Determination of Molybdenum by Controlled-potential Coulometry Application to the Analysis of Molybdenum-Tungsten- R henium Alloys L. P, Rigdon and J. E. Harrar Chemistry Department, Lawrence Radiation Laboratory, University of California, Livermore, Calif. 94550 A procedure has been developed for the determination of molybdenum by controlled-potential coulometry. The method is based on the reduction of Mo(VI) to Mo(V) at -0.25 V VS. SCE in a supporting electrolyte of 0.2M (NH4),C204 and 1.3M H2S04at pH 2.1. Solutions containing 0.2 to 10 mg Mo per ml may be analyzed with an accuracy and precision of 0.1%. A sample dissolution and pretreatment procedure is described which is especially suitable for the analysis of alloys containing tungsten and rhenium. Rhenium is removed by volatilization. Tungsten is not separated and does not interfere in the determination. Moderate amounts of CI-, C104-, NO,-, and P043- can be tolerated. Fe(lll), U(VI), and Cu(ll) interfere quantitatively in the coulometry, but small amounts can be removed during the sample pretreatment. Correction for Fe(lll), Cu(lI), Bi(lll), and Ti(IV) interference may be made by reoxidation at +0.25 V vs. SCE.
THEDEVELOPMENT of a reactor for space vehicle electric power applications at this laboratory has required analyses of several high-temperature alloys. In particular, procedures have been required for the determination of the major constituents of ternary alloys of molybdenum, tungsten, and rhenium. In evaluating various methods of analysis for these alloys, it appeared that a superior method could be developed for the determination of molybdenum based on controlled-potential coulometry. The generally high accuracy and precision of this technique at the milligram level and its avoidance of standard titrants were expected to be definite advantages. The polarographic and related controlled-potential coulometric behavior of molybdenum in various media is a subject of continuing interest, and has been reviewed by Souchay (1) and Rechnitz (2). Most previous coulometric studies, including some more recent work (3-7), have been concerned with the elucidation of oxidation states and mechanisms rather than analysis. Ibrahim and Nair (8) determined molybdenum by controlled-potential coulometry in hydrochloric acid solutions; however, their reported accuracy was only 1-3 with 10-50 mg quantities, and a complete interference study was not carried out. Palmer (9) developed a coulometric method of high sensitivity for molybdenum, based on its catalysis of the reduction of perchlorate ion. A disadvantage of this technique is the need for careful control of solution volumes, stirring conditions, and temperature in order to reproduce the electrolysis currents. In these respects the method is similar (1) P. Souchay, Talanta, 12, 1187 (1965). (2) G. A. Rechnitz, “Controlled-Potential Analysis,” Macmillan, New York, N. Y.,1963, p 58. (3) J. J. Wittick and G. A. Rechnitz, ANAL.CHEM., 37, 816 (1965). (4) J. T. Spence and G. Kallos, Inorg. Chem., 2, 710 (1963). (5) J. A. Shropshire,J. Electroanal. Chem., 9,90 (1965). (6) F. Pantani, Ric. Sci. 33(II-A), 641 (1963). (7) F. Pantani and M. Migliorini, ibid., 33(II-A) 1085 (1963). (8) S . H. Ibrahim and A. P. M. Nair, J . Madras Unic., 26B, 521 (1956). (9) H. E. Palmer, U. S. At. Energy Comm. Rept. HW-66057, 1960.
