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Barrel Plating Rhodium Electrode: Application to Flow Injection Analysis of Hydrazine Jun-Wei Sue, Annamalai Senthil Kumar, Hsieh-Hsun Chung, Jyh-Myng Zen* Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan *e-mail: [email protected] Received: October 13, 2004 Accepted: December 10, 2004 Abstract We introduce here the application of barrel plating technology for mass production of disposable-type electrodes. Easy for mass production, barrel plating rhodium electrode (Rh-BPE) is for the first time demonstrated for analytical application. Hydrazine was chosen as a model analyte to elucidate the electrocatalytic and analytical ability of the RhBPE system in pH 7 phosphate buffer solution. Flow injection analysis (FIA) of hydrazine showed a linear calibration range of 25 – 1000 ppb with a slope and a regression coefficient of 5 nA/ppb and 0.9946, respectively. Twenty-two replicate injections of 25 ppb hydrazine showed a relative standard deviation of 3.17% indicating a detection limit (S/ N ¼ 3) of 2.5 ppb. The system can be continuously operated for 1 day without any alteration in the FIA signals and is tolerable to the interference of oxalic acid, gelatine, Triton X-100, and albumin for even up to 100 times excess in concentration with respect to 400 ppb hydrazine. Since the fabrication cost of the electrode is cheap, it is thus disposable in nature. Furthermore, barrel plating technique can be extendable to other transition metals for application in many fields of research interest. Keywords: Rhodium, Barrel plating, Hydrazine, Electrocatalysis, Flow injection analysis

1. Introduction Economical and mass producing electrode fabrication techniques are highly in demand in electroanalysis to prepare chemically modified electrodes and chemical sensors suitable for various practical applications. So far, screen-printed electrodes have been given much attention in this regard. We introduce here an engineering process of barrel platting technology for mass production of disposable-type electrodes. The primary function of barrel plating is to provide an economical means to electroplate manufactured parts that meets specific finishing requirements [1, 2]. The technology can allow for electroplating small parts in large groups. It can accommodate a wide variety of shapes and sizes as well as different metals and alloys by immersing in a big tank which contains the necessary plating solution with a horizontal conducting bar as an interior cathode electrical contact under constant rotating of the “barrel”. The mechanical energy of the rotation produces a tumbling action that helps in the high degree of plating uniformity. Barrel plating is used most often for bulk finishing and by properly turning the barrel-rotation rate and applied potential or current one can easily control the quality of the surface films. It is the most efficient method for finishing bulk parts and any pieces that do not require individual handling. Barrel-plating rhodium electrode (Rh-BPE), as shown in Figure 1, was demonstrated in this study for trace analysis of hydrazine. Note that various metals, such as gold, silver, palladium, nickel, and tin are also capable of deposition by Electroanalysis 2005, 17, No. 14

barrel plating. Rhodium is well suited for plating of parts such as sliding electrical contacts that require protection from corrosion or galling [3, 4]. It is the hardest of all of the precious metals and provides the most wear resistant finish possible for the most demanding environments. The good stability and electrocatalytic ability of the rhodium-modified carbon electrodes was reported previously [5 – 7]. The main purpose of this study is therefore to validate the idea of using barrel plating technique to produce electrode for analytical application. Most important of all, the versatile barrel plating technique can be extendable to other transition metals for application in many fields of research interest. As per the world health organization (WHO) [8], the threshold limit value for hydrazine was lowered to the order of 10 ppb because of its hazardous effect. Consequently, trace detection techniques for hydrazine are very important in environmental, industrial, and biological analysis [9 – 11]. In this report, the advantages of using Rh-BPE are explored for trace detection of hydrazine. A simple and cheap homemade flow injection cell is specifically designed to couple with the Rh-BPE. The Rh-BPE was used as amperometric microsensors for the continuous detection of hydrazine under flow-injection conditions with a detection limit lower than 10 ppb. Note that the recently reported Rh-plated microelectrodes and carbon fiber systems for hydrazine analysis only showed a detection limit (DL) of 20 ppb in dilute H2SO4 [5, 6]. The developed method was finally applied to the determination of hydrazine in real water samples.

