Spot Electrolysis DENNIS H. EVANS Department o f Chemistry, Harvard University, Cambridge, Mass.
b An electroanalytical method in which the sample is placed directly on the electrode surface is proposed and tested. A small volume of sample solution is placed on the electrode and the solvent is allowed to evaporate leaving a spot on the electrode. A method of bringing supporting electrolyte in contact with the spot with simultaneous electrolysis is devised. The resulting current is a function of the quantity of electroactive material in the spot. Less than one nanoequivalent is detectable and the useful quantitative range is from 5 to 20 nanoequivalents. The method is tested with Fe(lll) and Cd(ll) reduction and hydroquinone oxidation.
H
IGH SENSITIVITY of voltammetry
can be increased in two ways. First, modifications can be made in the electrical circuit. For example, the electrical signal from the cell can be differentiated (5, 8 ) or a modified signal can be applied to the cell as in alternating current polarography ( 2 ) . The second approach is to increase the rate of movement of electroactive material to the electrode surface by introducing convective transfer. The electrode can be moved through the solution as with the rotated wire electrode (6) and the rotated disk electrode (3) or the solution can be moved past the electrode by simply stirring the solution or in the case of the tubular electrode ( I ) by pumping the solution through the electrode. Convective transfer as used in the above techniques increases the sensitivity by a factor of about twenty over methods in which transfer is limited by diffusion. A natural extension of the above methods would be one in which the electroactive material was already at or near the electrode surface so that the rate of mass transfer would not be so severely limiting. This could be attained by applying a small volume of sample solution to a solid electrode and allowing the solvent to evaporate leaving a spot of electroactive material adhering to the electrode. The electrode would then be mounted with the lower part submerged in supporting electrolyte solution but with the sample spot above the solution, After adjusting the potential of the electrode to a value where the electroactive material would 1520
ANALYTICAL CHEMISTRY
be reduced (or oxidized), the supporting electrolyte would be brought in contact with the sample spot and the resulting current would be measured. An investigation of the practicability of this general method is reported in this communication. The proposed method bears resemblance to stripping analysis (10) in which electroactive material is concentrated in or on an electrode by preelectrolysis. I n spot electrolysis, however, the sample is placed directly on the electrode and it need not be deposited by a prior electrochemical reaction. EXPERIMENTAL
Apparatus. The electrode used in this study is shown in Figure 1. The platinum wire served both as a mechanical support and a n electrical contact. The surface of the gold foil was etched lightly by dipping i t in aqua regia because sample spots of reproducible size and character are formed more easily on a slightly roughened surface. The solution level can be raised from the initial to the final level in several ways. I n the first method attempted, the electrode was mounted rigidly in the cell and additional supporting electrolyte was pumped into the bottom of the cell which slowly raised the solution level engulfing the sample spot. The slow passage of the solutiongas interface over the spot produced a very erratic and irreproducible current-
d2
3 Heavy Gold Foil ( I x 4 x 0.04cml
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_-,,Final
Solution Level
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Solution Level
-Sample Figure 1. ysis
spot
Electrode for spot electrol-
time curve though the results were more reproducible when the level was raised rapidly so that the spot was covered in 1-2 seconds. Therefore a second method was adopted which allowed very rapid coverage of the spot. The electrode was mounted rigidly with the gold foil extending downward into the cell which was itself mounted on a laboratory jack (Big Jack, Precision Scientific Co., Chicago). The level of the cell could then be raised with respect to the electrode by a quick turn of the screw control and the solution level would pass suddenly over the sample spot. The time required for a halfturn of the wrist which raised the cell 1 cm. was found to be 0.13 f 0.01 second by oscilloscopic measurements with the jack breaking and making contact in an electrical circuit. The cell was an ordinary polarographic H-cell. The auxiliary/reference electrode was a mercury pool covered by either mercurous sulfate or mercurous chloride, depending upon whether the supporting electrolyte which was used in both sides of the cell contained sulfate or chloride ions. A fritted glass disk separated the two compartments. The cell resistance with 0.2F potassium chloride as supporting electrolyte was 360 ohms. Voltage was applied to the cell with a manual polarizing unit ( 7 ) comprised of a 12-volt storage battery and 1000ohm potentiometer. A 100-ohm precision resistor (General Radio Co., Type 500-D) was placed in series with the cell and the current was determined by measuring the iR drop with a recording potentiometer (Sargent Model M R , full-scale response time of 1 second). This circuit does not provide potentiostatic control because it does not compensate for iR drop in the cell circuit, changes in the back e.m.f. of the cell and the polarizability of the auxiliary/reference electrode. This means that the potential of the gold foil electrode moves from its initial value on the plateau of the voltammetric wave of the electroactive material to less cathodic values (or less anodic values for oxidation reactions). It soon returns to the initial potential as the current decreases. I n experiments in which the potential was controlled, a potentiostat similar to that of Underkofler and Shain (11) was used and a platinum auxiliary electrode was included in the cell. Integration of the current-time curves was effected by photographing the current-time trace of a Tektronix type 502 oscilloscope using a Polaroid camera attachment with “PolaLine” Type 146-L
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2 0 nanomoies Fe(lIl)
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Figure 2.
