Anal. chem. 1 ~ 2 64, , 1785-1789
1785
Pulsed Voltammetric Detection of Arsenic( I I I) at Platinum Electrodes in Acidic Media Douglas G. Williams and Dennis C. Johnson' Department of Chemistry, Iowa State University of Science and Technology, Ames, Iowa 50011
The oxldatlon of AS(OH)~at Pt Is believed to occur by an anodlc oxygen-transfer mechenlsm that Involves anodlc dlrcharge of H20to form adsorbed hydroxyl radlcals (PtOH); however, anodlc converslon of PtOH to the Inert oxide (PtO) resultsIn severe parslvatlon of the electrode surface. Hence, a tranrport-llmlted response Is not obtalned at a Pt rotateddhk electrode by conventlonal h e a r scan or staircase VOC tammetry. The AS(OH)~response Is compared for several pulsed voltammetric waveforms. The preferred waveform corulsts of four potentlal steps wlth an actlvatlon step to produce a small quantlty of PtOH prior to the detectlon step which Is followed by porltlve and negatlve potentlal steps to achleve reproduclble cleanlng of the Pt surface. Thls waveform produces a transport-llmlted slgnal wlth mlnlmal background current.
INTRODUCTION The anodic detection of As(OH)3 by the reaction in eq 1 is an example of anodic oxygen-transfer reactions in which oxygen is transferred from HzO in the solvent phase to the As(OH),
+ H,O
-
OAs(OH),
+ 2H' + 2e
(1)
oxidation product(s). This reaction, as a model of 0-transfer reactions, has been studied a t Pt electrodes in acidic media using cyclic voltammetry,chronocoulometry,and square-wave hydrodynamically-modulatedvoltammetry.' Onset of the 0transfer reaction at Pt occurs at substantial overpotential and concomitantly with anodic discharge of H20 to generate adsorbed hydroxyl radicals (PtOH) as the first step in the mechanism for formation of inert surface oxide (PtO). It has been concluded that the hydroxyl radical is the intermediate state of oxygen in the process of transfer from HzO to OAs(OH)3,as indicated by eqs 2 and 3, converted to the inactive
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oxide (PtO), as shown in eq 4, which has the effect of passivating the P t surface for further detection of As(OH)3. At PtOH
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high overpotentials on the PtO-covered surface, oxidation of As(OH)3 recommences when HzO is discharged to produce adsorbed hydroxyl radicals as the first step in the mechanism for 0 2 evolution.' The application of cyclic voltammetry (CV) for detection of As(OH)3 at a P t rotated-disk electrode (RDE) has two highly objectionable characteristics: (i) The anodic signal (1) Cabelka, T. D.; Austin, D. S.;Johnson, D. C. J. Electrochem. SOC. 1984, 131, 1595; 1985, 132, 359.
for As(OH)~does not achieve a transport-limited value, undoubtedly because of the rapid formation of inert PtO, and (ii) whereas greatest sensitivity is obtained during the positive potential scan, a large background signal is obtained as a result of the formation of surface oxide (PtO) and the surface-controlled oxidation of As(OH)3previously adsorbed on the oxide-free Pt surface.' Here, we evaluate response characteristics for the voltammetric detection of As(OH)3using the three pulsed-potential waveforms shown in Figure 1. Normal pulsed voltammetry (NPV) is based on the waveform for normal pulsed amperometric detection (NPAD; Figure 1A). Applications of NPAD, alternately known as PAD, have been highly successful at Pt and Au electrodes for the sensitive anodic detection of alcohols, polyalcohols, and carbohydrates, following their separations by liquid chromatography (LC).293The NPAD waveform regulates the electrocatalytic properties of noblemetal electrode surfaces, and in fact, many compounds now detected with great sensitivity had been previouely considered to be electroinactive on the basis of futile efforts