159

J. Electroanal. Chem., 209 (1986) 159-169

Elsevier Sequoia S.A., Lausanne - Printed m The Netherlands

PULSED AMPEROMETRIC DETECTION OF SULFUR COMPOUNDS

PART I. INITIAL STUDIES AT PLATINUM ELECTRODES IN ALKALINE SOLUTIONS

THERESA Z. POLTA and DENNIS C. JOHNSON l

Department of Chemistry, Iowa State Unruersity. Ames, IA 50011 (l7.S.A )

(Received 21st November 1985; in revised form 10th March 1986)

ABSTRACT

Conventional voltammettic and amperometric techniques at Pt or Au electrodes have not been

considered applicable for quantitative detection of most sulfur compounds. This is the result of the

observed loss of electrode activity caused by accumulation of sulfurous adsorbates and surface oxides.

Pulsed Amperometric Detection (PAD) at Pt and Au is highly sensitive for the anodic detection of sulfur

compounds, orgaruc and inorganic, which adsorb on the electrode surface. The detection of thtourea by

PAD at Pt is described here, and IS concluded to correspond to the surface-oxide catalyzed oxidation of

the sulfur moiety of adsorbed tbiourea to sulfate. Results reported here for detection in a flow-injection

system (FI/PAD) produce hnear calibration plots of Ir vs. cb at low concentrations and linear plots of

l/1, vs. l/cb at high concentrations. The detection limit for thiourea is 0.38 ppm (i.e., ca. 17 ng/45 ~1

sample) for the FI system used.

INTRODUCTION

During the last two decades, the need for inexpensive, economical fuel cells produced a strong interest in the electrochemical oxidation of organic substances. These studies were motivated by the lack of understanding of mechanisms of the anodic oxidation reactions of most organic compounds at solid electrodes. Many organic materials were studied extensively, with the notable exception of those containing sulfur [l-4]. The loss of activity for noble-metal electrodes due to adsorbed carbonaceous radicals and polymeric films was observed for most com- pounds, with the most severe loss of activity observed for sulfur compounds [1,5]. Several workers have reported on preliminary experiments with simple organic sulfur compounds [6-131; however, relatively little definitive information is availa-

l To whom correspondence should be addressed.

0022-0728/86/$03.50 0 1986 Elsevier Sequoia S.A.

160

ble. As a result, anodic detection of sulfur compounds at solid electrodes is generally not considered to be viable for quantitative analysis.

Nicholson [6] studied the oxidation of several simple organic sulfides at platinum in methanolic HCl. Reproducible Z-E curves were obtained only when the electrode was cycled between the positive and negative limits for solvent decomposition for several minutes prior to each recorded scan. Although aliphatic sulfides gave anodic peaks suitable for analytical quantitation, the current due to oxidation of chloride ion obscured any peak for oxidation of aryl sulfides. Houghton and Humffray [7] investigated the diary1 sulfides and determined that single-sweep voltammetry was a valid technique for electroanalysis in acid media, provided halide ions were absent. A voltammetric procedure at platinum was also described by Drushel and Miller [8,9] for the estimation of aliphatic sulfides in petroleum fractions.

The oxidation of cysteine at platinum has been studied systematically [lo-131. Davis and Bianco showed the necessity of a special alternating current (ac) pretreatment to obtain reproducible i-E curves [lo] and they concluded that the main detection process was a one-electron oxidation to form the disulfide, cystine. However, the electrode reaction was found to be strongly suppressed by the oxide film (PtO) forming on the electrode surface. Pradac and Koryta studied the oxidation of cysteine on platinum and gold electrodes [ll-131. They confirmed the one-electron oxidation mechanism

RSH + RS’+ e- + H+

2 RS’+ RSSR (1)

on Pt [lo]; however, their results refuted the conclusion by Davis and Bianco that no anodic electrode process occurs for cystine. It was concluded that surface oxide participates in the oxidation process to form the disulfide; however, a possible mechanism was not described.

Subsequent studies have attempted to elucidate the electrochemical behavior of surface-bound sulfur [14-181 and surface oxide has been shown to participate in the oxidation of sulfur compounds. LouCka [14,15] postulated that during the adsorp- tion of sulfur, surface atoms were removed from the platinum lattice, exposing a sub-surface layer upon which a reactive oxide forms. It was concluded that this oxide participates in the oxidation of elementary sulfur to sulfur(V1). Najdeker and Bishop [16] proposed that the oxidation of sulfur occurs by the formation of platinum(W) sulfide, through several possible platinum oxide intermediates. They did not consider the fate of the oxygen from the platinum oxide. Another theory, suggested by Contractor and La1 [17,18], proposed that sulfur is chemisorbed as a sulfoxide onto platinum, perhaps in two forms which should be distinguishable by the number of platinum sites occupied per sulfur atom. No experimental evidence was provided to substantiate this claim.

