A N A L Y T I C A L CHEMISTRY, VOL. 50, NO. 3, MARCH 1978
tubes are suitable for general electrothermal atomic absorption analysis of trace elements in various matrices. They failed, however, in the determination of platinum group metals by effectively suppressing their atomization. While the tubes may not be suitable for a tantalum determination, a similar treatment with the second highest boiling niobium carbide, NbC. may make it possible. T h e developed procedure is simple and inexpensive. It uniformly affects the whole surface of the graphite tube. The extent of the carbide formation can be easily controlled by the number of sequential treatments and by the concentration of the tantalum solution.
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J. H. Runnels, R. Merryfield, and H. 8. Fisher, Ana;. Cbem., 47, 1258 (1975). K. C Thompson. K. Wagstaff, and K. C. Wheatstone, Ana/yst(London), 102, 310 (1977). R. Cioni, G. Ottonello, and A. Mazzucotelli, Anal. C,+im.Acta, 82, 415 (1976). D. D. Siemer, R. Woodriff, and B. Watne, Appi. Specfrosc., 28. 582 (1974). S. A. Clyburn, T. Kantor, and C. Veillon, Anal. Cbem., 46, 2213 (1974). K . C. Thompson, R. G. Godden, and D. R . Thomerson Anal. Cbim. Acta, 74, 289 (1975). D. C. Manning and R. D. Ediger, At. Absorpt. News/., 15, 42 (1976). H. M. Ortner and E. Kantuscher, Talanfa, 22, 581 (1975). T. Stiefel, K. Schulze, G. Tolg, and H. Zorn, Anal. (:him. Acta, 87, 67 (1976). A. K Holliday. G. Hughes, and S. M. Walker "Carbon" in 'Comprehensive Inorganic Chemistry", Vol. 1, J. C.Bailar, Jr., H. J. Emel6us, R. Nyholm, and A. F. TrotmanDickenson, Ed., Pergamon Press, Oxford. 1973, p 1211.
LITERATURE CITED (1) E L Henn, Anal Cbem , 47, 428 (1975) (2) E L Henn ASTM STP 618, 1977, p 54
RECEIVED for review October 3. 1977. Accepted November 23, 1977.
Contamination-Free Adjustment of pH during Trace Analysis J. E. Riley, Jr. Bell Laboratories, Murray Hill, New Jersey 07974
Determinations of trace elements in high purity materials often require dissolution of the sample followed by adjustment of the p H prior to chemical separations or preconcentration. These adjustment steps are sources of contamination even when efforts are made to use high-purity reagents (1-3). Very high purity reagents are produced by a number of excellent nonboiling distillation procedures (1-6). However, assuming reagents of acceptable quality are produced, storage for periods of time can result in contamination of the reagent by even extensively precleaned storage vessels ( 2 ) . Circumventing the problem of reagent contamination during storage can be accomplished by using gaseous reagents generated when needed for trace analyses. Gaseous acids and bases have been used occasionally in industrial applications (7, 8). Use of such reagents in t h e trace analysis laboratory could be more extensive (9). This paper reports a study of the application of gaseous reagents to trace analysis with data to exhibit the utility and advantages.
EXPERIMENTAL Equipment. Important considerations in the design of the apparatus are: (1) total amount of reagent to be transferred, (2) accuracy and precision of transfer, (3) speed of reagent transfer, and (4) number of samples to be treated. In the majority of trace analytical work in our laboratory, a relatively small number of valuable samples are processed. Precise control of pH in small volumes of solution (2-5 mL) is essential for quantitative recovery of trace elements during chemical separations. The apparatus in Figure 1 is basically an isopiestic distillation system. The container was a cut-off 2000-mL beaker placed on a glass plate with multiple samples accommodated around a single reservoir. Isopiestic distillation will proceed until stopped or until the volatile reagent has attained an equilibrium distribution in all solutions. Initially, for control, an extra sample solution with indicator was used in one of the beakers: but after several runs the transfer rate was characterized for the particular experimental arrangement and the indicator was omitted. To increase the speed of distillation, a small disk of aluminum with a radially bored hole for a small cartridge heater was placed under the reservoir. To minimize leaching by the sample solutions, only the reservoir and not the entire apparatus was heated. When used, this heater did little more than supply the heat of vaporization to the reagent reservoir. The reagent in the reservoir was not boiled since violent bubbling would transfer droplets to the open sample beakers. The gas stream apparatus (Figure 2) processed single samples quickly with exact control of pH. The rate of reagent generation 0003-2700/78/0350-0541$01 OO/O
was controlled by the temperature of the reservoir. There were two reagent delivery modes in this system, one for rapid transfer with coarse control (left side) and one "diluting" system for exact control of the final pH adjustments (right side). For the higher rate of transfer, the "wand" portion of the generator directed the output to the surface of the sample solution. Although transfer of reagent is more efficient and the sample is stirred by bubbling from the submerged tip of the capillary, this c o n i x t should be avoided unless great care is taken in choosing and cleaning the capillary in order to minimize leaching impurities into the sample. Exact control of the pH was achieved by metering into the sample small amounts of the reagent or reagent and pure carrier gas mixtures with a syringe. To avoid drawing sample solution into the capillary due t o a low flow of' highly soluble gaseous reagent, a constant stream of pure, filtered carrier gas (SP or He at
