Microcell for Voltammetry wit William b. Underkofler and Irving Shain, Department of ~ h ~ m University ~ ~ ~ ~of yWisconsin, , Madison, Wis.
HE hanging mercury drop electrode (HMDE) has been used in several analytical applications which are versatile and extremely sensitive (1, 3, 6, 8, IO). No particular effort has been made, however, to investigate the lower limit of sample volume required to perform convenient producible analyses. Since a cant reduction of sample volume would be of use in extending the sensitivity of methods such as stripping analysis (1, I I ) , a microcell was designed and tested in which analyses could be carried out with sample volumes on the order of 0.5 ml. One of the major problems in the design of such a cell results from the rather bulky components used with a hanging mercury drop electrode. The electrode itself is compact, as it consists of a drop of mercury (about a 0.05-cm. radius), hung on a small platinum wire sealed in glass tubing. The associated components, however, are a dropping mercury electrode capillary, and a scoop which is used to catch the drops of mercury and transfer them to the platinum wire. As a result, cells which have been used previously require about 100 ml. of sample solution. Another form of the hanging mercury drop electrode, which does not require bulky associated equipment. has been described by Kemula (3, 6). The mercury drop is formed on the end of a capillary connected to a small sealed mercury reservoir. A screw extends through the cap of the reservoir into the mercury. By rotating this screw, mercury can be forced through the capillary, thus forming the hanging mercury drop electrode. Such electrodes are very sensitive to temperature changes, however, and even for routine analytical work, the cell temperature must be controlled carefully. The inconvenience of jacketed cells or working in a thermostat seemed to outweigh the advantages of a less bulky electrode system. Thus, in this work, the hanging mercury drop electrodes used were of the type previously used in this laboratory (10). Electrodes of this type have been criticized by Kemula (3, 4, 6), who reported that when mercury drops are
1966
ANALYTICAL CHEMISTRY
hung on platinum wires, small m o u n t s of platinum are dissolved in the mercury, and that anomalous results are observed in some cases because of interactions between the plathum amalgam and the products of the electrode reaction. Such observations have not been confirmed in this laboratory, and no anomalous interactions could be traced to the presence of platinum in the hanging mercury drop electrode.
K
1Figure 1. C.
K.
R. W.
20mm-q Diagram of microcell Cell body Cell lid Reference electrode Working electrode
The possibility of such effects may have been minimized by the small area of the platinum wire exposed to the mercury, and by the fact that normally a new drop of mercury is used for each experiment. In any event, no loss of platinum from the wire has been noted even from electrodes which have been In use for periods up to a year. With hanging mercury drop electrodes hung on a platinum wire, most experiments could be performed without controlling the cell temperature. However, to reduce the volume of solution required, the capillary of the d r o p ping mercury electrode was not placed directly in the cell. The mercury drops were formed externally, caught in a special cup, and then transferred to the platinum wire. The cell was designed so that the cell body could be removed and replaced without disturbing either the contents (except for exposing the solution to oxygen) or the electrodes. Thus, new drops of mercury could be hung on the platinum wire as desired. Construction of the Microcell. The cell used is shown in Figure 1. The cell body was made from a standard taper 10/18 joint (Corning 6640) sealed off flat about 17 mm. below the grinding. The total volume was about 1 nil.; the working volume, 0.3 to 0.5 ml., was about tenfold larger than the microcells previously described (8) 7) for use with other electrodes. The lid of the cell was machined from a 1/&xh Teflon rod (Figure 1). Holes were provided for the hangiqg mercury drop electrode, the reference electrode, and a nitrogen dispersing tube. Also, a very small hole was provided as a nitrogen exit. The two electrodes were pressed tightly into the lid. The nitrogen inlet fitted loosely so that it could be raised above the level of solution after the oxygen had been removed. The upper part of the Teflon lid was made long enough so that it could be clamped conveniently. The entire assembly was supported in this fashion; the cell body required no additional support, provided the Teflon was properly machined to fit the standard taper joint. The hanging mercury drop electrode was constructed by sealing a short
Table 1.
Stripping Analysis of
kv. Peak" Height, ip i,/tC Av. Dev.,a @mP. ( X 106) % 2.06 3.9 5.15 5 2.12 4.5 0.530 5 2.07 5.0 0.155 15 5.00 X -4verage and average deviation of five replicat,e determinations on the same solution.
Concentration, C, Moles/Liter 5.00 x 10-8 5.00 x 10-7 0
1-25 Figure 2.