Most recently, Kapusta et al. [19] studied the mechanisms involved in sulfide oxidation and concluded that a surface layer containing platinum(IV) sulfide and sulfur is formed on the electrode. Unlike Najdeker and Bishop [16], Kapusta et al. did not propose participation by surface oxide in the anodic process.

161

Despite the work done at solid electrodes, mercury electrodes are still the most commonly used for electroanalytical determination of sulfur-containing compounds [20]. In addition to the dropping mercury and mercury pool electrodes, Bioanalytical Systems Inc. has recently commercialized a thin-layer, dual, mercury-amalgam electrode for chromatographic detection of thiols and disulfides [21-241. Mercury electrodes are preferred at present because of the observed loss of activity of solid electrode surfaces caused by adsorption of sulfur. However, mercury electrodes are useless for detection of numerous sulfur-containing compounds which do not form stable products with the mercury ions formed by the anodic dissolution of the electrode.

The surface activity of Pt electrodes was observed by Hughes et al. [25] to be

restored by alternate anodic and cathodic polarizations and they incorporated this pretreatment into a triple-step potential waveform for reproducible detection of simple alcohols in aqueous solutions at a Pt electrode [25]. This technique of Pulsed Amperometric Detection (PAD) has since been utilized to determine alcohols, polyalcohols and carbohydrates at Pt and Au electrodes [26-301 and amino acids [31], aminoglycosides [32], and even electroinactive adsorbates [33,34] at Pt elec- trodes. The PAD technique involves, generally, the application of a triple-step potential waveform consisting of a detection potential (E,), where oxidation of the adsorbed analyte occurs, followed by a large positive potential (E2) for oxidative cleaning of the electrode surface, and, finally, a large negative potential (E,) for reductive cleaning and adsorption of the analyte. The triple-step waveform is executed at a frequency of ca. l-2 s-l. In this paper, PAD is described for the reproducible anodic detection of sulfur compounds at a Pt electrode in aqueous alkaline solutions with emphasis on thiourea.

EXPERIMENTAL

Materials All chemicals were reagent grade. The supporting electrolyte was 0.25 M NaOH.

Water was distilled, deionized and filtered through an activated carbon column. Dissolved 0, was removed by saturation with N, from solutions in voltammetric studies at rotated electrodes; solutions were not deaerated for flow-injection detec- tion.

Instrumentation Current-potential curves (I-E) were obtained by cyclic and pulsed voltammetry

at a Pt, rotated ring-disc electrode (RRDE, Model AFDT06, disc: 0.460 cm2, ring: 0.060 cm2; Pine Instrument Co., Grove City, PA) using a PIR rotator (Pine Instrument Co.). Potentiostatic control for cyclic, linear scan voltammetry was achieved by a Model 173 potentiostat equipped with a Model 175 universal programmer (EG&G Princeton Applied Research, Princeton, NJ). The I-E curves were recorded on a RE0074 X-Y recorder (EG&G Princeton Applied Research). Potentiostatic control for the pulsed voltammetry was achieved by a UEM Pulsed

162

Amperometric Detector (Dionex Corp., Sunnyvale, CA). Current-time curves (Z-t) were obtained at a Pt, rotated disc electrode (RDE, Model AFMDlO, 1.0 mm2; Pine Instrument Co.). The electrode potentials for Z-E and Z-t curves are reported vs. the saturated calomel electrode (SCE).

The flow-injection (FI) system was based on a Model SHS-200 sample handling system (Fiatron Systems, Inc., Glendale, WI), which incorporates a multichannel peristaltic pump with a microprocessor-controlled sample injection valve. Ampero- metric detection was achieved at a Pt flow-through detector (Dionex Corp.) potentiostated by the UEM Pulsed Amperometric Detector. All potentials for flow-injection detection are reported vs. the silver-silver chloride electrode (SSCE). The dispersion coefficient, i.e., K = c,/cb, for the FI system was determined to be 0.090 using anodic detection of I- in 0.5 M H,SO, at constant potential.

