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ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
Table 11. Absorbance of 100 ppm Silicon in a 0.77 41 Neutralized Sodium Salt Solution vs. Silicon Absorbance of Same Solution with 1.17 M Excess Acid absorbance
acid used
at neutralization
at 1.17 M excess
nitric hydrochloric
0.229
0.256
0.234
0.240
0.266 0.264 0.266
0.281 0.296 0.333 0.375
formic
acetic propionic butyric
0.277
sorbance. As seen in Table I, a similar effect is observed with other metals. Sensitivity increases of 5-2070 were observed for other metals in organic acid fusionate solution. It is possible that an even greater increase could be obtained because the absorbances for the other metals may or may not have plateaued. The silicon absorbance did plateau with 55 mL (1.94 M) of acetic acid in the fusionate solutions. In a further study of this enhancement effect, solutions of 0.77 M sodium hydroxide, containing 100 ppm silicon, were neutralized with nitric, hydrochloric, formic, acetic, propionic, and butyric acids. The solutions neutralized with the organic acids all showed approximately the same absorbance. The
absorbances of the solutions neutralized with nitric and hydrochloric acid were lower. The same basic solutions were then acidified with a 1.17 M excess of each of the same acids. The silicon absorbance increased with increasing molecular weight of the organic acid. An enhancement effect was also observed with the excess nitric acid, whereas excess hydrochloric acid had no significant effect on sensitivity (Table 11). Surface tensions of the solutions were investigated as a source for the enhancement effect. As the molecular weight of the organic acid increased, surface tension of the solutions decreased. However, when a surfactant, Eastman Kodak, Photo-Flo 200 Solution, was added to the hydrochloric acid solution, its surface tension decreased by 50%; yet it had the same silicon absorbance. Although the flame chemistry of this system is not understood, acidifying basic fusionates with organic acids was shown to be useful for alleviating signal drift and increasing sensitivity.
LITERATURE CITED (1) McHard, J. A,; Servais, P. C.: Clark, H. A. Anal. Chem. 1948, 20, 325. (2) Grove, E. L. "Analytical Emission Spectroscopy", Vol. I, Part 11; Marcel Dekker: New York, 1972: Chapter 7, p 260.
RECEIVEDfor review April 19, 1979. Accepted September 18, 1979.
Modification of a Commercial Micrometer Hanging Mercury Drop Electrode J. E. Bonelli,' H. E. Taylor, and R. K. Skogerboe' U.S. Geological Survey, Box 25046, Denver Federal Center, Denver, Colorado 80225
The hanging mercury drop electrode (HMDE) has found widespread use in electroanalytical chemistry, especially for anodic and cathodic stripping voltammetry. The introduction of the micrometer HMDE in the late 195O's, (ref. 1-5), in which mercury is displaced from a reservoir by a screw driven plunger to form renewed and reproducibly sized electrodes, was a significant advance in convenience and performance. One of the most popular commercial versions of the micrometer HMDE is manufactured by Metrohm AG (Herisau, Switzerland), and is marketed in the United States by Sybron-Brinkmann Instruments Ltd., (Westbury, N.Y. 11590) and by EG&G Princeton Applied Research Corp. (Princeton, N.J. 08540) (Figure 1). The Metrohm E-410 HMDE employs a borosilicate glass mercury reservoir and capillary from which mercury is displaced by a screw driven plunger (see Figure 2). Although this electrode performs well when properly filled and assembled, it is difficult to fill the mercury reservoir and simultaneously exclude all air. The consequences of trapped air in the mercury reservoir are familiar to all who have used this electrode: excessive mercury thread withdrawal upon dislodging a drop, nonreproducible drop sizes, premature drop dislodgment, and, worst of all, changing electrode size during an analysis. Even a properly filled electrode soon admits air
Present address: Department of Chemistry. Colorado University, Fort Collins, Colo. 80523.
