Polarographic Determination of Molybdenum(V1) Tartaric Acid as Supporting Electrolyte E. P. PARRY and M. G. YAKUBIK' Department of Chemistry, University o f Connecticut, Storrs, Conn.
This investigation was undertaken to develop a rapid method for the polarographic determination of molybdenum(V1)in the presence of tungsten(V1). -4supporting electrolyte of tartaric acid has been found very suitable for such a determination. In the presence of tartaric acid, molybdenum(V1) is reduced in two steps, the total height of the two waves being proportional to the molybdenum concentration. Tungsten(V1) is not reduced under these conditions. -4systematic study of the system has shown that the reducible molybdate species must be adsorbed before reduction can occur. Because of this adsorption, the presence of indifferent salts decreases slightly the total wave height; the use of the method of standard additions is therefore most convenient for the determination. Methods for overcoming interference due to iron(II1) and chromium(111) are described. Nickel(II), cobalt(II), and other substances reduced at potentials more negative than -0.75 volt vs. the saturated calomel electrode do not interfere. The method is rapid, accurate, convenient, and subject to few interferences.
S
EVERAL methods for the polarographic determination of molybdenum(V1) have been described ( 1 , 4, 5, 12, 14). The majority of these methods require preliminary separation of many of the common elements associated with molybdenum before the polarographic method can be applied. il common interfering element is tungsten(V1). Recently Meites ( I O ) and Pribil and Blazek (I1 ) have described methods for the determination of molybdenum in the presence of tungsten, the former using a citrate buffer and the latter an ethylenediaminetetraacetic acid solution as supporting electrolytes. The present paper describes the use of tartaric acid as a supporting electrolyte for the polarographic reduction of molybdate. It is shown that molybdate produces two waves in 0.1M tartaric acid, the sum of the heights of the two waves being proportional to the molybdenum(V1) concentration. As tungsten does not give a polarographic wave in this supporting electrolyte, the reduction of molybdate was investigated systematically, and a method for the determination of molybdenum in the presence of tungsten has been developed. Methods for overcoming interferences due to large amounts of iron(II1) and chromium( 111) are given. Nickel, manganese, cobalt, and other substances that are reduced a t more negative potentials than -0.75 volt us. S.C.E. in this supporting electrolyte will not interfere in the determination.
water thermostat a t 30.0" f 0.1" C. The value of m2'3 t1'6 for the capillary used was 1.41 mg.2I3sec.-1'2. 911 chemicals in the experiments were of analytical reagent grade and were used without further purification. A Beckman Model G pH meter was used for pH measurement. The diffusion currents obtained with the Leeds and Northru Electrochernograph were compared against those obtained witE the manual instrument to check the usefulness of the automatic instrument in obtaining quantitative data. Several concentrations of lead nitrate in a supporting electrolyte of 0.5N potassium nitrate with methyl red as a maximum suppressor were prepared and the total current-Le., the diffusion current plus the residual current--R as measured on both instruments a t -0.6 volt us. S.C.E. after oxygen was removed. With the recording instrument, peak current values-i.e., the maximum of the current oscillations-were taken. Using the zero damping position on the Electrochemograph, the ratios of the current obtained with the manual instrument to the peak current obtained with the recording instrument were constant a t 0.799 & 0.003 for current ranges of 30, 20, 15, 10, and 7, and lead ion concentrations varying from 2 X 10-3 to 5 X 10-4M. Assuming the current values obtained with the manual polarograph represent true average currents, the peak current values obtained with the Electrochemograph represent closely the maximum current which flows a t the end of the drop life. Although the theoretical ratio of average current to maximum current is 0.857, Taylor, Smith, and Cooter ( I S ) found the ratio to be 0.81. It appears that the response of this recording polarograph is fast and faithfully reproduces the variation of current during the latter part of the drop life. I t is obvious that this instrument can be used to obtain precise quantitative data, and the procedure of measuring current peaks is recommended !$-hen using the instrument with no damping of the current oscillations. POLAROGRAPHIC BEHAVIOR OF MOLYBDENUM(V1) IN TARTARIC ACID
Molybdenum(V1) is reduced a t 'the dropping mercury electrode in 0 . 1 M tartaric acid (pH 2.0) producing two waves with half-wave potentials of -0.22 and -0.52 volt us. S.C.E. (Figure 1, a). Both waves are irreversible. The height of the first wave is approximately one half that of the second wave, indicating that molybdenum(V1) is probably reduced first to the +5 state then to the +3 state. The final rising portion of the curve a t - 1.1 volts represents the discharge of hydrogen. No constant diffusion current region is reached for the first reduction wave, and that reached for the second wave is constant only over a rather small potential range. As the potential becomes more
8.0.
