Anal. Chem. 1991, 63,957-963
Table I. Determination of Cobalt in Standard Reference Materials (n = 6) sample
found"
BCS 254 steel 0.0027% BHOlOl vanadium-iron ore 0.0145% BH1016 alloy steel peach leap
std value 0.0029% 0.0140%
0.085% 0.083% 0.260 p g / g 0.250 pg/g
RSD, % 5.0 5.2
5.0 4.2
" The average value of six determinations. *From Beijing Environmental Chemistry Institute. ACKNOWLEDGMENT This work was supported by the National Natural Science of Foundation of China, the aid from which is gratefully acknowledged. LITERATURE CITED Murray, R. W. In EEectroanalyNcel Cbemlstry; Bard, A. J., Ed.; Marcel Dekker: New York. 1984; Vol. 13, pp 191-368. Murray, R. W.; Ewing, A. G.; Durst, R. A. Anal. Cbem. 1987, 59, 379A-390A. Dong, S.; Wang, Y. Electroanalysis 1989, 7 , 99-106. Baldwln, R. P.; Christensen, J. K.; Kryger, L. Anal. Cbem. 1988, 58, 1790- 1798. Prabhu, S. V.; Baldwln, R. P.; Kryger, L. Anal. Cbem. 1987, 59, 1074-1078. Oyama, N.; Anson, F. C. J . Am. Chem. Soc. 1979, 107, 3450-3456. Oardea-Torresdey, J.; Darnali. D.; Wang, J. Anal. Cbem. 1988, 60, 72-76. Heineman, W. R.; Wleck, H. J.; Yacynych. A. M. Anal. Chem. 1980, 52, 345-346.
(9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29)
957
Price, J. F.; Baldwin, R. P. Anal. Cbem. 1980, 52. 1940-1944. Umana, M.; Waller, J. Anal. Chem. 1988. 58, 2979-2983. Ye, J.; Baldwin, R. P. Anal. Chem. 1988, 60. 1979-1982. Coury, L. A.; Birch, E. M., Jr.; Heineman, W. R. Anal. Cbem. 1988, 60, 553-560. Thomsen, K. N.; Baldwin. R. P. Anal. Cbem. 1989, 67, 2594-2598. Gerhart, G. A.; Oke, A. F.; Nagy, 0.; Moghaddam, B.; Adams, R. N. Brain Res. 1984, 290, 390. Wang, J.; Golden, T.; Tuzhi. P. Anal. Cbem. 1987. 59, 740. Ohnuhi, Y.; Matsuda, H.; Ohasha, T.; Oyama, N. J . Electroanal. Chem. 1983, 758, 55. Wang, J.; Ll, R. Anal. Cbem. 1989, 67, 2809-2811. Wang, J.; Lu, Z. J . Electroanal. Cbem. 1989. 266, 287. Cheek, G. T.; Nelson, R. F. Anal. Lett. 1978, A l l , 393-402. Cheng, K. L.; Ueno, K.; Imamura, T. CRC Handbook of Organic Analytcal Reagents; CRC Press: Boca Raton, FL, 1982; Chapter 4. Martin, C. R.; Frelser. H. Anal. Chem. 1981, 53, 902-904. Rubinstein, I.; Bard, A. J. J . Am. Cbem. SOC. 1980, 702, 5007-50 13. White, H. S.; Leddy, J.; Bard, A. J. J . Am. Cbem. SOC.1982, 104, 4811-4816. Martin, C. R.; Dollard, K. A. J . Electfoanal. C h m . 1983, 757, 127- 135. Wang. J.; Tuzhi, P.; Ll, R.; Zadeii, J. Anal. Lett. 1989, 22, 719-727. Qian, T. Ion-Exchenger and Its AppllcaNons; Tianjlng Sclnce and Technology Press: Tiangjing, 1984; pp 84-95. Bralnina, Kh. 2.; Roizenblat, E. M.; Belyavskaya, V. B.; Klva, N. K.; Fomicheya, Zav. Lab. 1967, 33. 274. He, R. Applications of Polarograpby; Indrustrial Press: Beljlng, 1963; p 27. Young, R. S. Separation Procedures in Inorganic Analysis; Zhang. G., Transl.; Shangahi Science and Technology Press: Shanghai, 1984; pp 192-21 1.