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T I M E , seconds
Typical current-time curves
for Fe(lll) reduction Supporting electrolyte: 1 F sulfuric acid. potential: -0.2 volt VI. S.C.E.
Initial
projection film followed by graphical integration of enlargements of these photographs. Procedure. Samples were applied to the electrode by transferring 5 to 50 pl. of sample solution to the surface with a calibrated microliter syringe (Hamilton, No. 705-N). For solvents which evaporated slowly, the electrode was placed on a hot plate whose temperature was 60' to 85' C. depending upon the solvent. The sample remained in a small spot adhering to the electrode. To duplicate exactly the size of the spots was not convenient though the average diameter was easily made to be within 10% of 3 mm. With the reduction of Fe(II1) doubling the spot size decreased the current by only 5% so small variations in spot size can be tolerated. The electrode was mounted with the lower part submerged in the deaerated supporting electrolyte which was allowed to become quiescent for 2 minutes while nitrogen was passed over the surface. The potential of the electrode was adjusted to the desired initial value and the cell was quickly raised with the jack and the resulting current was recorded. The electrode was then removed, rinsed with water, and dried before another spot was applied. Alternatively two or more spots could be applied in a vertical row and each spot could be electrolyzed in turn without removing the electrode from the cell. RESULTS AND DISCUSSION
Iron. Typical current-time curves for the reduction of Fe(II1) are shown in Figure 2. The sample solution was 1.07 X l O - 3 F FeNH4(S0&.12HzOin iF acetic acid. Sample volumes ranged from 5 to 20 pl. The supporting electrolyte was 1 F sulfuric acid and the initial potential was -0.2 volt us. S.C.E. which is 0.6 volt more cathodic than the formal potential for the Fe(II1) /Fe(II) couple in 1F sulfuric acid (4). The obaerved current for the reduction of Fe(II1) to Fe(I1) is initially
very large but it decays rapidly to a small, constant value. The current a t a given time after the beginning of the electrolysis was chosen as a convenient measure of the quantity of Fe(II1) in the spot. As is shown in Figure 2, 3 seconds was the time most frequently used. The lower curve in Figure 2 shows that a significant current is obtained when no Fe(II1) is on the electrode. This is due to charging of the electrical double layer of the electrode surface that suddenly enters the solution and the reduction of solution impurities such as oxygen. The latter source of the current can be seen in the lower curve of Figure 2 where the final constant current is about twice the current before movement of the cell. If the observed current minus the final current is integrated over time, the quantity of electricity due to charging of the double layer in the lower curve of Figure 2 is 80 pcoulombs. The rest potential of a gold electrode in 1F sulfuric acid was found to be $0.6 volt us. S.C.E. Thus the potential was changed from 4-0.6 volt t o -0.2 volt us, S.C.E. Because the increase in electrode area in contact with the solution was 2 sq. cm., the average double layer capacitance would have to be 50 pfarads/apparent sq. cm. in order for this current to be due entirely to double layer charging. This capacitance is not unreasonable for solid electrodes. Because double layer charging and reduction of impurities also occur when a sample is on the electrode, the currents observed with Fe(II1) should be corrected by subtracting the blank current observed when no Fe(II1) was on the electrode. Typical data for Fe(II1) reduction are summarized in Table I. The average deviation from the mean for three or four trials is given as a rough indication of reproducibility. The corrected currents are quite close to being proportional to the quantity of Fe(II1) in the spot, the average value of the ratio of corrected current to quantity of Fe(II1) being 6.06 f 0.06 pa./nanomole. The currents may be measured a t other times. For example, the average value of the ratio of corrected current to quantity of Fe(II1) at 6 seconds after the beginning of electrolysis is 3.00 f 0.10 pa./nanomole. When the quantity of Fe(II1) exceeds 25 nanomoles, the results become irreproducible. The shape of the current-time curves depends upon the initial potential. For example, compared to an initial potential of -0.2 volt vs. S.C.E., an initial potential of 0.0 volt produces a current which is smaller at the beginning and greater toward thp end of the electrolysis. This is because the electrical circuit does not provide potentiostatic control and the potential
Table 1.