P
6.0-
L
EXPERIMENTAL
All current-voltage curves in this research were obtained on a Leeds and Northrup E Electrochemograph. In those cases where data were required to test a wave for reversibility, a manual polarograph similar to that described by Lingane and Kolthoff (9) was used. All diffusion currents were corrected for residual currents. A single compartment polarizing cell similar to that described by Hume and Harris ( 3 )was used as the electrolysis cell. The reference electrode was a saturated calomel electrode (S.C.E.) which was connected to the electrolysis cell through a potassium chloride-agar bridge. The experiments were carried out in a
-0.1
-02
-0.3
-0.4
-05 -0.6 -0.7 V O L T S 'is. S C E
-08
-0.9
-IO
-1.1
Figure 1. Effect of pH on Reduction of 1.17 X 10-aM Sodium Molybdate in Tartrate Buffers
+
a. 0 1M tartaric acid p H 2 . b . 0.1M tartaric acid sodium hydroxidk, p H 2.3; c. 0.d6M tariaric acid and 0.04M sodium tartrate, p H 2.80. d 0.05M tartaric acid and 0.05M sodium tartrate p H 3.35e. 0.64M'tartaric acid and 0.06M sodium tartrate, p H 3.79; ). 0.1M so: dium tartrate, p H 7.77
1 Present address, Polychemicals Department, Research Division, E . I. d u P o n t de Nemours & Co., Wilmington, Del.
1294
1295
V O L U M E 26, NO. 8, A U G U S T 1 9 5 4
crease in current beyond about -0.6 volt to become much more sharp and pronounced and to be shifted to less negative potentials. Such a shift was also found by Laitinen and Onstott (8) in the systematic investigation of a somewhat similar adsorption phenomenon. Beyond the minima in Figure 2, b and c, the camphor itself begins to be desorbed a t these concentrations, and the adsorption and subsequent reduction of molybdate again occur so that the current gradually increases up to the value it would have in the absence of camphor. When the solution is saturated with camphor (Figure 2, d ) the wave is inhibited. Apparently under these conditions camphor is not completely desorbed until a more negative potential is reached than that a t which the liberation of hydrogen starts (> 1.1 volts). Saturated octyl alcohol also completely represses the wave. The potential range in which the desorption process (desorp tion wave) occurs appears to be independent of the temperature in the range from 25" to 35" C., and practically independent of the concentration of molybdate. However, a t higher concentrations of molybdenum( VI), the half-wave potential for the reduction of molybdate is shifted to slightly more negative potentials, and the desorption wave is also shifted slightly in this direction.
Table I. Diffusion Current Constant of Molybdenum(V1) in 0.1M Tartaric Acid Concentration of Molybdate, Ivlillimolar 0.0 0.186
0.372 0.930 1.86
3.72 a
Measured at -0.76 volt.
id (pa.) a t -0.65 Volt I , $ . S.C.E.