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DOI: 10.1002/elan.200403243

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The geometric surface area of the Rh-BPE was ca. 0.08 cm2. The content of Rh at the Rh-BPE was analyzed by ICP-MS after treating the electrode in aquo-regio to dissolve the overlayer plating and a value of 1.626 mg was obtained. The electrochemical impedance spectra (EIS) was measured at 10 discrete frequencies per decade from 10 mHz to 100 kHz at amplitude of 10 mV (rms) using the Autolab FRA frequency response analyzer with FRA2 module controlled by an IBM compatible PC. The acquired data was analyzed based on equivalent electrical circuits by a weighted nonlinear least square method using the fitting software, EQCRT, elaborated by Boucamp. Fitting constraints were imposed that further iterations will stop when the chi-square (c2) change is less than 0.001% compared to the previous iteration. The satisfaction of fit was assessed from minimum c2, correlation matrix, and relative error distribution plots. Less than 5% fluctuations between the experimental and fit data were assumed satisfactory in confirming the validity of the selected fitting circuit.

2.3. Electrochemical Detector Setup

Fig. 1. Pictures of the Rh-BPE.

2. Experimental 2.1. Chemicals and Reagents

The flow injection analysis (FIA) system consisted of a ColeParmer microprocessor pump drive and a Rehodyne 7125 sample injection valve (20 mL loop). Figure 1 shows typical pictures of the Rh-BPE working system used in this work. Series of Rh-BPEs were shown in the first picture to depict the uniformity of the system. A prototype homemade flowthrough cell (ca. 50 mL of volume capacity) was designed for analytical measurements as illustrated in Figure 2. It

Hydrazine sulfate (Wako), albumin (Sigma), gelatine (Showa), and oxalic acid (Aldrich) were used as received. All other compounds used in this work were ACS-certified reagents and used without any further purification. Aqueous solutions were prepared using double distilled deionized water. Unless otherwise stated the base electrolyte used in the study was made of 0.1 M, pH 7 phosphate buffer solution (PBS). Caution! Because of the carcinogenic activity, extreme care must be taken during handling hydrazine. Anhydrous hydrazine explodes during distillation if trace of air is present.

2.2. Apparatus Cyclic voltammetric and chronoamperometric experiments were carried out using a CHI 660 electrochemical workstation (CH Instruments, Austin, TX). The three-electrode system consists of an Rh-BPE working electrode, an Ag/ AgCl or Ag-wire semi-reference electrode, and a platinum or stainless tube auxiliary electrode. A screen-printed carbon electrode (SPE) (Zensor R&D, Taiwan) of geometric surface area of ca. 0.2 cm2 was used for comparative experiments. The Rh-BPE (1.25 mm diameter, 31 mm length) with an average weight of 392.2  0.5 mg (n ¼ 10) was a special order from Zensor R&D (Taichung, Taiwan). Electroanalysis 2005, 17, No. 14

Fig. 2. Diagram of the homemade flow injection electrochemical detector setup. G 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Fig. 4. Equivalent circuit mold for complex impedance plots. Rs, solution resistance; Cdl electrode/solution interface capacitance; Rct, charge transfer resistance due to the faradic reaction; W1, Warburg diffusion impedance due to the diffusion of the hydrazine in the solution.

