1010
Anal Chem 1980, 52, 1010-1013
Characteristics of Thin-Layer Cells with Nafion Separators Josip Caja,’ Andrzej Czerwinski,* Kenneth A. Rubinson, William R. Heineman, and Harry B. Mark, Jr.” Depariment of Chemistry, University of Cincinnati, Cincinnati, Ohio 4522 I
The characteristics of thin-layer cells employing Nafion porous cation-exchange membranes as separators have been examined. Two cell designs which utilize different methods of filling the thin-layer cavity are presented. A comparison of theoretical and experimental cyclic voltammograms shows that thin-layer behavior is obtained for sweep rates less than 5 mV/s with sample volumes in the 1 to 0.1 pL range. Coulometric determinations of cell volume agree to within f3 % with volumes calculated by measurement with a microscope. Some limitations of the cell resulting from the apparent large size of the Nafion pores, osmotic pressure, temperature effects, and Donnan and junction potentials are discussed. An application of this cell is demonstrated by a study of vitamin B,2a.
A preliminary report of a new thin-layer electrode and cell system was recently published (I). T h e cell utilizes a new porous cation-exchange membrane, Nafion (Du Pont), as a concentric separator surrounding t h e working electrode. Advantageous features of this electrode include simplicity and low cost of construction, ease of use, very small volume capacity (less than 1 FL), and low solution resistance. Thus, t h e cell eliminates some disadvantages in many other thinlayer cell designs (2-11). The paper describes t h e design, characterization, and limitations of this thin-layer electrode system. A typical a p plication of this cell is demonstrated by a study of vitamin B12.
EXPERIMENTAL The initial publication describing the Nafion thin-layer cell ( I ) employed simple capillary action to fill the thin-layer compartment (as well as the compartment with auxiliary and reference electrodes) with the sample solution. Discussed here are two different, more reliable methods of filling the thin-layer compartment with the sample. The cell designs for implementing these filling procedures are shown in Figure 1. The electrode assemblies of both are identical; the filling techniques differ. The assembly shown in Figure 1A has a 0.050-cm diameter wire working electrode (a) which is 3.0 cm in length with one end sealed into the body of a 1-mL plastic syringe (d). At the upper, exposed end of the wire, electrical contact was made with the potentiostat. An appropriate length of the pliable cation-exchange membrane tubing (b), Nafion type 811 (Du Pont, Wilmington, Del.), inside diameter 0.06 cm, was pulled over the wire and inserted into a small polyethylene adapter tube in the tip of the syringe and sealed with Parafilm. The void between the wire and the membrane tubing forms the thin-layer cavity. The lower end of the Safion tubing extends beyond (about 1 cm) the bottom tip of the wire working electrode. The sample solution is drawn with the syringe into the thin-layer cavity. The cell is then removed from solution, and the sample volume is positioned via the syringe between the bottom tip of the working electrode wire and the bottom tip of the syringe as shown in Figure 1. The second type of assembly, shown schematically in Figure IB, employs the “screw type” micropipet head, either plastic or metal, which is used in hanging On leave of absence from the Faculty of Technology, University of Zagreb, Zagreb, Yugoslavia. *On leave of absence from the University of Warsaw, bVarsaw, Poland. 0003-2700/80/0352-1010$01 OO/O
mercury drop electrode assemblies. The “screw type“ assembly was found to give more precise control in manipulation of the position of the sample solution placement. Given the above geometry, the thin-layer volume contained in a I-cm length of the sample solution is less than 2 pL. A concentric Teflon tube can be used to contain the auxiliary electrode and the reference electrode as described previously ( I ) . However, it is more straightforward to dip the thin-layer electrode into a small vial containing the electrolyte solution in which the concentric auxiliary electrode and a saturated calomel reference electrode (SCE) are immersed. The air bubble at the end of the Nafion tube prevents mixing and insulates the internal and external compartments. Gold, mercury-coated gold, and platinum wires were used as electrodes. The mercury-gold wire electrodes were made by the procedure used to make mercury-gold minigrid electrodes (22) with one exception. In the first step, Le., when the solution of saturated Hg(NO&, 0.5 M KC1,O.l M HCl was drawn into the thin-layer compartment, the solution in the auxiliary compartment was simply 0.5 M KCl, 0.1 M HCl. Cyclic voltammetry measurements were made with a Bioanalytical Systems, Inc., Model CV-1A unit. Chronocoulometric measurements were carried out using a Princeton Applied Research Corporation 173 potentiostat which was controlled by microcomputer instrumentation. All solutions were prepared from reagent grade chemicals (except for the vitamin Blh) and deionized, distilled water. Stock solutions of 0.01 M K,Fe(CN), in 0.5 M KC1, 0.01 M FeC13in 0.5 M KCl and 0.01 M K,Fe(CN), in 0.5 M H2S0, were prepared by weight and stored in the dark. The solutions employed in this study were prepared each day by dilution of the stock with supporting electrolyte. Vitamin Blh (Fluka,A.G. Puriss.) solutions were prepared by weight in supporting electrolyte ( 1 3 ) . The solutions were deaerated with N2 in the usual manner before use, and the experiments were carried out under a N2 atmosphere in a glove bag. In this study, the cell consisted of a glass vial (approximately 10-mL total volume) which contained supporting electrolyte, a Pt gauze auxiliary electrode, a saturated calomel reference electrode, and the thin-layer electrode. The thin-layer electrode contained the sample solution and was filled as described above. For each experiment, the Nafion tubing was soaked in deionized water for a t least 30 min before assembly. The gold wire working electrodes were pretreated by boiling in chromic acid cleaning solution for a t least 30 min and then washing thoroughly with deionized water before use. The working electrode background characteristics were tested by taking cyclic voltammograms in 0.5 M H2S04with a sweep rate of 50 mV/s. The background cyclic voltammograms were compared with literature data (14, It?)to determine the quality of the gold surface.