0 1 2 6
13 33 6-1 69 13 30 26 6 i a
id/CnL2'3t''c
5:Oi 5.04 5.10 5.05 5.08 Av. 5 . 0 7 i 0 . 0 2
negative, a decrease is observed in the diffusion current of the second wave (Figure 1, a). This is not a regular maximum a8 is shown by the effectof 0.01% solutions of gelatin and agar. Both of these maximum suppressors lower the total wave height by about one third and accentuate the decrease in current. The decrease has been attributed to a desorption of the reducible species from the dropping mercury a t the more negative potentials. 'I'hc total limiting current, after correction for the residual current, is proportional to the molybdate concentration (Table I). The total diffusion current values were measured a t a potential of -0.65 volt us. 3.0 S.C.E. except for 3.72 mM molybdate, where it was measured a t -0.75 volt. At the higher molybdenum concentrations, the potential a t which the limiting current plateau is reached is shifted slightly 2.0 in the negative direction. The diffusion coefficient for the reducible molybdate species calculated from these data assuming an electron transfer of 3, gives a value of 7.8 X 10-6 1.0 sq. cm. per second. This value is about 34% lower than the diffusion coefficient of the chromate ion (1.1 X sq. cm. per second) and indicates that the molybdate species in the solution is larger than -*-0 J - 7 x d */ the simple molybdate ion. Undoubtedly the molyb-0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 - 0 8 -0.9 -1.0 -I, date exists in the solution as a tartrato complex of V O L T S VS. S C E a polymerized molybdate species. The degree of Figure 2. Effect of Camphor on Molybdate Reduction Wave polymerization of the molybdate species does not 3.72 X l O - 4 M sodium molybdate i n 0.l.V tartaric acid: a. 0.001% camappear t o change with increasing concentration, as phor; b. 0.002% camphor; c. 0.005% camphor; d . saturated with camphor is usual for molybdate, since a constant value is obtained for the diffusion current constant. The total diffusion current was found to be directly proporThe half-wave potentials of the molybdate waves are shifted tional to the square root of the pressure on the dropping mercury toward more negative values as the pH of the solution is increased electrode. Moreover, the temperature coefficient of the d 3 u (Figure 1). In addition, the height of the second wave is lowered sion current was found to be low (1.0% per degree in the range considerably and the wave becomes more ill defined. The desorp25' to 35" '2.). Both of these factors indicate a diffusion-contion of the reducible species apparently becomes greater as the trolled process. p H is increased. Above a p H of 4 the second wave is hardly disThe decrease in the diffusion current with increasing negative cernible. S o reduction of molybdate occurs in neutral or alkapotential may be best explained by assuming that an adsorption line tartrate solutions. of the reducible species is necessary before reduction can occur. The pH of the solution should be between 1.5 and 2.3 for the With increasing negative potential a slight desorption occurs and determination of molybdate. Below the lower limit the disthe current becomes less because not all of the ions reaching the charge of hydrogen begins to interfere. -1bove a pH of 2.3, and electrode are adsorbed and reduced. To substantiate this hyin the presence of tungsten, no constant diffusion plateau is pothesis, the effect of camphor on the reduction wave was studied. reached because of the early discharge of hydrogen and the negaCamphor is strongly adsorbed on the dropping electrode and its tive shift in the molybdate wave, as described below. Even effect on the reduction of substances which are adsorbed before without tungsten present the wave begins to become ill defined being reduced is well known (6, 8 ) . much above this pH value. Figure 2 shows the effect of caml;hor on the wave for 3.72 X DETERWIV 4TION O F MOLYBDEVURI IN PRESEVCE OF 10-4M molybdate in 0.1X tartaric acid. With 0.001% camphor, TUXGSTET the concentration of camphor is so small that its adsorption is negligible and virtually the same curve is obtained for the molybTungsten is not reduced a t the dropping mercury electrode in date reduction as that without any camphor present. As the 0 . l M tartaric acid. Large amounts of tungsten, however, lower concentration of camphor is increased, however, the area availthe diffusion current of the molybdate wave slightly. Furtherable for adsorption of the molybdate species is decreased, causing more, the presence of tungsten causes the wave, due to the disthe total reduct'ion wave to decrease considerably and the decharge of hydrogen, to start a t more positive potentials and the
-
-
ANALYTICAL CHEMISTRY
1296 limiting current, due to the molybdate reduction, to be reached a t more negative potentials. However, diffusion current values could be obtained for the molybdate reduction with concentrations of tungsten up to about 0.05M (Figure 3). Provided the concentration of molybdate in the unknown could be adjusted by proper dilution to be in the neighborhood of 2 X 10-4LW,molybdate could be determined in the presence of about 250 times as much tungsten.
Table 111. Effect of Ferric Chloride on Molybdate Wave" Concentration of Ferric Iron, Moles 0.0 0.002 0.005 0.006 0.010 Q
Diffusion Current Diffusion Current (pa.) for Iron Wave for Molybdate a t 0.0 Volt u s . S.C.E. i d / C X 10s Wave a t -0.65 Volt ... 8.39 4.32 2 : is 8.38 11.00 2.20 8.39 13.02 2.17 8.39 21.89 2.19 8.38
1.17 X lO-3M sodium molybdate in 0.1.1.1 tartaric acid.
Table IV. Determination of Molybdenum in Presence of Iron" Molybdate Added Molybdate Found Error, %
M
3 . 7 2 X 10-4 9 . 3 0 x 10-4 1 . 8 6 X 10-8
5
4
3.81 9.11 1.89
x x x
10-4 10-4 10-8
+2.4 -2.0 +1.6
a Supporting electrolyte 0.1M tartaric acid and 0.05.V ferric chloride: ferric iron reduced with sulfur dioxide.