to polarography, and it appears to be most suitable only for trace analysis. Neither the electrolysis in hydrochloric acid nor that in perchloric acid would be useful for analyses of samples containing tungsten, without a prior separation of this element. The goal of the work reported here was the development of an analytical procedure that not only would be generally applicable to the determination of molybdenum, but also would be particularly suitable for the analysis of refractory alloys of the Mo-W-Re type. Accordingly, investigations were directed toward an appropriate dissolution and pretreatment procedure for these alloys, and experiments in coulometry were centered on complexing supporting electrolytes that would minimize interferences and required separations. Oxalate solutions have been advocated by Pantani (7, 10) and by Deshmukh and Srivastava (11) for the polarographic determination of molybdenum and tungsten, and initial work in this laboratory revealed these electrolytes to be the most promising for coulometry. The optimum conditions for the determination of molybdenum have been defined and the influence of a variety of potential interferences has been delineated. Special attention has been given to the effect of rhenium and its removal from solution by volatilization. EXPERIMENTAL
Apparatus. The controlled-potential coulometer used in this study was an operational amplifier-type instrument designed in this laboratory (12). The integrator was calibrated electrically, and its readout voltages were measured with a Non-Linear Systems (Del Mar, Calif.) Model 484A digital voltmeter. Log i m. t curves were obtained by applying the voltage signal from a resistor in series with the counter electrode, through a Pacific Measurements (Palo Alto, Calif.) Model 1002 logarithmic converter, to a Leeds and Northrup Model W recorder. A Burr-Brown (Tucson, Ariz.) 5 Hz, active low-pass filter was used to smooth the signal. Polarographic measurements were made with both a controlled-potential, fast-sweep differential polarograph (13), and an ORNL Model Q-1988-ES controlled-potential, dc derivative polarograph (14, 15). Details of the construction and operation of the mercury pool electrolysis cell are available elsewhere (16). The reference electrode used was a Beckman No. 39270, whose potential was checked occasionally against a laboratory prepared SCE.
(10) F. Pantani, Ric. Sci., 33 (11-A), 873 (1963). (11) G. S. Deshmukh and J. P. Srivastava, J . Anal. Chem. USSR, 15, 687 (1960). (12) J. E. Harrar and E. Behrin, ANAL.CHEM., 39, 1230 (1967). (13) F. B. Stephens, E. Behrin, and J. E. Harrar, U. S.At. Energy Comm. Rept. UCRL-50374, 1968. (14) M. T. Kelley, H. C. Jones, and D. J. Fisher, ANAL.CHEM., 31, 1475 (1959). (15) M. T. Kelley, D. J. Fisher, and H. C. Jones, ibid., 32, 1262 (1960). (16) J. E. Harrar, U. S. At. Energy Comm. Rept. UCRL-50335, (1967). VOL. 40, NO. 1 1 , SEPTEMBER 1968
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Reagents. Supporting electrolyte solutions were prepared from reagent grade H~Cz.04,HzS04,and NH40H. Batches of 200 ml were prepared by dissolving 5.05 grams of H2C204 in 150 ml of water, adding 12-16 ml of concd H2S04, and adjusting to pH 2.1 with NH40H. This solution was then diluted to 200 ml. Standard solutions of molybdenum were prepared with Kulite Tungsten Co. (Ridgefield, N. J.) high-purity molybdenum pellets. This material was nominally 99.95% pure and spectrographic analysis at this laboratory showed that it contained approximately 135 ppm of metallic impurities. The surface of the pellets was etched with a solution 2M in HzS04and 2M in HC1. The pellets were then washed with water, dried at 130 “C, and stored in a desiccator. One to 2.5 grams of the cleaned pellets were dissolved in 25 ml of aqua regia and 2.5 ml of H2S04. After the metal had dissolved, the excess acid was evaporated and the mixture was heated to copious fumes of SO,. Approximately 25 ml of water and enough 6N NaOH were added to dissolve the molybdic acid and bring the solution to pH 12. The basic solution was then digested 10-20 minutes and filtered through Whatman No. 41 paper to remove ’small amounts of precipitated metal hydroxides. The filtered solution was then diluted to volume to give a concentration of approximately 10 mg per ml. Calibrated micropipets were used to take aliquots of the stock solutions for coulometric analysis. Reactor alloy standards were prepared from Cleveland Refractory Metals high-purity rhenium powder and from Materials Research Corp. (Orangeburg, N. J.) tungsten wire which was nominally 99.999z pure. Spectrographic analyses confirmed that neither material contained significant amounts of metallic impurities. All other chemicals were