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Electrochemistry Communications 10 (2008) 277–282 www.elsevier.com/locate/elecom

Disposable barrel plating nickel electrodes for use in flow injection analysis of trivalent chromium Jun-Wei Sue, Chun-Yen Tai, Wan-Ling Cheng, Jyh-Myng Zen * Department of Chemistry, National Chung Hsing University, 250 Kuo-Kuang Road, Taichung 402, Taiwan Received 22 November 2007; accepted 6 December 2007 Available online 23 December 2007

Abstract We report a disposable barrel plating nickel electrode (Ni-BPE) coupled with a specifically designed electrochemical cell for use in flow injection analysis for the determination of trivalent chromium (CrIII). The response of the activated Ni-BPE was found very sensitive under alkaline condition with a systematic increase in current signal upon successive addition of CrIII. Under optimized conditions, the calibration curve showed a linear range up to 1 mM with a detection limit (S/N = 3) of 0.30 lM. Continuous hydrodynamic flow of 0.1 M NaOH at regular CrIII injections verify the reproducibility of the present system. The proposed method was successfully used to detect the amount of CrIII in environmental water to show its potential use in real applications. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Chromium; Nickel electrode; Disposable; Flow injection analysis

1. Introduction Trivalent chromium (CrIII) is an essential nutrient ion needed to maintain normal physiological functions. It is one of the essential trace elements in multivitamin with multimineral pharmaceutical formulations [1]. The detection of CrIII that exists in many biological and industrial materials is of great interest [2]. Many methods, such as atomic adsorption spectrophotometer [3,4], plasma mass spectrometry [5], spectrofluorimetry [6], chemiluminescence [7–9], spectrophotometry [10] and electrochemistry in potentiometric measurement [11–17], have been employed for the determination of CrIII. In general effective separation and preconcentration are both essential to obtain the best results, but the procedures are both complex and time consuming. Consequently, it is difficult to directly apply them in situ for monitoring CrIII in aqueous environments. Direct oxidation of CrIII is promising in this regard and has been studied at platinum, stannum and diamond *

Corresponding author. E-mail address: [email protected] (J.-M. Zen).

1388-2481/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.12.008

electrodes [18–21]. Notably, the oxidation of CrIII was examined at polycrystalline gold and Au(1 1 1) single crystal electrodes [22,23] and the following mechanism was proposed for the overall electrode reaction:  CrO ! e þ Hþ þ CrO2 2 þ OH 3

! e CrO2 3  CrO 3 þ OH

þ

þH þ 

CrO 3 þ

! e þH þ

ð1Þ ð2Þ

CrO2 4

ð3Þ

In this paper the electrochemical oxidation of CrIII is studied at an activated barrel plating nickel electrode (designated as Ni-BPE). It is anticipated the reaction mechanism of gold electrode might also be accessible at the activated Ni-BPE. Our previous study introduces an engineering process of barrel platting technology for mass production of disposable-type electrodes [24]. The primary function of barrel plating is to provide an economical means to electroplate manufactured parts that meets specific finishing requirements [25,26]. Since the fabrication cost of the Ni-BPE is cheap, it is thus disposable in nature. It was found that the activated Ni-BPE coupled with a specifically designed electrochemical cell allowed us to obtain

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a distinctive voltammetric profile towards the oxidation of CrIII in flow injection analysis (FIA). To validate the applicability, the developed method was applied to the determination of CrIII in real water samples. 2. Experimental 2.1. Reagents and chemicals Chromic chloride (RDH, min. 96%) and sodium hydroxide (Showa, min. 96%) were used as received. All reagents were of analytical-reagent grade and prepared with high resistivity (18 MX/cm) de-ionised water. Natural water samples were collected in a polyethylene bottle from a lake at the campus of Chung Hsing University and kept under refrigeration below 4.8 °C and were filtered by 0.22 lm membrane (Millipore Inc.) before detection.