RESULTS AND DISCUSSION Thin-Layer Behavior. Experiments were carried out t o test Rhether the electrode exhibited thin-layer behavior. T h e K3Fe(CN),/K,Fe(CN)6 redox couple was chosen t o test t h e thin-layer behavior of t h e Nafion membrane thin-layer electrode because of the extensive literature on this couple which has frequently been used t o test electrode behavior and thin-layer spectroelectrochemical cells (16, 17). Typical cyclic voltammograms for 1 X M K,Fe(Cx), in 0.5 M KC1 (a) and 0.5 M H2S04 (b) are shown in Figure 2. Thin-layer electrode theory (9) predicts t h a t t h e peak potentials for t h e forward and reverse scans for reversible reactions are the same and t h a t t h e half-peak widths should be 90 mV. Note in C 1980 American Chemical Society
ANALYTICAL CHEMISTRY,
VOL. 52, NO. 7, JUNE
1980
1011
Table 11. Comparison of Experimental and Theoretical Peak Currents as a Function of Sweep Rates. 1 mM [Fe(CN),]-3 in 0 . 5 M H,SO,. Volume =: 0.64 p L sweep rate, mV/s
ipc,
PA
i,
PA
__i,c, P A
1
0.6
0.6
0.60
2 4
1.2 2.3 4.3 6.7
1.1
1.20 2.40
10 20
40 a
theoreticala
experimental
2.2 3.8 6.2
6.00
12#.0
24.0
10
11
Calculated from Equation 3 in Ref. 9.
(1.0
10.6 104
Figure 1. Schematic drawings of the two types of thin-layer electrodes: (A) syringe type, (B) screw-type, with (a) wire working electrode, (b)
Nafion membrane, (c)sample solution, (d) body of the filler, (e) plunger, (f) screw
3'
!0.2
1
\\
\~
//
\\
0.0
-/o.o* l _ i -
?D4~~.~L--u04
.-.~
\
2!8
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/-#-
LO4
I
i""
b
>3\
i
108 anodic 11.0
Figure 3. i / i v s . potential ( E o - E ) for thin-layer cyclic voltammetry of a 1 X lo-', K,Fe(CN),-0.5 M KCI solution. The dashed line is t h e
calculated theoretical curve. The solid line is the experimental curve, sweep rate 1 mV/s
Figure 2. Typical current-potential curves for a 1 X M K,Fe(CN), solution in: (a) 0.5 M KCI, (b) 0.5 M H,SO,. Cell volume sample is 0.64 p L , sweep rate is 2 mV/s, E,, is the cathodic peak potential, E, is the anodic peak potential
Table I. Formal Reduction Potentials for 10 mM [Fe(CN),]-3 in 0 . 5 M KC1. Volume = 0.64 wLU sweep rate, mV/s 5
2.5 1 a
EpcrmV
Epa,mV
Eo,mVb
200 203 209
227 225 218
213 214
Potentials vs. SCE reference.
214
E o = /Epa + E p c ) / 2 .