Reduction of Fe(lll)
(Current measured at t = 3 seconds. Blank correction = 11.0 pa.) CorQuantity rected of Fe(III), current, pa. nanomoles Current, pa. 1.07 5.35 10.7 21.4
17.6 f 0.03 43.2 f 0 . 9 76.1 f 3.9 139 f 1
6.6 32.2 65.1 128
of the electrode deviates from the initial potential. This effect is more pronounced with less cathodic initial potentials. With potentiostatic control, the shape of the current-time curves is almost independent of the initial potential. The data in Table I pertain to samples in which an Fe(II1) salt is the only nonvolatile material. To determine the effect of other solid but nonelectroactive components, potassium chloride was added to the sample in a 100-fold molar excess. With 21.4 nanomoles of Fe(II1) the corrected average current a t t = 3 seconds was 77.3 I!= 5.0 pa. and with 10.7 nanomoles, it was 25.7 i 3.6 pa. Three changes can be noted. The current for the reduction of Fe(II1) in the presence of potassium chloride is reduced. The precision of the measurements decreases. The corrected current is no longer proportional to the quantity of Fe(II1) in the spot. The presence of the potassium chloride altered the appearance of the sample spot. With no potassium chloride the Fe(II1) salt crystallized in a narrow ring with very little material in the center. I n the presence of potassium chloride, the spot was comprised mainly of large, irregular crystals of potassium chloride distributed more or less evenly throughout the spot. The decreases in current and reproducibility are caused by the large quantity of potassium chloride mixed with the Fe(II1). Hydroquinone. As a n exltmple of an electrochemical oxidation, the anodic reaction of p-benzohydroquinone was studied. Sample solutions were prepared with ether as a solvent and spots were applied a t room temperature. With this solvent the sample material did not form in a narrow ring at the periphery of the spot but was more evenly distributed. I n 1F sulfuric acid the chronopotentiometric quarter-wave potential for hydroquinone oxidation is f0.45 volt us. S.C.E. and oxidation of the gold electrode commences a t 1.15 volt us. S.C.E. (9). An initial potential of 1.0 volt us. S.C.E. was chosen because it was considerably anodic from the quarter-wave potential but was not so
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VOL. 37, NO. 12, NOVEMBER 1965
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Figure 3. Typical current-time curves for hydroquinone oxidation Supporting electrolyte: 1 F sulfuric acid in 50% ethanol. Initial potential: 1 .O volt VI. S.C.E. (as.)
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anodic that the electrode itself would be oxidized. However when 1F sulfuric acid was used as supporting electrolyte, the results were not reproducible. Because the irreproducibility might be due to slow and erratic dissolution of the hydroquinone in the aqueous solution, the supporting electrolyte was changed to 50% ethanol, 1F sulfuric acid. Reproducible results were then obtained using an initial potential of 1.0 volt us. S.C.E. (as.). Typical current-time curves for hydroquinone oxidation are shown in Figure 3. I n accordance with polarographic convention, anodic currents are considered to be negative. The average deviation of a series of identical samples was about the same as with Fe(II1). The blank correction was -4.9 l a . at t = 3 seconds. Corrected currents as a function of the quantity of hydroquinone are presented in Figure 4. The currents a t t = 3
+
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seconds depend upon the initial potential. The currents are larger with an initial potential of $0.8 volt than they are at $1.0 volt vs. S.C.E. (as.), because the potential tends to move to the region of the quarter-wave potential in the former case resulting in less current early in the electrolysis and more current later. With an initial potential of $1.0 volt, however, the potential does not shift so far to cathodic regions and there is more current early in the electrolysis and less when i t is measured a t t = 3 seconds. The extreme case of this trend is represented by the data obtained with potentiostatic control a t f0.8 volt. Here most of the current flows in the first second of electrolysis, causing the current a t t = 3 seconds to be even lower. Thus, the simple electrical circuit which was used has an advantage over true potentiostatic control in that the sensitivity is greater when the current is measured a convenient length of time after the beginning of electrolysis. The currents are also no longer proportional to the quantity of electroactive material but increase roughly with the square of the quantity. For quantitative determinations a calibration curve like those in Figure 4 is necessary. Integrations of the current-time curves with potentiostatic control were carried out to see if the integral might be a more linear function of the quantity of hydroquinone in the spot. The current rose to a peak in 40-60 X lo-+ second, the time required to bring the spot into the solution. The peak currents ranged from l to 10 ma. depending upon the amount of hydroquinone present. The currents decreased rapidly after the peak, reaching less than 2% of the peak current in 0.5 second. The integrals were not reproducible, typical __ average deviations being i15%. The source of this variation may be the fact