RECEIVED for review September 26,1990. Revised manuscript received February 4, 1991. Accepted February 7, 1991.
Potentiometric Stripping Analysis at Microelectrodes in Various Solvents and Some Comparisons with Voltammetric Stripping Analysis J. F. Coetzee* and M. J. Ecoff Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
Potentlometric stripping analysls (PSA) dlffers from the betler known voltammetric strlpping anaiysls (VSA) In that the electrochemkally preconcentrated analyte Is stripped chemically, rather than electrochemically. We present here comparlsons of lower detectlon limits and other features of PSA and VSA at both macro- and microelectrodes conslstlng of thin films of mercury on glassy carbon, carbon fiber, and gold substrates. The posslblilty that certain amalgams and/or metals that undergo sluggish electrochemlcal oxidation would exhlblt more facile chemlcal oxidation was Investigated. I t was found that In fact the PSA of manganese and nickel Is analytically more favorable than the VSA of these metals. The appllcablllty of PSA to such electroposltlve elements as sodium and potassium, described by us before, was further Improved. Falr resolution of sodlum from potassium was obtained by solvent opthnlzatlon and the use of microelectrodes on gold substrates. The PSA of llthlum Is much less favorable than that of sodium or potasslum.
INTRODUCTION Stripping analysis (SA) involves preconcentration of an analyte by electrodeposition on an electrode, followed by 0003-2700/9 110363-0957$02.50/0
stripping of the analyte from the electrode either voltammetrically or chemically. As compared to direct voltammetry or several optical methods, this technique has somewhat restricted scope, but for those analytes to which it is applicable, its lower detection limits (LDLs) can be exceptionally favorable, owing, in part, to the preconcentration step. In addition, SA has multielement capability, allowing the determination of several elements in the same experiment. These and other advantages of SA, as well as its limitations, have been discussed (1).Voltammetric stripping analysis (VSA), in which the stripping current is monitored as a function of applied potential, is usually carried out by fast linear sweep voltammetry or differential pulse voltammetry and has been extensively applied (1).More recently, J. and R. A. Osteryoung (2,3)and Wojciechowski (4)have applied fast square wave voltammetry to SA, thereby greatly reducing the time required for the analysis as well as obviating the need for deaeration of the solution. In the second variant of SA, chemical or potentiometric stripping analysis (PSA),in which the potential of the working electrode is monitored as a function of time, dates back to 1961 when Bruckenstein and Nagai stripped thallium and lead amalgams with mercury(I1) ion (5). This approach has been greatly extended by Jagner to the determination of a number of metal ions in a variety of matrices, including beverages, blood, urine, and seawater 0 1991 American Chemical Society
958
ANALYTICAL CHEMISTRY, VOL. 63, NO. 10, MAY 15, 1991
(6-8). In an extensive series of studies typified by the indicated references, he investigated the effectiveness of a number of oxidants, including dissolved oxygen and especially mercury(I1) ion, for the chemical stripping of metal amalgams, as well as the influence of various parameters affecting the utility of PSA. Accuracies and LDLs of PSA and VSA appear to be similar, but PSA does have certain demonstrated or potential advantages. Among these are instrumental simplicity and, more significantly, certain benefits derived from the fact that the stripping process is monitored potentiometrically, so that the current passing through the working electrode is very low. The consequences are the following. (a) Problems associated with double-layer charging are less severe. (b) Electroactive substances, including dissolved oxygen, interfere less; this is particularly important with biological samples (9). It is to be noted, however, that stripping by square wave voltammetry is also unaffected by the presence of oxygen (3,4). (c) Problems associated with the iR drop in high-resistance media (e.g., certain nonaqueous solvents) are minimized. In addition to these demonstrated advantages of PSA, a potential benefit is that, for those redox couples exhibiting sluggish electrochemical charge-transfer kinetics, chemical charge transfer may be more facile. One of the objectives of the present study was to investigate this possibility for the oxidation of the amalgams and/or metallic states of manganese, cobalt, and nickel. A second objective of the present work was to verify the theory of PSA presented before (10). It was shown that, for an assumed parabolic concentration gradient of amalgam in a thin-film mercury macroelectrode on a planar substrate (e.g., glassy carbon), the transient potential, Et, varies with time, t, as in
