series terms do not involve transcendental functions and convergence is extremely rapid. A useful property of E(2) is that this function of the conjugate of 2 is equal to the conjugate of E(2).
E(x
and rn=2n-1
(AW
This expression permits evaluation of the real and imaginary parts of E(2) for all values of the argument x iy with an error of less than two parts in the sixth significant figure with seven terms in the evanescent series in Equation A10. For large x or y only the sums contribute significantly to The first two terms on the right-hand side of Equation A10 may be set equal to zero for x greater than two or the absolute value of y greater than about ten. Equation A10 is suitable for calculations on a small laboratory computer because the
+
&(a.
- iy) = E(x + i y )
(‘413)
Equation A1 3 may be derived from relationships presented elsewhere (21). This symmetry relation is useful because when the exponential error function complement with complex argument occurs in electrochemical theory, the complex arguments always appear in conjugate pairs. Useful approximations for the error function and Dawson’s integral with complex argument may be obtained from Equations AS and A8, respectively. Equations A5 and A8 may be written in terms of real variables after substitution of Equation A9. The resulting expressions permit evaluation of erf(2) and F ( 2 ) with complex argument, even on a small laboratory computer. RECEIVED for review March 3, 1969. Accepted June 13, 1969. Work supported by the United States Department of the Interior, Office of Saline Water.
Rapid and Specific Extraction of Aluminum From Complex Mixtures with N-Benzoyl-N.Phenylhydroxylamine Spectrophotometric Determination with 8-Quinolinol Robert Villarreal, John R. Krsul and Spence A. Barker Argonne National Laboratory, Idaho Division,Idaho Falls, Idaho 83401 A highly selective method for the separation and determination of aluminum has been developed and applied to uranium-based fuels, stainless steel, and various other materials. The procedure is based on the extraction of aluminum with N-benzoyl-N-phenylhydroxylamine (BPHA) into benzene from an ammonium carbonate solution containing several masking agents. Aluminum is back-extracted into 0.20M HCI, complexed with 8-quinolino1, and the colored complex extracted into chloroform and measured at 390 nm. Of 50 metallic elements tested none gave serious interference. Milligram quantities of oxalate, tartrate, phosphate, sulfate, cyanide, and other common anions do not interfere; citrate, fluoride, and EDTA do interfere. Beer’s law is obeyed from 0-50 pg AI in 10 ml of chloroform; the optimum range i s 5-30 rg AI. At the 0.01% level in uranium alloy and the 0.05% level in stainless steel, the relative precision of the method is *2% at the 2~ level. The specificity of the method makes it applicable to many types of samples.
INGENERAL, spectrophotometric determinations of aluminum are based on measuring the color formed by aluminum with various lake-forming reagents or 8-quinolinol. The nonselectivity of these chromogenic reagents for aluminum makes several separations necessary to remove interfering ions. Direct masking of interfering ions aids in reducing the number of preliminary separations required to determine aluminum with unspecific chromogenic reagents. Lake-forming chromogens require exacting conditions and, like 8-quinolinol, are subject to interfering ions even after addition of masking 1420
ANALYTICAL CHEMISTRY
agents. Ion-exchange separation of aluminum is slow and after separation, lengthy treatment of samples is required to establish the optimum conditions for a spectrophotometric procedure. Several methods for the determination of aluminum have been summarized by Sandell (I). Reactor fuels used in Experimental Breeder Reactor I1 (EBR-11) are uranium-metal alloys which may contain all the high-yield fission products along with common metallic impurities. To study the effect of trace impurities on reactor fuel characteristics, the determination of low-level aluminum in various fuels was necessary. Several methods were tried for the determination of microgram quantities of aluminum in fuel containing uranium alloying metals, fission products, and common impurities. The often erratic results were probably due to (a) incomplete separation of interfering ions, (b) interference of unknown ions, (c) aluminum contamination in different reagents, and (d) lengthy chemical procedures. Although several procedures have been reported for the determination of aluminum in specific alloys (2-8), no satis(1) E. B. Sandell, “Colorimetric Determination of Traces of Metals,” 3rd ed, Interscience, New York, N. Y., 1959, p 219-253. (2) W. Sprain and C. V. Banks, Anal. Chim. Acta, 6 , 363 (1952). (3) A. Claassen, L. Bastings, and J. Visser, ibid., 10, 373 (1954). (4) F. J. Miner, R. P. Degrazio, C. R. Forrey and T. C. Jones, ibid., 22, 214 (1960). ( 5 ) D. W. Margerurn, W. Sprain and C. V. Banks, ANAL.CHEM., 25, 249 (1953). (6) R. J. Hynek and L. J. Wrangell, ibid., 28, 1521 (1956). (7) H. B. Evans and Hiroshi Hashitani, ibid., 36, 2032 (1964). (8) R. T. Oliver and E. P. Cox, ibid., 41, lOlR (1969).