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-4ooc
I 0
that a large fraction of the total quantity of electricity flows during the same time that the spot is being brought into the solution. Therefore, variations in the movement of the cell may cause a larger variation in the integrals than in the currents measured several seconds afterward. Within the precision of the measurements the integrals did not appear to be proportional to the quantity of hydroquinone. Instead, relatively less hydroquinone was oxidized with large quantities (50% at 10 nanomoles) than with small quantities (90% at 2 nanomoles) . When an R C integrating circuit (time constant = 4.5 seconds) was used with the polarizing unit, the peak voltage across the capacitor was as reproducible as the current measured at 3 seconds, but the response was still not linear with respect to the quantity of hydroquinone-Le., like the true integral (see above), i t was relatively smaller with larger quantities. Although this mode of detection obviates the necessity for measuring the current on the steep portion of the current-time curve, a calibration plot is still necessary for quantitative work. The results were not dependent upon the presence or absence of dissolved oxygen in the supporting electrolyte nor on whether the application of the spot was carried out under a stream of nitrogen. Therefore, hydroquinone is not oxidized by oxygen under the conditions of the experiment. Cadmium. A gold electrode suffers the disadvantage of having a very limited cathodic range because the evolution of hydrogen occurs with very little overpotential. I n order t o take advantage of the extremely high hydrogen overpotential of mercury, a gold amalgam electrode was prepared by coating the gold electrode with mercury, wiping off the excess, and dipping the electrode in 1:3 nitric acid until the roughness of the gold amalgam surface was about the same as that of
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5 IO 15 20 QUANTITY OF HYDROQUINONE, nonomoles
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Figure 4. Corrected current at t = 3 seconds as a function of the quantity of hydroquinone 0
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Initial potential: +0.8 volt VI. S.C.E.[aq.) Initial potential: $1.0 volt VI. S.C.E.(aq.) Controlled potentlal: +O.S volt vs. S.C.E.(aq.)
ANALYTICAL CHEMISTRY
Figure 5. Corrected current at t = 3 seconds as a function of the quantity of Cd(ll)
the original gold surface. The hydrogen overpotential of this electrode is about the same as that of a mercury electrode. This electrode was used for the reduction of Cd(I1) using 0.2F potassium chloride as supporting electrolyte. The sample solution was comprised of Cd(1;0&.8 HzO in 95% ethanol. The spots were applied at a temperature of about 60’. The shape of the spot was intermediate between Fe(II1) and hydroquinone-Le., most of the material was in a ring but some was in the center. The initial potential was -1.1 volt us. S.C.E. and the blank correction was 7.5 pa. Corrected currents a t t = 3 seconds as a function of the quantity of Cd(I1) are presented in Figure 5. I n this case the potential had to be adjusted to -0.1 volt us. S.C.E. after each trial in order to reoxidize the cadmium from the amalgam before the electrode was removed and a new spot was applied. The roughness of the electrode slowly increased with time and the currents obtained with rough electrodes were greater than with smoother electrodes. General Characteristics of the Method. The physical processes involved in this method of electrolysis are quite complex. When t h e violent manner in which the sample is brought into contact with the supporting electrolyte is considered, t h e fact t h a t reproducible results can be obtained is surprising. The thickness of the sample spot is very small. For example, 20 nanomoles cm.9 of hydroquinone (1.6 X distributed evenly in a circular spot of 3-mm. diameter would be 0.2 micron in thickness, 1 nanomole would be 0.01