where D is the diffusion coefficient of the analyte ion as well as of the oxidant (e.g., mercury(I1) ion), T is the limiting stripping time, 1 is the thickness of the mercury film, and 4 = t / T . In the present study, the validity of eq 1was tested over a wide range of 4, using computerized PSA. This work is concerned mainly with the use of microelectrodes in PSA as well as in VSA. The advantages of microelectrodes (defined here operationally as disk or cylindrical graphite or noble metal fibers with diameters in the range 8-60 pm) in direct voltammetry are by now well established (11). Two of the most significant advantages are the smaller charging current densities, allowing very fast potential sweeps, and (somewhat paradoxically) also the smaller faradaic currents, thereby minimizing iR drops and therefore the need for supporting electrolytes and/or highly polar solvents. These and other advantages have been discussed by R. M. Wightman, A. M. Bond, R. W. Murray, J. Osteryoung, and others (11-13). Most of these advantages also apply to uoltammetnc stripping analysis; in addition, nonplanar diffusion enhances mass transport of the analyte sufficiently that forced convection during the deposition step becomes unnecessary, which is an advantage for fragile electrodes or small volumes of solution. The utility of microelectrodes in VSA has been demonstrated (4,14,15). As far as PSA is concerned, the main advantages are the smaller capacitance background and the greater resolution of stripping signals (IO). The advantages of cylindrical graphite fibers over glassy carbon macroelectrodes in the PSA of cadmium, copper, and lead have been demonstrated (16). The PSA as well as the VSA of cadmium, zinc, and lead from graphite fibers has been studied (17). Jagner et al. investigated the applicability of various types of microelectrodes in the PSA of flow systems; included were
platinum and gold fibers, as well as both Ndion-modified and unmodified graphite fibers (18). In addition, Jagner e t al. studied constant-current stripping analysis from microelectrodes (19). Finally, we have described results obtained in the PSA of sodium, potassium, and other electropositive elements at thin-film mercury macroelectrodes in certain nonaqueous solvents (particularly dimethyl sulfoxide) and their mixtures with water (20). Resolution of sodium from potassium was poor. In this work, attempts to improve resolution by solvent optimization and by using microelectrodes are described. EXPERIMENTAL SECTION Solvents and Other Chemicals. The water used in the aqueous electrochemical experiments and in the preparation of stock solutions was purified as described before (21). NMethylpyrrolidinone (NMP, Aldrich Chemical Co., 99% purity) was used as received or after vacuum distillation. Propylene carbonate (PC, Burdick & Jackson, high purity) was used as received with a reported water content of 4.4 mM. Tetraethylammonium perchlorate (TEAP) and tetrabutylammonium perchlorate (TBAP) were prepared as described elaewhere (22). The salts used for the preparation of stock solutions were of reagent quality and used as received. Caution: Solutions of perchlorates in organic solvents are potentially explosive and should be handled accordingly. Instrumentation. Potentiometric stripping analysis was performed employing the same ISS-820 ion scanning unit (Radiometer, Copenhagen) and other instrumentation described elsewhere (20). Computerized PSA was done by interfacing an Apple IIe computer equipped with a fast A/D-D/A converter to the ion scanning instrument. The 16-channel A/D converter (Model AI13, Interactive Microware, Inc.) had a resolution of 12 bits. Custom software and software purchased from Interactive Microware, Inc. (Lab Data Manager),were used for data collection and processing. To aid in the timing of data collection, an extemal trigger was built to initiate the data acquisition process. Details of the trigger can be found elsewhere (23). The maximum real-time data acquisition rate of the A/D converter was specified by the manufacturer