Electrochemical Behaviour of a Gold Film- Glassy Carbon Electrode for Determination of Chromium (VI)

A gold nanostructured film modified glassy carbon electrode (Au film /GCE) was developed for the determination of chromium (VI) in water sample. GCE was immersed into HAuCl 4 solution (10 -3 M) and electrodeposition of thin gold layer was conducted at –0.4 V (vs Ag/AgCl) for 10 min. The strong affinity between Au and Cr species resulted with increasing of Cr (VI) signal, compared with the bare glassy carbon electrode. The electrodepositing time, type of supporting electrolyte, pH, the scan rate, modulation amplitude, and modulation time were optimized using differential pulse anodic stripping voltammetry (DP-ASV). The calibration graph using accumulation time of 120 s was linear from 10 to 120 µgL -1 with a sensitivity 1.3 x 10 -2 µA/µgL -1 . Under optimum experimental conditions, a good correlation coefficient R 2 =0.9971, and a low detection limit 5.5 µg/L Cr (VI) was obtained. The signal was reproducible with a relative standard deviation ±4.5 %. The developed Au film /GCE sensor was applied for the Cr (VI) determination of in sewage water samples. electrode Au film /GCE was prepared by electrochemical method, in order to improve analytical performance of Glassy Carbon Electrode (GCE) for quantitative determination of Cr (VI) in water. The influence of several physicochemical (pH, type of supporting electrolyte) and instrumental (deposition time, scan rate, modulation amplitude) parameters on the DP-ASV response has been evaluated. From the estimated results this sensor could be an alternative method for a sensitive detection of Cr(VI) in water sample with DP anodic stripping voltammetry.


Introduction
Chromium is a metallic element usually found in aquatic systems in two different oxidation states, Cr (III) and Cr(VI) with completely different natures. Chromium (III) in trace amounts is an essential supplement in the body and combines with various enzymes to transform proteins, sugar and fat, while chromium(VI) which, depending on the pH, is available as CrO4 2− , HCrO4 − , H2CrO4 or Cr2O7 2− is toxic, mutagenic and carcinogenic. The oxidation state of chromium has a remarkable effect on its toxicity and also its bioavailability (Kachoosang and Compton 2013). Therefore, as Thacker et al 2012 reported, determination of chromium (VI) is much more important than chromium (III), and the design of an analytical method which is capable of selective measurement of chromium (VI) is of great importance. According to El-Shahawi et al. 2005 and 2008, the chemical speciation of chromium in environmental samples is necessary for accurate assessments of the pollution source and levels. Chromium(VI) is usually determined by UV-VIS spectrophotometry by means of diphenylcarbazide in acidic solutions (Rudel and Terytze 1999).
Electrochemical techniques are considered to be powerful methods. Voltammetric methods are advantageous in speciation of chromium because they offer a low detection limit and the determinations can be carried out without any additional separation step. In particular, adsorptive stripping voltammetry (AdSV) is of interest due to its high sensitivity and selectivity according to Liu et al. 2007 Liu et al. 2008. In this work, a gold thin film modified glassy carbon 3 electrode Aufilm/GCE was prepared by electrochemical method, in order to improve analytical performance of Glassy Carbon Electrode (GCE) for quantitative determination of Cr (VI) in water. The influence of several physicochemical (pH, type of supporting electrolyte) and instrumental (deposition time, scan rate, modulation amplitude) parameters on the DP-ASV response has been evaluated. From the estimated results this sensor could be an alternative method for a sensitive detection of Cr(VI) in water sample with DP anodic stripping voltammetry.

Preparation of the Aufilm/GCE
The Aufilm/GCE sensor was prepared by electrodeposition. Initially the glassy carbon electrode was rinsed with water to remove any residual polishing material. Than the surface of the electrode is polished gently with the Al2O3 powder with particle sizes conducted at the potential of -0.4 V (vs Ag/AgCl) for 10 min. Thereafter the stripping step was performed till +0.6 V in stirring condition for 25 s.