factory method was found for EBR-I1 fuel. The present method was then developed and has been successfully applied to the determination of aluminum at the 0.005% level in uranium-metal alloys. N-Benzoyl-N-phenylhydroxylamine (BPHA) was introduced as a reagent for the precipitation of aluminum at pH 4-6 by Shome (9). Although BPHA is not specific for aluminum, by use of a carbonate medium and by addition of various masking agents, a procedure was developed which is highly selective for aluminum. The specificity of the separation should make it widely applicable for the determination of aluminum. In this method aluminum is extracted with BPHA into benzene from an ammonium carbonate solution containing thioglycolic acid, hexametaphosphate, cyanide, and hydrogen peroxide as masking agents. The aluminum is back-extracted from the benzene into 0.20M HC1. The separated aluminum is complexed with 8-quinolinol in an ammonium carbonate solution, and the colored complex is extracted into chloroform and measured at 390 nm. Under the conditions of the procedure, Beer's law is obeyed over the range 0-50 pg in 10 ml of CHC13with an optimum range of 5-30 pg. Common anions found to interfere were fluoride, citrate, and EDTA. Of 50 metallic elements tested, no serious interference in the procedure was found. EXPERIMENTAL
Equipment. Absorbance measurements were made with a Beckman Model B spectrophotometer in matched 1-cm silica cells against deionized water. Conditions. All experiments and measurements were conducted at room temperature (25 "C). Reagents. All chemicals used were analytical reagent grade. Deionized water was used throughout the experiment. 2 z BPHA. Dissolve 2 grams of N-phenylbenzohydroxamic acid (Eastman Organic Chemicals, Catalog No. 7297) in 100 ml of either ethanol or acetone. This solution is stable for at least one month if kept in the refrigerator. 80 THIOGLYCOLIC ACID (TGA). Eastman Organic Chemicals, Catalog No. 2249. AMMONIUM CARBONATE SOLUTION.20 solution, stored in a polyethylene container. AMMONIUM CARBONATE-SODIUM HEXAMETAPHOSPHATE BUFFER. Dissolve 5 grams of sodium metaphosphate (NaPO& and ca. 200 grams of ammonium carbonate in about 900 ml of water in a 1-liter volumetric flask. Transfer the buffer solution into a 1-liter separatory funnel and extract three times with a 2 z solution of 8-quinolinol in CHC13. Discard CHC13 extracts and wash the buffer solution twice with CHCls. Store the buffer solution in a polyethylene container. 1 . 2 z HZOL Dilute 4 ml of 3 0 x HtOz solution to 100 ml with water. KCN. 5 % in water. ~-QUINOLINOL: 2 z in ethanol. ALUMINUM (1 mg/ml): Dissolve 1 gram of high-purity aluminum wire in 50 ml of 6M HC1 and dilute to 1-liter with 1M HCl. Sample Preparation. Samples may be dissolved by any appropriate technique; however, borosilicate glassware contains aluminum which may be leached by H F or alkaline and acid fusion dissolutions. Analytical reagent grade acids also contain varying amounts of aluminum contamination and an awareness of the level of contamination or selection of acids which have been repurified of trace metals is necessary. Analytical Procedure. Pipet an acidified sample containing 5-30 pg A1 into a 60-ml separatory funnel and add 1 ml of
z
z
(9) S. C . Shome, Analyst, 75, 27 (1950).