83
EFFECT O F OTHER IONS VOLTS
vs SCE
Figure 3. Reduction Wave for 1.32 X 10-3M Sodium Molybdate in 0.1M Tartaric Acid and 0 . O S M Sodium Tungstate
'sionI n current the presence of a constant amount of tungstate the difiuof molybdate is proportional to its concentration. It should be possible, therefore, to use the method of standard additions ( 7 ) for the determination of molybdenum in the presence of tungsten. Table I1 shows some results obtained by this method. The diffusion current was corrected for residual current by extrapolation.
Table 11. Determination of Molybdenum(V1) in Presence of Tungsten(V1) Using Method of Standard Additions XIolybdate Added, Millimolar 0.372 0.186 0.372 1 18 0 235 0.372
Concentration Tungstate, Molar 0.00 0.01 0.01 0 01 0 05 0.05
Molybdate Found, Millimolar 0.379 0.181 0.379 1.18 0.232 0 369
Error, % +19 -2.7 $1 9 0 0 -1 3 -3 8
__~
The presence of the alkali chlorides, sulfates, or perchlorates causes a decrease in the total diffusion current. For example, in the absence of anything but 0.1M tartaric acid the total diffusion current a t -0.65 volt for 1.17 X 10-3M sodium molybdate was 8.39 pa. I n the presence of 0.2X sodium chloride and 0.1X tartaric acid it was 8.13 pa.; with 0.1X sodium sulfate and 0 . M tartaric acid it was 7.71 pa., and with 0 . 2 X sodium perchlorate and 0.1M tartaric acid it was 8.13 pa. Since the reduction current is governed by the adsorption of the reducible species on the electrode, large amounts of such electrolytes might be expected to decrease the area available for the adsorption or cause a decrease in the rate of adsorption. Hence, the effect of such salts on the diffusion current is not too surprising. It was found that i d / C for the molybdate reduction was still constant in the presence of a constant amount of these nonreducible salts. Thus, the effect of the presence of such salts can he circumvented by use of the method of standard additions, and the determination of molybdate can be made in concentrations of these salts up to 0.5M. Sitrate interferes causing a rather large increase in current and distortion of the wave, probably caused by the catalytic reduction of nitrate under these conditions ( 4 ) . It therefore must be removed.
B possible application of the proposed method would be for the determination of molybdenum in steel. I n view of such an application the effect of iron, chromium, manganese, nickel, copper, and vanadium was investigated. Iron(II1) is reduced in 0.1M tartaric acid to iron(I1) producing a &-ell-definedwave with a half-wave potential of 0.12 volt us. S.C.E.The diffusion current is proportional to the concentration of iron(II1) so that both molybdate and iron can be determined simultaneously in solutions containing up to about 20 times as much iron as molybdate (Table 111). With larger amounts of iron the reduction of molybdate begins to be masked, and the precision for the molybdate determination is considerably decreased. I t was found that sulfur dioxide, when bubbled through tartaric acid solutions containing iron( 111)and molybdate, quickly reduced the iron without affecting the molybdate. Since iron(I1) does not interfere with the molybdate wave, the determination of molybdenum may be made after reducing iron(II1) in this manner and removing the excess sulfur dioxide by boiling. Table IV gives some results obtained in this manner. For freshly prepared solutions containing chromium( 111) chloride in 0.1M tartaric acid, a single polarographic wave ie produced wbich starts a t about -0.6 volt us. S.C.E. and therefore would interfere with the molybdate wave. On standing, however, the green chromium(111)solution is transformed slowly into a violet form which is reduced a t a more negative potential. The green chromium complex is probably the dichlorotetraquochromium(II1) ion [Cr(H20)4C12+]and the violet complex the hexaquochromium(II1) ion [Cr(HlO)s++-] ( 2 ) . The rate of transition from one form to the other is slow, so that chromium will interfere with the determination of molybdate unless the solution is allowed to stand for a long period before electrolysis. The transformation of chromium(II1) to an innocuous form may be hastened by passing sulfur dioxide through the tartaric acid solution