micron in thickness. Because the electrode surface is not smooth, the average thickness is even lower -Le., the sample thickness can be as small as a few molecular layers. With the electroactive material in such close physical proximity to the electrode surface, a large fraction of the material reacts. When the supporting electrolyte contacts the sample spot, dissolution and electrolysis begin simultaneously. The sample must dissolve before it can be reduced or at least solvent molecules and ions of the supporting electrolyte must be in the vicinity to provide electrical conduction. If dissolution is the slow step in the process, the current obtained may depend upon the physical character of the spot-e.g., crystal size. Thus reproducible results would be more di6cult to obtain. With hydroquinone a change from aqueous to 5001, ethanol supporting electrolyte was necessary to obtain reproducible results. When the reductions of p-nitrobenzoic acid, ethyl pnitrobenzoate, and p-nitrophenol were attempted a t the amalgam electrode, no reproducible results were obtained in any of several ethanol, ethanol-water, and methanol supporting electrolytes. These phenomena may be caused by slow and erratic dissolution. Increasing the temperature to 40” C. did not improve the reproducibility. At still higher temperatures the alcohol tended to condense on the cool electrode and the spot was partially rinsed away before the electrolysis began. Spot electrolysis offers the advantage of high sensitivity. Less than 1 nanoequivalent of electroactive material
(usually less than 0.1 pg.) is detectable and 5 to 20 nanoequivalents can be determined with a n accuracy of a few per cent. The instrumentation required is very simple. Analyses require very little time, only about three minutes being necessary to prepare and electrolyze a sample spot. The major savings in time over conventional electroanalytical methods accrue from the fact that deaeration of each sample i s unnecessary. LITERATURE CITED
(1) Blaedel, W. J., Olson, C. L., Sharma,
L. R., ANAL.CHEM.35,2100 (1963). (2) Breyer, B., Bauer, H. H., “Alternating
Current Polarography and Tensammetry,” Interscience, New York, 1963. (3) Galus, Z., Olson, C., Lee, H. Y., Adams, R. N., ANAL. CHEY. 34, 166 (1962). (4) “Hydbook of Analytical Chemistry, L. hleites, ed., pp. 5-9, RlcGraw Hill, New York, 1963. (5) Kelley, M. T., Jones, H. C., Fisher, D. J.. ANAL.CHEM.31. 1475 (1959). stry,” 2nd ed., p. 245, Interscience, New York, 1958. (8) Perone, S. P., hlueller, T. R., ANAL. CHEM.37, 2 (1965). (9) Peters, L). G., Lingane, J. J., J . Electround. Chem. 2, l(1961). (10) Shain, I., “Treatise on Analytical
Chemistry,” I. M. Kolthoff and P. J. Elving, eds., Part I, Sec. D-2, Chap. 50, Interscience, New York, 1963. (11) Underkofler, W. L., Shain, I., ANAL. CHEM.35, 1778 (1963).
RECEIVEDfor review June 1, 1965. Accepted September 1, 1965. Division of Analytical Chemistry, 150th Meeting ACS, Atlantic City, N.J., September 1965
Test of the Theory of Glass Bead Columns in Gas Liquid Chromatography S. J. HAWKES,’ C. P. RUSSELL, and J. C. GlDDlNGS Departmenf of Chemistry, University of Utah, Salt lake City, Utah The plate height theory of glass bead columns in gas liquid chromatography has been tested over a wide practical range, including bead diameters from 0.004 to 0.023 cm. and liquid loadings from 0.1 to 3.9%. Theoretical values are calculated completely independent without reference to plate height data from any chromatographic system. The agreement is very good (usually within 10% for the largest beads). The conformity deteriorates for the smallest beads, the theory predicting plate height terms too small by factors of from 2 to 4 at liquid loadings less than 1%. Possible reasons for the divergence
are discussed. The agreement is also very poor for height liquid loadings as expected from theory and observation.
T
HE glass bead column used in gas liquid chromatography is perhaps the best present-day system for comparing chromatographic theory and experiment. The “diatomaceous earth” columns have a complex liquid distribution which cannot yet be defined in detail (7). It is reasonable t o believe that the capillary column, despite its apparent simplicity, suffers the same disadvantage (3). (If the inside column
wall is “rough,” the liquid configuration will again be complex; while if the wall is smooth, the liquid film will be unstable and probably not maintain a uniform character for a long time.) The glass bead surface is very smooth and well defined geometrically. There is no instability problem, since most of the liquid accumulates around the contact points and is not expected to adhere t o the bead surface as a uniform layer (I). I n addition, the CI term is relatively large and easily discernable. This Present address, Department of Chemistry, Brigham Young University, Provo, Utah. VOL. 37,
NO. 12,
NOVEMBER 1965
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