80% TGA. The order of addition of reagents is important and should be followed. Add ammonium carbonatesodium hexametaphosphate buffer until the solution is basic (add 1-5 ml excess) and add 1 ml of KCN and 1 ml of 1.2 H202. Dilute the sample to ca. 30 ml with water, add 1 ml of 2 % BPHA, mix the solution, and allow to stand 5 min. Extract the A1-BPHA complex into 15-20 ml of benzene by shaking for 1 mih. Allow the phases to separate, and drain off and discard the aqueous phase. Wash the organic phase by shaking with 10-15 ml of water. Drain and discard the wash water. Back-extract the aluminum by shaking the benzene phase with 15-20 ml of 0.20M HCl for 1 min. Drain the 0.20M HC1 into a clean separatory funnel, dilute to ca. 30 ml with water, and add 1 ml of 2% 8-quinolinol. Make the solution basic with the ammonium carbonate solution and extract the Al-8-quinolinol complex by shaking with 10 ml of CHC13 for 1 min. Suspended water droplets in the CHCl, may be removed by centrifuging the CHC13 phase or by passing the CHC13 through clean surgical cotton. Measure the color intensity in a 1-cm cell at 390 nm. Run blanks and standards with samples as described below. RESULTS AND DISCUSSION
A1-BPHA Extraction-Back-Extraction. Aluminum is quantitatively precipitated by BPHA from pH 3.6 to 10 but BPHA is more selective in basic solution. A pH of 8-9.5 is readily achieved with ammonia carbonate which also forms strong soluble complexes with many elements. The AlBPHA complex is extracted into benzene within l min. The benzene is washed with water to remove any entrained sample or residual sample on the walls of the separatory funnel. Aluminum is easily back-extracted within 1 min from the benzene phase at any acid concentration greater than 0.05M HCl. An acid concentration of 0.20M HC1 was selected since many metals that react with BPHA in the absence of masking agents--e.g., Fe, Zr, Mo-are not back-extracted at this acidity. The 0.20M HCl solution is easily neutralized and made basic with less than 2 ml of ammonium carbonate solution. Effect of Other Ions-Masking. In general, metal complexes have higher stability constants in basic solutions. The specificity of this method was possible because metals that are precipitated by BPHA could be selectively masked in the basic ammonium carbonate solution. The interference of various ions was determined by adding known quantities of test ion to aluminum standards and blanks and noting the effect on the final absorbance. Many analytical reagent grade chemicals have aluminum contamination and had to be purified for these tests. One hundred and fifty mg of Ue+ did not interfere in the method. Ten mg each of Ni2+,Sb5+, Sr, Cr6+, Na, MOB+,Th, Li, Fe3+,K, Pbzf, Mg, Cs; one mg each of In, Be, Cd, Sn4+,Mn7+,Rh, Rua+, Zr, Pt4+,Ba, La, Y ,Pd, Ce4+,Ca, Coz+,V5+,Re7+,Zn, CuZf, Nb5+, Bis+, mixed rare earth elements; and 0.5 mg G a gave no interference in the method. Most of the test ions added in their higher oxidation state were immediately reduced by TGA. Greater quantities of most of these elements were not investigated for interference. Most of the test ions did interfere in the absence of the recommended masking agents. Interference due to oxidation of BPHA by strong oxidantse.g., Mn04-, Cr2072-,Ce4+, etc.- was not a problem because they were rapidly reduced by TGA. Thioglycolic acid forms a precipitate with Cu but completely redissolves on addition of ammonium carbonate. Thioglycolic acid complexes many metals including In, Pb, VOL. 41, NO. 11, SEPTEMBER 1969
1421
Bi, Sb, Fe, Cr, Zr, Co, Sn, Cd, and Ga. Cyanide forms complexes with most of the transition elements and is especially useful for complexing Fe as the ferrocyanide complex after it has been reduced by TGA. Hydrogen peroxide forms complexes with Ti, V, Cr, and their congeners. Hexametaphosphate prevents precipitation of Sc, Y,and the rare earth elements by ammonium carbonate and also prevents precipitation of thorium peroxide after addition of H202. Ammonium carbonate serves as a complexing agent for uranium, for other actinide elements, and for some transition metals. The order of addition of masking agents is important and must be followed because some masking agents are added to prevent precipitation of elements by other