2.2. Apparatus and instrumentation Cyclic voltammetric and chronoamperometric experiments were carried out using a CHI 900 electrochemical workstation (Austin, TX, USA). The three-electrode system consists of a Ni-BPE working electrode, an Ag/AgCl of reference electrode, and a platinum or stainless tube auxiliary electrode. The Ni-BPE (1.25 mm diameter, 31 mm length) with an average weight of 392.2 ± 0.5 mg (n = 10) was a special order from Zensor R&D (Taichung, Taiwan). The geometric surface area of the Ni-BPE was 0.012 cm2. Scheme 1 shows typical pictures of the Ni-BPE working system used in this work. It consists of a Ni-BPE working electrode, a stainless tube counter electrode (outlet), an Ag/AgCl reference electrode. The flow injection analysis (FIA) system was equilibrated in 0.1 M NaOH carrier solution at +0.5 V until the current became constant. The

Scheme 1. Scheme and pictorial representation of the proposed flow injection electrochemical detector setup.

J.-W. Sue et al. / Electrochemistry Communications 10 (2008) 277–282

Ni-BPE surface was activated by cycling from 0.6 to 0.2 V in 0.1 M NaOH solution. The CrIII oxidation peak signal in FIA was uniformly taken as a quantitative parameter. 3. Results and discussion 3.1. Electrochemical behavior of CrIII at the Ni-BPE Fig. 1A illustrates the cyclic voltammograms obtained at a fresh Ni-BPE during consecutive scans. It can be seen that no significant oxidation process was detected on the first scan; whereas a characteristic oxidation peak at 0.47 V and reduction wave at 0.38 V was observed. This phenomenon can be attributed to multilayer formation and reduction of nickel oxide (i.e., NiO + OH ? NiO(OH) + e (or Ni(OH)2 + OH ? NiO(OH) + H2O + e) at the NiBPE [27]. This continued to grow with further scans, and reached saturation eventually. The initial electrochemical oxidation of 400 lM CrIII was then investigated at the activated Ni-BPE in 0.1 M NaOH solution (Fig. 1B). Upon

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CrIII addition, there is an increase in the anodic peak current together with a slightly decrease in the cathodic peak current. This represents a mediated catalytic oxidation which the cathodic peak current is decreasing. Note that the electrocatalytic function toward the oxidation of CrIII can only be observed at the activated Ni-BPE. Based on the Pourbaix diagram, CrIII occurs in the form of CrO 2 in solutions above pH 12 and the overall electrochemical oxi dation of CrIII proceeds via the reaction: CrO2 4 þ 3e þ  þ 4H CrO2 þ H2 O [22]. Meanwhile, a new oxidation wave emerged at 0.20 V upon addition of CrIII and also continued to grow with further additions of CrIII. This could be attributed to the oxidation of CrIII to CrVI. Even so, this process was found to be chemically irreversible in the potential range studied. Based on the above results, when applying a potential of either 0.30 or 0.50 V to the Ni-BPE, the increase in the oxidation current can be used for the amperometric determination of CrIII. Since the two current responses for CrIII are originated from different reaction mechanisms, the applicability of these two detection potentials should therefore be evaluated first. 3.2. Parameters optimization in FIA

Fig. 1. (A) Cyclic voltammograms at a Ni-BPE in 0.1 M NaOH (50 cycles from 0.2 to 0.6 V at a scan rate of 100 mV/s). (B) Typical cyclic voltammograms of the activated Ni-BPE in 0.1 M NaOH before (a) and after (b) the addition of 400 lM CrIII at a scan rate of 5 mV/s.