Figure 2 that the peak potentials do have a slight separation (about 20 mV) a n d the half-peak widths slightly exceed the theoretical values also. T h e Eo' values agree well with literature values with these two experimental conditions (17, 18). As can be seen in Tables I and 11, the peak-voltage separation is a function of sweep rate. This separation arises from a combination of both the finite resistance of the Nafion membrane and the uncompensated resistance which results from the placement of the reference electrode outside the thin-layer cavity far from the working electrode surface (19). T h e E,' values are independent of sweep rate, however, as is expected.
A quantitative comparison between the shape of theoretical and experimental reduction-oxidation curves for the cyclic voltammograms of K,Fe(CN), are shown in Figure 3. T h e theoretical curve (solid line) is calculated on the basis of known thin-layer relations ( 2 , 9 ) . The current axis is normalized by plotting the ratio of i/z, vs. (Eo - E ) with E, of the theoretical curve defined as Eo, and i/i, at E , defined as 1.00. T h e experimental curve (dashed line) was obtained for a 1 x M K,Fe(CN)6 solution in 0.5 M KC1 using a sweep rate of 1 mV/s. T h e agreement is quite close a t slow sweep rates. A comparison of calculated theoretical peak currents ( 2 ) and experimentally obtained values as a function of sweep rate are shown in Table 11. This shows that thin-layer behavior is obtained when sweep rates are 5 m V / s or less. Diffusion effects influence the current response for faster sweep rates. S a m p l e V o l u m e D e t e r m i n a t i o n s . In order to use this thin-layer electrode for quantitative analysis or the coulometric determination of n-values in redox reactions, the precise volume of the sample solution in the thin-layer cavity must be known. It was found, using known micropipetted volumes and repetitive sets of experiments using a new Nafion tube for each set, that the inside diameter of the various Nafion samples was uniform (0.064 cm). Thus, a certain constant length aliquot of solution in the tubing represents a constant volume. For example, a 2.0-mm solution lengi h represents 0.65 HL. The solution lengths were measured with a graduated ocular on a microscope. An aliquot of sample solution is drawn up into the tube keeping the aliquot between the electrode tip and the tube end. T h e length of the aliquot is measured with the microscope and the volume calculated. T h e sample solution aliquot is then drawn into the thin-layer cavity (keeping it in between the electrode tip and bottom of the
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1:
ANALYTICAL CHEMISTRY, VOL. 52, NO. 7, JUNE 1980
cathodic
1 anodic
\
-10
\3
i /
/
Figure 4. Thin-layer cyclic voltamrnogram of a 1 X lo-‘ M FeCI, 0.5 M HCI solution. (a)Nafion membrane pretreated in satd. CaCI,, (b) Nafion membrane pretreated in 0.5 M HCI. Sample volume is 0.64 pL. Sweep rate is 2 m V / s syringe body, (d) as shown in Figure 1). In order to test the validity or limitations of the above method of estimating the thin-layer electrode sample volume, the volumes of typical K,Fe(CN), sample solutions in the cavity were experimentally determined by coulometry ( 1 7) and compared to values obtained by the above estimation method with the microscope. T h e potential step employed was from +0.007 L7 to +0.150 V vs. SCE using a M K3Fe(CN)6solution in 0.5 M H2S0,. T h e volume of the sample solution is calculated using the equation ( I 7):
Q
- Qb
= nFVC
I
Figure 5. Thin-layer cyclic voltammograms on a gold electrode for (a) 1X M vitamin B,2a in 0.04 M + 0.5 M Na,SO,, pH 5.15; (b) supporting electrolyte only. Sample volume is 0.64 pL. Sweep rate is 5 m V / s
(1)
where Q = total charge measured, C; Qb = charge due to electrolyte blank, C; n = number of electrons transferred per molecule; F = Faraday number, 96 500 C/mol; V = volume of thin-layer cell, L; C = concentration of electroactive suhstance, mol/L. It was found that repetitive coulometric volume measurements compared to those measured by the geometric estimation agree within 3%. Adding in the inherent errors of the electronics, the calculation of volume based on the geometric estimate is considered better t h a n =k8% for any single run. System Properties and Limitations of Its Use. As mentioned previously, Nafion is a porous cation-exchange membrane. Thus, in theory small cations diffuse freely through the membrane while large cations, large neutral molecules, a n d all anions should not pass through. These latter classes should be amenable to study with this thin-layer electrode. Insofar as the contained ions are charged, a Donnon potential