LITERATURE CITED
(1) Frumkin,
A., “Polarography,” Kolthoff, I. M., ed., Vol. I, p. 143, Interscience, New York, 1952. (2) Howorth, R. C., Mann, F. G., J. Chem. Soc. (London) 1943, 603. (3) Kolthoff, I. M., Lingene, J. J., “Polarography,” 2nd ed. p. 509, Interscience, New York, 1952.
(4) Malik, W. U., Haque, R., ANAL. CHEM.32, 1528 (1960). (5) Malik, W. U., Ha ue, R., 2. Anal. Chem. 180, 425 (19SI’f. (6) Malik, W. U., Haque, R., Naturwiss. 49, 346 (1963). (7) Malik, W. U.,Kafoor Khan, H. A., Ind. J. Chem. 2 (11) 455 (1964).
(8) Schwartz, A. M., Perry, J. M.,
“Surface Active Agents and Detergents,” p. 212, Interscience, New York, 1949.
Composite Graphite-Mercury Stripping Voltammetry SIR: The technique of anodic stripping voltammetry is well suited, in terms of sensitivity and convenience, to determinations of trace quantities of electro-reducible metal ions in natural media. Total deposition of the metal ions of interest is advantageous where unknown complexing agents are present and may cause a very slow attainment of equilibrium. A wide variety of experimental procedures and electrode materials have been used (4) in anodic stripping methods. However, the full potentialities of the technique are not easily realized because the shape of the anodic current peak is greatly dependent upon the electrode. Thus, bulk mercury electrodes, usually in the form of a hanging drop or a pool, cause excessive tailing and poor rezovery after long deposition times. Graphite or carbon electrodes have the disadvantage of relatively low hydrogen overvoltage and the peaks are broad. We report a variation of the graphite electrode which gives very sharp current peaks, allowing high sensitivity and resolution. This electrode consists of mercury deposited on graphite. The deposited metals are then in a mercury solution with a high surface-to-volume ratio and as such are not subject to variable activity effects, as are solid deposits. The advantages of using mercury plated platinum electrodes (2) appear to be increased with this composite mercury-graphite electrode.
Table 1.
-1.0
-0.8
-0.6
-0.4 -02 E vs. S.C.E.
0
0.2
Figure 1. Anodic stripping curve of aerosol sample in 0.1M KCI, pH = 5.0 Plating time 70 minutes a t Sweep rate 40 mv./recond
- 1 .O
volt vs. S.C.E.
EXPERIMENTAL
The unit used for the stripping analysis was built around Heathkit operational amplifiers after the circuit of Enke ( 1 ) . A synchronous motor with a magnetic Teflon-covered stirbar was used for stirring. Saturated calomel reference and platinum counter electrodes were separated from the solution to be analyzed by porous Vycor plugs (Corning Glass Co.) in tubing made of Teflon (Du Pont). The cell was made of Vycor to prevent contamination by lead from borosilicate glass, and nitrogen was introduced Apparatus.
Determinations of Lead by Anodic Stripping Voltammetry
a
19.3 19.3 174
21.5“ =IC 0.5 20.8 178
Average of three successive determinations on one sample.
1594
ANALYTICAL CHEMISTRY
WAHIDU. MALIK PURAN CHAND Department of Chemistry Roorkee University Roorkee, India The authors are grateful to C.S.I.R. (India) for the award of a fellowship t o one of us (P. C.) to carry out %is work.
Electrode for Anodic
14ml. sample volume Coulombs X 106 Amount Pb added Coulombs Amount Pb Moles x 106 Peak area Blank found 1 . 0 0 x 10-10 1 . 0 0 x 10-10 9 . 0 x 10-10
(9) Vogal, A. I., “Practical Organic Chemistry,” p. 790, Longmans, Green, London, 1947.