mm+
Mercury drop transfer eup
piece of platinum wire (0.016-inch diameter) in the end of a 3-mm. soft glass tube. The end of the electrode was polished on a metallurgical polishing wheel (600X Alundum grinding powder), until the end of the platinum wire formed a smooth, flat surface with the glass. The platinum then was etched back a few thousandths of an inch with aqua regia and plated with mercury. This procedure ensured that no platinum was exposed t o the solution and eliminated the possibility of extraneous hydrogen reduction curr e d s on the platinum surface. One, two, or three drops of mercury from the dropping mercury capillary could be hung on the electrode prepared as above. The reference electrode was constructed by sealing a short asbestos fiber in the end of a 3-mm. soft glass tube. This tube contained saturated potassium chloride and also a silver/ silver chloride electrode which was formed by anodizing a length of silver wire (0.040-inch diameter). Contact was made directly to the silver wire which protruded through the top of the cell. The nitrogen dispersing tube was constructed by sealing a bundle of very fine capillaries in the end of 3-mm. glass tubing, drawing i t out slightly, and then breaking it off to produce a nozzle of fine holes. For stripping analysis, reproducible stirring is required. Although Nicholson (9) has reported the successful use of a controlled flow of nitrogen to provide this stirring for stripping analysis in larger cells, attempts to use the method in this work were not successful. The cause was probably the small size of the cell, which prevented setting up uniform convection. Therefore, small magnetic stirring bars were constructed by sealing '/d-inch lengths of iron wire in 1-mm. glass tubing. A small horse shoe magnet, mounted in a Bargent synchronous rotator, was used to provide reproducible stirring.
admiurn in 0.1M Potassium Chloride
Preelectrolysis Time, t , Minutes
The cup for transferring mercury drops (Figure 2) was machined from Teflon. In use, the cup was filled with the indifferent electrolyte and held with the tip of the dropping mercury capillary immersed in the solution. The first one or two drops of mercury were allowed to fall into the outer portion of the cup, and then the one or two drops to be used for the hanging mercury drop electrode were caught in the center (raised) portion of the cup. The main disadvantage of this procedure was that it was necessary to remove the cell body in order to hang a new drop of mercury on the platinum wire. This, in turn, made i t necessary to deaerate the solution each time the hanging mercury drop electrode was replaced, However, deaeration times of the order of 3 minutes or less were sufficient when analyzing solutions a t the lO-4M concentration level, because of the small volume of the cell. All data were obtained uith a modfied Sargent Model XV Polarograph, The rate of voltage scan was 33.3 mv. per second. The cell resistance was on the order of 500 ohms when the cell was filled with 0.1M potassium chloride. To remove oxygen from the cell, high purity nitrogen was used without further purification. To prevent evaporation of the solution, the nitrogen was bubbled through a test tube containing the indifferent electrolyte before i t entered the cell. RESULTS
To test the reproducibility of the procedure used to form the hanging mercury drop electrode, replicate determinations using fast sweep polarography (voltammetry with linearly varying potential) were performed on a solution containing 2 X l O - * M Cd+Z in 0.1M potassium chloride. For each determination, the cell body was removed, a new drop was hung, the cell body containing the solution was replaced, and nitrogen was bubbled through the solution for 3 minutes. Then the nitrogen dispersing tube was raised above the surface of the solution, and 30 seconds was allowed for the solution to come t o rest before starting the voltage scan. The average deviation of five replicate determinations performed in this manner was less than
0,6010, indicating that mercury drops of reproducible size could be collected from the capillary. To test the cell with very dilute samples, stripping analyses were performed on cadmium solutions in the concentration range of 5 X 10-6 to 6 X 10-8M. The results, Table I, indicate that stripping analyses can be performed conveniently with very small volumes of solution, The range of error probably could be reduced somewhat by placing the cell in a thermostat, but it was felt that the 3 to 5% error level was adequate for most applications at these concentrations. More serious errors ordinarily arise in sampling. For a 15-minute electroiysis, the most dilute sample was depleted by about 10%; but, since most of this is reoxidized into the solution on the anodic scan, replicate stripping analyses can be performed readily on the same solution, even with the small sample volume. These results re-emphasize the remarkable sensitivity of stripping analysis with the hanging mercury drop electrode, since for the 5 X 10-8M solution of Cd+2, the sample contained a total of 2.5 X lo-" mole or 2.8 X 10-9 gram of cadmium. LITERATURE CITED
(1) DeMars, R. D., Shain, I., ANAL. CHEY.29, 1825 (1957).
(2) Iwamoto, R. T., Adams, R. N., Lott, H., Anal. Chim. Acta 20, 84 ( 1959). (3) Kemult: W., in "Advances in Polarography, I. S.Longmuir, ed., Vol. 1, p. 105, Pergamon Press, Kew York, 1960. (4) Kemula, W., Galus, Z., Kublik, Z., Bull. Acad. Polon. Sci., Sdr. Sci., Chim., Geol. et Geograph. 7 , 723 (1959). (5) Kemula, W., Kublik, Z., Anal. Chim. Acta 18, 104 (1958). (6) Kemula, W., Kublik, Z., Galus, Z., Nature 184, 1795 (1959). (7) Lord, S. S.,O'Neill, R. C., Rogers, L. B., AXAL.CHEW24, 209 (1952). ($) Martin, X. J., Shain, I., Ibid., 30, 1808 (1958). (9) Nicholson. RT. ll., Ibid., 32, PO58 (1960) (10) Ross, J. W., DehIars, R. D,, Shain, I., Ibid., 28, 1768 (1956). (PI) Shain, I., Lenilisoll, J., Ibid., 33, 187 I
(1961).
WORKwas supported in part by fun& re-
ceived from the United States Atomic Energy Commission under Contract Nu. AT (11-1)-64, Project No. 17. VOL. 33, NO. 13, DECEMBER 196% e
1967