Fig. 3. Typical CV responses of the Rh-BPE without (b) and with (c) 5 mM hydrazine in pH 7 PBS. Comparative CV response in the presence of 5 mM hydrazine at SPE (a). Scan rate ¼ 50 mV/s.

consists of ah Rh-BPE working electrode, a stainless tube counter electrode (outlet), an Ag wire semi-reference electrode, a silica-gel gasket (2 mm in thickness), a highly transparent polymethyl methacrylate (PMMA) sheet (5 mm diameter), and a holding electrode panel (2 mm in thickness). The cell can be easily assembled using silica gel, stainless steel (i.d., 0.9 mm; o.d., 1.6 mm), silicon (i.d., 0.5 mm) tubes, and clamping holder. Note that the thin film of gasket system (1.25 mm) prepared by appropriately pressed using silicon gel in-between two polished-PMMA plates is essential for the system. Improper alignment of the gasket can be noticed by a leakage of the carrier buffer. The FIA system was first equilibrated in pH 7 PBS carrier solution at þ 0.7 V until the current became constant. It usually took a few minutes. The hydrazine oxidation peak signal in the FIA was uniformly taken as a quantitative parameter. All the experiments were performed at room temperature (25 8C).

3. Results and Discussion 3.1. Electrocatalytic Behavior Figures 3b and c show the typical CV responses of the RhBPE without and with 5 mM hydrazine in pH 7 PBS. As can be seen, no hydrazine response was observed at a bare SPE in the potential window of 0.0 – 1.0 V (Figure 3a); while the Rh-BPE showed a catalytic hydrazine oxidation current starting from 0.3 V with a maximum at ca. 0.7 V. Note that the electrocatalytic potential of the Rh-BPE is fairly close to that reported earlier for the cases of Rh-modified carbon fiber microelectrodes except with a less complicated cyclic voltammogram [5, 6]. The catalytic oxidation with low potentials might occur through mediation by electrogenerated Rh3þ species at the Rh-BPE. The anodic oxidation current (ipa) increases regularly with increasing scan rate (v) in CV studies. A log(ipa) versus log(v) plot shows a slope value of ca. 0.5 indicating a diffusion-controlled mechanism for the oxidation reaction Electroanalysis 2005, 17, No. 14

Table 1. Electrochemical impedance spectral ( EIS) data for the oxidation of 2 mM hydrazine at different working electrodes at an applied potential of 0.7 V (vs. Ag/AgCl) in pH 7 PBS. Parameter

SPE

Rh-BPE

Equivalent circuit Rs/W Cdl/F Rct/W W1

Rs( Cdl[ RctW1]) 200.10 4.95  107 5.21  106 1.38  103

Rs( Cdl[ RctW1]) 60.40 1.33  106 3.02  103 8.24  104

(plot not shown). The fact that no hydrazine trace was observed after the medium exchange experiments from hydrazine to blank electrolyte denotes the absence of any adsorption process on the working system. Useful information was obtained from the ac impedance studies of the Rh-BPE and SPE in the presence of 2 mM hydrazine at an applied potential of 0.7 V (vs. Ag/AgCl). As can be seen in Figure 5, an obvious lowering in the working resistance value was noticed for the EIS response of the RhBPE. It should have something to do with the difference in electron-transfer rates at the respective electrodes. The charge transfer resistance (Rct) was determined by a built-in FRA2 fitting program using the Boukamp equivalent (RandlePs) circuit of Rs[Cdl(RctW1)], where Rs is the resistance corresponding to the solution phase system, Cdl and W1 are double layer capacitance and Warburg diffusion impedance for hydrazine (Figure 4). Table 1 summarizes the results corresponding to the analysis. As can be seen, Rct value observed at the Rh-BPE is about 3 factor lower than that of SPE. Note that smaller Rct would indicate an increase in exchange current (I8). The values of I8 and apparent heterogeneous rate constant (khapp) can be calculated from the following equations [12]: I8 ¼ (RT/nF)(1/Rct)

(1)

I8 ¼ nFAkhappC*

(2)

In the above equations, C* is the analyte concentration, n is the number of electron involved in the reaction, and other symbols have their usual significance. The I8 and khapp for hydrazine oxidation (N2H4 ! N2 þ 4Hþ þ 4e) were calculated as (1.23 nA and 2.04  109 cm/s) and (2.12 mA and 3.4  106 cm/s) on SPE and Rh-BPE, respectively, at G 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Fig. 5. Complex plan EIS plot for hydrazine (2 mM) oxidation on SPE (a) and Rh-BPE (b) at an applied potential of 0.7 V (vs. Ag/AgCl). Insert Figure is the enlarged response of the Rh-BPE system.