2.0 2.0 2.0
19.5 18.8 176
through a fine-tipped needle made of Teflon. Reagents. All water used was double-distilled under nitrogen first from alkaline KMn04 and finally from H3P04. Reagent grade mercury was dissolved in double-distilled nitric acid for the mercury additions. Lead standards more dilute than 10-4M were made up every few days and always assigned to the same flask. Nitrogen was prepurified and passed through a distilled water and a glass wool trap before being introduced to.the cell. Procedure. A spectroscopic grade graphite electrode (Fisher Scientific Co.) was impregnated with a good grade of molten paraffin wax under vacuum until the evolution of gas from the electrode ceased. Approximately 1 sq. cm. of the electrode was then scraped clear of wax and polished with 4/0 emery paper. The electrode was then placed in the solution to be analyzed and between 10-7 to 5 X lo-* mole of mercury was added to the cell. To plate out the mercury the electrode was held a t -200 mv. us. S.C.E. for 10 minutes with vigorous stirring, during which time the oxygen was also flushed from the cell. The electrode was then regulated to the desired plating potential. RESULTS AND DISCUSSION
The electrode exhibits a hydrogen overvoltage of 550-600 mv. and in media of pH 4-5, can be used up to -1.3 volts vs. S.C.E. I n similar media, waxed graphite alone can be used up to about -0.8 volt vs. S.C.E. The stripping peaks are quite sharp (100-150 mv., Figure 1) and for lead, the only metal investigated thus far, show total recovery of plated material (Table I) with very little tailing. The cell can accommodate samples of 4-20 ml. and the half time for total plating is 6-13 minutes, depending on electrode area, stirring rate, and volume. For a 1 sq. cm. area electrode, 15 ml. of solution, and a stirring rate of 120-160 r.p.m., the half time is 10 j=1 minutes. Under these conditions, the sensitivity
of the electrode a t the 10% precision level is approximately 1 microcoulomb or 3 x 10-l0M, and the range over which the total plating method is applicable is 5 X lo-? to 10-loM. Above 5 x 10-7M the electrode is overloaded and tailing is severe. Below 10-10,1.1, the current peaks become indistinguishable from noise and background currents. The electrode can be used repeatedly as long as the mercury is not stripped off. Microscope examination of the electrode shows about one tenth of the mercury as droplets of about 0.01-mm. diameter; the rest is apparently in the form of much smaller droplets beyond the resolving power of the microscope used. From the high overvoltage, the
sharpness of the peaks, and the lack of tailing, it is proposed that the plating and stripping occurs chiefly on these droplets, since they are assumed to have formed a t the most active sites on the graphite-wax surface. The electrode has been used for a series of 23 determinations of lead in aerosols in Boston, Mass., air (3) and is currently being applied to complex natural water samples from the Boston area. LITERATURE CITED
(I) Enke, C. G., Baxter, R. A., J. Chem. Educ. 41, 202 (1964). (2) Gardiner, K. W., Rogers, L. B., ANAL. CHEM.25,1393 (1953). (3) Lininger, R. L., M. S. Thesis, hlassa-
chusetts Institute of Technology, Cambridge, 1965. (4) Shain, Irving, “Treatise on Analytical Chemistry,” Part I, Vol. 4, I. M. Kolthoff, P. J. Elving, eds., Interscience, New York, 1963. WAYNE R. MATSON DAVIDK. ROE DAYTON E. CARRITT~ Department of Chemistry and Laboratory for Nuclear Science Massachusetts Institute of Technology Cambridge, Mass. Department of Geology and Geophysics, WORKsupported in part by the National Science Foundation under Grant GP-486, in part by the Office of Naval Research under Contract Sonr-1841(74), and in part by the Atomic Energy Commission under Contract AT(30-1)-905.