as 20 kHz. The software employed limited the rate to a value somewhat less than 20 kHz but still sufficient to capture the shortest stripping events employed (20 ms). Whenever necessary, we digitally subtracted the typically small capacitance background from the analytical stripping curve in a fashion similar to that used by Jagner (24). Differential pulse anodic stripping voltammetry (DPASV) was performed with a standard PAR Model 174A electrochemical analyzer (EG&G Princeton Applied Research, Princeton, NJ). An Omnigraphics 2000 recorder (Houston Instruments, Houston, TX)was interfaced to the PAR 174A. Electrodes. Various types of carbon fiber microelectrodes similar to those described by Bond (25)were fabricated. The cylindrical carbon fiber electrodes (CCFEs) were constructed by placing a single carbon fiber that had been cleaned with acetone into the tip of a disposable glass pipet. The fiber was sealed into the pipet with epoxy (Epotek 353 ND), which then was cured at 60 "C for 1 h. Electrical contact between the carbon fiber and a copper wire was made by introducing a few drops of mercury into the pipet tip. The fiber was then cut to the desired working electrode length (typically 0.5 cm). The carbon fibers were obtained from Union Carbide (Thornel type P, Grade VSB-32) with a reported diameter of 11 fim. Gold electrodes of cylindrical geometry were constructed in an analogous manner except that electrical contact was made by silver epoxying (Epotek 410E) the gold fiber to copper wire. Gold fibers were obtained from Goodfellow Metal in both 10-and 60-fimdiameters. Gold electrodes of planar geometry (Bioanalytical Systems) were employed with two different geometric areas. The macroelectrode had a geometric area of 0.050 cm2,while the microelectrode (BAS l0-wm AUE) had a diameter of 10 fim. A glassy carbon electrode (Radiometer Model F 3500) with a geometric area of 0.070 cm2was also employed as a working electrode. In all experiments, the counter electrode was a 1-cm2platinum foil. Potentials were measured against a Radiometer Model K4040 ceramic doublejunction saturated calomel electrode (SCE) when water was the
ANALYTICAL CHEMISTRY, VOL. 63,NO. 10, MAY 15, 1991
670
gI
650
> E
959
1
t
630 I D
610ID1 -0.8
'
'
-0.4
I
I
0
I
I
0.4
'
0.0
- )/JB]
Lo* [ ( 1 d,
Figure 1. Test of eq 1 for potentiometric stripping analysis: stripping of cadmium from a thin-film mercury macroelectrode on a glassy carbon substrate ( A = 0.070 cm2). Conditions: analyte, 1.1 X lo-' M CdCi,; supporting electrolyte, 1 X lo-, M HCI; oxidant, 2 X lo4 M HgCI,; solvent, water: E, = -1.0 V vs SCE; t , = 2 min. Curve before background subtraction (X); after background subtraction (0).
solvent and against a Pt/(13- + I- in PC) electrode (26)when PC or NMP was the solvent. In the nonaqueous experiments, both the reference electrode and salt bridge assembly were terminated with a ceramic pin. The salt bridge assembly contained the same solvent and concentration of supporting electrolyte as the test solution. Procedures. The working electrodes (excluding those of cylindrical geometry) were cleaned before each set of experiments with a polishing grade of alumina (Fisher, 0.05-pm particle size) and were then washed successively with 0.1 M nitric acid, water, and acetone. Deaeration of the analyte and plating solution was carried out in the absence of the working electrode with ultrapure nitrogen. In the nonaqueous experiments, the nitrogen was first passed through a drying chamber and was then presaturated with the solvent in question. Thin-film mercury electrodes were prepared by placing the working electrode in the electrochemical cell containing an aqueous solution of mercury(I1) chloride, and the PSA potentiostat was set to -0.6 V vs SCE. At least four 1-min depositions were employed to apply the mercury film on the working electrode, interrupting the applied potential for a few minutes after each deposition. The potentiostat was then set to the desired working electrode potential, and deposition-stripping cycles were carried out with the background electrolyte until a reproducible background was established. In the PSA experiments, the optimum concentration of oxidant (usually mercury(II) chloride) was determined emDiricallvand tmicallv varied between RESULTS AND DISCUSSION Testing the Theory of Potentiometric Stripping Analysis. Equation 1 predicts that for a thin-film macroelectrode a plot of E , vs log ((1- 4)/41/2]will be linear with a slope of 59.2f n