[hydrazine] ¼ 2 mmol/cm3 by taking n ¼ 4. The observed results are in agreement with mediated oxidation of some biological compounds on chemically modified electrodes [12, 13], which further validates the proper electrocatalytic function of the Rh-BPE.

3.2. Analytical Performance To adapt the Rh-BPEs as amperometric detectors for monitoring hydrazine under flowing conditions, a flow cell described in experimental section was employed. The effect of hydrodynamic flow rate (Hf) on the detection of hydrazine was first optimized at an applied potential (Eapp) of 0.7 V (vs. Ag semi-reference electrode) (Figure 6A). The FIA signals were found to increase gradually at slow flow rates and reached a maximum at around Hf ¼ 1.5 mL/min presumably as a result of the formation of proper hydrodynamic electrode/electrolyte interface. The effect of Eapp on the detection of hydrazine was next optimized. Figure 6B shows an increase trend in the window of 0.0 – 0.7 V (vs. Ag semi-reference electrode) with ca. 4 times increase in the FIA signal was observed at 0.7 V over that of 0.0 V. The results were consistent with the CV behavior as shown in Figure 3. Since erratic FIA signals were observed at Eapp > 0.7 V, these potentials were therefore not suitable for analytical application. One of the most important practical features of using the Rh-BPEs as a detector for hydrazine under flowing conditions is their ability to yield reproducible measurements with no need of electrode pretreatment. To prove this, continuous hydrodynamic flow of carrier buffer at regular hydrazine injections were performed to verify the reproducibility of the present approach. As shown in Figure 7A, the setup can be operated continuously for up to 20 h without any obvious alteration in the detecting current signals (RSD ¼ 4.7%). Despite the hydrodynamic conditions, the Electroanalysis 2005, 17, No. 14

Fig. 6. Effect of A) hydrodynamic flow rate (Hf) and B) applied potential (Eapp) on the detection of 400 ppb hydrazine on the RhBPE with 0.1 M, pH 7 PBS as carrier solution.

stability of the Rh-BPE in the carrier stream is sufficiently good to allow a correct performance of the amperometric sensor. Furthermore, the detection procedure was also carried out for 4 different freshly assembled electrochemical detector setups. A RSD value of 3.7 % (n ¼ 22) was obtained for the case of 400 ppb hydrazine indicating that the fabricating process of the Rh-BPE was reliable, and the reproducible amperometric responses can be achieved with different Rh-BPE constructed in the same manner in large groups. Overall, all these results clearly verify the appreciable stability and workability of the proposed system to hydrazine analysis. The effect of potential interferents on the hydrazine amperometric response was checked under flow-injection conditions. Figure 7B shows the interference effect of oxalic acid, gelatine, Triton X-100, and albumin with 10 and 100 excess in concentration over 400 ppb hydrazine. Note that the previously reported CoPc and CoPc/Nafion systems were found to have serious electrode poisoning effects from these interferents [14]. In contrast, as shown in Figure 7B, no alteration in FIA signals were observed in the presence of these interferents at the Rh-BPE. The Rh-BPE is effective to prevent the interference from these compounds for even up to a 1 : 100 ratio. Moreover, the amines such as cyclohexylamine and morpholine normally used as corrosion inhibitors in hot-water heating systems, and inorganic anions and cations such as Cl, SO42, PO43, Ca2þ, Naþ, and Mg2þ commonly present in this type of water also show no interference to the hydrazine response (data not enclosed). As to the presence of hydrazine derivatives such as 1,2-dimethylhydrazine and methyl hydrazine, the G 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Fig. 7. A) FIA response for the oxidation of 400 ppb of hydrazine at different time intervals. B) FIA detection of 400 ppb of hydrazine with and without oxalic acid (a), gelatin (b), Triton X-100 (c) and albumin (d) at different ratio. FIA conditions: Eapp ¼ 0.7 V vs. Ag semi-reference electrode and Hf ¼ 1.5 mL/min.

