Anal. Chem. 1985,
563
57. 563-564
of the uranyl complex are present. From the comparison of the mean value of the peak area, obtained from five extractions from the same nitric solution, with those of the calibration curve, a concentration of 650 30 ppm in the original sample is deduced. These preliminary results, even though are to be confirmed on a wider range of real samples, indicate that the proposed method could be effective and highly selective in the determination of uranium at parts per million concentration levels even in the complex matrices. Registry No. H,DIB, 73818-26-5;UO,DIB, 74091-79-5;CuDIR, 86305-88-6;U, 7440-61-1.
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Flgure 3. Chromatogram of the solution obtained by extraction with H,DIB in dichloromethane of a nitric solution of a uranium-containing ore; column LiChrosorb RP2, mobile phase methanol-water 65/35; detection UV, A = 345 nm; 10-pL injection.
arbitrary units and x is the nanograms injected; r = 0.9996 for n = 8. The relative standard deviation of the slope is 1.55%. This curve was compared with that obtained when starting from aqueous solutions of uranium in the same range of concentration (1-10 ppm) and extracting them with the same volume of dichloromethane solutions of ligand. A stoichiometric ratio of ligand to uranium of about 5 to 1 is maintained in the extraction. This ratio does not cause interference in the chromatographic separation, because of the high separation between ligand complex and because of the low absorptivity of the ligand a t 345 nm. Under these conditions the experimental data fit the equation, expressed in the same units as the previous one, y = 2.97 X 102x- 4.52 X lo2;r = 0.9995, relative standard deviation of the slope 2.1 % for n = 8. A t test was carried out on the slopes of the two curves (22)to verify if the dependence of the detector response on the quantities injected was significantly different following the two procedures. The value of the slope of the curve obtained by starting from dichloromethane solutions of the complex was assumed as the "true" value. This test indicates that the slopes of the two curves are not significantly different a t 95% confidence level. T o verify the effects of the presence of a complex matrix, the method was applied to the determination of the uranium content in a reference ore (SY-3, GRS from Geological Survey of Canada, certified concentration 760 A 38 ppm). The nitric solution (pH 2) of the reference sample, corresponding to 1 g of sample in 100 cm3, was used for the extraction. A 2-cm3 aliquot of this solution was extracted with an equal volume of dichloromethane solution of the ligand M) by equilibrating the two phases for a few minutes using a Vortex stirrer; 10 p L of the organic phase was injected. Figure 3 shows the corresponding chromatogram obtained using a methanol-water mixture 65/35 (v/v) as the mobile phase and UV detection at 345 nm. In spite of the complexity of the matrix, under these conditions only the peaks of the free ligand and
LITERATURE C I T E D "Gmelin Handbook of Inorganic Chemistry"; Springer-VerLag: Berlin, 1982: Uranium Supplement Volume A7. Aznarez, J.; Paiacios, F.; Vidal, J. C. Analyst (London) 1983, 108, 1392. Uesugi, K.; Nagahiro, T.; Mlyawaki, M. Anal. Chin?. Acta 1983, 148.
315. Ohshita, K.; Wada. H.; Nakagawa, G. Anal. Chim. Acta 1983, 149. 269. Kenney-Wallace, G. A.; Wilson, J. P.; Farrell, J. F.; Gupta. B. K. Talanta 1981, 28, 107. Zhi-Lin, W.; Chi-Ke, C.; Xia-Nian, L.; Fu-Xin, T.: Xun-Xi, P. Anal. Chlm. Acta 1984, 160, 295. Lynch, T. P.; Taylor, A. F.; Wilson, J. N. Analyst (London) 1983, 108, 470. Sielfwerbrand-Lindh, C.; Nord, L.; Danielsson, L. G.; Ingman, F. Anal. Chlm. Acta 1984, 160, 11. Johri, K. N.; Bakshi. K. Chromatographia 1972, 5 ,309. Qureshi, M.; Sethi, 8. M.; Sharma, S. D. Sep. Sci. Technol. 1980, 15. 1685. Upadhyay, R. K., Tewari. A. P. J. Indian Chem. SOC. 1979, 56,972. Upadhyay, R. K.; Tewari, A. P. Sep. Scl. Technol. 1980, 15, 1793. Srivastava, S. P.; Dua, V. K.: Gupta, V. K. 2. Anal. Chem. 1977, 286,255. Srivastava, S. P.; Dua. V. K.; Gupta, V. K . Anal. Lett. 1979, 12,169. Srivastava, S. P.; Dua, V. K.; Gupta, V. K. Z. Anal. Chem 1978, 292,415. Mangia, A.; Pelizzi. C.; Pelizzi, G. Acta Ctystallogr., Sect. B 1974, 8 3 0 . 2146. Paolucci. G.; Marangoni, G.; Bandoli, G.; Clemente, D. A. J . Chem. SOC., Dalton Trans. 1980, 1304. Lorenzini, C.; Pelizzi, C.; Pelizzi, G.; Predieri, G. J. Chem. Soc., Dalton Trans. 1983, 721. Casoli, A.; Mangia, A,; Predieri, G. Anal. Chlm, Acta, submitted for publication. Palla. G.; Mangia. A,; Predieri, G. Ann. Chlm. (Rome) 1984, 74, 153. Marangoni, G.; Paoiucci, G. J. Chem. Soc., Datton Trans. 1981, 357. Mack, C. "Essential of Statistics"; Plenum Press: New York, 1967; p 142.
