1. Spectrophotometric determination of Zn(II)) by quantitative displacement with copper(II) from diethyldithiocarbamate (DDTC) complex
Md. Amzad Hossain, Mohammad Nasir Uddin and Md. Abdus Salam
Department of Chemistry, University of Chittagong, Bangladesh
ABSTRACT
Procedures for the determination of zinc(II) by quantitative displacement of diethyldithiocarbamate (DDTC) from its complex with copper(II) are described and the absorbance was measured at 435 nm after back extraction of Cu(DDTC)2 with CHCl3. Reproducibility within 2% and detection limits of 0.25 ppm Zn(II) have been obtained, and linear calibration ranges up to 18 ppm. In the presence of a suitable masking agent (0.1M EDTA, NH4SCN) very good selectivity was achieved. Method was successfully applied to the food nutrients, pharmaceutical, fertilizer and biological samples.
Reagents, standard solutions
High-purity chloroform, various acids, salts and reagent grade Na-DDTC (Merck) were used. The standard stock solutions (1000 μg mL－1) were prepared by dissolving appropriate amount of each salt in water. The pH values in the range of were adjusted with acetate buffers by mixing proper amounts of acetic acid and sodium acetate. Doubly distilled deionized water was used throughout. Suitable portions of these solutions were mixed to get the desired pH.
Instrumentation
Shimadzu (Kyoto, Japan) (Model-1800) double beam UV/VIS the recording spectrophotometer for the measurements of absorbance and a Jenway (England, U.K) (Model-30100) pH meter for the measurements of absorbance and pH were used.
General procedure
To determine mercury, 1 mL of 2×10-4 M Zn(II) and 1 ml of the reagent 6×10-4 M DDTC solution were mixed in a 25-ml standard flask along with 0.5 mL 0.05M H2SO4. Zinc complex was extracted carefully with the addition of 10 mL CHCl3. pH 5 was maintained with addition of acetate buffer. 1 mL 2×10-4 M Cu(II) solution was added when immediate yellow colour was formed and extracted to the organic phase after vigorous stirring for 10 min. The absorbance was measured by a spectrophotometer at 435 nm against a blank.
Zn(II) + DDTC
Extraction in CHCl3
Cu(II)
Zn(DDTC) Cu(DDTC)2
Absorbance at 435 nm
INTRODUCTION
Diethyldithiocarbamate (DDTC) is commonly used for spectrophotometric determination of a number of transition metal ions including Cu(II). The complexes are insoluble in water and are extracted for measurement. Zn(II) gives white insoluble complexes which are less stable than the yellow Cu(DDTC)2 complex. Therefore, zinc can be indirectly determined by a displacement of (DDTC) from Cu(DDTC)2 complex with Cu(II) by measuring the absorbance. The method is based on the quantitative replacement of Zn(II) ions in the solid Zn(DDTC)2 phase by Cu(II) ions present in aqueous phase. The obtained solid Cu(DDTC)2 phase was dissolved in chloroform and the absorbance was determined spectrophotometrically at 435 nm. The aim of this work was to investigate the optimum conditions for the selective spectrophotometric determination of Zn(II) using DDTC reagent in the food nutrients, pharmaceutical, fertilizer and biological samples. Better reproducibility and very good selectivity was achieved using a suitable masking agent (0.1M EDTA, NH4SCN).
OPTIMAL CONDITIONS
Primarily M acid concentration was maintained with the use of H2SO4 for the formation of zinc complex. The study of the effect of pH on the color intensity and best extraction showed that the maximum color was obtained in the wide range of pH Analytical studies were therefore, carried out at pH 5.0. A 15 fold molar excess of DDTC was necessary for complex and constant color development. Excess of the reagent has no effect on the absorbance of the complex. The absorbance of the solution was measured at different time intervals to ascertain the time stability of the color complex it was observed that the color development is instantaneous and remains constant for more than 48 hours.
METHOD VALIDATION
Method was validated in terms of analytical performance parameters; precision, accuracy, limit of detection, limit of quantitation and linearity range.
Calibration curve, linearity and sensitivity
The calibration graph was constructed with seven standard solutions containing 0.01 – 20 μg mL–1 of zinc according to the general procedure. The curve was constructed by plotting absorbance against corresponding concentrations. The linearly ranges, regression equation and correlation coefficient were obtained by the method of least squares. Beer’s law was obeyed over the concentration range of 0.1 – 18 μg mL–1. The molar absorptivity coefficients of the solutions were × 10-4 L mol−1 cm−1. The results are presented in Table 1.
EXPERIMENTAL
Absorption spectra
The absorption spectra of the solution containing zinc(II) or copper(II) complex against the reagent blank and that of the reagent solution against the corresponding buffer blank were recorded in the wavelength region nm. Typical spectra are presented in the (Fig 1) showing absorption maximum of Zn(II) complex at 435 nm. Whereas the reagent itself does not show any considerable absorbance at this wavelength. Hence, 435 nm was chosen for further studies.
Table 1 Optimization and validation parameters for the proposed method
Precision
Within-day repeatability (RSD) was determined in six replicates at three concentrations levels in standard samples in the same day. The procedure was repeated on six consecutive days, in standard samples at same concentration levels, to determine between-day repeatability.
Sensitivity
Sensitivity of the method has been tested by examining the Limit of Detection (LOD) and Limit of Quantification (LOQ) values. The calculation method is based on the standard deviation of the response (Sxy) and the slope of the calibration curve (a). The limit of detection were calculated from calibration graph by the formula; LOD=3·Sxy/a, and the limit of quantification; LOQ=10·Sxy/a. The results are presented in Table 1.
Effect of foreign ions
The reaction selectivity was investigated by determining zinc(II). The tolerance limit was taken as ± 10% change in absorbance. Copper(II), nickel(II), cobalt(II), cadmium(II), iron (III) and iron(II) ions interfered seriously at all proportions. However, selective extraction of zinc from tiocyanate solution in HCl 0.5 mol L-1 medium followed by its back extraction with an ammoniacal solution solves this problem.
Optimization parameters
Studied range
Optimal range
Acid concentration for Zn(II) complex formation
5.010-4M -7.010-2M -
1.0 10-3M
Metal:reagent (molar excess)
1:1-1:60
1:15
Time effect / min
10 min – 48 hrs
up to 48 hrs
Extraction pH
5.0
Mathod Validation Parameters
Composition M:L
1:2
Molar absorptivity, 
1.945 104 M
Linearity range
g mL-1
Regression coefficient, R2
0.9971
Limit of Detection, LOD
0.29 g mL-1
Limit of Quantification, LOQ
0.97 g mL-1
Standard Deviation, SD
0.03
Relative Standard Deviation, RSD
4.98
Linearity Equation
y = x 0.079 A B C
Figure Absorption Spectra of (A) Cu(II)-DDTC, (B) Zn(II)-DDTC and (C) DDTC
CONCLUSION
UV-VIS spectrophotometry is a useful, widespread, and more economical technique compared to AAS and ICP-AES procedures. On the other hand, its selectivity is lower and the interfering effects are said to be considerably higher. Very good selectivity was achieved In the presence of a suitable masking agent (0.1M EDTA, NH4SCN). The proposed method may be successfully employed for the spectrophotometric determination of Zn(II) ions in the samples of different origin.
Future Perspectives
The extraction may be avoided by using non-ionic surfactants to form micellar solutions of the DDTC complexes in aqueous medium. Zu(II) has been determined with DDTC in the presence of Triton X-100 as the solubilizing agent by the displacement reactions of Zn(II) with Cu(DDTC)2.
(g/mL)
Figure Calibration curve for the determination of Zn(II)
34th Bangladesh Chemical Conference, 2011 held in Dhaka University on th December, 2011