low constant for cobalt in mild acid solutions, using chloroform as the extractant. For the 25-ml. aliquot of the aqueous sample solution taken for extraction, two preliminary 5-minute extractions, each using 50 ml. of a 0.7% (weight/ volume) solution of dithizone in chloroform, followed by two 1-minute extractions, each with 25 ml. of chloroform, were sufficient to reduce copper to a level where it no longer interfered. The separation was not specific, since in each case approximately 0.3% of the cobalt was removed by the first dithizone extraction and approximately 10% of that remaining was removed by the second. Because of this, the addition of the dithizone extraction t o the method of analysis used by Claassen and Daamen would not give accurate results. However, the use of the dithizone extraction with the isotope dilution principle proved to be ideal.
obtained by the present method is 0.7%. Results of four determinations using the ingot iron sample are also shown in Table 11. The National Bureau of Standards provisional certificate of analysis for this iron sample lists a cobalt content of 0.006%. The results of the present method are in complete agreement, ACKNOWLEDGMENT
The authors are grateful to the United States Air Force for making it possible for one of them (W.D.R.) to undertake this study a t The Ohio State University. LITERATURE CITED
(1) Am. Soc. Testing Materials, Philadelphia, “ASTM Methods for Chemical
Analysis of Metals,” p. 120, 1950.
( 2 ) Ibid., pp. 239-40; (3) Claassen. A,. Daamen,’ rl., Anal. Chim. . Acta 12, 547 (1555).
(4) Sandell, E. B., “Colorimetric De-
RESULTS
The results obtained in four determinations with the copper-nickel-zinc alloy are shown in Table 11. The National Bureau of Standards certificate of analyses for this alloy lists a cobalt content of 0.136y0 (an average of nine values ranging from 0.13 to 0.147%). On the basis of this average value, the relative error of the mean of the values
termination of Traces of Metals,” 3rd ed., pp. 135-76, Interscience, S e w York, 1959. (5) Schwarzenbach, G;, “Complexometric Titrations,” pp. 18-9, Interscience, Kew York, 1957. (6) Sporek, K. F., ANAL.CHEJI.33, 754 (1561). RECEIVED for review March 2, 1961. Accepted November 6, 1961. Taken in part from the M.S.thesis of William D. Ralph, Jr., The Ohio State University, 1960.
Spectrophotometric Determination of Ruthenium with 2-Nitroso-1-naphthol D. L. MANNING
and OSCAR MENISI
Analytical Chemistry Division, Oak Ridge Nafional Laboratory, Oak Ridge, Jenn.
b A sensitive spectrophotometric method for the determination of ruthenium is based on the color reaction of ruthenium with 2-nitroso- 1 -naphthol in 3M hydrochloric acid saturated with sulfur dioxide. The colored species exhibits maximum absorbance a t 600 mp. Simple salts of ruthenium such as ruthenium chloride can b e determined in the presence of uranium without a prior separation. Beer’s law is obeyed over the concentration range, 1 to 10 pg. Ru per ml. The relative standard deviation is about 5%. The molar absorptivity of the colored species is approximately 6500.
R
one of the most abundant elements formed during nuclear fission, is of interest in the homogeneous reactor program because of its 94
UTHENIUM,
ANALYTICAL CHEMISTRY
effect upon the stability of solutions of uranyl sulfate and upon the corrosion rate of austenitic steel systems. Numerous chromogenic reagents for the spectrophotometric determination of ruthenium are recorded in the literature (f-3, 6-7). Color reactions with ruthenium are frequently carried out in weakly acidic solutions a t a controlled pH, after the ruthenium has been separated from other substances by a distillation process. Jacobs and Yoe (S), however, introduced another reagent that can be used in a strong mineral acid solution without a close control of p H after distillation. A new chromogenic reagent, 2-nitrosoI-naphthol, is proposed for the colorimetric determination of ruthenium. The feasibility of utilizing various naphthols for the colorimetric determination of ruthenium was reported by Ryan
(5), but no experimental details were presented. I n this laboratory, ruthenium, in a medium of hydrochloric acid saturated with sulfur dioxide, reacts with 2-nitroso-1-naphthol to form a blue complex which exhibits maximum absorbance a t 600 mp. This reagent and that proposed by Jacobs and Yoe can be utilized in a strong mineral acid solution without close pH control. Ruthenium can also be determined without prior separation in the presence of milligram amounts of uranium and microgram quantities of the corrosion products of steel such as iron, nickel, and chromium when 2-nitroso-1-naphthol is used as the chromogenic reagent. 1 Present address, Nuclear Materials and Equipment Corp., A4pollo,Pa.