RESULTS AND DISCUSSION

Preliminary characterization of several sulfur compounds was performed using cyclic voltammetry at the disk of the Pt RRDE in 0.25 M NaOH. Aqueous base was chosen as an electrolyte to keep the volatile compounds studied in the ionic form, and because future work is expected using an anion-exchange column with NaOH as the eluent to affect separation in a liquid chromatographic procedure. Anodic response was observed for numerous inorganic and organic sulfur compounds as given in Table 1. The voltammetric response appeared surprisingly similar for these compounds and thiourea was chosen as a representative compound for further study due to its low volatility and relatively low toxicity. A typical Z-E curve for thiourea

TABLE 1

Representative sulfur compounds detected by PAD in alkaline media. Conditions: 0.25 M NaOH, 0.67

ml min-’

Classification

Inorgamc:

Examples studied d

SCN-

s,o:-

S2-

(32

Organic: thiol R-SH R = C,H,

thioamide f

R = NH, (thourea)

R-CNH, R=CH,

R’ S dithiocarbamate ;N-C; R=R’=C,H,

R S

;7 sulfoxide R-S-R’ R = R’ = CH, (DMSO)

a Waveform: E, = 0.60 V (300 ms) “detection”; E, = - 1.00 V (800 ms); E3 = 0.70 V (50 ms).

163

I/PA -

I -1.0 -0.6 -0.2

I2 1 d6 I 0.

E/V(vs SCEI

Fig. 1. Current-potential curves for thiourea at a Pt RDE by cyclic voltammetry in 0.25 M NaOH.

Conditions: $I = 6.0 V min-‘, o = 94 rad s-l: concentrations: (a) 0.00 mM, (b) 1.00 mM.

is shown in Fig. 1. The cathodic peaks for the negative scan in the region -0.6 to -0.9 V in the absence of thiourea correspond to the production of adsorbed hydrogen atoms. Evolution of H,(g) occurs at E < -0.9 V. The anodic peaks observed on the positive scan in the same potential region correspond to oxidation of adsorbed hydrogen atoms. The anodic wave at E > -0.3 V and the cathodic peak at -0.3 V in the absence of thiourea are caused by surface oxide formation and reduction, respectively. Evolution of O,(g) occurs at E > 0.6 V in this media.

In the presence of thiourea, the cathodic and anodic processes for adsorbed hydrogen are completely suppressed, indicating that thiourea is adsorbed at the electrode surface under these conditions to such an extent that no free Pt sites are available for hydrogen adsorption. Oxidation of thiourea produces an anodic wave on the positive potential scan in the region -0.1 to 0.6 V. The oxidation is severely inhibited by the surface oxide formed during the positive scan, as noted by the dramatic decrease in current upon reversal of the scan at 0.6 V. Yet, if a surface oxide is formed during the positive scan with thiourea present, a reduction peak for the oxide at -0.3 V is expected on the negative scan (see residual curve for absence of thiourea). No oxide reduction peak is observed in the presence of thiourea, however. A possible explanation for the lack of an oxide reduction peak is that, simultaneous to oxide reduction, an anodic process for thiourea is occurring and the net effect (lanod + Icath) is that only a small cathodic current is obtained in the region where the large cathodic oxide peak is expected.

The FI/PAD system was applied to investigate the possibility of an anodic process for thiourea in the region -0.2 to -0.6 V. The waveform was as follows: E, (350 ms) was varied in the region - 0.2 to - 0.6 V, the current was sampled at the end of this period; E, = 0.70 V (100 ms) to clean the electrode surface oxidatively; E, = - 1.00 V (800 ms) to reduce the Pt surface and allow adsorption of analyte. When this waveform was executed, anodic peaks were detected for thiourea. A mechanism consistent with these results and the literature [35] is the two-electron oxidation of thiourea to its corresponding formamidine disulfide at E,

164

in the region of platinum oxide reduction,

NH2 TH 2 H,N-C=S -+ [H,N-C-S12 + 2 H+ + 2 e-

(E < - 0.4 V). It is concluded that the adsorbed disulfide is the species which has been responsible for previous observations of serious loss of electrode activity. Exhaustive electrolysis, at a large Pt-gauze electrode with E = 0.6 V for 5 h, yielded enough SOi- to be readily detected as insoluble BaSO, following addition of Ba(NO,),. It is concluded on this basis that the adsorbed disulfide is oxidized to urea and sulfate by a surface oxide-catalyzed process on the position scan for E > 0.1 V.

The FI/PAD experiment was repeated with DMSO and no anodic process was detected in the potential region -0.1 to -0.9 V, although the adsorption of DMSO is evident from I-E curves by suppression of the hydrogen peaks at the Pt electrode. These results are consistent with the conclusions for thiourea, as no dimer containing a sulfur-sulfur bond can easily be formed from DMSO.