State
to the mercury reservoir, necessitating frequent cleaning and refilling. A simple and inexpensive modification to the Metrohm E-410 HMDE is described here to eliminate these problems. The Metrohm E-410 HMDE is modified to accommodate a new borosilicate glass mercury reservoir-capillary and screw driven plunger tip from the recently introduced EG&G Princeton Applied Research Corp. (PAR)Model 302 Universal Mercury Electrode (UME). The necessary parts are available from PAR a t low cost ($70). The Model 302 UME is a combination dropping mercury electrode (DME) and HMDE. In the HMDE mode, the plunger is driven by a motor down the top tapered section of the mercury reservoir bore, displacing excess mercury and air, until a sealing gasket on the plunger contacts the walls of the reservoir. From this point downward, the reservoir bore is constant, and the plunger very effectively displaces mercury via the capillary to form new and highly reproducible electrode surfaces (Figure 2). In contrast, the original configuration of the Metrohm E-410 HMDE relies on an O-ring seal on the upper rim of the borosilicate glass mercury reservoir-capillary to contain the mercury and exclude air. Leaks are possible both between the plunger and O-ring and between the O-ring and reservoir rim. The following replacement parts available from PAR are required to make the modification: One PAR G104 UME Capillary; one PAR 2517-0736-29 UME metering rod plunger.
This article not subject to U.S. Copyright. Published 1979 by the American Chemical Society
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Flgure 2. Plunger and capillary reservoir configurations
Figure 1. The Metrohm E-410 hanging mercury drop electrode
About one hour of machine shop time is required to fit the UME plunger tip to the existing plunger shaft. The modification is performed as follows: (1)the electrode is first disassembled (6); (2) about 8 mm of the existing plunger shaft is removed; (3) the stub is then faced off flat with a lathe, center drilled, and then drilled and tapped to a depth of 3 mm for #2-56 machine screw; (4) the new plunger tip is installed by simply screwing it on (thread locking sealant may be used); (5) the existing plastic capillary bushing is enlarged with a reamer to accommodate the slightly larger G104 capillary; and (6) the red silicone rubber capillary bushing is discarded where relevant. The following procedure is recommended for filling the electrode with mercury. Fill the capillary reservoir with freshly pinholed mercury so that the meniscus is roughly even with the top of the reservoir. Assemble the electrode loosely with the plunger withdrawn all the way. Holding the electrode vertically, carefully drive the plunger down into the reservoir until the sealing gasket contacts the reservoir bore; some slight increase in turning resistance may be felt. Drive the plunger down into the reservoir 1or 2 additional full turns, and then assemble the electrode body tightly. The initial loose assembly of the electrode body provides self-alignment of the plunger and capillary reservoir. The modified electrode should exhibit superior performance in all respects to an unmodified electrode; that is, negligible
mercury thread withdrawal upon dislodging drop, reproducible drop size, ability to withstand vigorous solution stirring, and constant electrode size over a wide range of applied potentials. Because of the larger diameter of the new plunger, however, the electrode drop size per unit micrometer rotation is somewhat larger than in the original configuration. The performance of the modified electrode with respect to drop size reproducibility was demonstrated by determining the weights (proportional to volumes) of 12 successive 4 division drops extruded from a modified electrode immersed in an 0.2 M ammonium citrate supporting electrolyte solution at a constant applied potential of 0.0 V vs. Ag/AgCl (sat. KC1). The calculated mean drop surface area (assuming sphericity) of 0.0320 cm2 with a relative standard deviation of 1.4% compared favorably with the 3 to 4% claimed hy PAR for the unmodified electrode (7).
LITERATURE CITED Randles, J. E. 9.; White, W. Z.Elektrochem. 1955, 59, 669. Kemula, W.; Kublik, 2. Anal. Chim. Acta 1958, 18, 104. von Sturrn. F.; Ressel. M. hesenius' Z.Anal. Chem. 1962, 186, 63. Riha, J. I n "Progress in Polarography", Vol. 2; Zurnan, P., Kolthoff, I . M., Eds.; Wiley-Interscience: New York. 1962; p 383. (5) Vogel, J. J. J. Electroanal. Chem. Interfacial Electrochem. 1964, 8, 82. (6) "Model 9323 Hanging Mercury Drop Electrode Operating and Service Manual", MDL 9323-5C-9/74-MG; Princeton Applied Research Corp.: Princeton, N. J. 08540, 1974. (7) "Model 315 Automated Electroanalysis Controller Operating and Service Manual", MDL 315; 8/74-A; Princeton Applied Research Corp.: Princeton, N.J. 08540, 1974; p 111-8.
(1) (2) (3) (4)
RECEIVED for review August 17, 1979. Accepted September 21,1979. The use of brand names is for identification purposes only and does not imply endorsement by the U.S.Geological Survey.