will arise which will be time-independent but which will shift the apparent measured potentials. In addition, as the permeation of cations and anions is slow but not stopped (owing to the nonideal behavior of the membrane), time-dependent junction potentials will appear. These will die out only when the electrolytes in the inner and outer compartments are the same. T o minimize these effects, the inner and outer compartments should contain the same solution. Difficulties due to the Donnon potential are minimized by using a large excess of background electrolyte. Given the permselectivity of the membrane, even if the electrolytes are the same on both sides, one could expect to see osmotic concentration changes. However, in most instances, for dilute (millimolar or less) electroactive species, the rate of change of concentration observed was small enough not to be a problem even with slow scan experiments. In one experiment, the membrane was soaked in concentrated calcium chloride solution. This was done to investigate
-1.2
I-
L
Figure 6. Thin-layer cyclic vobmmcgrams on a mercurygold electrode for (a) 1 X M vitamin B,2a in 0.04 M -t 0.5 M Na2S04,pH 5.15; (b) supporting electroiyte only. Sample volume is 0.90 pL. Sweep rate is 5 mV/s the effects of loading the membrane with electroinert cations; this might suppress conduction of small electroactive cations. The results are shown in Figure 4a. This treatment does slow the loss of small cations but not to any practical extent. One problem with the system stability is changes in temperature around the body, (d) in Figure 1, of the thin-layer electrode which moves the sample solution (c) up or down the thin-layer cavity. Thus, if care is not taken, part or all the sample solution may move out of the thin-layer cavity. These “thermometer” effects would not, however, occur with the open ended cell described in the original report ( 1 ) . With this particular membrane under the conditions used, there was no evidence of any problems resulting from leaching of organic material from the membrane. Vitamin BlZa Studies. Vitamin BlZawas taken as an example to test the applicability of the Nafion thin-layer electrode system. The vitamin’s electrochemical behavior has been well studied (13, 20, 21) a t both gold and mercury-gold electrodes. Vitamin BIza(aquoCo(II1)cobalamin) is a monopositive ion at pHs below 8 and has a molecular weight of over 1200. Its size is sufficiently large to prohibit diffusion through the Nafion membrane. Figures 5 and 6 show the thin-layer M solution of cyclic voltammetric behavior of a 1 X
Anal. Chem. 1980, 5 2 , 1013-1020
vitamin B12ain 0.5 M Na2S04.0.04 M phosphate buffer ( p H 5.15) on gold and Hg-Au wire electrodes. T h e gold wire working electrode shows the well defined, irreversible Co(III)/Co(II) curves around 0.0 V. This wave is not as well defined a t the Hg-Au electrode, but the totally irreversible Co(II)/Co(I) wave a t -0.8 V is easily seen. This thin-layer electrochemical behavior of vitamin B12acorresponds exactly to that described in the literature (13, 20, 21). In this case less t h a n 0.2 pg of BIzawas employed in the experiment.
LITERATURE CITED (1) Caja, J.; Czerwinski, A.: Mark, H. B., Jr. Anal. Chem. 1979, 57, 1328. Hubbard, A. T.; Anson, F. C. Anal. Chem. 1964, 36, 723. McClure, J. E.; Maricle, D. L. Anal. Chem. 1967, 39, 236. Christensen, C. R.; Anson, F. C. Anal. Chem. 1963, 35, 205. Sheafer, J. C.: Peters, D. G. Anal. Chem. 1970, 4 2 , 430. Anderson, L. B.: Reilley, C. N. J . Nectroanal. Chem. 1965, 70, 295. Propst, R . C. Anal. Chem. 1971, 4 3 , 994. Murray, R. W.; Heineman, W. R.; O'Dorn, G. W. Anal. Chem. 1967, 39,
(2) (3) (4) (5) (6) (7) (8)
1066. (9) Hubbard, A. T. Crif. Rev. Anal. Chem. 1973. 3 , 201.
1013
(IO) LeCuire, J. M.; Pillet, Y . J . Electroanal. Chem. 1978, 97, 99. (11) Tom, G. M.; Hubbard, A. T. Anal. Chem. 1971, 4 3 , 671. (12) Meyer, M. L.; &Angelis, T. P.: Heineman, W. R. Anal. Chem. 1977, 49, 602. (13) Kenyhercz, T. M.; Mark, H. B., Jr. J . Hectrochem. SOC. 1976, 123, 1656. (14) Will, F. G.; Knorr, C. A. Z.Hecfrochem. 1960, 6 4 , 270. (15) Ferro, C. M.; Calandra, A. J.; Arvia, A. J. J . Electroanal. Chem. 1974, 5 0 , 403. (16) Nowak. R. J.; Kutner, W.: Mark, H. B., Jr. J . Electrochem. SOC.1978, 125, 232. (17) DeAngelis, T. P.: Heineman. W. R . J , Chem. Educ. 1976, 53, 594. (18) "Handbook of Chemistry and Physics"; CRC Press: Cleveland, Ohio, 1976: 57th Ed., D-141. (19) Harrar, J. E.: Shain, I . Anal. Chem. 1966, 38, 1148. (20) Kenyhercz, T. M.; DeAngeiis, T. P.; Norris, B. J.; Heineman, W. R.; Mark, H. B., Jr. J . Am. Chem. SOC.1976, 98, 2469. (21) Lexa, D.: Saveant, J. M.; Zickier, J. M. J . Am. Chem SOC.1977, 107, 2786.