containing the molybdenum and chromium. A color transition from the green form to a blue-gray color occurs after a few minutes and a new chromium species, probably a sulfite complex, is formed 7%hich does not interfere with the molybdate wave. The half-wave potential of the wave for the sulfite complex of chromium(II1) is even more negative than that for the hexaquochromium(II1) complex. Further study of this system would be interesting. Seither cobalt nor manganese produces waves in 0.1M tartaric acid before the discharge of hydrogen. Xickel produces a single wave, but the wave does not begin until a more negative potential is reached than that a t which the molybdate wave is read. Thus nickel, cobalt, and manganese will not interfere in the molybdate determination. Copper, which produces a single wave in 0.1M tartaric acid
V O L U M E 26, NO. 8, A U G U S T 1 9 5 4 (half-wave potential of 4-0.15 volt us. S.C.E.), causes a slight decrease in the molybdate limiting current but does not interfere in the proposed determination when present in concentrations up to 2mM or a maximum copper to molybdenum concentration ratio of about 10 to 1 . Both species can be determined up to this limit, since the diffusion current of copper is proportional to its concentration. At higher copper concentrations a maximum appears in the copper wave which masks the molybdate reduction. An effective maximum suppressor could not be found which eliminated this maximum and did not seriously lower and distort the molybdate wave. Thus, the concentration of copper in the unknown cannot be larger than about 10 times that of molybdate. Vanadium(V) produces three waves in 0.1~11tartaric acid with half-wave potentials of about +0.4, 0.0, and -0.6 volt tis. S.C.E., caused by reduction to the +4, +3, and +2 oxidation states respectively. Since the half-wave potential for the third reduction wave is very close to the potential where the molybdate wave is read (-0.65 volt) the presence of vanadium will interfere in the determination of molybdenum unless its concentration is about that of the molyhdste. PROCEDURE
The appropriate amount of unknown solution to give a final molybdate Concentration between 1.5 X and 4 X lO-3M is placed in a beaker. The volume of this solution should not exceed 30 ml. Five milliliters of lJ4 tartaric acid is then added. The p H of the solution should be between 1.5 and 2.3. If chromium or a large amount of iron is present, the solution is heated to 80" to 90" C. while sulfur dioxide is carefully bubbled through the solution. After 10 minutes the passage of sulfur dioxide is stopped, and the solution is gently boiled for a few minutes to remove excess sulfur dioxide. The solution is then cooled to room temperature, transferred quantitatively to a 50-ml. volumetric flask, and diluted to volume. After thorough mixing, a suitable aliquot is placed in the polarograph cell, oxygen is removed by passage of air-free nitrogen, and temperature is allowed to reach equilibrium. The polarogram is then recorded. A known volume of standard molybdate solution is then added, and air free nitrogen is bubbled through the solution again for 2 to 3 minutes. The second polarogram is then recorded. It is best to add a volume of molybdate which will approximately double the wave height. The current is read a t the plateau occurring between -0.6 and -0.75 volt. (The exact potential range in
1297
which this plateau is obtained changes somewhat with change in concentration of molybdate, tungstate, and various indifferent salts.) The concentration of the molybdate is calculated by the equation: Crnolybdate
where i T' u
iVCstnd
Ai(V 3- v )
+ iu
= original diffusion current of unknown = volume of original unknown solutions = volume of standard molybdate added
Cstnd. =
Ai
=
concentration of standard molybdate
= increase in current caused by standard addition ACKZTOWLEDGM ENT
The assistance of Robert H. Moss in preparing some of the illustrations is gratefully acknowledged. LITERATURE CITED
(1) Haight, G. P., Jr., ANAL.CHEX. 23, 1505 (1951). (2) Hamni. R. E., and Shull, C. lI.,Jr., J . Am. Chem. SOC.,73, 1240
(1951). . (3) Hume, D. N., and Harris, W. L., ISD.ENG.CHEM.,A N ~ LED., 15, 465 (1943). (4) Johnson, hI. G., and Robinson, R. J., AB.