Electrochemical Measurements
The electrochemical measurements were performed using modified Aufilm/GCE and unmodified GCE electrode applying differential pulse anodic stripping voltammetry

Sewage Water Analysis and Recovery
Sewage water samples were collected near an industrial factory of leather processing and cleaning. Firstly, the water sample was purified by a vacuum filtration process as the sample contained many solid suspensions. 2 mL of filtered sewage water were introduced into electrochemical cell containing 15 ml HCl 0.1M. Recovery experiments were performed by spiking standard Cr (VI) stock solution to the water sample following by the determination of analyte using DP-ASV experiments.

Electrochemical response of Cr(VI) on Aufilm/GCE sensor
The effect of the gold film in the electrochemical detection of chromium (VI) was studded using the differential pulse anodic stripping voltammetry in a solution 0.1M HCl (pH=1) containing Cr (VI). The successive voltammograms were recorded between -0.2 to +0.8 V using a scan rate 100 mV/s, step potential 4 mV, amplitude 20 mV and modulation time 5 ms. Although the anodic peaks of Cr (VI) using modified and bare electrode in the electrochemical cell contained 100 µg L -1 of Cr (VI) appeared at potential +0.3 V the highest peak was obtained with modified electrode. This can be attributed to the strong affinity between Au and Cr.

Optimization of experimental parameters
The optimization of DP-ASV instrumental parameters influencing in the current response of analyte it is very important step in the development of electroanalytical methodology.
The effect of different parameters such as pH, type of supporting electrolyte, and accumulation time was investigated in order to optimize analytical performance of   Some experimental condition parameters such as scan rate and modulation magnitude were optimizing in order to obtained the best signal with high sensitivity toward analyte Cr VI. With increasing of modulation amplitude is observed that the peak current increases gradually up to 50 mV, and then it remained constant but the form of the peak becomes wider. It was choosing 20 mV as the most suitable amplitude to give the highest and sharper peak. Also by increasing the scan rate from 50 to 150 mV/s, peak current increased proportionally up to 100 mV/s. With increasing the scan rate more than 100 mV/s no effect to the peak was observed. To obtain the highest signal of Cr VI scan rate 100mV/s was chosen in further experiments. According to the results presented above in all subsequent experiments regarding evaluation of analytical parameters and real sample analysis were used optimal conditions listed in table below.
In table 1 are summarizes the operating conditions optimized for the DP-ASV analysis of Cr(VI) using a Aufilm-GCE.

Analytical Parameters
The analytical performance of modified sensor Aufilm/GCE for the determination of   For comparative purposes in table 2 are listed various publications which report the analytical performance of the modified electrodes for Cr (VI) analysis. By the data reported the sensitivity of the method studied in this work shows advantages relating with sensitivity and low detection limit compared with reported electroanalytical methods referred. Regarding other analytical application procedures such as reproducibility, linearity, simplicity of the modification procedures is comparable with other methods.

Analysis of a real sample
The modified glassy carbon electrode (Aufilm/GCE) was applied to Cr(VI) determination   Table 3, together with the total contents of chromium found by Aufilm/GCE in sewage solution. The concentration of Cr in analyzed samples taken near an industrial factory of leather processing and cleaning resulted 150 ± 0.6 µgL -1 much higher than the allowance level of Cr in surface water according to U.S. EPA, 1998 determinations in the presence of excess of Cr(III) and also can be easily adopted for routine analysis of chromium(VI).

Conclusions
A gold thin film-glassy carbon electrode (Aufilm/GCE) is studied using DP-ASV for determination of chromium (VI) in liquid discharges of the leather processing industry.
This method has advantages regarding the low cost for the fabrication of modified sensor, simple preparation, wide linear calibration zone, sensitivity and lowest limit of detection. It has been shown that, after proper optimization of the experimental conditions, the electrode prepared was suitable for the determination of Cr(VI) with high sensitivity and good reproducibility. The method developed was applied to real samples which gave satisfactory results and recovery of 97.8% obtained for a concentration of 88 µgL -1 . We can recommend that the proposed sensor can be easily applied for analysis of chromium (VI) in industrial and environmental polluted water.