mV/decade a t 25 "C. Figure 1 provides a test of this prediction for the stripping of cadmium from a thin-film mercury macroelectrode. The slope of the line after digitally subtracting the background signal was 29.7 mV/ decade. At short stripping times, however, deviations from eq 1 occur. For example, in the stripping of lead from a similar electrode for various lead concentrations and deposition times, the slope of the plot gradually increased from 29 to 42 mV/decade for stripping times decreasing from 250 to 20 ms. We attribute this trend to the fact that eq 1does not take into account the effect of the double-layer capacitance on the analytical signal. PSA, like VSA, is not totally free from errors associated with the double layer. In PSA, charge is being consumed from the reaction of the oxidant, not only with the amalgam but also with the double-layer charges. The influence of this perturbation of the stripping curve will be greatest at the shortest stripping times. Potentiometric Stripping Analysis at Carbon Fiber Microelectrodes. In principle, microelectrodes should offer
Time ( s e c )
-
Figure 2. Computerized potentiometric stripping curves of lead from a thin-film mercury microelectrode on a cylindrical carbon fiber substrate (diameter = 11 pm, length = 0.5 cm). Conditions: supporting M HgCI,; Ed = -0.9 V vs electrolyte, none added; oxidant, 1 X SCE; t , = 10 min. Solution quiescent during deposition as well as during stripping. Curve a, background; b, 2 X lo-' M Pb(N03), added; M Pb(N03), added. Curves are offset for clarity. c, 4 X
significant advantages over macroelectrodes a t very low analyte concentrations because the smaller double-layer capacitance should have less influence on the transient potential-time function. Four types of carbon fiber microelectrodes were tested: single fibers of cylindrical geometry, single fibers of planar geometry (disk), bundles of fibers of cylindrical geometry, and bundles of fibers of planar geometry (23). Background potentiograms for these microelectrodes as well as glassy carbon macrodisk electrodes were obtained for a mercury deposition time t d of 30 s a t -0.9 V vs SCE in a M HN03+ quiescent aqueous solution containing M HgC12. The most reproducible background signal was obtained with the single fibers of cylindrical geometry, and this type of electrode was used exclusively in subsequent studies. Lead(I1) ion was chosen as the test analyte. Reproducibility of the limiting stripping time, T , was good for quiescent solutions, less so for stirred solutions in which the cylindrical electrode tended to vibrate. As a typical example, for a quiescent solution containing 9.7 X lo-' M Pb(I1) ion and 3 X M HgC12,with t d = 4 min, 7 = 10.24 f 0.01 s. The sensitivity of PSA is defined as the slope of the T vs concentration plot, which was found to be linear over a wide range. Sensitivities of various electrodes ranged from 800 to 900 s/mM for a deposition time of 4 min and a mercury(I1) chloride concentration of 2 X M. The dependence of T on t d for quiescent solutions also was found to be linear over a wide range. This is in agreement with the findings of Golas and J. Osteryoung ( I @ , who reported that the rate of accumulation of anal@ on a microcylinder by cylindrical diffusion is equivalent to that at a macrodisk rotated a t ca. lo00 rpm. Even though the current does not reach a steady state, its time dependence is small enough that the amount of analyte accumulated is linear in deposition time over reasonable ranges of experimental parameters. At the lowest analyte concentrations (
> #
W
Time(sec1
-
NMP'
Figure 10. Potentiometric stripping curves of sodium and potassium from a thin-film mercury electrode on a glassy carbon substrate ( A = 0.070 cm2) in distilled N-methyipyrroiklinone as solvent. Condffions: supporting electrolyte, 0.1 M TBAP oxidant, 5 X lo4 M HgCI,; Ed = -2.90V vs Pt/(If I- In PC);2, = 2 min. Curve a, background; b, 8 X 10dM NaCladded; c, 8 X 10"M NaCl+ 8 X 104MKCIadded. Curves are offset for clarity.
+
-
Na
K
L
.\
m-s-.-
f 4t
Na K Li Na K
Experimental conditions: 10-6-104 M NaCl or KC1 + 2 X lo-' M HgCl,, and 10-3-10-4 M LiC104 + 1 X M HgC12; 0.1 M Bu4NC104as supporting electrolyte; thin-film mercury on 0.07-cm2 planar glassy carbon electrode; Ed = -2.90 V vs Pt/(I< + I- in PC) reference electrode; t d = 2 min. *Represents the sensitivity of the determination; units: s/mM. e Correlation coefficient. Purified solvent. e Commercial solvent used without Durification.