reagent grade. Sample Dissolution and Pretreatment Procedures. I. For Mo-W-Re alloys and other materials containing Re. A. Add 20 ml of water and 5 ml each of H2S04,HF, and H N 0 3 to a sample containing 0.1 to 1.0 gram of Mo in a 400-ml Teflon beaker. Cover the beaker with a lid and warm until the sample dissolution is complete. B. Remove the Teflon beaker cover and rinse it with water. Evaporate the solution slowly in the uncovered beaker, then remove the fluoride by heating to copious fumes of SOs. C. Cool and transfer the solution to a 500-ml Berzelius tall form beaker, and add 5 ml of ”0,. Cover the beaker with a Speedyvap watch glass and evaporate slowly to fumes of SO,. D. Increase the heat to copious fumes of SOs to volatilize the Re, and continue the fuming until 1 to 2 ml of H2S04remain. E. Add sufficient 6 N NaOH to dissolve the solid material and bring the solution to pH 11. F. Digest on a warm hot plate for 10-20 min and filter through Whatman No. 41 paper if any insoluble residue or metal hydroxide precipitates are present. Wash the filter paper several times with dilute NaOH. G. Transfer the solution to a suitable volumetric flask to give a concentration of 1-10 mg Mo per ml. 11. For samples not containing Re, but requiring H F for dissolution, Omit steps C. and D. in Procedure I. 111. For samples neither containing Re nor requiring H F for dissolution. Dissolve the material in a 500-ml Berzelius tall form beaker with a suitable solvent in the presence of 2-3 ml of H2S04. Evaporate to copious fumes of SO3 to remove excess nitrates and halides, if used in the dissolution. Continue with step E. of Procedure I. IV. For samples requiring an acid solution after dissolution. Carry out the required steps A-D of dissolution Procedure I. After the fuming, dissolve the mixture in water and adjust the pH of the solution to 2.1 with NH40H. Transfer the solution to a suitable volumetric flask, adding a solution of H2C204and H2S04,to give a final concentration of 0.2 to 0.3M oxalate, 1.0 to 1.5M H2S04,and 0.2 to 10 mg Mo per ml. Allow the solution to equilibrate for 4 hours before carrying out a coulometric determination. Coulometric Analysis Procedure. Introduce mercury into 1642
ANALYTICAL CHEMISTRY
Figure 1. Fast, linear sweep polarogram of Mo(V1) in oxalate solution 1.0 x lO-3M Mo(V1) in 0.2M (NH4)2C204, 1.3M H2S04, pH 2.1, sweep rate 1.0 V per sec, initial potential 0.1 V L’S. SCE, horizontal scale 0.1 sec per division, vertical scale 10pA per division
+
the electrolysis cell so that its surface is level with the upper surface of the stirrer disk. Pipet a 0.1-1-ml aliquot (containing 1-10 mg of Mo) of sample solution into the cell, and add enough supporting electrolyte to bring the total volume in the cell to 5-6 ml (the pH of the resulting solution should be between 1.5 and 2.5). Larger aliquots and more dilute solutions may be analyzed provided the electrolyte concentrations and pH are properly adjusted. Turn on the stirrer and deoxygenate the solution for 7 minutes. Electrolyze the solution at -0.250 V L‘S. SCE; 15-50 mA of initial current should be observed. Read the integrator output voltage and terminate the electrolysis when the current has decreased to 20 PA. Ten to twenty minutes are required to reduce 1-10 mg of Mo. Determine the background correction by carrying out an electrolysis on the supporting electrolyte for the same length of time; the correction should not exceed the equivalent of 0.010 mg of Mo. RESULTS AND DISCUSSION
Sample Dissolution and Pretreatment. Experiments conducted on the dissolution and pretreatment of pure molybdenum samples, and samples from which rhenium was separated, revealed that care must be taken to avoid the loss of molybdenum. Dissolution and fuming or samples in Griffin-type, 50- or 100-ml beakers resulted in losses ranging from 0.1-OSW. A similar loss of molybdenum has been reported to take place in all the published methods for the volatilization of rhenium, and has been attributed to mechanical carry over (17). It was found here that the use of Berzelius type tall form beakers (500 ml, at least 14 cm high) eliminated this problem. For dissolutions involving HF and removal of the excess fluoride, 400-ml Teflon beakers were required. In experiments to determine the best conditions for removal of rhenium, it was found that fuming with HzS04 was effective, while fuming with HC104 was not. Samples containing up to 1 gram of rhenium have been handled by the recommended procedure, and the rhenium was removed to a level at which it did not interfere in the coulometric analysis. (17) C . L. Rulfs in “Treatise on Analytical Chemistry,” Part 11, Vol. 7, I. M. Kolthoff and P. J. Elving, Eds., Interscience, New York, N. Y., 1961, p 515.
