maximum time. The graphical log plot slope determination does not offer any error estimates, but the weight of the data is more evenly distributed. Since deviations from simple first-order behavior are mathematically equivalent to increasing the number of exponential terms, the if(t> method should not be used to quantitatively detect these deviations. Practically this graphical me-thod should be less sensitive to deviations; which usually occur near the end of the reaction, than the graphical log plot. The most important data for the tf(t)plot are found
around 37 completion and the data beyond the maximum can be ignored; this is not true for log graphs. ACKNOWLEDGMENT Data used in Figure 1 were kindly supplied by Kenneth Brown. RECEIVED for review May 8, 1970. Accepted September 8, 1970.
Microdetermination of Molybdenum by Anodic Stripping at stant Current Using the Hanging Mercury Drop Electrode Philippe Lagrange, and Jean-Paul Schwing lnstitut de Chimie, I rue Blaise Pascal, 67-Strasbourg9 France ANODICSTRIPPING at constant current or with continuously varying potential has been widely used for the determination of small concentrations of metal ions plated into a mercury cathode at controlled cathode potential (1-3). Molybdenum cannot be plated into a mercury cathode and gives in acidic media soluble reduced species. However, we could show that at pH 5 the reduction of Mo(VI) at a mercury electrode (4, 5 ) gives a solid product (MoOz.2HzO) forming a thin film that shows the properties of a semiconductor. Our aim was to achieve a method for the microdetermination of molybdenum based upon the accumulation of Moog. 2 H z 0 on the mercury drop, through cathodic reduction, followed by anodic stripping a t constant current. Only a few examples of anodic stripping of a precipitate have been studied until now ( I , 2) as a method for microanalytical determinations. The following factors have been shown to be of importance in this study: the concentration of molybdenum; for a given concentration, the preelectrolysis time, Le., the mass of M o O z . 2 H z 0 deposited; and the intensity of the anodic stripping current. The stripping time is measured as shown in Figure 1, which gives the potential of the working electrode GS. the time at constant stripping current. EXPERIMENTAL The cell comprises nitrogen inlet and outlet tubes, a reference electrode (Ag-AgCl, 3M NaCl), an auxiliary electrode and a working electrode (hanging mercury drop) prepared as indicated by Gerischer (6), and Ross, De Mars and Shain (7, 8). The weight of each mercury drop is 0.00760 gram. (1) G. Charlot, J. Badoz-Lambling and B. Tremillon, “Les reactions
Clectrochimiques,” Masson et Cie, Paris, 1959.
(2) P.Delahay, “New Instrumental Methods in Electrochemistry,” Interscience Publishers, New York, 1954. (3) J. J. Lingane, “ElectroanalyticalChemistry,” Interscience Publishers, New York, 1958. (4) P. Lagrange and J. P. Schwing, C. R. Acad. Sci., Ser. C,263,
848 (1966). (5) P. Lagrange and J. P. Schwing, Bull. Soe. Chim. Fr.,536 (1968). (6) H. Gerischer, Z . Plzys. Chem. (Leiprig),202, 302 (1953). (7) J. W. Ross, R. De Mars and I. Shain, ANAL.CHEM., 28, 1768 (1956). (8) R. De Mars and I. Shain, ibid., 29, 1825(1957).
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Thermoregulation at 25 =t 0.1 “C is achieved by a water circulation device. During the cathodic deposition of MoOz.2Hz0(as well as during the anodic stripping), the solution is reproducibly stirred with a magnetic stirrer, the stirring rate of which is controlled. The cell is coated with a silicone coating in order to avoid the adsorption of molybdic anions on glass, a process which is quite strong as we have previously shown (9, IO). The electric circuit allows the working electrode to be maintained at -0.80 volt us. the reference electrode during a preset preelectrolysis time and, immediately thereafter, to apply to this electrode a constant anodic stripping current. During the anodic stripping, the potential of the working electrode is recorded, leading to the curve shown in Figure 1. The solutions contain variable amounts of molybdenum (2 x to 10-3 mole/liter), introduced as NazMoOd solution (Merck, p.a.); they are 0.02M in CH3COOH, 0.02M in CHaCOONa9and 3M in NaCl. The pH of these solutions is near p H 5 where the predominant species is probably the paramolybdic anion Moi02d6-. RESULTS AND DISCUSSION
Preelestrolysis time. Solutions containing 10-4 mole/liter of Mo(V1) have been electrolyzed at -0.80 volt us. Ag-AgCl, 3M NaCl electrode during increasing lengths of preelectrolysis time and the stripping times have been measured for a current intensity of 10 PA. (Figure 2). The quantity of electricity required for the stripping is very nearly proportional to the preelectrolysis time as long as the preelectrolysis time remains less than 260 seconds for lO-4M solutions. This result means that, in order to observe direct proportionality between the preelectrolysis time and the quantity of electricity necessary for complete stripping, the thickness of the deposit has to be limited. Integration of the preelectrolysis current corresponding to the reaction Mo(V1) 2e --I.Mo(IV) has shown that the allowable maximum thickness corresponds to about 100 monomolecular layers of MOO:! 2H20. The
+
-
(9) P. Jost, Institut de Chimie de Strasbourg (France), personal communication,1970. (10) G. Goldstein and J. P. Schwing, Bull. SOC.Chihim. Fr., 728 (1967).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
I
L
0
8
12
t (sec)
16
Figure 1. Potential-time curve. Concentration of solution: 10-4 mole/liter of Mo(V1); preelectrolysis time: 200 sec; stripping time: 14.90 sec; anodic stripping current: 10 pA
1w
2m Preelectmlysis time
300
(sec)
Figure 2. Effect of preelectrolysis time on quantity, Q, of electricity necessary for dissolution. Concentration of solution: mole/liter; anodic stripping current: 10 pA quantity of electricity necessary for the complete stripping of such a deposit is about 180 microcoulombs (Figure 2) for the electrode we used. Intensity of Stripping Current. Solutions 10-4M in Mo(VI) have been electrolyzed during a constant length of time of 120 sec, and the deposit has been stripped at variable currents between l and 30 PA (Figure 3). We have established that for a current above 3 pA, the stripping time is effectively inversely proportional to the stripping current. When the anodic stripping current is below 3 PA-this giving a current density of 0.92 pA/mm2-the stripping time is no longer inversely proportional to the stripping current. Thus, the determination of Mo(V1) must be done with an anodic stripping current density above 0.92 pA/mm2. Concentration of Molybdenum. The influence of the concentr ation of Mo(V1) on the quantity of electricity required for the stripping has been studied, the previous conditions being fulfilled. We have made sure that the supporting electrolyte by itself did not give a measurable stripping time, even after a preelectrolysis time of 2400 sec. In Table I we give, as a function of the molybdenum concentration, the quantity of electricity, q, necessary for the stripping of the Moo2 . 2 H 2 0 deposited during one time unit of preelectrolysis. As q is not perfectly proportional to the concentration of Mo(VI), it is advisable to construct a calibration curve especially for the most dilute solutions. For concentrations below 5 x 10-6A4, the V ( t ) curves no
Figure 3. Effect of magnitude of anodic stripping current I , on stripping time. Concentration of solution: mole/liter; preelectrolysistime: 120 sec Table I. Influence of Concentration of Mo(V1) on Quantity of Electricity Necessary for Stripping Preelec- Stripping Av of MoVI concn, trolysis current, stripping qa PA time, sec (KYsec) (mole/liter) time, sec 10 15.0 7.50 10- 3 20 10 12.9 3.68 35 5 x 10-4 10 15.0 1.50 2 x 10-4 100 10 15.0 0.750 10-4 200 11.0 10 0.367 5 x 10-5 300 10 17.2 0.143 2 x 10-6 1200 10 16.1 0.0671 10-5 2400 3 12.9 0.0323 5 x 101200 a q is the quantity of electricity necessary for stripping of the deposit formed during one time unit of preelectrolysis. longer show an inflection point sufficiently marked for measuring the stripping time. For concentrations above 10-sM, the chosen preelectrolysis time would have to be very short in order to keep the thickness of the deposited M o o z 2Hz0 film beneath 100 monomolecular layers; therefore, it is advisable to dilute these solutions to a known ratio in order to use preelectrolysis times sufficiently long for precise determination of the preelectrolysis time. The relative precision of the method is =kl.7%for 10-4M solutions and =k5 for 5 x 10-6Msolutions. The deposit at the mercury electrode consists of Mo(1V). A comparison between the quantity of electricity required for the formation of the deposit and that necessary for the stripping has shown that the first one was approximately twice the second one. Thus, it is probable that the deposit of Mo(1V) is reoxidized to Mo(V) which is soluble in this solution. This method is useful because it allows one t o determine Mo(V1) with a sensitivity that is ten times greater than that of ordinary polarography. a
ACKNOWLEDGMENT
The authors thank L. B. Rogers for helpful discussion of this paper. RECEIVED for review May 7, 1970. Accepted September 14, 1970. This work was supported in part by the Centre National de la Recherche Scientifique E.R.A. No. 166.
ANALYTICAL CHEMISTRY, VOL. 42,
NO. 14, DECEMBER 1970
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