Anal. Chem. 1995,67, 208-211

Flow lnjection Amperometric Detection of Hydrazine by Electrocatalytic Oxidation at a Perfluorosulfonated lonomer/Ruthenium Oxide Pyrochlore Chemically Modified Electrode Jyh-Myng Len* and Jen-Sen Tang

Depatfment of Chemistry, National Chung-Hsing University, Taichung, Taiwan 402, Republic of China

A Nafiotdrutheniumoxide pyrochlore (PbzRu~,Pb,O7-~)modi6ied glassy carbon electrode exhibits excellent electrocataIytic activity in the oxidationof hydrazine in neutral media. The catalyst is synthesized directly inside the Nafion thin lilm matrix, which is spin coated onto a glassy carbon electrode. Hydrazine is detectedby flow injection analysis at the modi6ied electrode with excellent sensitivity. b e a r calibration curves are obtained over the 5 x 10-5-6 x M range with pH 8.0 ammonia-ammonium chloride buffer solution as the carrier. The detectionlimit is 0.048 ng. The practical analytical utility is illustrated by selective flow injection measurements of hydrazine in the presence of oxalic acid and surface-active materials. Sensitive methods are needed for the detection and determination of hydrazine due to its importance in industry and pharmacology. In practice, the most useful techniques for monitoring easily oxidizable species are the flow-through amperometric detection methods for liquid chromatography. However, electrochemical determinations are difficult at ordinary carbon electrode due to a large oxidation overpotential. One promising approach for minimizing overvoltage effects is the use of electrocatalytic chemically modified electrodes (CMEs). Generally, the CMEs are constructed by incorporating complexes with good catalytic activity toward the electroxidation of hydrazine into a suitable polymer matrix coated on glassy carbon electrode (GCE) for flow injection analysis For example, Wang et al.' described a trifunctional electrode coating based on a mixture of cobalt phthalocyanine (CoPC) and cellulose acetate for the detection of hydrazine (detection limit, 0.64 ng) and other compounds in FIA. Wang and Liz later substituted cellulose acetate with Nafion to prepare a multifunctional CME for the detection of hydrazine (detection limit, 5.7 ng) and hydrogen peroxide in FIA. In another paper, Hou and Wand described a Nafon film coated on top of a Prussian Blue-mod~edGCE for the detection of hydrazine in FIA with a detection limit of 0.6 ng. We report here another interesting oxide catalyst, ruthenium oxide pyrochlore (PbzRuz-xPbx07-y),for the construction of the CME and for the determination of hydrazine in FIA.

The electrocatalytic activity of PbzRuz-xPb,O,-y has been exploited for oxygen reduction reactions in our previous studies.5~6 Because of the good catalytic activity of Pb~Ruz-~Pb~07-~ toward the electroreduction of molecule oxygen, our laboratory later developed a novel way of constructing the Nafion/ruthenium oxide pyrochlore CME for the determination of dissolved ~ x y g e n .The ~ catalyst is synthesized directly inside the Nation thii film matrix, which is spin coated onto a GCE. The catalytic reduction of dissolved oxygen on this CME results in fast and sensitive determination of dissolved oxygen in both acid and alkaline environments. In this work, the application of the Naiion/ruthenium oxide pyrochlore CME is further pursued for the detection of hydrazine in FIA. The electrode exhibits excellent stability and thus offers great potential for electrochemical monitoring of flowing streams. As Wang et a1.ls2have demonstrated the improved stability, scope, and selectivity of amperometric measurements resulting from the multifunctional operation of their CMEs, similar advantages will also be demonstrated by our CME in this study. EXPERIMNTAL SECTION

