Coulometric Generation of Molybdenum(V1) from a Molybdenum Electrode-Titration of Lead(l1) G. Stephen Kelsey and Hurd W. Safford Department of Chemistry, University of Pittsburgh, Pittsburgh, Pa. 15260
The earliest report of coulometric generation of an anionic species from an elemental electrode was by Monnier and Zwahlen ( I ) who generated Cr(V1) as CrrOj2- and Cr04?- in acidic and basic media, respectively. Subsequently, Kostromin applied the generation of Cr(V1) to the determination of a number of transition metals and rare earths (2-4). In addition to chromium, vanadium , as V03-, has been generated from a vanadium electrode in 0.1M NaOH ( 5 ) . The current-potential and current-efficiency data reported here indicate that molybdenum metal is a suitable generant for Mo(V1) over a fairly broad pH range and can be used as a titrant source.
EXPERIMENTAL Apparatus. Molybdenum electrodes were fabricated by sealing glass tubing around lengths of %-inch diameter rod (99.95% purity, A. D. Mackay, Inc., New York, N.Y.). T h e area of each, without regard for roughness, was 2 cm2, Electrical contact was secured by sealing a copper lead wire in a hole drilled in t h e molybdenum. Both a Sargent Model IV Coulometric Current Source a n d a :?OO-volt constant current generator, with output continuously variable from 0 t o I5 mA, were used in conjunction with a Sargent Model N X digital p H meter for t h e recording of E = f(i) curves. A Heath Model EUA-19 Polarography Module, operated manually with current output recorded on a Sargent Model S G R potentiometric strip chart recorder, was used t o obtain i = / ( E ) curves. T h e Sargent current source was standardized using a precision IO-ohm resistor (L&N, 10.01"/0, oil immersion type) and a Sargent potentiometer. All electrochemical studies were carried out in Metrohm EA 880 Universal Titration Vessels. Current-potential experiments and amperometric titrations employed a saturated calomel electrode (SCE), with 'J' tip, as d e scribed by Lingane ( 6 ) .T h e counter electrode used in coulometric titrations and i-E studies was a platinum electrode of about 1.5 em2 area, placed in an isolation tube. Titrant for amperometric titrations was delivered from a Sargent Automatic Constant Rate Burette calibrated by the weight method. Reagents. Standard solutions of Mood2- were prepared by dissolving molybdenum rod of 99.95% purity in "0.1, evaporating t o dryness, dissolving t h e Moos in a minimum of 1M NaOH, a n d diluting with deionized water. Such Mo04'- solutions were stable for about three weeks. Lead solutions were prepared from stock made by dissolving 99.999% pure lead shot (A. D. Mackay, Inc.) in nitric acid. evaporating to dry Pb(NOJ2, and taking u p in dilute " 0 3 . All other chemicals used were reagent grade. Deionized water was used to prepare all solutions a n d prepurified nitrogen was used to purge them prior to all experiments. Procedure. Any necessary procedural details will be found in the pertinent discussion sections t h a t follow.
(1) D. Monnier and P. Zwahlen, Helv. Cbim. Acta., 39, 1865 (1956). (2) A . I. Kostromin, A. A. Akhmetov, and L. N. Orlova, Zb. Anal. Kbim., 25, 189 (1970). (3) A . I . Kostromin and A A. Akhmetov, Zb. Khim., 1969, Abstr. No. 20G110 (4) A . I. Kostromin and A . A . Akhmetov, lssled. Elecktrokbim. Magnefokbim. Elecktrokbim. Mefod. Anal., 1969 (2),221, (5) A. I. Kostromin, P. K. Agasyan, and V. N. Basov, Zh. Anal. Kbim., 25, 21611970). (6) J . J . Lingane. "Electroanalytical Chemistry," 2nd ed., Interscience, New York, N.Y. 1958, p 362.
Figure 1. E = f
( I ) Current-potential curves in NaN03 solutions
r)
20.B
-.
_lp---~-_-J-p-_--
20
10
63 CURRENT
80
. . l
00
.--L ~
I23
I40
I60
3EkSiTY. iY4/CMzl
Figure 2. Current efficiency vs. current density for Mo(VI) generation in N a N 0 3 solutions (A)O.O5M;(B)O.lM;(C) l.OM;(O, 2.5M
RESULTS AND DISCUSSION Current-Potential Characteristics. Current-potential curves were obtained both as E = f(z) and z = ! ( E ) in various concentrations of NaC104, NaN03, and NaOH, under stirred conditions. In all cases, the curves obtained by the two methods were identical within experimental error. The anodic characteristics of molybdenum in various concentrations of NaOH are essentially identical to those of vanadium in the same medium ( 5 ) .Figure 1 represents the i-E behavior in NaNOj solutions and is identical uiih that observed in NaC104 solutions of the same concentrations. While the I-E behavior in dilute solutions would indicate possible passivation and concomitant reduction in curient efficiency, later current efficiency studies showed the opposite to be true. Unlike the generation of V(V) in hydroxide medium ( 5 ) , the current efficiency in 0.05M NaOH for Mo(V1) remained essentially constant a t 90 f 2% over the current density range of 20 to 160 mA/cm'.