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Figure 2. Controlled-potential coulometric determination of Mo(V1) as a function of control potential 4.819 mg &io in 0.2M (NHa)zCzOa, 1.3M H,SOa, pH 2.1. Numbers on curve :time of electrolysis to reach 20 PA, unless otherwise noted
Figure 3. Controlled-potential coulometric determination of Mo(V1) as a function of pH of electrolyte 4.819 mg Mo in 0.2M (WI)ZCZOI, 1.3M HzSOI,E = -0.25 V us. SCE. Numbers on curve: time of electrolysis to reach 20 pA unless otherwise noted
An important variable in the analysis is the pH of the solution that is added to the coulometry cell. The presence of isopolymolybdates in noncomplexing acid solutions (18) apparently retards the formation of the oxalate complexes. For example, when a molybdenum solution of pH < 2 is mixed with the supporting electrolyte, several hours are required for the oxalate complexes to form and the solution to reach equilibrium. If coulometry is attempted immediately, some oxidation of the mercury by the uncomplexed molybdenum occurs, the electrolysis of the Mo(V1) at -0.25 V cs. SCE is extremely slow, and low results are obtained in the determination. No such difficulty is encountered with solutions that are alkaline prior to the coulometric analysis. For solutions that are to remain acidic throughout the pretreatment, it is necessary that they be made up to volume in the supporting electrolyte and equilibrated 4 hours before coulometric analysis. Polarography of Molybdenum in Oxalate Media. Because the primary objective of this study was to establish the optimum conditions for coulometry, a complete investigation of the electrochemistry of molybdenum in oxalate solutions was not carried out. However, a brief study revealed some interesting features of this system. Figure 1 shows a fast sweep polarogram of a solution of Mo(V1) in the supporting electrolyte recommended for coulometric analysis. Use of both fast sweep polarography and derivative dc polarography has indicated that over the pH range of 0.5-4.0, the Mo(V1) reduction is resolved into four waves, or two principal waves composed of doublets. Thus, as it is in several other electrolytes (3, 19, 20), the reduction of Mo(V1) appears to be complicated by a multiplicity of electroactive species. Also similar in this electrolyte is the presence of two different species of Mo(VI), giving rise to the first pair of waves. Evidence for two species of Mo(V1) is the fact that Mo(V1) is reduced coulometrically in a one electron process with 100% current efficiency at -0.25 V cs. SCE. At low values of pH, the doublet character of the principal
waves becomes less distinct, which probably accounts for the fact that this has not been reported previously for oxalate media (6, 11, 21, 22). In the absence of oxalate, a first wave is visible which appears to be identical to the first wave of Figure 1. As the concentration of oxalate is increased and the pH of the solution is raised, the second wave of the first doublet grows. Thus the second wave is caused by a reducible Mo(V1)-oxalate species, while the first wave is probably due to a species not involving oxalate. Upon controlledpotential electrolysis at -0.25 V us. SCE, both species, which are in sluggish equilibrium, are quantitatively and rapidly reduced to Mo(V). The nature of the Mo(V) species also is complicated. During the controlled-potential electrolysis, the initially colorless solution becomes very dark brown for the first few minutes and then grows to a light amber color as the reduction of Mo(V1) goes to completion. The dark brown solution can be rapidly reoxidized at +0.25 V cs. SCE, while the final