Chemicals and Reagents. Nafion perfluorinated ion-exchange powder, 5 wt % solution in a mixture of lower aliphatic alcohols and 10%water, was obtained from the Aldrich Chemical Co. (Milwaukee, WI). Lead nitrate (Pb(NO&) and ruthenium chloride (RuCkxHzO) were also obtained from Aldrich. Hydrazine sulfate (Wako), albumin (Sigma), gelatin (Showa), and all the other compounds (ACScertiiled reagent grade) were used without further purification. Aqueous solutionswere prepared with doubly distilled deionized water. Caution! Care must be taken, as hydrazine sulfate may reasonably be anticipated to be a carcinogen. Anhydrous hydrazine explodes during distillation if traces of air are present. Apparatus. Electrochemistry was performed on a Bioanalytical Systems (West Lafayette, IN) BAS lOOB electrochemical analyzer. A BAS Model VC-2 electrochemical cell was employed in these experiments. The threeelectrode system consisted of a Nafionhuthenium oxide pyrochlore CME working electrode, a Ag/AgCl reference electrode (Model RE5, BAS), and a platinum wire auxiliary electrode. The flow injection system consisted of a carrier reservoir, a Cole-Parmer Masterflex microprocessor pump drive, a Rheodyne

(1)Wang, J.; Golden, T.; Li, R Anal. Chem. 1988,60,1642. (2)Wang, J.; Li, R Talanta 1989,36,279.

(5) Zen, J.-M.; Manoharan, R; Goodenough, J. B. J. Appl. Electrochem. 1992, 22, 140.

(3) Wang, J.; Lu,2.Electroanolysis 1989,1 , 517. (4)Hou, W.; Wang. E. Anal. Chim. Acta 1992,257, 275.

(6) Zen, J.-M.; Wang, C.-B. J. Electrochem. SOC. 1994,141, L51. (7) Zen, J.-M.; Wang, C.-B. J. Electround. Chem. 1994,368,251.

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Analytical Chemistty, Vol. 67, No. 1, Januaty 1, 1995

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Figure 3. Hydrodynamic voltammograms of 1 x 10-5 M hydrazine at the bare GCE (a) and the Nafion/ruthenium oxide pyrochloreCME (b). The carrier was pH 8.0 ammonia-ammonium chloride buffer solution; flow rate, 1.O mUmin.

Model 7125 sample injection valve (2@,Lloop), interconnecting Teflon tubing, and a BAS Model CC-5 thin-layer electrochemical detector with a BAS MF 1000 dual GCE. A BAS lOOB electrochemical analyzer was also used in the flow injection experiments. Electrodes. The preparation of the Nafionhthenium oxide pyrochlore CME generally followed the procedures described previou~ly.~ In brief, the GCE or the dual GCEs for FIA were first polished with the BAS polishing kit and rinsed with deionized water and then further cleaned ultrasonically in 1:lnitric acid and deionized water successively. Nafion, a perfluorosulfonated ionexchange polymer, was spin coated onto a cleanly polished GCE. Ruthenium oxide pyrochlore particles were then synthesized in the Nafion matrix by treatment of Ru3+,Pb2+exchangedpolymer in alkaline aqueous solution with purging of 0 2 at 53 "C for at least 1 day. The optimal conditions used to prepare the CME, i.e., a dipping solution of Pb2+:Ru3+1.5:1, a coating solution of 4.38 wt % Nafion, and a spin rate of 2800 rpm, for the dissolved oxygen experiments were also used in this work. Procedure. Dependence of the anodic peak current of hydrazine on pH at the Nafion/ruthenium oxide pyrochlore CME was first evaluated using the following buffer mixtures: NaZHP04

