Colorimetric Determination of Iron in Plutonium Metal Using a Nitrobenzene Extraction Technique C. E. Plock a n d C. E. Caldwell Dow Chemical Co., R o c k y Flats Division,P. 0. Box 888, Golden, Colo. 80401 THE SPECTROPHOTOMETRIC DETERMINATION OF IRON as the ferrous orthophenanthroline complex ( I ) has limitations because of the interference from colored solutions and because of the reaction of orthophenanthroline with other cations (2). In the past, the method used in the Rocky Flats Plant plutonium laboratory was based on the formation of the orthophenanthroline complex of ferrous iron in the presence of plutonium (3). The absorbance was corrected for the contribution of plutonium(II1). The precision of the orthophenanthroline method was generally poor, and the relative standard deviation between initial samples and resubmitted samples was on the order of = k 5 % for metals with 750 to 1500 ppm of iron and f15 % for metals with 150 t o 500 ppm of iron. These errors in the iron results caused a larger standard deviation than desired in plutonium assay results because of the additive effect of iron in the plutonium titration (4) that is used at the Rocky Flats Plant. In this investigation, it was found that the standard correction for plutonium(II1) interference was not only low, but that there is doubt that a valid standard correction can be applied. At low concentrations of plutonium, iron has been determined spectrophotometrically using orthophenanthroline without any apparent interference from plutonium (5-9, but at high concentrations, plutonium will interfere, causing high results (7, 8). To eliminate the plutonium interference, a number of separation techniques have been investigated (713). The application of a solvent extraction technique (14) for the determination of iron in plutonium appeared to be the best approach t o the problem and was, therefore, investigated. The results of that investigation are described in this paper. (1) W. B. Fortune and M. G. Mellon, IND.ENG. CHEM.,ANAL. ED.,10, 60 (1938). (2) G. F. Smith and F. P. Richter, “Phenanthroline Indicators,” G. F. Smith Chemical Co., Columbus, Ohio, 1944. (3) E. C. Anderson, er al., U.S. At. Energy Comm. Repf. LA-416
(1945). (4) C. E. Caldwell, L. F. Grill, R. G. Kurtz, F. J. Miner, and N. E. Moody, ANAL.CHEM.,34, 346 (1962). ( 5 ) W. Allen, U.S. Ar. Eliergy Comm. Rept. CN-1656 (1944). (6) N. E. Moody, Rocky Flats Division, Dow Chemical Co.,
unpublished work, 1960. (7) C. E. Pietri and J. A. Boglio, U . S. At. Energy Comm. Rept. NBL-159, Semi-Annual Progress Rept. for Period July 1959 to December 1959, p. 73. (8) K. S. Bergstresser, U . S. At. Energy Comm. Rept. LA-739 (1949). (9) C. F. Metz and G. R. Waterbury, “Treatise on Analytical Chemistry,’’ Part 11, Vol. 9, I. M. Kolthoff and P. J. Elving, Eds., Interscience, New York, 1962, p. 358. (10) Utiited Kingdom At. Energy Authority Rept. IGO-AM/W-183
(1958).
(11) G. A. Bell, P. R. Stanwin, and D. G. Boase, United Ki/zgdom A t . Eiiergy Authority PG Rept. 709(W) (1 966). (12) Uiiited Kiiigdom At. Energy Autlioriry PG Rept 729(W) ( 1966).
(13) B. Pichotin and P. Chasseur, CEN Rept CEA-R 2969 (1966). (14) D. W. Margerum and C. V. Banks, ANAL.CHEM.,26, 200 ( I 954).
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ANALYTICAL CHEMISTRY
EXPERIMENTAL
Apparatus. Absorbance measurements were made with a Beckman DU quartz spectrophotometer using matched 1.OO-cm silica cells. The absorbance measurements were made at 515 mp. Safety Precautions. Because of the toxicity of plutonium, extreme care must be exercised in its handling. All work with plutonium was performed in glove boxes. Reagents. The hydroxylamine hydrochloride solution (5.473 was prepared by dissolving 108 grams of N H 2 0 H . HCl in water, adding 60 ml of glacial acetic acid, and diluting to 5000 ml with water. The orthophenanthroline solution (0.273 was prepared by dissolving 2 grams of recrystallized orthophenanthroline in 25 ml of 95 ethanol. This solution was then combined with a solution which contained 80 grams of sodium acetate trihydrate. The combined solutions were diluted t o 1000 ml with water. All other reagents were prepared in the normal manner from analytical grade materials. Sample Procedure. Plutonium metal (100 mg) was dissolved in 1 ml of 6M HCl. After dissolution, 5 ml of “,OH. HCl were added, and the sample was permitted t o sit for 30 minutes. After 30 minutes, 5 ml of orthophenanthroline were added, followed by 10 ml of sodium acetate buffer (28.5x of CH3COONa.3H20) and 1 ml of 1M NaC104. The p H of the solution at this point should be between 3.5 and 4.5. Twenty-five milliliters of nitrobenzene were added using an automatic filling and dispensing pipet. The sample was then shaken for approximately 30 seconds, and the two phases were permitted t o separate. After the phases had separated, 5 to 15 ml of nitrobenzene, which now contained the ferrous orthophenanthroline complex, were transferred to a test tube which contained approximately 2 grams of anhydrous Na2S04. The N a 2 S 0 4was used to dry the nitrobenzene. The nitrobenzene was then transferred to a silica cell, and the absorbance was determined at 515 mp using a reagent blank as the reference solution. Calibration Procedure. A calibration curve for iron was prepared over the range 2 t o 200 pg/25 ml of nitrobenzene using the above procedure. Iron solutions of eight different concentrations were prepared, and the absorbance of each was determined. A plot of the absorbance ( A ) 6s. the iron concentration (C), pg/25 ml, was linear and passed through the origin when extrapolated. The average relative standard deviation of the slope ( A / C ) was 2.25 %. The calibration curve was checked three times in the period of 5 to 7 weeks after the original curve was prepared. In each case, each point was within 2 % of the value given by the original curve. RESULTS AND CONCLUSIONS
It was necessary to determine if a quantitative extraction of iron was possible in the presence of plutonium. For this, 48 weighed aliquots of a plutoniun~metal sample were analyzed using the nitrobenzene extraction technique. The average result was 229 i 6 ppm. Ten weighed aliquots of this plutonium metal sample were spiked with an iron standard t o determine the recovery of the nitrobenzene extraction. The average relative error with
the plutonium ranging from 49.2 to 163.7 mg and the iron ranging from 21.0 t o 46.9 pg was less than 0.1 pg with a standard deviation of 0.6 pg. Some plutonium was extracted along with the iron into the nitrobenzene. The quantity of plutonium ranged from 4 to 120 pg/25 ml. This was determined by radiometric analyses. The average quantity of plutonium in nitrobenzene for 18 analyses was 40 pg/25 ml. The plutonium in such low concentrations did not interfere with the determination of iron. The precision of the nitrobenzene extraction technique was determined by the analyses of two sets of plutonium metal samples. In one set, 27 metal samples were analyzed for iron using independent submission methods. The differences of these independent submissions were treated statistically. The analytical standard deviation of the differences was i 17 ppm for metals with an average of 600 ppm of iron. On the other set, 24 metals were analyzed as above, and the 33 ppm for an standard deviation of the differences was average iron concentration of 1280 ppm. Iron results obtained for eight metal samples by the orthophenanthroline aqueous method have been compared with those results obtained by the nitrobenzene extraction technique (Table I). Each of the results in Table I is the average of five or more aliquots. It is apparent from the data that better precision is obtain-
*
Table I. Comparison of Iron Results Obtained by the Aqueous Method and the Extraction Technique
Sample number
Aqueous
1 2 3 4 5 6 7 8
345 345 140 1198 695 174 1225 73 1
method
Standard deviation
Extraction technique
Standard deviation
+55
23 1 71 63 1049 580 71 1074 630
i.5
+46 i25 i.60 it48 c35 i48 i35
+2 +2 +21 iI4 2Cl f20 3: 16
able when the nitrobenzene extraction technique is used. It is also apparent that the results of the extraction technique are consistently lower by 75 t o 275 ppm.
*
ACKNOWLEDGMENT
The authors thank L. F. Grill and F. J. Miner for their helpful discussions. RECEIVED for review May 5, 1967. Accepted July 31, 1967. Work performed under U. S. Atomic Energy Commission Contract AT(29-1)-1106.
Measurement and Significance of the Hammett Acidity Function in Non-Hydroxylic Solvents William N. Sanders and Jerry E. Berger Research Laboratory, Shell Oil Co., Wood Ricer, Ill.
62095
THEHAMMETT ACIDITY function (Ho)
was developed to extend the p H concept into nonaqueous systems (1-3) and Ho values have been reported for numerous acids in various media (4-6). Two reviews cover the subject thoroughly (7, 8). Hammett (3) stated that valid Ho measurements would be restricted t o solvents of high dielectric constant, and the greatest utility and success of the Hammett acidity function has been in solvents of high dielectric constant. Nevertheless, attempts have been made to apply acidity functions t o media of low dielectric constant. Carboxylic acids have been studied in aqueous ethanol ( 9 ) , ammonium salts in aqueous ethanol (1) L. P. Hammett and A. J. Deyrup, J . Am. Cliem. SOC.,54, 4239 (1932). (2) Zbid.,p. 2721. (3) L. P. Hamrnett, “Physical Organic Chemistry,” McGraw-Hill, New York, 1940, Chap. IX. (4) K . N. Bascombe and R. P. Bell, J. Clrem. SOC.,1959, 1096. ( 5 ) J. E. B. Randles and J. T.Tedder, Zbid.,1955, 1218. (6) Thor. L. Smith and J. H. Elliott, J . Am. Cliem. SOC.,75, 3566 (1953). (7) M. A. Paul and F. A. Long, Cliem. Rev., 57, l(1957). ( 8 ) Zbid.,p. 935. (9) E. Grunwald and B. J. Berkowitz, J . Am. Clrem. SOC.,73, 4939 (195 I).
received attention (IO), and glacial acetic acid was the solvent in several investigations (11-13). Although benzene and chlorobenzenes have been used as solvents in studying acid-base reactions ( 1 4 , relatively little work has been reported concerning Hovalues in hydrocarbons. The purpose of this investigation was to test the applicability of the Hammett indicator method in systems containing acids dissolved in solvents of very low dielectric constant (e.g., hydrocarbons). Detailed presentations of the Hammett theory can be found in papers by Bruckenstein (13) and Kolthoff and Bruckenstein (12, 15). EXPERIMENTAL Reagents. All of the indicators used were readily available and are listed in Table I. The acids studied and their sources are listed in Table 11. Stock solutions of acids and indicators were prepared by weight. Purified toluene (percolated over silica gel) was used as the solvent in most cases. Because of solubility problems, phenylphosphonic acid and chloromethylphosphonic acid were studied in methyl ethyl ketone (MEK), and tri-p-tolylphosphate in chloroform. (10) (11) (12) (13) (14) (15)
B. Gutbezahl and E. Grunwald, Zbid.,75, 559 (1953). P. S. Noyce and W. A. Pryor, Zbid.,77, 1397 (1955). I. M. Kolthoff and S. Bruckenstein, Zbid.,78, 1 (1956). S. Bruckenstein, Ibid.,82, 307 (1960). L. E. I. Hummelstedt and D. N. H u m , Ibid.,83, 1564 (1961). I. M. Kolthoff and S. Bruckenstein, Zbid.,78, 10 (1956). VOL. 39, NO. 12, OCTOBER 1967
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