solution cannot. A polarogram of Mo(V) resulting from the complete electrolysis of Mo(V1) at -0.25 V us. SCE exhibits waves corresponding to the second doublet of Figure 1 ; however the waves are only one thirtieth the magnitude of those of the polarogram of the Mo(V1) solution. Moreover, when Mo(V1) is electrolyzed at potentials cathodic of the second doublet, there is a very slow decay of the current after an initial rapid decrease. The effects indicate that the initial electrode reaction product is transformed into a considerably more inert species at a rate comparable to that of the electrolysis. Coulometric Analysis. The choice of control potential and solution pH for the coulometric analysis in 0.2M (NH& C204-1.3MH2SO4was based on the results shown in Figures 2 and 3. Maximum accuracy and minimum electrolysis times are obtained at -0.25 V us. SCE and pH 2.0; however, an accuracy within COS^ at the 5-mg level is attainable in the range of -0.15 to -0.32 V us. SCE and pH values of 1.0 to 2.7, at the expense of considerably increased electrolysis times at the ends of these ranges. At a potential of -0.25 V us. SCE and pH values less than 1.0, and at a pH of 2.1 at po-
(18) F. A. Cotton and G. Wilkinson, “Advanced Inorganic Chemistry,” Interscience, New York, N. Y.,1962, pp 784-5. (19) I. M. Kolthoff and I. Hodara, J. Electroanal. Chem., 4, 369 (1 962). -\ -
(20) D.’L. Manning, R. G. Ball, and 0. Menis, ANAL.CHEM., 32, 1247 (1960).
w. Zahnow and R. J. Robinson, J. Electroanal. Chem.,3, 263 (1962). (22) F. A. Uhl, Fresenius’ 2. Anal. Chem., 110, 102 (1937). (21) E.
VOL. 40, NO. 11, SEPTEMBER 1968
1643
0
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4.837 mg Mo in 0.2M (NH~)zCZO~, 1.3MH2.504, pH 2.1 E = -0.25 V CS. SCE
tentials more cathodic than -0.32 V us. SCE, high results are obtained and a steady background current is observed because of the further reduction of Mo(V). At the recommended pH and control potential, the oxalate concentration may be varied between 0.2 and 0.3M and the H&04 may be varied between 1.0 and 1.5Mwithout effect on the results. Table I. Analyses of a Standard Solution of Molybdenum by Controlled-Potential Coulometry Molybdenum, mg Taken Found 0.966 0.966 4.823 4.819 9.645 9.657
n 6 6 5
Re1 std dev, 0.20 0.12 0.06
Re1 error,
2
0.0 -0.08 -0.12
Table 11. Tolerances of Diverse Substances in the Determination of Molybdenum (Determination of 5 mg of Mo) Amount to cause 0 . 5 2 re1 Substance Added as error, mg Bi(N0d3 0.025 Bi(111) CdClz >5 Cd(I1) >5 Cr(II1) CrC13 KzCr2O7 0,020 Cr(V1) 0.020 Cu(I1) CuSOa Fe2(S04)3 0.020 Fe(II1) Hg(N03)z 0.025 Hg(II) Pb(N0a)z >5 Pb(I1) 1 .o Re(VI1) Rez07a >5 Sn(IV) SnC14 UOz(C2H~0z)z 0.05 U(V1) 0.50 V(IV) VOCI, V(V)
V205a
Ti(IV) Au(II1) Ce(1V) Pt(1V) WWI) c1CIO4NOaPOaa-
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0.50
0.15 0.05
0.15 0.05
>15 >18 >50
>6.5 >5 0.5 >5
Table I indicates the precision and accuracy that were obtained in the analysis of pure molybdenum solutions. Table I1 summarizes the results of tests to determine the effects of various possible interferences. As expected from their polarographic half wave potentials, several elements such as Cu(II), Fe(III), Bi(III), and U(V1) constitute serious interferences in the coulometric determination. However, minor amounts of most such potential interferences can be separated from Mo(V1) in the solution pretreatment procedure. Although not investigated in detail, it also appeared that a reasonably accurate correction for such species as Fe(III), Bi(III), Cu(II), and Ti(1V) could be obtained, after coreduction with Mo(VI), by reoxidation of their reduced forms. Mo(V) is only slightly reoxidized:
