Determination of Aluminum by Potentiometric Titration with Fluoride Elizabeth W. Baumann Sacannnh River Laboratory, E. I . du Pont de Nemours and Co., Aiken, S. C. 29801
BECAUSE ALUMINUM is widely used as a component of nuclear reactor fuel assemblies, aluminum must sometimes be determined in highly radioactive solutions. Such determinations require small samples and simple procedures that can be used in shielded facilities with restricted visibility. We needed a method to determine microgram quantities of aluminum in the highly radioactive feed solution for the Tramex process for recovering transplutonium elements ( I ) . The usual procedure of back-titrating excess EDTA with a visual colorimetric indicator (2) is difficult in a shielded or remote location because the end point color change is difficult to detect. A method was developed to determine aluminum remotely by potentiometric titration using a technique similar to that for determination of lithium (3). The aluminum is titrated with fluoride in ethanol using a lanthanum fluoride electrode (4). In the presence of sodium ion, insoluble cryolite (Na8AlFJ or a similar compound is formed. The ratio of six fluorides to one aluminum favors determination of small quantities of aluminum.
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Figure 1. Curve for titration of aluminum with fluoride in ethanol (Solution: 50 pl of 0.25M A1 (NO&, 0.5 ml of 1M NaOH, 0.5 ml o f lMHNOa,15 ml of ethanol, 1 ml of pH5 buffer)
EXPERIMENTAL
Equipment. An expanded scale pH meter was used with a saturated calomel reference electrode and a fluoride-selective electrode (Model 94-09, Orion Research Inc., Cambridge, Mass.). A 24-11microburet with a micrometer control and a precision “Teflon” (Du Pont) plunger (Cat. No. 7846, ColeParmer Instrument and Equipment Co., Chicago, Ill.) was modified by replacing the glass reservoir with one of polyethylene or Teflon so the NaF titrant did not contact glass. Reagents. Reagent-grade chemicals were used. Standard 0.1M NaF solution was prepared determinately from NaF that was dried several hours at 120 “C. The pH 5 buffer solution consisted of 60 ml of glacial acetic acid and 270 g of NaC?Ha02.3H20diluted to one liter in deionized water. (1) H. J. Groh, C. S. Schlea, J. A. Smith, R. T. Huntoon, and F. H. Springer, Nuclenr Applicatio/rs, 1 (4), 323 (1965). (2) L. Meites, “Handbook of Analytical Chemistry,” McGrawHill, New York, 1963, pp 3-76 ff. (3) E. W. Baumann, ANAL.CHEM.,40, 1731 (1968). (4) M. S . Frant and J. W. Ross, Jr., Science, 154, 1553 (1966).
Procedure. When the sample contained interfering metal ions, they were precipitated in 1 M NaOH and separated from the soluble NaA102 by filtration or centrifugation. In the fluoride titration, 20 mV for a 20941 addition of 0.1MNaF. Table I compares determinations by the fluoride and EDTA procedures in a solution of aluminum nitrate and in a simulated Tramex process solution. The results represent the average of four or more determinations. Bias between the two methods is within the precision of the determination. The relative standard deviation is 1 to 2 % for both methods at 10 micromoles of Al(II1). Table I1 compares determinations in Tramex process solutions. General agreement between the methods is obtained for the six samples. The presence of radioactivity does not affect the fluoride titration. Successful potentiometric titration of aluminum with fluoride depends upon removal of other ions that form complexes or insoluble compounds with fluoride. In the present application, where aluminum is a major constituent, NaOH precipitation is practical because the aluminum occluded on the metal hydroxide precipitate is negligible. However, other mixtures may require different purification procedures to avoid losses of aluminum. The method is based on the postulated formation of an aluminum hexafluoride that is insoluble or undissociated in
1.47, 1.47 1.87, 1.87 1.73, 1.73 1.88 2.03 2.08 a
1.48 1.88 1 S 6 , 1.73 1.90 1.85, 2.06 2.08
Original solution diluted 1 :100 in 1M NaOH; 500-1000
p1
of supernate titrated.
ethanol. The slight solubility of cryolite in water is reduced in ethanol. In aqueous solution, Tananaev and Lel‘chuk (5) found that the solid phase consists of two double salts: A1F3.3NaF and 4A1F3.11NaF. In both of these salts, the F/Al ratio is very close to six. The agreement between the fluoride and the EDTA methods indicates that this value can be used for the equivalence factor. RECEIVED for review July 10, 1969. Accepted October 20, 1969. Information contained in this article was developed during the course of work under Contract AT(07-2)-1 with the U. S. Atomic Energy Commission. (5) I. V. Tananaev and Yu. L. Lel’chuk, Zhur. Anal. Kliim., 2, 93 (1947); C.A., 43, 5695c.
Quinhydrone Electrode Drift George Dahlgren and Melvin J. Goodfriend, Jr. Department of Chemistry, Unioersity of Cincinnati, Cincinnati, Ohio 45221 REPORTS of a slow decay in the potential of quinhydrone OS. silver-silver chloride electrodes are as old as the cell system itself (]). Literature references usually ascribe this emf drift to chlorination of the p-benzoquinone component of quinhydrone ( 2 3 ) . However, others either report no problem with drift (4) or report drift only in cells containing certain buffer ions (5). We have observed a linear emf decay with time in cells containing acetate buffers. Possible sources of the drift such as poor electrode behavior or diffusion of quinhydrone between the half cells were eliminated and the actual source of the problem appears to be a nucleophilic attack by acetate ion on the p-benzoquinone component of quinhydrone. The reaction was followed by changes in the UV spectrum of the p-benzoquinone with time and this rate correlated with changes in the emf of the cells. ~
(1) D. J. G. Ives and G. J. Janz, “Reference Electrodes,” Academic Press, New York, N. Y . , 1961. (2) H. S. Harned and D. D. Wright, J. Amer. Chem. Soc., 55, 4849 (1933). (3) H. S. Hovorka and W. C. Dearing, ibid., 57, 446 (1935). (4) G. Dahlgren and F. A. Long, ibid., 82, 1304 (1960). ( 5 ) K. C. Rule and V. K. LaMer, ibid.,60, 1974 (1938).
EXPERIMENTAL Materials. Materials were the best grade available and were used without further purification except in the case of quinhydrone. Several different recrystallization procedures were used with quinhydrone and all gave identical results. Solutions were prepared from deionized-distilled water which had been freed of dissolved carbon dioxide and oxygen by flushing with pure nitrogen. Apparatus. A semi-micro cell similar to the one used in previous studies ( 4 ) was constructed from borosilicate glass. The silver-silver chloride electrodes were of the thermal type described by Bates (6). However, the finished helices were inserted in rubber plugs which were then sealed in 10/30 borosilicate points with silicone rubber. Electrodes prepared in this way had longer life because the channeling in platinumglass seals observed by many investigators (2) could not occur. Prior to each emf run sets of like electrodes were intercompared and were usually found to be within 10.03 mV of each other, even after two months use. Potentials were measured using a Leeds and Northrup K-3 potentiometer and Model 9834 null detector. (6) R. G. Bates, “Determination of pH, Theory and Practice,” John Wiley and Sons, Inc., New York, N. Y . , 1964, p 281.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
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