at detection with a constant applied potential. In applications oqNPAD for anodic reactions, the electrodepotential is stepped through three characteristic values: (i) a detection potential of &et, applied for a total time period of tdet, with digital sampling of the amperometric current during the period t, following a holding period of th, i.e., tdet = t, + th; (ii) an oxidation potential of E,, >> E d e t , applied for a period of to,, to achieve rapid formation of surface oxide (PtO) with oxidative desorption of adsorbed impurities, reactants, and/or detection products; (iii) a reduction potential of Er4 ca. 0.5 V as a result of the anodic discharge of HzO to form PtOH and PtO. The positive scan of E d e t was terminated at 1.20 V because the evolution of 0 2 for &et > ca. 1.3 V results in a rapid increase in anodic current. In the presence of As(OH)3,some As(OH)3 is adsorbed at the oxide-free Pt surface with the result that the onset of oxide formation is suppressed to ca. 0.65 V.' The onset of As(OH)3 oxidation and oxide formation occur concurrently to produce a peak response at ca. 0.9 V. The decline of anodic signal for E d e t > 0.9 V undoubtedly is a consequence of the potential dependence of the kinetics for formation of inert oxide (PtO). Effectively, the fractional surface coverage by inert PtO formed at each valueEdetduring the period t h , prior to the measurement of the amperometric signal, increases as &et is increased. Figure 3 contains the chronoamperometric (i-t) response obtained from 2.0 mM As(OH)3for four values of &et. Also shown is the steady-state transport-limited current (0.616 mA) calculated for the Pt RDE using the values 1.027 X loe5 cm2 5-1 for the diffusion coefficient' and 1.014 X 10-2 cm2 s-1 for the kinematic viscosity of 0.1 M H z S O ~At . ~E d e t = 0.8 V, the response is stable over the 5000-ms period monitored but is substantially below the transport-limited value. Hence, we conclude that the conversion of active PtOH to inert PtO is relatively slow at this potential; however, there is not sufficient PtOH formed to activate the entire electrode ~~
(6) CRC Handbook of Chemistry and Phyarcs, 60th ed., Weast, R E d , CRC Press Boca Raton, FL, 1979, p D-271
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 17, SEPTEMBER 1, 1992
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Detection Time (ma) Figuo 9. Chronoemperometrlcresponseby NPAD for 2.0 mM As(OHI3 at a Pt RDE in 0.1 M H2S04as a function of &,. Conditions: E,, = 1.4 V, 6,= 100 ms; Ersd = 0.2 V, f,& = 500 ms; w = 1600 rev mln-l. &, (V): (A) 0.80; (B) 0.90: (C) 1.00; (D) 1.20.
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D e t e c t i o n P o t e n t i a l ( V v a . SCE) Flguro 5. RPV response for As(0Hh as a function of concentration at a Pt RDE in 0.1 M H2S04. Conditions: Eht = stepped, t,, = 200 ms; ,Erd = 0.2 V, Csd = 500 ms; E,, = 1.2 V, ,&, = 100 ms. 0 (mM): (A) 0.00; (B) 0.40; (C) 0.80; (D) 1.20; (E) 1.60; (F) 2.00.
the presence of As(OH)3. This can be explained on the basis of the initial inhibition of oxide formation by adsorbed As(OH)3followed by a very rapid growth of oxide following desorption of As(OH)3. It is also possible that oxidation of the adsorbed As(OH)3contributes to ineta Plots of inatvs wl/* were examined for a variety of values of Edetand t h in the NPAD waveform; however, no combination of values gave in,, values in agreement with the theoretical transport-limited value over the range of rotation speeds represented in Figure 4.
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Flguro 4. Levlch plots for the background (A), total (B), net (C),and transport-ilmited (D) NPAD response for 0.4 mM As(OH), at a Pt RDE In 0.1 M H2S04 with &, = 1.00 V. Conditions: E,, = 1.2 V, 4, = 100 ms; End = 0.2 V, hsd = 500 ms.
surface. As Edetis increased, the initial current for As(OH)3 is made larger; however, the rate of decay of the anodic current is also increased due to the more rapid formation of inert PtO which passivates the electrode surface. Plots are shown in Figure 4 of anodic current vs square root of rotation speed (w1l2,rev1l2 rnin-W for 0.400 mM As(OH)3using Edet= 1.0 V. Clearly, under these conditions, a substantial background current (ibkd) is obtained (curve A). Hence, Figure 4 shows plots of the residual current (curve A), total current (ibt; curve B) in the presence of As(OH)3, the net current corrected for the background (&et = ibt - ibkd; curve C),and the theoretical transport-limited current (curve D). Values of in,, (curve C) are significantly larger than the theoretical values (curveD), and this observation is considered as evidence that ibkd values obtained for the absence of As(OH13 are smaller than the actual background current in
Reversed Pulsed Voltammetry (RPV). RPV is performed using the RPAD waveform in Figure 1B. As explained above, it is expected that RPV will result in a significant decrease in the background current as compared to NPV. Furthermore, As(OH)3 adsorbed at Erd is expected to be quickly desorbed anodically at E,, with the result that the total anodic current a t Edet< E,, will have minimal contribution from oxide formation and oxidation of adsorbed As(OH)3. The RPV response for As(OH)3is shown in Figure 5 as a function of concentrations in the range 0-2.0mM. In the RPV waveform, the oxidation step (Eox)immediately precedes the detection step (Edet).Hence, the degree to which the formation of surface oxide is stopped by the step from E,, back to Edetcan be expected to improve as the difference E,, - Edet is increased. This is illustrated in the background response (curve A), and we conclude for the waveform parameters used that ibkd is minimal for E,, - Edet > ca. 0.2 V. This conclusionis also in agreement with results discussed by Gilroy.5 Values of E,, > 1.5 V were avoided to prevent accumulation of Odg) a t the electrode surface. As expected, a substantial cathodic peak is observed in all curves for dissolution of surface oxide a t Edet< ca. 0.6 V. It is of great significance in Figure 5 that the As(OH)~ response (curves B-F) is independent of Edetin the range ca. 0.7-1.1 V. This is evidence that the RPV waveform results in uniform activity of the Pt surface throughout this potential region. Values of ibkd and ibt were determined as a function OfW1/2USingEdet=1.OVintheRPADwaveform. Theibt-w1/2 plot for 0.400 mM As(OH)3 was virtually linear with the following regression statistics: slope = (3.24f 0.03)X 10-3 mA r e d 2 min1’2, which is in good agreement with the theoretical value of 3.08 X mA r e d 2 for a mass
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 17, SEPTEMBER 1, 1992
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Sq. Fit. R o t a t i o n S p e e d ( r e v / m l n ) Flgurr 6. Levich plots of the RPAD responses for 0.4 mM ANOH), = 1.00 V. as a function of hXat a Pt RDE in 0.1 M HzSO4 with Conditions: Erd = 0.2 V, &, = 500 ms; E,, = 1.4 V, &, = varied. (ms): (A) 100; (B) 1000; (C) 5000.
transport-limited response; intercept = (18.4 f 2.3) X 10-3 mA, which corresponds closely with ibkd = (19.2 f 1.7) X mA measured in the absence of As(OH),; sy = 1.64 X 10-5 mA and r = 0.99957. These observations have great significance for applications of RPAD to LC and FIA because the observed baseline response will correspond to the true background for the detection peaks. The RPV response is a sensitive function of the choice of to,, as is illustrated in Figure 6 by plots of inet vs wl/*. For to, = lo00 ms (curve b) and 5000 ms (curve c), sufficient time is allowed at E,, for conversion of a substantial amount of the active PtOH to inactive PtO. The consequence of long to, is that a substantial fraction of the electrode surface is passivated with the resulting negative deviation of inetfrom the transport-limited response. Activated Pulsed Voltammetry (APV). APV is performed with the waveform in Figure 1C for activated pulsed amperometric detection (APAD). An