Coulostaiic Anodic Stripping with a Mercury Electrode AKIKO ARAMATA and PAUL DELAHAY Coates Chemical laboratory, louisiana State University, Baton Rouge, l a .
b Coulostatic anal,@ is combined with anodic stripping with a hanging mercury electrode. Experimental results are given for zinc determination down to 5 X 1 O-8M,,and potentialities for application to lower concentrations are indicated. A (differential instrument for the measurement of potentialtime curves is described.
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of the coulostatic method with anodic stripping for a mercury electrode was previously pointed out (1, 3, 5 ) and was discussed quantitatively for the plane electrode (2). The metal to be analyzed is plated on a mercury electrode and the decay of potential during anodic oxidation, immediately after plating, is determined under coulos,tatic conditions. Experimental results are reported here for the hanging mercury drop with metal plating in a stirred or iinstirred solution. HE COMBINATION
EXPERIMENTAL
One pulse is applied to the cell, CL, in series with potentiometer, P2, and the other pulse to adjustable capacitor, c3. The voltage applied to the differential amplifier of the cathode-ray oscilloscope, CRO, is equal to zero a t time t = 0 after coulostatic charging provided c3 is properly adjusted. The compensation circuit previously used (5) is then not necessary. Coincidence in the closing of the contact, 2, of relay RL is essential for satisfactory operation. Capacitor c4 was connected in parallel with the cell to slow down the decay of potential (6). Cell. An all-glass H-cell was used with a hanging mercury drop prepared according to ROSS,DeMars, and Shain (8). The arm of the cell without the hanging drop was connected to a saturated calomel electrode with a bridge with 1M potassium nitrate. Both arms of the 13-cell contained the solution being analyzed and were purged with nitrogen. Contamination of the solution in the arm with the hanging drop by diffusion from the other arm was thus prevented. This arrangement as well as the plating technique
Instrument. Potential-time curves were recorded with a differential instrument (Figure 1:1. Operation is similar t o t h a t of the instrument previously described (1, 6 ) except that two charging pulses w e now supplied by capacitors cl and c2, respectively.
were similar to those of Deblars and S h a h (7). The cell resistance was approximately 40,000 ohms, and the time constant for the discharge of capacitor el was 0.02 second. The initial voltage acrom cl, prior to discharge across the cell, was 4 volts, and discharge of el thus was essentially completed within less than 1 my. before 0.25 second) a t which the time (t measurements were taken along potential-time curves. Solutions. Analytical reagents were used as supplied. Purification of the supporting electrolyte (potassium nitrate) was unnecessary as its concentration was only 1mM. Water twice-distilled over potassium permanganate was used for solution preparation.
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RESULTS AND DISCUSSION
Plating from Stirred Solution. Curves showing the decay of potential under coulostatic conditions are given in Figure 2 for a constant plating time of 200 seconds. The blank is I
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Figure 1. Instrument for differential recording of potential-time curves c1. ct. 0.5-pf. capacitor, General Radio, Type 5 0 5 X ca. 0 to 1.1 1 1-pf. decade (capacitor box, General Radio, Type 141 9-K c,. Same capacitor box as :)j odjusted a t 1 pf. in this work CL. Cell CRO. Cathode-ray oscillosccipe, Tektronix 5 3 5 with D amplifier PI. 10,000-ohm Helipot potentiometer with 0- to 45-volt adjustment of span voltage Pz. 100-ohm Helipot potentiometer with 0- to 2-volt adjustment of span voltage Ri, R2. 22 megohms RL, triple pale relay
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Figure 2. Tracings of oscillograms showing variations of potential with time for oxidation of zinc after plating on a mercury drop for 200 seconds from solutions of zinc ions at different concentrations Supporting electrolyte. 10-3M KNOa adjusted at p H 4 with HNOa Temperature. 2 Z 0 C. Electrode area. 0.0346 sq. cm. Plating a t E = 1.1 volts VI. S.C.E. for 200 seconds Potential E at t = 0 after application of coulostatic pulse = 0.05 volt VI. S.C.E.
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VOL. 35, NO. 9, AUGUST 1 9 6 3
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very low because the coulostatic pulse brought the potential during anodic oxidation in the range ( E = 0.05 volt vs. S.C.E. a t t = 0) in which oxygen and most other impurities are not reduced and few impurities are oxidized. Conditions are thus more farorable in general than in direct coulostatic analysis because of decrease in the blank correction, but the above procedure is only applicable to analysis of a single metal. There is, in addition, an increase in sensitivity as in the ordinary stripping method. It was previously pointed out (2) that plots of AE against t 1 1 2 for the potential decay curves are essentially linear for a plane electrode when the plating time is sufficiently long. The concentration of metal in mercury before stripping is fairly uniform and the plot of AE us. t l ' Z is thus linear. Conditions in this respect are even more favorable for the hanging spherical electrode than for the plane electrode, as shown by Shain and Leivinson (9), and experimental plots of A E w tl'zivere indeed linear within experimental errors except for small values of 1 (Figure 3). Values of AE for a given time, t, in the linear region of such plots mere essentially proportional to the concentration of zinc in solution. Variations of potential during coulostatic stripping at a given time, t, also increased with the plating time (Figure 4), and the sensitivity was increased accordingly. Plating times up to 1 hour have been used in direct anodic stripping ( 7 ) . Plating from Unstirred Solution. Results for 10-7 to 5 x 10-idI Zn were similar to those obtained above except
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Figure 4. Plot of A € vs. t1/2 after blank correction for 1.5 1 O-'M Zn*2 for different plating times
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Same conditions otherwise as in Figure 2
that values of AE for comparable conditions were smaller. For instance, AE for a plating time of 200 seconds was 8 mv. for t = 1 second and 10-7M Zn for the unstirred solution whereas aE = 42 mv. for the same conditions but with plating in stirred solution. Results were also less reproducible than for plating with a stirred solution.
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CONCLUSIONS
Combination of anodic stripping with coulostatic analysis greatly minimizes the blank correction when stripping occurs in a range in which oxygen and other impurities hardly interfere. Sensitivity is also enhanced, as for direct anodic stripping, but the difficulty resulting from the double layer charging in the latter method is eliminated. rlpplication to lower concentrations than those used in this work appears feasible by selection of a sufficiently long plating time, Measurements could be simplified by use of the direct-reading instrument for coulostatic analysis recently developed (4, 6). LITERATURE CITED
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(1) Delahay, P., ANAL. CHEM. 34, 1262 (1962). ( 2 ) Ibid., p. 1662. (3) Delahay, P., Anal. Chim. Acta 27, (1962). lid., p. 400. elahay, P., Ide, Y., ANAL.CHEM.34, 1580 (1962). (6) Delahay, P., Ide, Y., Ibid., 35, 1119 (1963). (7) DeMars, R. D., Shain, I., Ibid., 29, 5 (1957). oss, J. D., DeMars, P. D., Shain, 68 (1956). winson, J., Ibid., 33, 187
Figure 3. Plot of AE vs. t'IZafter blank correction for data of Figure 2
RECEIVED for review February 26, 1963. Accepted May 21, 1963. Research supported by the Office of Naval Research under contract Nonr-1575(04).
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ANALYTICAL CHEMISTRY
