Table Ill. Spectrophotometric Determination of Uranium in Presence of Various Organic Bases Organic case, Absorbance, (0.2M) pK 67 pg. U Triethanolamine 3 75 0 655 Aniline 9 42 0 560 Ethylenediamine 4 07 0 490 Pvridine 8 77 0 530 nITributylamine 3 58 0 660 Quinoline 9 20 0 330 KOH 2.0
analysis a t this laboratory. None of the cations tested interfered. Absorbance Curves of t h e Complex. Cranium forms a red complex ion with 1-(2-pyridylaso)-2-naphthol in basic ethanol solution. T h e absorbancy curve. of a solution containing 6.44 p.p.m. of uranium against a reagent blank, and of a reagent blank against ethanol, are sho\+n in Figure 1. The maximuni absorbance of the complex is 555 mp where the reagents do not have an appreciable absorbance. The absorbancy curve for a solution containing 1 1i.p.m. of plutonium and P A S is also shown in Figure 1. The maximum absorbance of the plutonium complex ion occurs a t 520 mp. This maximum can be used to indicate if the uranium was separated quantitatively from the plutonium. Effect of pH. Cheng ( 2 ) made a study of the efTect of p H on the UP A S complex in aqueous solution and found that a maximum absorbancy occurs a t p H 10. Table I11 shows the absorbancy of the PAX complex with 6.4 p.p.m. of uranium when various bases are used t o develop the color.
Addendum.
Tributylamine and triethanolamine both give approximate p H values of 10 in ethanol and a maximum color intensity. Some organic bases, although giving the proper pH values, inhibit a maximum color development. This may be due to competition with the PA?; for the formation of uranium complexes. Effect of Concentrations of Reagents. The absorbance of the uranium complex ion is independent of the volume ratio of ethanol to cyclohexane above 1 : 1. -1 triethanolamine concentration greater than 0.5M inhibits color development. Although this effect is small, the triethanolamine concentration should be constant in the solutions. A value of 0.15.11 was used in these experiments. Order of Mixing. The triethanolamine must not be added to a uranium-cyclohexane-TOP0 mixture unless P.1N is present. The color does not develop to its maximum intensity unless the P A 9 is added prior to the triethanolamine. Effect of Time. The color will develop a t room temperature in 15 minutes, if the triethanolamine is freshly prepared. T h e color is stable from 15 to 60 minutes after mixing. If the triethanolamine solution is several days old, the color does not develop with as high an intensity as that of fresh material. Therefore, it is best to prepare the triethanolamine solution on the day it is used. Adherence to Beer’s Law. A series of solutions containing varying amounts of uranium, 2 ml. of triethanolamine buffer, 1 ml. of P A N , and sufficient ethanol to give a total volume of 10 ml. were prepared. The absorbances of these solutions mere
measured a t 555 mp against reagent blanks and plotted against uranium concentrations. The results show that Beer’s law is followed between 1 and 20 p.p.m. of uranium. The relative standard deviation in duplicate measurements was 2%. The molar absorbancy index measured from the standard calibration curve was 22,000. Cheng found the value in aqueous solution to be 23,000 ( 2 ) . The method was specifically designed for the separation and determination of uranium in plutonium. However, in view of the fact that no interfering cations were found, the method can be applied to a wide variety of mixtures containing traces of uranium. ACKNOWLEDGMENT
The author thanks P. L. Redding for his technical assistance in development of this procedure. LITERATURE CITED
(1) Booman, G. L., Rein, J. E., “Treatise on Analytical Chemistry,” Uranium, Part 11. 1-01, 9. I. 51. Kolthoff and P. J. Elving,’ eds., pp. 1-164, Wiley, New York, 1962. ( 2 ) Cheng, K. L., ANAL.CHEM.30, 1027 (1958). (3) Rodden, D. J., Ibid., 25, 1598 (1953). (4) White, J. C., Ross, W. J., “Separation bv Solvent Extraction with Tri-noctylphosphine Oxide,” NAS-NS 3102, February 1961.
J. BALTISBERGER RICHARD Monsanto Research Corp. Mound Laboratory’ LLIiamisburg, Ohio l Mound Laboratory is operated by Monsanto Research Corp. for the U. S. Atomic Energy Commission under Contract No. AT-33-1-GEN-53.
Compositional Analysis of n Component Systems
by the X-Ray Absorption Method SIR: I t has come to my attention in a private conversation with K. Heinrich, Du Pont Experimental Station, Wilmington, Del., that in the first paragraph of my paper, “The Compositional Analysis of n Component Systems by X-Ray Absorption Method,” -INALYTICAL CHEMISTRY, 36, p. 1857, August 1964, there is a sentence which needs clarification. I n the first para-
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graph, I state, “The wavelengths can be chosen completely a t random; with knowledge of the element,s present, however, more accurate results can be obtained by a judicious choice of the n wavelengths.” Although true theoretically, this was not meant to imply that in a practical case n wavelengths should be chosen indiscriminately because in the measurement of absorption coef-
ficients sufficient precision is not attainable. One should try to pick the wavelengths such that as many absorption edges are straddled as possible. Such a judicious choice will lead to the best practical results. ROBERT LEFKER U. S. Army Electronics Command Fort hlonmouth, N.J.
