INTRODUCTION:

Rapid industrialization and abnormal population growth has enhanced water pollution. Monitoring the metal ions and organic compounds in aquatic environment has been a subject of great concern over the last few decades and will continue to be so, as increasing number of metal ions in increasing amounts and a diverse array of organic compounds form a part of an industrial effluent. Metal ions are the most common electroactive species present in the industrial effluents. ‘Heavy metals’ is a general collective term applied to the group of metals such as Pb, Cr, Cu, Ni and Zn which are commonly associated with pollution and toxicity problems. Some of these elements may be micronutrients for many living organisms and are required in small amounts for normal healthy growth, but any metal ion in large amount will always cause acute or chronic toxicity.

Trace determinations of metals by voltammetric methods with modified electrodes has been reported. ^{1, 2, 3}. Complexation of the metal ions in waste water was also studied by differential pulse cathodic stripping voltammetry ^4-5. Simultaneous determination of Zn(II) and Ni (II) was also studied in the presence of crown ethers by D.C. polarography ⁸. Metals present in industrial effluents and sludge samples have been separated and concentrated by using other techniques like electrodialysis, coulometry and photocatalysis ⁹.

However, less work has been done in the area of environmental chemistry especially on separation and quantitation of electroactive species present in industrial effluents.

Objective

The main objective of the study was to provide a simple, rapid, efficient, precise and economical method for the

simultaneous determination of Ni(II) and Zn(II) from an industrial effluent using sampled D. C. polarography. The developed method has been validated as per ICH guidelines ^6-7.

MATERIALS AND METHODS:

Introduction to the workstation

All the measurements were performed on a fully automated computerized electroanalytical workstation, an electrochemical system PG STAT 30 with 663 VA electrode stand manufactured by Metrohm. It includes 3 electrode system viz. hanging mercury drop electrode as a working electrode, saturated calomel electrode as a reference electrode and platinum electrode as an auxiliary electrode

Reagents

Merck A.R grade NiSO₄.7H₂O, ZnSO₄.7H₂O and KSCN were used.

Preparation of standard solution

28 mg of NiSO₄.7H₂0 and 28.75 mg of ZnSO₄.7H₂O were accurately weighed and dissolved in minimum amount of double distilled water and made up to the mark in a 100 mL volumetric flask. The solution so prepared contained 58.69 μg/mL of Ni and 65.39 μg/mL of Zn respectively. All the other standard solutions containing both Ni and Zn were prepared using this stock solution.

Voltammetric Method

18 mL of distilled water and 2.0 mL of1 M KSCN were placed in the dry, clean cell. The solution was purged with pure nitrogen gas for 120s. The potential scan between 0.0 V to -2.0 V vs. S.C.E was applied. The operational parameters were as follows: 1] Scan rate- 60 mVs^-1. 2] Pulse amplitude- 50mV. After recording a polarogram of the blank, 1.0 mL of standard solution of Ni and Zn were added in succession and polarograms were recorded and peak currents were measured and calibration curves were prepared.

Preparation of Sample Solution

The sample was an effluent from an electroplating industry. The sample solution was centrifuged and filtered through Whatman paper no. 41. 50 mL of the sample was evaporated to dryness and extracted with water containing 0.2 mL of conc HCl and diluted to 50 mL in a volumetric flask with distilled water. Polarograms for the sample solutions were recorded under the same conditions used for the calibration curve. The amount of Ni and Zn were calculated from the measured peak currents and using the equation of the calibration curve. The equation of the calibration curve for Ni was y=33.416x + 7.0976 and for Zn was y = 25.943x + 0.1264 where y is the current in nanoamperes and x is the concentration in μg/ml.

ANALYTICAL METHOD VALIDATION^6-7

System Suitability

System suitability tests were carried out to ensure reproducibility of the instrument. The system suitability test was carried out by recording polarogram for Ni and Zn at one concentration (11.738 μg/ml for Ni and 13.078 μg/ml for Zn) with five replicates and the mean current was used for the calculation. The % RSD in both cases was found to be less than 2%.

Specificity

The specificity of method was confirmed by comparing the polarograms of the combined standard solutions containing Ni and Zn with the sample solution. The peak potentials recorded for the sample solution were found to be identical to those obtained for the combined standard solution of Ni and Zn. The addition of the standard solutions of Ni and Zn to the sample solution did not change the characteristics of differential pulse polarogram but enhanced the peak current. This confirms the specificity of the method.

Robustness

The robustness of the method was examined by observing the consistency of the peak height and the peak shape with the deliberately made small changes in the experimental parameters. It is a measure of the capacity of the method to remain unaffected by small, but deliberate variations in method parameters and provides an indication of its reliability during normal usage. To determine the robustness of the proposed method, the following variations were made in the analytical parameters. The Scan rate was changed by ± 0.5 mVs^-1and the Pulse amplitude ± 1.0 mV These parameters were deliberately changed one at a time and the effect of these changes on the peak shape and peak currents were studied. The proposed method was found to be robust.

Linearity and Dynamic range

The linearity for Ni and Zn in a solution containing the two was determined. In the concentration range, 2.795 μg/mL to 18.214 μg/mL for Ni and 3.114 μg/mL to 20.293 μg/mL Zn, a good linearity was obtained. The linear working range selected for Ni was 5.335 to 18.214 μg/mL and Zn was 5.945 to 20.293 μg/mL. The equation of the calibration curves is presented in (Table1).

