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Anal. Chem. 1981, 53,906-908
Cyclic Voltammetric Estimation of Applied Potential for Electrochemical Detectors in Liquid Chromatography Sir: Care is required in selecting the applied electrode potential of constant-potential amperometric detectors for liquid chromatography (LC) and flow injection applications. T o maximize analytical response and minimize background interference, the applied potential should be held at the minimum value at which current reaches the limiting-current plateau of the analyte (Eplateau). The analytical potential can be determined directly from hydrodynamic voltammetry (HDV), in which current is measured vs. applied potential, point by point, for samples injected into the stream flowing through the electrochemical detector. Alternatively, Eplateau can be estimated indirectly from other techniques, e.g., cyclic voltammetry (CV), which may be carried out on a quiet solution, possibly in a separate electrochemical cell. HDV measurements can precisely identify Eplakau under analytically useful flow conditions but can be tedious for measurements a t low concentrations (I1pM),requiring up to several hours for completion, due to the time required for background current to stabilize after each change of electrode potential (e.g., up to 10 min for a potential change of 50 mV), especially at higher applied potentials. This background current presumably reflects slow changes in the electrode surface redox state. CV measurements in a separate cell can be made much more rapidly, but Eplakau will generally have a magnitude greater than the CV peak potential (E,) under typical measurement conditions for slow electron transfer reactions, frequently encountered with many candidate molecules for electrochemical LC detection schemes. Reported here is a simple empirical procedure for estimating Eplateau from independent CV measurements. The procedure is demonstrated effective for a number of phenolic compounds exhibiting a wide range of electron-transfer rates. With this procedure, the time required to find an optimum applied potential for liquid chromatography with electrochemical detection (LCEC) can be significantly shortened. EXPERIMENTAL SECTION Equipment, The CV, LC, and HDV systems were as previously described (I-3), except that some measurements were performed on an LC system fitted with stainless steel plumbing and a 15 cm long x 4.6 mm i.d. reversed-phase column packed with a 5 pm diameter particle C-18 packing (Bioandyticd Systems, Inc., West Lafayette, IN). All potentials are reported vs. Ag/ AgC1/3.5 M KC1. Chemicals. All chemicals were reagent grade and used without further purification. Water was purified as previously described ( I , 2), except that sulfuric acid was not added for the final distillation. All eluents were passed through either a 5-bm Teflon filter (aqueous eluents) or a 0.5-pm Fluoropore (Millipore Corp., Bedford, MA) filter (organic eluents). RESULTS AND DISCUSSION Both E, (CV) and Eplah,(HDV) for slow electron transfer reactions shift to larger overpotential as the rate of voltage scan (CV) or mass transport (HDV) increases (4, 5). The magnitude of each shift depends inversely on the number of electrons transferred, n, and the electron transfer coefficient, a. Under typical test conditions, Eplateau can be expected to be shifted to significantly higher overpotentials than E, values due to the higher mass transport rate under HDV conditions (5, 6). The magnitude of the shift of Eplateau beyond E, increases as the rate of electron transfer decreases, and as the residence time in the flow detector decreases relative to the
time scale of the CV scan rate (Le., as flow rate increases or scan rate decreases). It is well-known that in the limit of a simple, rate-limiting electron transfer, the wave shapes for both CV and HDV become independent of mass transport conditions. Limiting values for the CV diagnostic parameter IE, - E,/zl and the HDV diagnostic parameter - E1/41 in the slow electron transfer limit are both inversely proportional to (an) ( 4 , 7), where E, and E,jz are the potentials at which CV current on the initial scan is at its peak and half of its peak values, and E3/4and are the potentials at which HDV current is three-fourths and one-fourth of the mass transport limiting value. Since (an) appears in the denominator of both wave-shape expressions, as well as the expressions for the shifts of Eplateau and E,, experimental values of IE, - EPjzlfrom CV can be used as an empirical guide for the expected HDV wave shape and shift, without the need to determine an experimental value for (an). Figures 1and 2 illustrate experimental CV and HDV results for 2,4-dichlorophenol and phenol, respectively. All CV scans were obtained at a fixed scan rate of 50 mV/s in an external cell. All HDV curves were obtained a t a flow rate of 0.96 mL/min (corresponding to a linear flow rate of ca. 3 cm/s), for a 6.4 mm diameter Kelgraf (1-3) electrode in a 125 pm thick rectangular flow channel. Voltage (iR)drop due to passage of current through the uncompensated