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Table I. Data and Results of Titrations of Pb(I1) Metal taken, mmoles" 0.03183 f 0.00007 0.02193 f 0.00005 Time t o end point, sec= 210.0 f. 0 . 0 141.2 f 3.2 Pb(I1) ion found, mmolesn 0.03183 f 0 . 1 % 0.02188 f. 2 . 1 % Current efficiency, 75 e 9 1 .o 93 . O Relative error, % 0.0 -0.2 Value is the average and standard deviation of five trials. Equation 3.
+
4H20
-
Moo4'-
+
8H'
+
6e
(1)
In basic medium, the reaction is very favorable and, in fact, the highest current efficiencies were obtained in sodium hydroxide, even a t very low current densities. However, since this basic medium would preclude the successful determination of lead (precipitation of Pb(OH)2), further investigation was focused on NaN03 solutions. Some formation of molybdenum blue was observed in 1M and 2.5M NaNOa solutions a t intermediate current densities. It is possible that the acidic nature of these solutions (pH 5 to 6), inhibited the complete oxidation of molybdenum to the +6 state, according to the equilibrium in Equation 2 (9).
H2Mo04,,,
+
2H'
ie
e MOO,'
+
2 H 2 0 (2)
pH Effects. The current efficiency for generation in 0.1M NaN03 was studied over a pH range of 3.2 to 9.5 and a slight dependence was observed. Subsequent analyses of lead solutions were carried out between pH 4.3 and 5.6, where current efficiency fluctuated less than 0.25%. Effect of Generation Time. The current efficiency for generation of Mo(V1) was found to be a linear function of time of current application, over the range of 10 to 230 sec(7)
J. J. Lingane, C. H. Langford, and F. C. Anson, Anal. Chim. Acta., 16, 165
(1957). (8)G. H. Aylward, Anal. Chim. Acta., 14, 386 (1956). (9) J.J. Lingane, "Analytical Chemistry of Selected Metallic Elements," Reinhold Publishing Corp., New York, N.Y., 1966, p 88.
1586
0.001929 i= 0.000009b 12.0 f 0 . 1 0.001941 f 0.8% 96.8 +0.6
These data are based on three trials in 50% ( v h ) ethanolic 0.1M NaN03.
Current Efficiency. The applicability of the interpretation of current-potential curves in terms of current efficiency, as outlined by Lingane (7), was checked by chemical analysis of the cell solution following generation of Mo(V1) a t various current densities. The method of analysis, the amperometric titration of MOO**- with Pb2+,as described by Aylward (8), was used without modification. Figure 2 represents those generations conducted in NaN03 with total coulombs constant at 24.12. In nitrate solutions of lower concentration, where passivation would be expected to reduce current efficiency a t high current density, a plateau is observed. Another deviation from predicted behavior occurred in higher concentrations of NOS- where current efficiency decreased to well below 100% a t high current densities. While the mechanism of molybdenum dissolution is not known, it undoubtedly involves hydrolysis, and so the high ionic strength would be expected to slow down step(s) involving solvent molecules. The net dissolution reaction is given in Equation 1.
Mo
0.01058 f 0.00003 6 7 . 1 f. 1 . 7 0.01064 i= 3 . 2 % 95.2 $0.6
Calculated from
onds, a t 96.43 mA. Equation 3 was derived from current efficiency data for several generation times in 0.1M NaN03. C E ( % ) = -0.0295k 97.2
+
Where conditions such as electrode purity, differ from those stated herein, the slope and intercept of Equation 3 may differ, and need to be established before applying the method to analysis. The decrease in current efficiency with time may be attributed to the formation of insoluble oxidation products on the electrode surface, which impede the diffusion of soluble species into the bulk solution. Under sustained current application, a black, amorphous coating was observed on the molybdenum anode, which was easily wiped off a t the end of an experiment. Beneath this coating, a blue, adherent layer was often observed. Coulometric Titration of Pb(I1). To demonstrate the applicability of the coulometric generation of Mood2-, the precipitation of lead was chosen. After the sample solution was made 0.1M with respect to sodium nitrate, it was purged with nitrogen for ten minutes. The molybdenum electrode used to follow the titration potentiometrically was sanded lightly before use. A Beckman SCE was inserted in an isolation tube filled with O.1M NaN03, and served as reference electrode. After each generation increment, one minute of equilibration time was allowed before the potential was recorded. The smoothing of the potentiometric data obtained a t such equilibration times, permitted use of first derivative plots for location of the end point. Titrations carried out at extended equilibration times showed no change in end-point location, within experimental precision, but were marked by some scatter, which made first derivative plots difficult to interpret. Only the lowest level of lead required two minutes equilibration time between titrant generations, and the size of the generation increment was adjusted so that about ten additions were made on either side of the end point. The results of the titrations are given in Table I. The precision of the end point a t the lowest lead concentration was made possible by using a 50% (v/v) ethanol solution of 0.1M NaN03. The precision of the two intermediate concentrations could also likely be improved by use of an ethanolic medium, but it was decided that the accuracy obtained was sufficient. Thus, accurate determinations of Pb(I1) can be achieved, even though the current efficiency for molybdate ion generation is not 100% under the conditions described herein.
RECEIVEDfor review March 18, 1974. Accepted May 13, 1974.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 11, SEPTEMBER 1974