The anodic current for thiourea in the region -0.1 to 0.6 V was found to be dependent on the rate of potential scan (Fig. 1). A plot of current vs. scan rate was approximately linear with a zero intercept, indicating that the oxidation is strictly a surface-controlled process. However, the current was found also to be slightly dependent on rotation speed. Thus, we conclude that mass transport, as well as adsorption, is a factor in determining the anodic response for thiourea; however, the rate of oxidation of the transported thiourea is controlled by the rate of surface oxide generation and, hence, is a linear function of scan rate.

Amperometric detection of thiourea at a constant electrode potential is not successful for analytical quantitation because of rapid fouling of the electrode surface by the proposed dimerization product for E -c -0.1 V, as well as the formation of PtO for E > -0.1 V. The anodic current response with and without thiourea is shown as a function of time in Fig. 2 (curves a and b). Literature [34] reports that the rapid decay of current as shown is either due to oxide formation, with desorption of an electroinactive adsorbate, or oxide formation with oxide-cata- lyzed oxidative desorption of an electroactive adsorbate. We conclude that the second process is the more likely explanation. Also shown in Fig. 2 (curves c and d) is the current response of the electrode upon application of PAD with and without thiourea. It is easily noted that the signal response is steady in both cases. The significance of a steady background signal obtained with PAD is that the anodic current signal for the analyte can be amplified and the constant background electronically offset.

The selection of an appropriate potential waveform for maximum sensitivity for thiourea with PAD was made on the basis of the I-E curve in Fig. 1. E, was chosen corresponding to the region of oxide reduction and analyte adsorption. E, was chosen greater than E, to decrease the background current, as the rate of oxide formation at E3 is greater than at E,. A suitable value for E, was determined from

165

!I 0

b

I I I 0.0 0.4 0.6 ’

hme/mm

I ’ I I I I

I I I

02 06

E /V(vs SCEI

Fig. 2. Current-time curves of thiourea at a Pt RDE in 0.25 M NaOH. Conditions: w = 94 rad s-‘.

Concentrations (mM): (a,c) 0.0, (b,d) 0.50. DC detection: (a,b) E = 0.60 V vs. SCE. Pulse detection: (c,d)

E, = 0.60 V (350 ms), E, = - 1.00 V (800 ms), E, = 0.70 V (50 ms).

Fig. 3. Current-potential curves for thiourea at Pt by pulsed voltammetry m 0.25 M NaOH. Conditions:

E, vaned (350 ms), E, = - 1.00 V (800 ms), E, = 0.70 V (50 ms); w = 94 rad s-l: concentrations (mM):

(0) 0.0. (X) 1.0.

hydrodynamic voltammograms shown in Fig. 3. The anodic signal was measured for 50 ms after a delay period of 300 ms to allow the background current to decay to a reasonably small value before sampling the faradaic signal for the oxidation of adsorbed thiourea. Upon comparison of the cyclic and pulsed voltammetric curves, it is noted that the largest difference between the anodic current signal with and without thiourea occurs at 0.6 V for the pulsed experiment, unlike the cyclic experiment where the maximum occurs at ca. 0.4 V. These results confirm the necessity of performing the FI/PAD experiment to optimize the design of the waveform for maximum sensitivity.

Application of the waveform with E, = 0.60 V (350 ms), E, = - 1.00 V (800 ms), and E, = 0.70 V (50 ms), for flow-injection detection of thiourea, gave reproducible peaks (rsd < 3% for 10 injections) with an anodic peak current found to be linear with concentration for 0.10-0.50 mM, as shown in Fig. 4. For reference, three

166

r

b

Fig. 4. Detection peaks for thourea in 0.25 M NaOH by FI/PAD. Pulse detection: (a-f) E, = 0.60 V

(350 ms), E, = - 1.00 V (800 ms), E3 = 0.70 V (50 ms); V, = 0.67 ml min-‘; V, = 45 pl. DC detectlon:

(ar) E = 0.60 V; V, = 0.60 ml min -‘. V = 40 ~1. Concentrations: given m figure, in mM. , s

injections of 0.50 mM thiourea were also made for a constant electrode potential of 0.60 V. After three injections, no detectable peaks were observed. The detection limit for thiourea using the PAD waveform in this FI system was determined experimentally to be 0.38 ppm (i.e., 17 ng in the 45 ~1 sample; S/N = 3).

Calibration plots for thiourea (1r vs. cb) are shown in Fig. 5 for adsorption time values of 200, 1000 and 8500 ms. The response is approximately linear for very dilute solutions and/or low values of tads, e.g., cb < 0.60 mM for tads = 1000 ms (see Table 2 for linear regression statistics). However, curvature can be severe at high cb

t , I I I I

60 t

I I I I I I

0.0 2.0 4,o 6.0 8.0 10.0

c x lO‘+/M

Fig. 5. Calibration curves (I, vs. c) for tluourea in 0.25 M NaOH by FI/PAD. Conditions: V, = 0.65 ml

-‘. V, = 43 ~1. Waveform: (A) E, = 0.60 V (300 ms), E, = - 1.00 V (8500 ms). E, = 0.70 V (50 ms);

;“B’; E, = 0.60 V (300 ms), E, = - 1.00 V (1000 ms), E, = 0.70 V (50 ms); (C) E, = 0.60 V (300 ms).