anticipated shortcoming of the RPAD waveform (Figure 1B)results because the step to E,, is responsible for both activation and oxidative cleaning of the electrode surface. Furthermore, it is unlikely that a single set of values for E, and to, can be optimal for both of these functions. The APAD waveform (Figure 1C) combines the advantages of NPAD and RPAD by allowing independent selection of parameters for the activation (Eact and tact) and oxidative cleaning (E,, and to,) functions. Figure 7 contains the APV response for 0.0 and 2.0 mM As(OH)3as a function of Ea& for tact = 100 ms; the positive scan of E d e t is terminated at E,,* The response for E d e t > 0.80 V corresponds to a plateau for 1.10 V IEactI1.25 V, and ibkd is very small in comparison. For Eact2 ca. 1.2 V, the current at Edet = 0.9 V steadily decreases below the transport-limited value of 0.616 mA. This undoubtedly is the result of surface passivation by increasingly greater quantities of PtO formed during tact for the increasing values of E,,*. The optimum values for the activation process are chosen as Eact= 1.20 V and tact= 100 ms. Figure 8 contains typical APV results for As(OH)3 concentration in the range 0.0-2.0 mM using E,,, = 1.20 V and tact = 100 ms. The current response is independent of E d e t throughout the range 0.8-1.1 V and is proportional to concentration. It is also apparent for the residual response
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Oetection P o t e n t i a l (V v s . SCE) Figwe 7. APV response for 2.0 mM As(OHk at a Pt RDE in 0.1 M HzSO4 as a function of €am Conditions: &, = stepped, t,, = 200 ms; &,, = 0.2 V, t,d = 500 ms; EOx= 1.4 V, 4, = 100 ms; €aol = varied, ran = 100 ms. €.,(V): (A) 1.00; (B) 1.10; (C) 1.20; (D) 1.30; (E) 1.40; (F) 1.50.
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(curve A) that ibkd is virtually zero for Edet in the range 0.7-1.1 V. Hence, we conclude for tact= 100 ms, that the difference E,,, - E d e t must exceed ca. +200 mV to bring about cessation of the oxide formation process with the step to Edet. Values of ibt were obtained vs w1l2 for 0.400 mM As(OH)3 with an APAD waveform described by Ea& = 1.20 V (tact= 100 mS), E d e t = 1-00v ( t h = 200 mS), E,, = 1.5 v (tox = 100 ms), and Erd= 0.20 V (trd = 500 ma). The ibt-uN2 plot was linear with the following regression statistics: slope = (3.02 f 0.02) X 10-3 mA rev1/2 min1/2, which is in good agreement with the theoretical value of 3.08 X mA revV2 for a mass transport-limited response; an intercept of (10.7 f 1.8) X 10-3 mA, which is in only fair agreement with ibkd = (4.5 f 0.5) X 10-3 mA obtained in the absence of As(OH),; sy = 1.3 X mA and r = 0.99968. Increasing tactminimized
ANALYTICAL CHEMISTRY, VOL. 64, NO. 17, SEPTEMBER 1, 1992
the intercepts at high As(OH)3 concentration; however, the ibt-w1/2 plots exhibited increasinglynegative deviations from linearity for large w1/2, probably because of the increased surface passivation by the accumulated PtO. Calibration data were obtained using the APAD waveform described above with w = 1600 rev min-l and As(OH)~ concentration in the range 0.01-2.00 mM. The ibt-Cb plot was virtually linear with the following regression statistics: slope = (3.095 f 0.002) X 10-l mA mM-1, which is in excellent agreement with the theoretical value of 3.083 X 10-1mA mM-1 for a mass transport-limited response; intercept = (16.2 f 6.8) X 10-3 mA, which is in only fair agreement with the value mA measured in the absence of of ibkd = (4.54 f 0.51) X As(OH)3; sy = 1.36 X 10-3 mA and r = 0.99998. Using the value 0.51 X 10-3 mA as the characteristic noise for this determination, the limit of detection (LOD)is estimated as 3(0.51 X 10-3 mA)/(3.095 X 10-1 mA mM-1) = 4.9 X 10-3 mM (i.e., ca. 5 pM).
CONCLUSION The APAD waveform is preferred because it combines the advantages of the NPAD and RPAD waveforms for oxide-
1789
catalyzed anodic detection mechanisms at Pt. These advantages include: (i) activation of the electrode surface by formation of PtOH prior to the detection process, (ii) cessation of the oxide formation process by the negativestep in potential from Eactto E d e t Eaetand Er4