I
\
H
0.2s
L
'0
0.1
0.2
0.3
>
0.4
W
I
Mole Fraction of W a t e r
Flgwe 11. Dependence of limiting stripping time of sodium on water concentration in mixtures of distilled N-methyipyrroiidinone and water. Conditions: Supporting electrolyte, 0.1 M TEAP; oxidant, 1 X lo3 M HgCI,; anaiyte, 2 X lo4 M NaCi; electrode, as for Figure 10; E, =
-2.90V vs Pt/(13-
+ I- in PC);
fd
= 1 min.
ethanol (EtOH) and 2-propanol (2-PrOH). Surprisingly, well-defined stripping signals could be obtained in the two alcohols. The best results were obtained in NMP, possibly because this cyclic amide is a good hydrogen bond acceptor (Kamlet-Taft p parameter = 0.77), thereby lowering the reactivity of water and other protic impurities present in the solvent. Typical stripping curves of sodium and potassium at a thin-film mercury electrode on a glassy carbon substrate are shown in Figure 10. Unfortunately, the two metals are not resolved. Limiting stripping times of sodium are proportional to concentration over the range tested (6 x 104-5 X M), as they are to deposition time (1-4 min). The ability of the solvent to tolerate substantial concentrations of water is illustrated in Figure 11. It is clear that the amalgam remains stable for a considerable time even when up to 17 mol '70water is present. It is possible that a contributing factor to this stability is the presence of a passivating film of sodium hydroxide, which is highly insoluble in aprotic solvents, even when some water is present. If such a film indeed is present, it evidently does not interfere seriously with either the electrodeposition or the chemical stripping of sodium. When the concentration of water is low, relatively high concentrations of amalgam can be tolerated and precision is high; for example, for five replicate deposition-tripping cycles with 8.7 X M NaCl and t d = 4 min, T = 73.23 f 0.14 s. An estimate of the LDL a t the 20 level is 3 X lo-' M for t d = 4 min. The PSA of lithium ion is of particular interest owing to its relevance to the operation of rechargeable lithium batteries. Results were disappointing, however. In Me2S0, stripping signals could not be obtained for deposition potentials as
Timelsec)
-
C
I
Figure 12. Potentiometric stripping curves of sodium and mixtures of sodium and potassium from a thin-film mercury electrode on a cyiindricai goid fiber substrate (diameter = 60 pm, length = 0.3 cm) in propylene carbonate as solvent. Conditions: supporting electrolyte, 1 X lo3 M TBAP; oxidant, 2 X M HgCI,; E, = -2.50V vs Pt/(13I- in PC); t , = 30 s. Deposition and stripping carried out in quiescent solution. Curve a, background (due to 5 X lo-' M Na+ impurity, determined by standard addition); b, 1 X lo-' M NaCi added; c, 1 X lo-' M NaCi 1 X lo-' M KCI added. Curves are offset for clarity.
+
+
negative as -2.99 V vs Pt/(I,- + I- in PC) and for any reasonable concentration or deposition time. This was not unexpected in view of Butler's previous finding that the standard electron-transfer rate constant for the Li+-Li(Hg), couple in Me2S0 is only 3 X cm s-l (32). In NMP, stripping signals could be obtained but at much lower sensitivities than for sodium or potassium ions. The results are summarized in Table I. Attempts were made to improve the resolution of mixtures of sodium and potassium ions by replacing the macroelectrode used so far (TFME-GC) with a microelectrode of the type TFME-CCF. I t was found, however, that the mercury film adhered poorly to the carbon fiber at the very negative potentials needed for the electrodeposition of the alkali metals. This problem, caused in part by the lowering of the surface tension of mercury at very negative potentials, was also encountered with the TFME-GC and even with the hanging mercury drop electrode, but it was much more severe with the TFME-CCF. There may be additional complications with such electrodes in nonaqueous solutions. For example, Wehmeyer and Wightman successfully deposited mercury on (planar) carbon fibers in aqueous solution but found it virtually impossible to transfer the fibers to nonaqueous media without loss of mercury (33). We obtained much better adhesion of mercury (owing to the formation of stable amalgrams) on a planar gold macroelectrode (diameter = 0.30 cm) or on gold fibers with diameters of 10 pm (planar, disk configuration: TFME-PGF) or 60 pm (cylindrical configuration,
ANALYTICAL CHEMISTRY, VOL. 83, NO. 10, MAY 15, 1991
063
L
0-
-2.0
-2.5
0 b
.