E, = - 1.00 V (200 ms), .Ej = 0.70 V (50 ms).

167

TABLE 2

Linear regrewon statistics for data plotted in Figs. 5 and 6. Uncertainties calculated at the 90%

confidence level

Fig. Equation of

response

Curve Range of cb/M r2 a/@ b/PA M-’

5 I,=a+bcb A 1.0x10-5 to

1.0x10m4 0.9998 - 0.38 + 0.10 2.08 f 0.03 x 10 5

B 1.0x 10K5 to

1.ox1o-3 0.9982 0.51 k 0.44 4.16*0.15x104

C 1.0x10-5 to

1.ox1o-3 0.9982 0.29 kO.19 1.78kO.62~10~

a/PA- b/M PA-

6 l/I, =

a + b(l/cb) A l.OXlO~~ to 0.9993 3.4*3.7x10-3 5.30+0.11x10-6

1.0x 10-3

B 1.0x10-~ to 0.9998 -1.7+0.9x10-2 2.27 f 0.03 x 10 - ’

1.ox1o-3

C 1.0x10-~ to 0.9995 -5.9*27.2x10-3 4.39xO.O8~10-~

1.ox1o-3

and large tads, as shown by curve C for tads = 8500 ms. This behavior was anticipated because all previous studies using PAD [25-341 have reported similar non-linear response at high concentrations of analyte. Non-linear calibration curves previously have been attributed to a response mechanism whereby the anodic peak signal (I,) is proportional to the peak surface coverage (0,) by adsorbed analyte. The Langmuir isotherm describes this behavior for low values of ZZ, in the form:

ep = zp/&mx = Kc,/( 1 + Kc,) (3) where K is the equilibrium constant for the adsorption and Zr, = Z,,,,, at t9 = 1.0. The value of cp in the FI system can be related to the bulk concentration ( cby of the sample injected by the dispersion coefficient (K) according to

K = C,/Cb

Hence, eqn. (3) is rewritten as

(4)

0, = zp/&mx = K'Cb/(l + K’cb) (5) where K’ = KK, the dispersed adsorption equilibrium constant for the FI system. For dr, -=K 1 (i.e., small cb or small tads), K’cb -=zc 1 and eqn. (5) can be simplified to

6’,, = Z,/Z,,,,, = K ‘cb (6)

Thus, at short adsorption times, plots of Zp vs. cb should be linear; however, for large tads and large cb, response must be linearized by plotting l/Z, vs. l/cb. These conclusions seem to be supported by the data for thiourea shown in Figs. 5 and 6 (see Table 2 for linear regression statistics). The matter of calibration and the dependence of response on tads will be considered further in Part II [36].

l

_

_

A

0.0 - _

I I I / I / I I I 0.0 2.0 4.0 6.0 8.0 10.0

10.’ c-’ / tv-’

Fig. 6. Calibration curves (l/I, vs. l/cb) for thiourea m 0.25 M NaOH by FI/PAD. Conditions:

vr = 0.65 ml min-‘, V, = 43 pl. Waveform: (A) E, = 0.60 V (350 ms). E, = - 1.00 V (8500 ms),

E, = 0.70 V (50 ms); (B) E, = 0.60 V (350 ms), E, = - 1.00 V (1000 ms), E, = 0.70 V (50 ms); (C)

E, = 0.60 V (350 ms), E, = - 1.00 V (200 ms), E, = 0.70 V (50 ms).

CONCLUSION

The PAD technique at Pt electrodes has been demonstrated to be useful for the reproducible anodic detection of numerous sulfur compounds. The results presented here for thiourea are characteristic of all compounds studied. Linear calibration curves of Zp vs. cb are obtained for low concentrations and short adsorption time. For high concentrations and long adsorption time, linear calibration curves are obtained by plotting l/Ir, vs. l/cb. Detection limits are better than with other methods [37]. Thus, pulsed amperometric detection of sulfur compounds at noble- metal electrodes is expected to gain popularity in the future, especially in effluent streams for liquid chromatography.

169

ACKNOWLEDGEMENTS

The financial support of National Science Foundation (Grant CHE-8312032) and Dionex Corporation, Sunnyvale, CA is greatfully acknowledged.

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