Solvent Extraction and Spectrophotometric Determination of Nickel in High Purity Tungsten or Tungsten Trioxide SIR: A spectrophotometric method for determining nickel was required to evaluate solvent extraction and ion exchange separation procedures for preconcentrating nickel and other metallic impurities from high-purity tungsten. The dimethylglyoxime extraction method described by Sandell (3) was selected for this purpose. Although this method was initially used to determine nickel after its separation from tungsten, it appeared that, with some modification the method could be applied directly to solutions containing large quantities of tungsten, If the extraction could be made quantitative in the presence of 10 grams of tungsten, a method for determining less than 10 p.p.m. of nickel in tungsten, would be provided. In a method by Rohrer (6)for determining nickel in tungsten powder, nickel was extracted as the dimethylglyoxime complex from solutions prepared by dissolving 0.5-gram to 1.0gram samples in hydrogen peroxide. The extracted nickel was then determined spectrophotometrically as the diethyldithiocarbamate complex. Previous experience in our laboratory had shown that hydrogen peroxide is a satisfactory reagent for dissolving tungsten samples in the form of fine powders. However, samples in the form of pellets or rods are only slowly dissolved in hydrogen peroxide. Also, when samples of large particle size are dissolved in hydrogen peroxide, a higher oxidation state of tungsten is formed, which interferes with the reduction of nickel. Penner and Inman (1) determined nickel in 0.5-gram samples of tungsten by dissolving the samples in hydrofluoric and nitric acids, complexing the fluoride with boric acid, and extracting the nickel as the dimethylglyoxime complex into chloroform. Application
of this technique up to 10-gram samples of tungsten resulted in precipitation of the tungsten, thereby making extraction impossible In the present method, tungsten metal samples are dissolved in hydrofluoric and nitric acids, the sample solution is then evaporated to dryness, and the resulting tungsten trioxide is dissolved in sodium hydroxide. The method is therefore suitable for determining nickel in any physical form of tungsten metal and for determining nickel in tungsten trioxide. Sodium hydroxide, rather than ammonium hydroxide, was used to dissolve the tungsten trioxide and to make necessary pH adjustments because sodium tungstate is more soluble than ammonium tungstate. Use of sodium hydroxide made it possible to retain 10 grams of tungsten in solution. EXPERIMENTAL
Apparatus. All absorbance measurements were made on a Beckman Model B spectrophotometer equipped with a blue sensitive phototube, using 5.00-cm. cells. A Radiometer Type PHM22 p H meter equipped with a combination electrode suitable for insertion into separatory funnels was used to make p H measurements. Separatory funnels of 250-ml. and 30-ml. capacity with plastic stoppers and plugs of Teflon were used. Reagents. All reagents were of analytical grade. Single stage distilled water was used for preparing reagent solutions and throughout the procedure. Standard Nickel Solution. Dissolve 0.5000 gram of high-purity nickel in 30 ml. of 1: 1 nitric acid and dilute to 1 liter. Dilute an aliquot of this solution to prepare a working solution containing 5 pg. of nickel per milliliter.
Preparation of Calibration Curve. Transfer 0, 1, 2 , 3, 5, 7 , and 10 ml. portions of the working solution to 25-ml. volumetric flasks and dilute to 20 ml. Add 1 ml. of saturated bromine water and let stand 15 minutes. Add 2 ml. of concentrated ammonium hydroxide and cool to under 30’ C. Add 1 ml. of 0.25M solution to the sodium salt of dimethylglyoxime and dilute to volume with water. Let stand 10 minutes, then without delay read the absorbance a t 445 m p using the nickel-free standard to set the spectrophotometer a t zero absorbance Procedure. X reagent blank is carried through the entire procedure t o provide a correction for nickel impurities introduced by the reagents. Weigh 10-gram portions of tungsten metal samples into 250-ml. beakers of Teflon. Add 10 ml. of hydrofluoric acid (48%). Then add concentrated nitric acid in small quantities t o dissolve the sample. Approximately 10 ml. of nitric acid is required. Transfer the sample solution to a 125-ml. platinum dish and evaporate to complete dryness on a hot plate to convert the tungsten to tungsten trioxide. Samples of tungsten trioxide are analyzed by starting a t this point. Add 20 ml. of 0.5X citric acid solution, stir to break up lumps of tungsten trioxide, and add 30 ml. of 5M sodium hydroxide solution. Warm the dish on a hot plate to dissolve the tungsten trioxide. Transfer the resulting solution to a 250-ml. separatory funnel, using a few milliliters of 1:l hydrochloric acid to wash the dish. Dilute to approximately 125 ml. Add 25 ml. of freshly prepared 6 X hydroxylamine hydrochloride solution. Ignore any noncoagulating white precipitate which forms a t this point. Add 10 ml. of 0.25-If solution of the sodium salt of dimethylglyoxime. -4djust to pH 9 with VOL. 37, NO. 12, NOVEMBER 1965
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