g
-2.0 E , V vs. Pt /(I;
y
L
-2.5
+I-)
Flgure 13. Differential pulse stripplng voltammograms of sodium and potassium and theh mixtures at thin-film mercury electrodes on gold substrates In propylene carbonate as solvent. Solution stirred during
deposition except for the last 30 s;solution quiescent during stripping. A Planar macroelectrode (diameter = 0.30 cm); supporting electrolyte, 0.1 M TBAP E = -2.50 V vs Pt/(13-4- I- in PC); t d = 5 min; scan rate = 5 mV s-f: puise amplitude = IO mv. Curve a, 5 x 106 M KCI; b, 5 X lo-' M KCI 4- 5 X lo-' M NaCI. B: Cylindrical electrode (diameter = 120 pm, length = 0.3 cm): supporting electrolyte, 1 X M TBAP Ed = -2.50 V vs Pt/(13- I- In PC); t d = 3 mln; scan rate = 5 mV s-': pulse amplitude = 10 mV. Cwve a, background (due to Na+ Impurity); b, 2 X 10" M NaCl added; c, 2 X lo-' M NaCl + 2 X lod M KCI added. All curves are smoothed and offset for clarity.
+
length = 0.3 cm: TFME-CGF). Potentiometric stripping curves for sodium and mixtures of sodium and potassium at the latter electrode in PC as solvent are shown in Figure 12. Resolution of the two metals is fair, the difference in limiting stripping potentials being ca. 100 mV. However, resolution becomes inadequate when the two concentrations differ by a factor of more than 3. No significant improvement resulted from replacing the TFME-CGF with the TFME-PGF. For solutions containing sodium ion as the only analyte, reproducibilities of limiting stripping times were better with the cylindrical microelectrode (rsd 1% for 1 X lo4 M Na+) than with the planar microelectrode (4%) or the planar macroelectrode (4%). The voltammetric stripping of alkali metals from these microelectrodes was studied in some detail. Some comparisons were also made with thin-film mercury on gold macroelectrodes, as shown in Figure 13. Resolution of the stripping voltammograms of sodium and potassium is much better at the microelectrode, the two peak potentials differing by 110 mV. Nevertheless, the resolution of the two metals becomes inadequate when the two concentrations differ by a factor of more than 3, as it is in such samples as human blood serum. Additional improvement in resolution was obtained at the 10-pm-diameter TFME-PGF, as shown in Figure 14, owing to a slight narrowing of the peaks. The peak width at halfheight for sodium was 95 mV, in close agreement with the value expected for a pulse amplitude of 10 mV. The reproducibility of peak currents was also better at the 10-pm disk than at the 60-pm cylinder. For example, for five replicate determinations of 2 X lo4 M Na+, the relative standard deviation was *2.5% at the latter electrode and *l% at the former. We conclude that the stripping determination of sodium and potassium is best carried out at thin-film mercury electrodes on gold microsubstrates in NMP or PC as solvent. Resolution of stripping signals of the two metals can be obtained provided the concentrations are not very different. Reproducibilities, lower detection limits, and resolution of
-2.5 -2.0 E,V vs. Pt /(I;
+ 1-1
Figure 14. Differential pulse strlpplng voltammograms of sodlum and sodium-potassium ion mixtures at a thin-film mercury planar microelectrode on a gold disk substrate (diameter = 10 pm). Conditions: supporting electrolyte, 1 X lo3 M TBAP; Ed = -2.50 V vs Pt/(131- in PC); t d = 5 min; scan rate = 5 mV s-l;pulse amplitude = IO mV. Solution quiescent during deposition as weii as during stripping. Curve a, background (due to Na+ Impurity); b, 1 X 10" M NaCl added; M KCI c, 2 X lo-' M NaCl added; d, 2 X lo-' M NaCl 2 X added. Curves are smoothed and offset for clarity.
+
+
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RECEIVED for review November 28,1990. Accepted February 22, 1991. This work was supported by the National Science Foundation under Grant No. CHE-8408411.
