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
e
1595
5M sodium hydroxide solution and 1:l hydrochloric acid. Extract three times with 5-ml. portions of chloroform, shaking 1 minute each time; combine the chloroform extracts in a 30-ml. separatory funnel. Discard the aqueous phase. Add 5 ml. of 1.5M hydroxylamine hydrochloride solution adjusted to pH 9 with NHIOH to the small separatory funnel containing the chloroform extracts, shake for 1 minute, and drain the chloroform phase into a 20-ml. beaker. Add 2 ml. of chloroform to the remaining aqueous phase, shake for 30 seconds, and drain the new chloroform phase into the 20-ml. beaker containing the previous chloroform phase. Discard the ammoniacal wash solution. Transfer the combined chloroform phases to the 30-ml. separatory funnel previously used and extract three times with. 5-ml. portions of 0.5N hydrochloric acid, shaking 1 minute each time. Collect the aqueous phases in a 50-ml. beaker. Discard the chloroform phase. Evaporate the dilute hydrochloric acid solution to remove any droplets of chloroform and to reduce the volume to less than 15 ml. Transfer to a 25-ml. volumetric flask keeping the volume less than 20 ml. Add sufficient saturated bromine water to produce a permanent yellow color (1 to 3 ml. is required) and let stand 15 minutes. Continue according to the procedure for the preparation of the calibration curve, beginning with the addition of 2 ml. of concentrated ammonium hydroxide. RESULTS AND DISCUSSION
The large quantity of hydroxylamine added in each determination was found to be necessary. This reagent functions both as a reducing agent and as an acid shifting the pH of the solution t.0 approximately 6. Low results were obtained when 2 grams of hydroxylamine were used and sufficient 1: 1 hydrochloric acid was added to adjust the pH to 6.
Table 1.
Added
Tungsten Metal B, 10 grams
ANALYTICAL CHEMISTRY
Nickel, p.p.m. Total Found
0.33 0.17 0.22
0.28
1.29
1.0
1.28
1.19 1.39
3.0
3.28 5.28
3.25 5.36
0 1.0 2.1
LITERATURE CITED
Av.
0.21 0.48
0
5.0
0
of a series of lighter aqueous phases requires that the chloroform phase be temporarily removed from the separatory funnel. The following technique should be used to prevent loss of the aqueous phase in the stopcock or stem of the separatory funnel. Drain the chloroform phase into the 20-ml. beaker in which it was originally collected, and drain the aqueous phase into a 50-ml. beaker. Return the chloroform phase to the separatory funnel and drain part of it back into the 20-ml. beaker to flush any aqueous phase from the stopcock. Return this portion of the chloroform to the separatory funnel and rinse the 20ml. beaker with the next portion of dilute hydrochloric acid to be added. The revised procedure was applied to 10-gram samples of high-purity tungsten to which known amounts of nickel had been added. A reagent blank was carried through the entire procedure with each group of samples to correct absorbance readings for nickel contributed by the reagents. In addition, the absorbance of each reagent blank was measured a t 445 mp, using distilled water to zero the spectrophotometer, to determine the nickel content of the combined reagents. The results are given in Table I. Because no analyzed samples of tungsten in this range of nickel content are available, the method was evaluated on a standard addition basis. The total quantity of nickel present in each test is calculated from the cyantity added and the quantity found when no nickel was added. The total quantity of nickel found in the reagent blank ranged from 4 to 6 pg., equivalent to 0.4 to 0.6 p.p.m. in 10-gram samples. A series of tests in which the reagents used in the dissolution and extraction portion of the procedure were independently varied showed that most of the nickel in the reagent blank was contributed by the sodium hydroxide. Therefore, the same quantity of sodium hydroxide should be used in each member of any set of determinations.
Determination of Nickel in Tungsten Metal Samples
Sample Tungsten Metal A, 10 grams
1596
In both the Sandeil and the Roher procedures, dimethylglyoxime is added in the form of an alcoholic solution. This reagent was initially used in the present investigation. However, an aqueous solution of the sodium salt of dimethylglyoxime produced less emulsion a t the interface between the aqueous and chloroform phases and was, therefore, used in the final procedure. In most published procedures for the extraction of the nickel dimethylglyoxime complex into chloroform, the combined chloroform extracts are washed with dilute ammonium hydroxide to remove any co-extracted copper which would interfere with the final results. An ammonium hydroxide wash was also used in early tests of the present procedure. The results of these tests on synthetic samples containing from 1 to 25 p.p.m. of nickel had an average standard deviation of 5% but showed a negative bias equal to approximately 10% of the nickel present. A series of tests to determine where small quantities of nickel might be lost showed that the ammonium hydroxide wash contained sufficient nickel t o account for the negative bias. On the assumption that oxidation during the washing step could cause such loss, the dilute ammonium hydroxide wash solution was replaced by the ammoniacal hydroxylamine hydrochloride wash solution. No nickel was found in portions of this solution which had been used t o wash chloroform extracts containing nickel. Use of the ammoniacal hydroxylamine hydrochloride wash solution eliminated the negative bias in subsequent tests. This wash solution was also found to be as satisfactory as dilute ammonium hydroxide for removing copper from the chloroform extracts. During the back extraction of the nickel from the chloroform phase into 0.5N hydrochloric acid, the collection
...
10.2 11.3
9.2 10.5 11.8
(I) Penner, E. M., Inman, W. R.,
Talanta 10, 997 (1963). (2) . . Rohrer, K. L., ANAL.CHEM.27, 1200 (1955). (3) Sandel!, E. B., “Colorimetric,, De-
termination of Traces of Metals, 3rd ed., p. 672, Interscience, New York, 1959.
THOMAS E. GREEN
College Park Metallurgy Research Center Bureau of Mines U. S. Department of the Interior College Park, Md. TRADEnames are used for information only and endorsement by the Bureau of Mines is not implied.
.