for review December 3, 19'79. Accepted March 17, 1980. This research was supported in part by National Science Foundation Grants NSF C H E 76-04321 and C H E 77-04399. RECEIVED
Determination of Nickel by Differential Pulse Polarography at a Dropping Mercury Electrode Carmen J. Flora and Evert Nieboer Department of Chemistry, Laurentian University, Sudbury, Ontario, Canada, P3E 2C6
Dimethylglyoxime (DMG) sensitized the differential pulse polarographic determination of NiZf. The detection limit was 2 ppb and the linear response in the concentration range 0-85 ppb could be extended to ppm levels by simply omitting DMG. Actual analyses of tap and lake water and plant tissue are reported. Characterization of the electroactive process included an examination of the degree of reversibility and the effect on the peak current of pH, buffer composition, DMG concentration, and the presence of other metal ions.
Electrothermal atomic absorption spectroscopy is being proposed by the IUPAC Subcommittee on Environmental and Occupational Toxicology of Nickel as the standard technique for trace nickel determinations in biological samples (1, 2 ) . Other t h a n atomic absorption (AA) procedures ( 1 , 3, 4 ) , relatively few independent ultramicrogram methods for nickel are available. We have been successful in developing a sensitive differential pulse polarographic ( D P P ) procedure for the determination of nanogram quantities of this metal. T h e basic methodology of this polarographic approach is outlined in this report, as well as its application to the analysis of tap a n d lake water. T h e merits of this new method are further demonstrated by the evaluation of nickel concentrations in nickel uptake studies involving living plant tissue. Reference will also be made t o its application to the analysis of human blood and urine. Previously characterized polarographic techniques suitable for the determination of submicrogram levels of nickel lack sensitivity ( 5 ) or suffer from serious interferences, in addition t o being time consuming (6). Furthermore, the standard anodic stripping voltammetric procedures a t a mercury electrode employed (7) for other metals (e.g., Cd, Pb, Cu, Zn) are not applicable to nickel because of the irreversibility of 0003-2700/80/0352-1013$01 OO/O
the N i Z f / N i couple. While investigating derivative polarography of nickel at a dropping mercury electrode in ammoniacal tartrate and citrate buffers, we discovered t h a t the addition of butane-2,3-dione dioxime (common name is dimethylglyoxime, DMG) enhanced the peak current by a factor of about fifteen. This enhancement resulted in excellent peak characteristics and afforded a detection limit of 2 ppb. On completion of the characterization of this DMG sensitized reaction, we became aware that a group of Russian researchers had previously noted this enhancement phenomenon (8, 9 a n d references therein). However, their reports lack detail on instrumentation, instrumental parameters, and experimental procedures. Equally important is that these earlier references appear to have been overlooked by analysts interested in trace nickel analysis (e.g., 10, 1 1 ) . T h e current work is intended to rectify these deficiencies and illustrates that the differential pulse polarographic analysis of nickel described complements even the most sensit,ive AA methods.
EXPERIMENTAL Reagents. Double-distilled deionized water (DDI) was pre-
pared by demineralizing laboratory distilled water (Corning LD-2a Demineralizer) and distilling it in a Corning Mega-Pure still. Standard nickel solutions were prepared from NiC'12.6H20(BDH AnalaR) and were standardized against EDTA (BDH AnalaR, disodium salt dihydrate) using murexide indicator (12). Mineral acids (HCl and HN03) were of Baker Analyzed grade. Ammoniacal tartrate and citrate buffers were prepared from aqueous ammonia (CIL Reagent, 28-29%), (+)-tartaricacid (BDH Anal&) and citric acid monohydrate (Baker Analyzed),respectively. DMG (BDH AnalaR) reagent solutions in 95% ethanol usually were 1% w / v . All other chemicals were reagent grade. Used mercury (Fisher ACS Reagent) was cleaned by passing a fine spray (gravity fed) through a 1-m column of 1 M "0,. This purification step was repeated nine times using fresh acid after every third run. Further purification was achieved by bubbling prepurified O2 for 12 h through mercury which was covered with 1070NaOH and C 1980 American Chemical Society