resistance of the HDV cell never exceeded 5 mV and was typically substantially smaller. The data of Figures 1and 2, and additional data for a series of phenolic compounds, summarized in Table I, allow a simple empirical generalization about the relationship between E, (HDV) and E, (CV), Le. IEplateau - EpI 4lEp - Ep/2l (1) under the specific conditions used in this study. The HDV data listed in Table I were acquired simultaneously in the liquid chromatographic mode for a group of phenolic compounds, which were readily separated on a pellicular C18 column ( 2 , 2 )in an eluent mixture consisting of 0.1 M, pH 5 acetate buffer in water/methanol solvent mixed 19:l by volume. The detector potential was varied stepwise for a series of replicate injections of phenolic sample mixtures. In addition, HDV experiments for several compounds were also repeated individually in an eluent mixture consisting of 0.1 M, pH 5 acetate buffer in water/methanol solvent mixed 3:l by volume. The simple relationsip exhibited in eq 1 between Eplakau and E, enables use of CV to establish a reasonable first approximation of the minimum applied plateau potential for LC experiments, for reactions with a wide range of electrontransfer kinetics. The average deviation between E, values predicted from CV by eq 1and experimental values was +0.01 f 0.11 V for the seven compounds examined. In only two cases was the CV-predicted Eplateau underestimated by 0.10 V or more. In all other cases, the predicted potential would assure maximum or nearly maximum possible current response. Thus a minimal number of additional potentials need be investigated to maximize response for a given compound. Use of eq 1 to predict HDV results from CV results automatically compensates for the difference in peak sharpness between relatively rapid (e.g., Figure 1)and slow (e.g., Figure 2) electron-transfer reactions and between reactions with differing numbers of electrons transferred. Table I illustrates
0003-2700/81/0353-0906$01.25/00 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 6, MAY 1981
Table I. Data from Hydrodynamic Voltammograms and Cyclic Voltammograms of Selected Phenols" cv HDV compound
E,,
v
E , - E,/%
0.82 0.63 0.78 0.22 0.72 0.83 0.75
phenol p-methoxyplienol resorcinol 3,5-dichloroliydroquinone
Eplate,u, b
V
V (predicted)
0.10 0.12
1.22
0.08 0.04
1.11
1.10 0.38 1.20 1.15 0.99
0.12 3,4-dimethylphenol 0.08 0-hy droxyet hylresorcinol 0.06 2,4.-dichlorophenol a Flow rate: 0.96 m;l/min: ca. 3 cm/s linear flow rate past detector electrode. empirical relationship given by eq 1. Measured by HDV.
Eplat+u
EgfateaurC exptl)
deviation, V (predicted exptl)
1.20 0.90 1.25
+ 0.02
t 0.21
0.40
-0.15 -0.02
1.20 1.25 1.00
-0.10 -0.01
0
Computed from CV data using
-10
1
b Ep~ataau
E plateau
I
?-
I
10
-
I
8 -
I ha) 0.5
0.7
0.9
1.1
1.3
E(V)
Figure 1. CV (a) and HDV (b) of 2,4-dichlorophenol. CV: 1 mM 2,4-dichlorophenol in 40% methanol, 0.1 M, pH 5 acetate buffer. Single scan starting at 0 V (scan rate 50 mV/s). E, = 0.75 V; = 0.69 V. HDV: 3 pM 2,4-dichlorophenoi in 25% methanol, 0.1 M, pH 5 acetate buffer (flow rate 0.96 mL/min), ,Eplateau = 1.00 V. this point for the monohydric phenols (typically involving one-electron oxidation (8)and relatively large Ep- EpI2values) vs. polyhydric phenols capable of forming quinones upon oxidation (typically involving two-electron oxidation (8) and somewhat smaller Ep- Epl2values as for 3,5-dichlorohydroquinone and p-methoxyphenol). Several precautions are necessary in using the empirical relationship of eq 1. The relationship hold8 for a specific set of HDV mass transport conditions and CV scan rate. Use of other flow conditions or scim rate than specified here will tend to modify the multiplicatiive factor but not the general form of the empirical relationship shown in eq 1. In general, increasing HDV flow rate or decreasing CV scan rate will tend to increase the multiplicative factor, and vice versa. The appropriate multiplicative factor for use with a specific LC detector and flow rate can best be established empirically by obtaining both CV and HDV results for a test compound. Subsequent LC detector potential settings for other compounds can be estimated from CV measurements. In all cases,
6 -
-
2 0
907
908
Anal. Chem. 1981, 53,908
due to large background currents or other sources will further shift the observed Ephhuand must also be taken into account. At higher applied potentials (typically >+1V), it may be necessary to select a potential less positive than Eplateau to minimize background current and noise. The pH dependence of the CV and HDV behavior of some of the phenols investigated indicated the presence of complicating chemical reactions, consistent with earlier observations (9). The consistency of the experimental results with the empirical relationship is therefore encouraging. When these precautions are heeded, the empirical relationship established here'is very useful in allowing rapid initial estimation from a single CV scan of the applied voltage required for detection of a compound of interest at an electrochemical LC or flow injection detector.