masking agents-e.g., U is precipitated by hexametaphosphate in the absence of carbonate. Up to 100 mg of formate, acetate, oxalate, borate, phosphate, perchlorate, nitrate, sulfate, cyanide, bromide, iodide, and triethanolamine, and 50 mg of tartrate, gave no interference with the method. Fifteen milligrams of HzOz did not interfere but quantities greater than 20 mg gave negative interference. Because H202in a basic medium oxidizes some of the BPHA and prevents formation of the Al-BPHA complex, the specified amount of BPHA (20 mg) must be added to each sample. Fluoride, EDTA, and citrate interfered by masking the aluminum. Five milligrams of fluoride gave about 10% negative interference when polyethylene separatory funnels were used in the procedure. The presence of fluoride in samples contained in borosilicate glass gave high results because of the aluminum leached from the glass walls. Color Development. The AI-8-quinolinol complex forms rapidly at pH 9 in the ammonium carbonate solution and is extracted into CHCI, within a 1-min shaking time. The stability of the colored complex is 1 hr when the CHCl, is drained directly into a 1-cm cell, 2 hr when drained through clean surgical cotton, and 3.5 hr when the CHC13 is first centrifuged and then transferred to a 1-cm cell. The A1-8quinolinol color was measured at 390 nm, the peak wavelength of the absorption spectrum determined under the conditions of the procedure. Sensitivity. The absorptivity of the Al-8-quinolinol complex measured under the conditions of the procedure at 390 nm was calculated from a Beer’s law plot to be 262 liters/g-cm. The corresponding molar absorptivity (ionic molar absorptivity) is 6800, and Sandell‘s sensitivity index is 0.004 y/cm2. The colored species obeys Beer’s law over the range 0-50 fig A1 in 10 ml of CHC1,. The optimum concentration range is 5-30 pg A1 as taken from a Ringbom plot. AI-BPHA Complex, The composition of the AI-BPHA complex was determined from both a continuous variations plot and a mole ratio plot to have an AI-to-BPHA ratio of
1422
e
ANALYTICAL CHEMISTRY
1 :3, in agreement with Shome (9),who determined the composition of the dried Al(BPHA)3 precipitate. Precision. The relative precision of the method at the 15-pg level for a standard aluminum solutlon was Zk2x at the 2a level. For the determination of aluminum at the 0.01 % level in uranium alloy and the 0.05 Z level in stainless steel, the relative precision of the method at the 20 level was Zk2Z. In the analysis of stainless steel for AI at levels less than 0.01 Z, the quantity of Fe in the sample limits the precision of the method because of incomplete masking with TGA-KCN. Discussion. The method described is suited for the determination of microgram quantities of aluminum in uranium alloys, stainless steel, and various other materiaIs. The unique feature of the described method is the highly selective separation of aluminum with BPHA, while the selection of 8-quinolinol as a chromogenic reagent was merely due to its suitability. The determination of aluminum after it has been separated into the 0.20M HCl may be completed by addition of more sensitive or less sensitive chromogenic reagents. Fluorescence methods might easily follow the BPHA separation for the determination of submicrogram quantities of aluminum. The highly selective separation achieved by this method should make it widely applicable for the determination of aluminum. ACKNOWLEDGMENT
The authors thank Ronald DiFelici for preliminary work
on the procedure and Earl Ebersole for his technical assistance and help in preparing the report. RECENED for review May 5 , 1969. Accepted June 26, 1969. Work performed under auspices of U. S. Atomic Energy Commission.
Correction Preparative Thin-Layer Chromatography a.nd High Resolution Mass Spectrometry of Crude Oil Carboxylic Acids In this article by Wolfgang K. Seifert and Richard M. Teeter [ANAL.CHEM.,41, 786 (1969)l on page 786, column 2, the final sentence should begin “Zone 2 (Table I and. . . . . . .” On page 794, Table VII-A, footnote 6 should read “To skeletal structures found in petroleum.” The final sentence of the manuscript, page 795, should read “Presented in part at the Gordon Research Conference on Geochemistry, New Hampshire, August 1968.”
