points (triplicate samples at each of three concentrations) shown in each graph. Similar results are obtained by taking the average standard deviations of the three NiEDTA solutions. Comparative data for the two spectrometers are shown in Table 11. The standard error of estimate for the modified spectrometer (0-000056 A.U.) indicates that species with an absorbance of 0.0002 may be detected with 99% confidence. The modified spectrometer has beem shown to possess
ultra-high stability and high precision necessary for the kinetic measurements discussed at the beginning of this report. The instrument is currently being used to measure precisely the rates of enzyme catalyzed reactions.
RECEIVED for review February 3, 1969. Accepted February 17,1969. This work was supported in part by PHS Research Grant No. G.M. 13326-03and in part by a grant from the Indiana Elks Association to F’urdue University.
Electrolytic Dissolution of Tantalum, Niobium, and Other Refractory Metals Abraham Aladjem Soreq Research Center, Yavne, Israel INthe analysis of tantalum, niobium, zirconium, tungsten, and other refractory metals, the specimens are usually prepared for analysis by dissolution in a mixture of nitric and hydrofluoric acids followed by several evaporation-dissolution stages. This procedure is cumbersome and laborious, and a new electrolytic dissolution technique developed by us offers greater simplicity and dispenses with the use of hazardous reagents; it would be especially valuable in cases in which the presence of the fluoride ion is objectional. Electrolytic dissolution techniques have found a limited use in analytical chemistry, notably in the case of uranium ( I ) . EXPERIMENTAL
Reagents. A 5 solution of NH4C1 in methanol is used as the electrolyte: analytical-grade reagents are used, without special precautions to ensure the absence of moisture, because the electrolyte tolerates a moisture content of up to 4 % without adverse effects. Procedure. The dissolution is carried out in borosilicate glass containers, without separation of anolyte from catholyte. Tantalum or platinum wire 0.4 mm in diameter is used as the cathode. The metal to be dissolved serves as the anode, either on a suitable graphite support, or directly connected to the leads. Dissolution occurs with a current efficiency of over 50% and is accompanied with an evolution of O2on the anode. N o metal deposition takes place on the cathode. The initial anode-to-cathode surface ratio should be about 1 :1 but naturally it decreases in the course of the electrolysis. The anode current density is not critical and may range from 3 to 150 mA/sq cm, but a density of 20-30 mA/sq cm is needed to provide reasonably short dissolution times, and densities beyond 40-50 mA/sq cm are to be avoided because of excessive heating. At current densities of the order of 40 mA/sq cm the time needed to dissolve a tantalum or niobium foil 0.1 mm thick is about 10 minutes; to avoid precipitation (probably of the oxychloride) at least 50 ml of the solution must be used for every gram of metal taken. It is preferable to use a power supply with controlled-current dc output instead of constant voltage because the conductivity of the solution increases with increasing temperature, and as some heat is evolved in the dissolution the temperature of the electrolyte may rise and the current density may rapidly increase beyond the optimum range, causing boiling of the electrolyte. (1) R.P.Larsen, ANAL.CHEM., 31, 545 (1959).
The electrolyte temperature should be kept below the boiling point but it is not critical except to avoid excessive losses of solvent by evaporation. A clear solution should be obtained at the end of the dissolution, the only possible residue being very thin (30-40 A) flakes of the natural oxide covering the metal; in most cases these flakes are also dissolved in the process. RESULTS AND DISCUSSION
The method has been used to dissolve tantalum, niobium, tungsten, zirconium, vanadium, hafnium, titanium, and their alloys-e.g., Ni-14% Ta, Ta-Nb-and no difficulty was experienced with materials from different sources (Fansteel, Heraeus, Metallwerke Plansee) and of varying degrees of purity. Although the method is especially suitable for the dissolution of massive samples or wires which may be connected directly to the leads, crushed samples or even finely divided powders may also be dissolved by supporting them on a graphite plate or preferably on a heavily-anodized (to an oxide thickness of 4000-5000 A) tantalum dish, which is not attacked at current densities below 50 mA/sq cm provided that the applied voltage does not exceed 10-12 volts. In some cases the analysis may be continued directly in the methanol solution (preliminary experiments show that colorimetric measurements are possible in such solutions), but in general the methanol is evaporated and the residue is dissolved in the aqueous medium needed for subsequent analytical steps-e.g., HC1 or H 2 S 0 4 solutions-and separation or determination of impurities is carried out by conventional methods. It should be recalled that some of the refractory metal chlorides are rather volatile; hence, care should be taken not to overheat the residue from the methanol evaporation stage -[.e., the evaporation should be carried out over a water bath. ACKNOWLEDGMENT
The author thanks Yitshak Marcus of the Hebrew University, Jerusalem, for his valuable comments. RECEIVED for review November 5, 1968. Accepted December 30, 1968. VOL. 41, NO. 7 , JUNE 1969
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