ACKNOWLEDGMENT The authors are grateful to Darryl Anderson of 3M Co. for supplying the Kel-F used in this work. LITERATURE CITED (1) Anderson, J. L.; Chesney, D. J. Anal. Chem. 1980, 52, 2156-2161. (2) Chesney, D.J.; Anderson, J. L.; Weisshaar, D. E.; Taliman, D. E. Anal. Chim. Acta 1981, 124, 321-331. (3) Anderson, J. E.; Taiiman, D. E.; Chesney, D. J.; Anderson, J. L. Anal. Chem. 9978, 50, 1051-1056. (4) Nicholson, 8.S.; Shain, I. Anal. Chem. 1964, 36,706-723.
(5) Blaedel, W. J.; Schieffer, G. S. J. Nectroanal. Chem. 1977, 80, 259-271. (6) Jordan, J; Javick, R. A. Nectrochim. Acta 1982, 6, 23-33. (7) Gaius, Z."Fundamentals of Electrochemical Analysis"; Haisted Press: New York, 1976; p 249. (8) Adams, R. N. "Electrochemistry at Solid Electrodes"; Marcel Dekker: New York, 1969; pp 363-369. (9) Vermilllon, F. J., Jr.; Pearl, I. A. J. Electrochem. Soc. 1964, f f f , 1392- 1400.
James L. Anderson* Department of Chemistry University of Georgia Athens, Georgia 30602
Duane E. Weisshaar Dennis E. Tallman* Department of Chemistry North Dakota State University Fargo, North Dakota 58105 RECEIVEDfor review August 22, 1980. Resubmitted January 22, 1981. Accepted February 9, 1981. This work was supported by the U S . Department of the Interior, Office of Water Research and Technology [Grants B-043 (J.L.A.), B-055 (D.E.T.), A-062 (D.E.T. and J.L.A.)], and the U S . Environmental Protection Agency [Grants R808084-01-0 (J.L.A.) and R803727-01-1 (D.E.T.)].
Measurement of Hydrogen Sulfide and Hydrogen Polysulfides in Sulfur Sir: Buyers of Claus process sulfur are showing increasing interest in the problem of monitoring hydrogen sulfide (present largely as hydrogen polysulfides) in sulfur. This is because of growing awareness of potential safety problems associated with the shipping and handling of this sulfur. The trace amounts of hydrogen polysulfides slowly weather off to give toxic (sometimes even explosive) amounts of hydrogen sulfide in the air space above the liquid sulfur in tank cars and trucks ( I ) . Because of this safety concern, we have done some work with analytical methods for measuring HZS and H2S, in liquid sulfur and would like to share our findings with the rest of the industry to aid in dealing with this safety problem. The main problem in doing a successful determination of all the free hydrogen sulfide and bound hydrogen sulfide (H2S,) is in assuring a complete breakdown of the hydrogen polysulfides during the course of the analysis. A method by Tuller (2) uses PbS as a catalyst to accelerate this breakdown while the H2Sis continuously removed from molten sulfur by a nitrogen sparge. The H2S is carried to zinc acetate absorption bottles where it is trapped as ZnS for later iodometric analysis. Tuller explicitly makes the point that a catalyst is required for the breakdown of hydrogen polysulfides. We would like to reemphasize the importance of using a catalyst. Failure to use a catalyst may give analytical results that are only a small fraction of those obtained when full degassing is achieved with a catalyst. Two small changes in the Tuller method were found to be helpful in our work. The Tuller method specifies a 2-h sparging period, but we found that somewhat higher values
could be obtained by using a 2.5-h degassing period. In addition, we found that the use of continuous magnetic stirring of the sulfur sample makes a significant difference in the degree to which the breakdown of hydrogen polysulfides may be taken to completion. For example, strict adherence to the Tuller method (using only occasional swirling of the sulfur) gave values of 296 and 265 ppm for the same sample, but addition of continuous power stirring gave 408 ppm for the same material. Although some concerns may arise that adding PbS to the sulfur might somehow cause an increase in the H2S content, we have been able to determine that this is not the case. We carried out the Tuller method of analysis (with added power stirring) on samples of sulfur that were completely degassed (by aging) before the analysis was begun. The results show no detectable H2S whether lead sulfide was added or not. We recommend that anyone wishing to monitor HZS and hydrogen polysulfides in sulfur either use this modified Tuller method or carefully check the method being used to make sure it correlates with this method. LITERATURE CITED (1) Donovan, J. R. Chem. Eng. Prog. 1982, 58, 92. (2) Tuller, W. N. "The Analytical Chemistry of Sulfur and Its Compounds"; Karchmer, J. H., Ed.; Wiiey-Interscience: New York, 1970.
T. H. Ledford Exxon Research & Development Laboratories Baton Rouge, Louisiana 70821 RECEIVED for review November 3,1980. Accepted January 19, 1981.
0003-2700/81/0353-0908$01.25/00 1981 American Chemical Society