To adapt the Ni-BPE as an amperometric detector for monitoring CrIII under flowing conditions, the detection signals at 0.30 and 0.50 V after one hour of operation were recorded for evaluation. As shown in Fig. 2A, the response of CrIII by detecting at 0.3 V resulted in a 50% decrease in response after one hour of operation. The relative standard deviation of five continuous injections also increases from 3% to 11% in this period. On the other hand, a much stable signal was observed by detecting at 0.50 V (Fig. 2B). The response of CrIII showed only 7% decrease in response after one hour of operation with a very consistent relative standard deviation of 0.6% for five continuous injections. Based on the above results, the advantage of using the peak of +0.47 V for CrIII determination is obvious. A detection potential through the mediated catalytic oxidation of NiO(OH) is thus used for analytical application in the subsequent studies. The effects of applied potential (Eapp) and hydrodynamic flow rate (Hf) on the detection of CrIII are next optimized. An increase trend in the FIA signal was observed in the window of 0.300.50 V. The fact that more increase was observed at 0.50 V over that of 0.30 V is in consistent with the CV behavior. Since a relatively higher deviation of FIA signal was observed when Eapp > 0.55 V, these potentials are not suitable for analytical application. The effect of Hf on the detection of CrIII was optimized at Eapp = 0.50 V and the FIA signals were found to increase gradually at slow flow rates and reached a maximum at around Hf = 400 lL/min, presumably as a result of the formation of proper hydrodynamic electrode/electrolyte interface. Therefore, the detection of CrIII at Eapp = 0.50 V and Hf = 400 lL/min was chosen as instrument setting in the following investigation.

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Fig. 2. (A) Typical FIA responses at a detection potential at 0.3 V for 300 lM CrIII at the activated Ni-BPE in 0.1 M NaOH mobile phase with a flow rate of 0.4 mL/min. (B) Typical FIA responses at a detection potential at 0.5 V.

3.3. Analytical performance One of the most important practical features of using the Ni-BPE as a detector for CrIII under flowing conditions is their ability to yield reproducible measurements. The setup can be operated continuously for 10 injections of 300 lM CrIII without an obvious alternation in the detecting current signals (RSD = 1.13%). The detection procedure was also carried out for 10 different freshly assembled electrochemical detector setup. A RSD value of 4.8% was obtained for the case of 500 lM CrIII indicating that the fabricating process of the Ni-BPE was reliable. Overall, all these results clearly verify the appreciable stability and workability of the proposed system to CrIII analysis. Under the optimal conditions of Hf = 400 lL/min and Eapp = 0.5 V, the calibration plot was linear up to 1 mM with a current sensitivity and a regression coefficient of 8.67 nA/lM and 0.998, respectively

(Fig. 3). Signal-to-noise (S/N = 3) characteristic on the detection of CrIII resulted in a DL of 0.30 lM. The flow-injection method with amperometric detection at the Ni-BPE can thus be used for CrIII determination at low concentration levels without preconcentration procedure. Finally, real sample analysis was demonstrated by spiking 100 and 300 lM of CrIII directly into test water samples for recovery test. The analysis was done by preparing the sample solution from a water aliquot in 0.1 M NaOH and by injecting it directly into the carrier. From a practical point of view, the experimental procedure is extremely simple and is thus an important advantage over previous methods. The recoveries for three runs were found to fall in the window of 96.11 ± 0.35 and 101.88 ± 2.35 for spiking 100 and 300 lM of CrIII, respectively, indicating the proposed system is suitable for CrIII analysis in real water samples.

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Fig. 3. (A) Typical FIA responses for various concentration of CrIII. (B) Linear calibration curve with a linear range up to 1 mM with r2 = 0.998.

4. Conclusion

References

The newly developed system was successfully demonstrated for CrIII determination using an activated Ni-BPE by FIA. Good electrocatalysis towards CrIII oxidation was observed with a distinctive voltammetric profile in alkaline condition. The system was successfully applied in the detection of CrIII in natural water. Since the electroanalytical setup is cheap, it offers a new platform to diverse applications.

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Acknowledgements The authors gratefully acknowledge financial support from the National Science Council of Taiwan. This work is supported in part by the Ministry of Education, Taiwan under the ATU plan.

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