+ NaH2P04(PH 4-7), N&Cl+ NH40H (PH 8-10), and NaOH + CzHsNO2 (PH 10-12.4). The Nafionhthenium oxide pyre chlore CME was equilibrated in the test buffer solution containing hydrazine before measurement. The hydrazine quantitationwas achieved by measuring the current of the oxidation peak. For the rest of the batch-type experiments, a pH 8.0 ammoniaammonium chloride buffer solution was used. Hydrazine sulfate solution for use in flow injection was prepared with the mobile phase, which is a pH 8.0 ammonia-ammonium chloride buffer solution. The flow rate was 1.0 mL/min. RESULTS AND DISCUSSION Electrochemistry in a Conventional Cell. The cyclic voltammograms in a conventional cell obtained for a bare GCE, a Nafioncoated GCE, and a Nafion/ruthenium oxide pyrochlore CME immersed in pH 8.0 ammonia-ammonium chloride buffer solution are shown in Figure 1. As can be seen, no oxidation peak was observed for both the bare GCE and the Nafioncoated GCE in the potential range of -1.00 to +1.00 V vs Ag/AgCl (curves a and b), whereas a well-defined oxidation peak with a peak potential of +0.60 V was observed with the Natlon/ruthenium Analytical Chemistry, Vol. 67, No. 1, January 1, 1995

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oxide pyrochlore CME (curve c). The overvoltage of hydrazine oxidation at the bare GCE is greatly decreased when the Nafion/ ruthenium oxide pyrochlore CME is used. The catalytic oxidation currents observed were proportional to the concentration of hydrazine. Since the experiments were done without deoxygenation, the reduction peaks appearing in the negative potential range were due to the presence of oxygen. The electrochemistry of hydrazine at the CME is strongly dependent upon pH. This is illustrated by examination of the influence of pH on the height of the anodic peaks in the Osteryoung square-wave voltammetry of hydrazine. Figure 2 contains a plot of peak current vs pH. As can be seen, the sensitivity is enhanced by working in neutral media, with a maximum appearing at pH 8. Based on these results, the FIA experiments were performed with pH 8 ammonia-ammonium chloride buffer solution as the carrier. Flow Injection Amperometric Detection of Hydrazine. Figure 3 shows the hydrodynamic voltammograms obtained for hydrazine sulfate via flow injection amperometric detection with the GCE and the Nafion/ruthenium oxide pyrochlore CME. The experiments were performed by injecting 1 x M hydrazine sulfate and varying the potential between +0.1 and +1.1Vvs Ag/ AgCl. Clearly, the unmodified electrode (GCE) does not permit the convenient determination of hydrazine sulfate over the entire potential range (curve a). The Nafiodruthenium oxide pyrochlore CME, in contrast, exhibited more sensitive peak-shaped results, with a maximum response at +0.7 V (curve b) . A potential of +0.7 V was therefore selected for the detection of hydrazine sulfate at the Nafion/ruthenium oxide pyrochlore CME in the subsequent experiments. One of the advantages of the Nafionhthenium oxide pyre chlore CME is its permselectivity. Baldwin and co-workers* demonstrated the ability of CoPC-modified electrodes to yield a (8) Korfhage, K M.; Ravichandran, IC;Baldwin,

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substantial lowering of the overvoltage for the oxidation of hydrazine and oxalic acid. However, because of an overlapping response, it is not possible to measure hydrazine selectively in the presence of oxalic acid. Wang and Liz later reported that the CoPC/Nafion film effectively rejects oxalate from the surface, hence allowing selective measurement of hydrazine. Since the Nafiodruthenium oxide pyrochlore CME was fabricated on the basis of a similar concept, the rejection of oxalic acid was also expected. Cyclic voltammograms for a solution containing oxalic acid in the absence (curve a) and presence (curve b) of hydrazine are presented in Figure 4 to illustrate the electrocatalytic and discriminative features of the Nafion/ruthenium oxide pyrochlore CME. As can be seen, the Nafion/ruthenium oxide pyrochlore CME excluded the oxalate anion from the surface (curve a) while maintaining the catalytic response of hydrazine (curve b). The utility of the CoPC/Nafion coatings for minimizing electrode fouling caused by adsorption of surface-active compounds was also explored by Wang and Id2. Their results showed that the catalyst was not protected against gelatin and albumin and that the extent of electrode poisoning was similar to that observed for plain CoPC-coated electrodes. The Ndon/tuthenium oxide pyrochlore CME, however, was found to effectively prevent the interference from gelatin and albumin. Osteryoung squarewave voltammetry was applied for the detection of 6.67 x M hydrazine with and without the presence of 50 ppm gelatin or 50 ppm albumin. The decrease in signal in gelatin and albumin was only 4.8% and 4.6%,respectively. The selectivity of the Nation/ ruthenium oxide pyrochlore CME in FIA was further demonstrated by detecting hydrazine in the presence of oxalic acid, gelatin, and albumin, as shown in Figure 5. Several points are to be noted from these data. First, comparing the peak current M oxalic obtained for 1 x M hydrazine containiig 1 x acid (Figure 5a) to that of 1 x M hydrazine alone (Figure 6c),no loss of catalytic activity was observed. This result again indicates effective exclusion of oxalic acid in FIA as well as in the