Antonella Casoli Alessandro Mangia* Giovanni Predieri Istituto di Chimica Generale ed Inorganica Universiti di Parma Via M. D'Azeglio 85 43100 Parma, Italy
RECEIVED for review August 6, 1984. Accepted October 12, 1984. This work was financially supported by CNR (Italy) Grant 83.00233.03.
Flow-Injection System for the Rapid and Sensitive Assay of Concentrated Aqueous Solutions of Strong Acids and Bases Sir: We frequently use a flow-injection system when optimizing the electrode potential wave forms for pulsed-amperometric detection (PAD) of carbohydrates and amino acids in liquid chromatography (1-3). For cases where a substantial background signal is present due to formation of surface oxide
on a Pt electrode, i.e., large positive value of detection potential, we have noted the necessity of exactly matching the concentration of NaOH in the samples and the carrier stream to eliminate blank peaks. The exact match of pH is not critical for application of PAD to liquid chromatography because the
0003-2700/85/0357-0563$01.50/00 1985 American Chemical Society
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Figure 1. Detection peaks for injection of alkaline solutions in 0.251 M NaOH: conditions, v , = 1.0 mL min-’, V , = 50 pL; wave form, E, = 550 mV vs. SSCE (225 ms), E, = -1300 mV (200 ms), E, = -200 mV (200 ms); pCNaOH, (A) = 0.900, (B) = 0.700, (C) = 0.600, (D) = 0.500, (E) = 0.300; carrier stream: pCMOH= 0.600.
solvent peak is separated from the analyte peaks. The decay of anodic current with time (i-t) corresponding to oxide formation following a positive change in applied potential is adequately described for Pt electrodes by ( 4 ) i = cq/t where c is a constant and 7 is the applied overpotential for oxide formation. In the absence of surface active species, 7 a t a constant applied potential ( E ) is a function of the hydrogen ion activity (aH)as described by q =
E - E,
+ 0.0591paH
where E, is a constant and p -log,,. Hence, for the evaluation of i a t a fixed t , a plot of i vs. paH is predicted to have a slope of 0.0591clt. It is noteworthy that amplification of the slope can occur by selection of a very small value for t. We estimate that a useful application of this phenomenon is for frequent assay, by the flow-injection technique, of production streams of pure acids and bases, especially for highly concentrated strong acids and bases for which the H+-selective glass electrode is lacking somewhat in sensitivity. To illustrate, typical PAD peaks are shown in Figure 1 for eight successive injections of 50-pL aqueous samples of NaOH into a carrier reference stream of 0.251 M NaOH. The apparatus consisted of the UEM PAD, with Pt flow-through cell, the CMA-1 chromatographic module, with column removed, and the PMA-1 pumping unit (Dionex Corp., Sunnyvale, CA). The flow rate (uf) was 1.0 mL min-’. Values of the potential and duration, Le., E ( t ) ,for each step in the triple step wave form are given in the figure caption. Measurement of i was made during the last 50 ms of the time period for El. El is selected to correspond with the rapid formation of the lower oxide (Le., “PtOH”) on the detector electrode. Injection of eight samples of the reference solution into the reference stream produced no peaks (central portion of Figure 1). A plot of anodic peak height (Ai) vs. PCbaeeis shown in Figure 2. The value of Ai = 0 corresponding to the injection of the
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Figure 2. Calibration curves for alkaline (A) and acidic (B) samples: conditions, vt = 1 .O mL min-’, V , = 50 kL; wave form, (A) as in Figure 1, (B) E , = 1400 mV vs. SSCE (500 ms), E , = -250 mV (150 ms), E, = -150 mV (250 ms).
reference solution, pcb = pCb,rebis indicated by the arrow. The observed linearity (slope = 133, sXy= 1.0 at 90% confidence) did not extend to values of ApCbaBemuch beyond one-half decade, probably because of activity effects. We point out, however, that the application for quality control is most likely for the purpose of noting the occurrence of a small difference in concentration between the reference and injected solutions rather than for the quantitative determination of that difference over an extended range of concentration. We note also, as an alternate procedure, that the sample solution can be pumped as the carrier stream with occasional injections of standard reference solution. The technique is equally applicable for concentrated solutions of strong acids and data are shown for H2S04in Figure 2 using 1.0 M as the concentration of the carrier reference stream. The linearity of the plot for H2S04(slope = 72.9, sXy = 4.7 at 90% confidence) is not nearly so great as that for NaOH, undoubtedly because pCacid# paH due to the second ionizable proton of H2S04. We estimate the detection limits ( S I N = 2) to be approximately dpC = 0.005 for the two cases illustrated. Registry No. NaOH, 1310-73-2;H2S04,7664-93-9. LITERATURE CITED (1) Hughes, S.;Johnson, D. C. Anal. Chim. Acta 1981, 132, 11-22. (2) Polta, J. A,: Johnson, D. C. J. Liq. Chromatogr. 1983, 6. 1727-1743. (3) Austin, D. S.;Polta, J. A,; Polta, T. Z.; Tang, A. P.-C.: Cabelka, T. D.; Johnson, D. C. J. Electroanal. Chem. 1984, 168, 227-248. (4) Gilroy, D. J . Eleotroanal. Chem. 1976, 7 1 , 257-277.
John A. Polta In-Hyeong Yeo Dennis C. Johnson*
Department of Chemistry Iowa State University Ames. Iowa 50011
RECEIVED for review August 20, 1984. Accepted November 5 , 1984.