determination would require a previous separation. Again, this is similar to that reported earlier for the cases of Rhmodified carbon fiber microelectrodes [5]. Under the optimal conditions of Hf ¼ 1.5 mL/min and Eapp ¼ 0.7 V, the calibration plot was linear in the window of 25 – 1000 ppb with a current sensitivity and a regression coefficient of 5 nA/ppb and 0.9946, respectively (Figure 8A). Replicate injections (n ¼ 22) of 25, 100, 500, and 1000 ppb hydrazine yielded RSD values of 3.17, 2.88, 1.90, and 0.90%, respectively (Figure 8B). Signal-to-noise (S/ N ¼ 3) characteristic on the detection of 25 ppb hydrazine resulted in a DL of 2.5 ppb. The DL is much lower than that of Rh-carbon fiber electrode and comparable to those of some chemically modified electrodes [5, 6, 14, 15]. The flowinjection method with amperometric detection at the RhBPE can thus be used for hydrazine determination at low concentration levels. It should be mentioned that the detection potentials can be applied in the range of 0.3 – 0.7 V and these are among the lowest values found in the literature for the amperometric determination of hydrazine. Finally, real sample analysis was demonstrated by spiking 40 – 200 ppb of hydrazine directly into test water samples for recovery test. Figure 9 shows typical examples for real sample analysis of two water samples. The analysis was done Electroanalysis 2005, 17, No. 14

Fig. 8. A) The responses of the electrochemical detector setup for increasing [hydrazine] at Eapp ¼ 0.7 V vs. Ag semi-reference and Hf ¼ 1.5 mL/min in 0.1 M, pH 7 PBS carrier solution. B) Repeated FIA injections for various [hydrazine]. Sample loop ¼ 20 mL. Table 2. Hydrazine detection in different water samples (n ¼ 5). Sample

Added (ppb)

Found (ppb)

Recovery (%)

Deionized water

40 80 120 160 200 40 80 120 160 200 40 80 120 160 200

39.1 79.9 121.2 158.3 200.3 40.1 78.5 122.0 159.9 199.4 39.8 79.9 123.4 162.0 196.7

97.6 99.9 100.1 99.0 100.1 100.2 98.2 101.7 99.9 99.7 99.5 99.8 102.8 101.2 98.3

Pond water

Ground water

by preparing the sample solution from a water aliquot in PBS and by injecting it directly into the carrier. From a practical point of view, the experimental procedure is G 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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interference from common co-existing chemicals of oxalic acid, gelatine, Triton X-100, and albumin with 100 times excess in concentration. The application of barrel plating technique to produce an electrode for analytical application represents a basic model for many future chemical-, biochemical-, and immuno-assays.

5. Acknowledgement The authors gratefully acknowledge financial support from the National Science Council of Taiwan.

6. References

Fig. 9. Typical FIA results of hydrazine real sample analysis. Hydrazine-containing real water samples were prepared by spiking a known concentration directly into the system. Other FIA conditions are similar to those of Figure 8A.

extremely simple and is thus an important advantage over previous methods. Table 2 summarizes the results obtained for deionized, pond and ground waters. All the recoveries were found to fall in the window of 97.6 – 102.8%, indicating the proposed system is suitable for hydrazine analysis in real water samples.

4. Conclusions A low cost and easy for mass production Rh-BPE was for the first time used as an electrochemical detector in couple with a specifically designed flow-through electrochemical cell for sensitive hydrazine analysis. The Rh-BPE showed a welldefined electrocatalytic signal at low potential for the hydrazine oxidation in neutral PBS medium. The system was highly stable in FIA response and can tolerate the

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