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batch-type experiments as mentioned earlier. Second, the addition of 50 ppm gelatin and 50 ppm albumin to the solution of 1 x M hydrazine containing 1 x M oxalic acid (Figure 5b) caused only about 5% peak depression. Such behavior is also in accordance with the squarewave voltammetric behavior described earlier. Third, the data in Figure 5a-e indicate that the Nailon/ ruthenium oxide pyrochlore CME responds rapidly to dynamic concentration changes that characterize the analytical flow system (with fast increase and decrease of the current). Note also that, as opposed to the requirement of hydrolysis for the cellulosebased CME,’ no hydrolysis time is required for our CME in the above measurements. Overall, the bioanalytical utility of such permselective/electrocatalyticbehavior is obvious. The concentration dependence of the Nafionhthenium oxide pyrochlore CME was evaluated for successive injections of hydrazine solutions of increasing concentration, 0.5-50 pM (operating potential, f0.7 V). Figure 6 contains sets of FIA curves for various concentrations of hydrazine. The calibration graph, as shown in the inset of Figure 6, was h e a r for injected amounts of hydrazine from 16 to 0.19 ng. The correlation coefficient was 0.9990, and the slope of the calibration graph was 0.1318pA/pM. Similar injections of a 0.5 pM hydrazine solution were used to estimate the detection limit. The signal-to-noise characteristics (S/N = 3) indicated a detection limit of 0.15 pM, Le., 0.048 ng, in the 2@pLsample. Finally, the stability of the Nafion/ruthenium oxide pyrochlore CME was also evaluated. A series of 25 repeated injections of 10

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TIME (SEC) Figure 6. Flow injection amperometry of different concentrations of hydrazine at the Nafiodruthenium oxide pyrochlore CME. (Inset) The calibration plot. The concentrationsof hydrazine are as follow: (a) 5 x 10-5, (b) 2.5 x 10-5, (c) 1 x (d) 5 x M. Other conditions are the same as in Figure 5.

pM hydrazine yielded a relative standard deviation of 5%,with the last injection producing a peak current almost identical to that exhibited by the first one. Moreover, the CME shows an excellent long-term stability. No decrease in hydrazine response was observed after the electrode was stored in 1 M NaOH solution for more than 2 months. CONCLUSIONS This study has demonstrated that the Nafion/ruthenium oxide pyrochlore CME can be applied to the detection of hydrazine in FIA with excellent sensitivity. Significant advantages have been achieved by combiniig the electrocatalytic function of ruthenium oxide pyrochlore with the chargeexclusion and preconcentration features of Nafion. The reliability and stability of the CME offer a good alternative for extending voltammetric techniques in FIA to hydrazine. ACKNOWLEDQMENT The authors gratefully acknowledge financial support from the National Science Council of the Republic of China under Grant NSC 842113-M-005015. Received for review August 22, 1994. Accepted October 19, 1994.@ AC940830T @Abstractpublished in Advance ACS Abstracts, November 15, 1994.

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