Limit of Detection and Limit of Quantitation

The limit of detection (LOD) and the limit of quantification (LOQ) for Ni and Zn were fixed at signal to noise ratio of 3:1 and 10:1 respectively. Twenty replicates of the blank solution were recorded and the mean current value at the peak potential of Ni (i.e. at -0.67 V) and Zn (i.e. at -0.98 V) were calculated. The concentration at which the peak current was found three times of mean blank current was taken as the limit of detection and the concentration at which peak current was found to be ten times the mean blank current was selected as the limit of quantification. The LOD and LOQ of Ni and Zn were 1.431 μg/mL and 2.795 μg/mL _.and 1.595 μg/mL and 3.114 μg/ml respectively.

Intraday and Interday Precision

The variability of the method was tested with the intra-day and inter-day precision. It was checked by recording the polarograms of standard solutions of Ni and Zn in the concentration ranges 5.335 μg/mL to 18.214 μg/mL for Ni and 5.945 μg/mL to 20.293 μg/mL for Zn. Intra-day precision was tested by recording the polarograms at an interval of four hours and inter-day precision twice a day with a gap of three days. The mean % RSD for intra-day and inter-day precision for Ni was found to be 0.89% and 1.56% and for Zn 0.66% and 1.85%, respectively.

Quantitation / Determination

The validated method was used for the determination of Ni and Zn. Polarograms were recorded under the optimum experimental conditions for the sample solution. Resulting peak currents for Ni and Zn were measured and the amount of Ni and Zn was calculated using calibration curve equations. The results are presented in (Table 2).

Accuracy (Recovery)

The recovery technique was used to evaluate the accuracy of the method. The method of standard addition was employed for the purpose. A fixed volume of the standard Ni and Zn solution was added to the sample solutions and the mixed solutions so obtained were analyzed by the proposed method. The percentage recovery was determined at different percentage levels i.e. the added amounts ranging from 25% to 175% of the amount present in the sample. The results of the recovery analysis for Ni and Zn are presented in (Table 3)

Result and Discussion

The present study provides determination of Ni and Zn from the industrial effluents using the technique of sampled d. c. polarography. The method was validated as per the ICH guidelines (Table 1-3). Before validation, optimization of the conditions i.e. pH, supporting electrolyte, scan rate and pulse amplitude were optimized. The polarographic response of the sample for Ni and Zn in different supporting electrolytes has been studied. With KCl as the supporting electrolyte the sample showed only one peak for both Ni and Zn. However, two separate peaks were produced with KSCN as the supporting electrolyte.

Table 1: Method Validation Parameters forNickel and Zinc

Parameters	Values
Parameters	Ni	Zn
System suitability (n=5) %RSD	0.6%	0.5%
Linear working range (μg/ml)	5.335 to 18.214 μg/ml	5.945 to 20.293 μg/ml
Slope (m) ^a)	33.416	25.943
Intercept(c)^a)	7.0976	0.1264
Correlation coefficient (R²)	0.9994	0.9993
LOD (μg/ml)	1.431 µg mL^-1	1.595 µg mL^-1
LOQ (μg/ml)	2.795 µg mL^-1	3.114 µg mL^-1
Intraday precision (n=5)	0.89%	0.66%
Interday precision (n=5)	1.56%	1.85%
Recovery	98% to 102%	98% to 102%

^a)Of the equation y = mx + c, where y is peak current, m is the slope, x is the concentration and c is the intercept

Table 2: Results of Quantitation Studies for Nickel and Zinc

Name of Metal ions	Ni	Zn
Conc in µg/ml	2131.13	1014.97
% RSD (n=5)	0.77	0.66

Table 3. Results of recovery studies for Nickel and Zinc

Standard	Level	Conc. of std [μg/ml]	Conc. of std Found [μg/ml]	Recovery (% )
Nickel	25%	2.668	2.663	99.81
	50%	5.103	5.077	99.49
	75%	7.336	7.362	100.35
Mean				99.88
% RSD				0.43
Zinc	60 %	2.972	2.956	99.46
	120%	5.686	5.692	100.11
	170%	8.174	8.121	99.35
Mean				99.64
%RSD				0.41

Figure-1. Sampled D. C. Polarograms of Nickel and Zinc

Figure-2. Linearity Graphs for Standard Ni and Zn

ACKNOWLEDGEMENT:

Authors thanks Department of Chemistry and Ruia College for providing us with the necessary facilities.

REFERENCES

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2) Voltammetric study of lead and its trace determination has been done using stripping methods with the help of modified electrodes. K.Honeychurch, J. P. Hart, D. C. Cowell and D. W. M. Arrigan, Electroanalysis, 14, 177-185, (2002).

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9) Seperation and Quantitation of metals present in industrial effluents and sludge samples by electrodialysis, coulometry and photocatalysis. G. Ramachandraiah; S. K. Thampy; P. K. Narayanan; D. K. Chauhan; N. Nageswara Rao; V. K. Indusekhara, Separation Science and Technology, Volume 31, pages 523 – 532, (1996).

Received on 27.01.2012 Modified on 13.02.2012

Asian J. Research Chem. 5(5): May 2012; Page 582-585