1. INTRODUCTION:
Thiosemicarbazones have been widely used as analytical reagents. The analytical applications and biological activity of thiosemicarbazones have been reviewed1-5. These reagents have been used as analytical reagents in spectrophotometry, flurometry, atomic absorption spectrophotometry and as indicators. Thiosemicarbazones with transition metal complexes have great medicinal value. These compounds are used in the treatment of diseases like influenza6, protozoa7, small pox8-9 and certain kinds of tumour related problems10-12. These reagents are known for their antitubercular activity13. In the treatment of cancer, the active species is the metal chelate of thiosemicarbazone14-15. Metal chelates of these reagents are used as pesticides16 and fungicides17 in agriculture. Thiosemicarbazones with transition metal complexes possess antifungal, antimicrobial and antiviral activity18-20. Both cadmium and cobalt are known to cause hypertension and cardiac pathology21-28.
In view of the fact that, quantities of these elements ingested by even heavy coffee drinkers but no firm conclusions can be drawn about their toxicity from this source. In this article, the authors present second order derivative spectrophotometric method for the simultaneous determination of cadmium and cobalt.
2. EXPERIMENTAL:
i) Preparation of FFTSC: The reagent furfuraldehyde thiosemicarbazone was prepared by simple condensation of furfuraldehyde with thiosemicarbazide by adopting the standard procedure. The structure of the compound is given below,
Furfuraldehyde thiosemicarbazone
The m.p. of FFTSC is 131-1350C, Yield = 8.10 gm, %Yield = 85.3%.
The structure has been established based on IR, mass and NMR spectra.
ii) Preparation of buffer solutions:
Buffer solutions are prepared using HCl, CH3COOH and NaOAC in acidic medium and NH4OH, NH4Cl in basic medium.
iii) Preparation of metal and reagent solutions:
The standard Cd(II) and Co(II) solutions were prepared using analytical reagent grade CdSO4 and CoCl2.6H2O. Appropriate quantity of FFTSC is dissolved in DMF for making 0.01 M reagent solution.
Procedure:
a. Preparation of standard derivative spectrum:
A series of solutions containing varying concentrations of Cd(II) and Co(II) are taken in 25 ml volumetric flasks along with 20 fold concentration of FFTSC reagent, so that it is sufficient to form complexes with both the metal ions. To this 10 ml buffer solution of pH 9.0 is added and made up to the mark with distilled water. The amplitudes of these solutions were measured between 360-425 nm against reagent blank. Shimadzu 160A UV-visible spectrophotometer (Japan) equipped with 1 cm quartz cell was used in these investigations for making amplitude measurements. A pH meter ELICO L1-120(Hyderabad) is used to make pH measurements.
b. Preparation and analysis of samples:
50 g of the biological material (tea leaves, coffee beans, human air, tobacco and rice) was heated in a 500 ml conical flask with 40 ml of conc.HNO3 on a steam bath and shaken vigorously until a fine emulsion was formed. The heating was continued with the gradual addition of 6% H2O2 (40 ml). The aqueous phase was then transferred to the beaker. The extraction was repeated twice with further addition of 20 ml con.HNO3 and 20 ml 6% H2O2. The combined extracts were evaporated to dryness. The residue was dissolved in minimum amount of dil.HCl and transferred into a 50 ml standard flask quantitatively. The contents were made up to the mark with distilled water. The solution is further diluted as required. A known aliquot of the sample solution, 10 ml of basic buffer solution of pH 9.0 and 5 ml of FFTSC reagent were taken into a 25 ml volumetric flask and mixed well. The derivative spectra of analytes in the sample were recorded using the same procedure. According to the concentrations and amplitudes of standard solutions, the contents of the sample can be calculated from derivative spectrum obtained from sample.
RESULTS AND DISCUSSION:
a. Derivative spectra of Cd(II) and Co(II) complexes:
The zero order spectrum of a mixture containing cadmium and cobalt in presence of FFTSC results only one peak, no resolution takes place and hence simultaneous determination is not possible and it is presented in the figure-1. Hence we have made an attempt to use a 1st order derivative spectrum for possible simultaneous determination of the two metal ions, but no fruitful results are obtained. Finally we made an attempt to use a second order derivative spectrum for possible simultaneous determination of the two metal ions as shown in the figures-2 and 3 consist of two peaks and valleys corresponding to the two metal ions. The 2nd order derivative spectra are recorded in one case keeping Cd(II) concentration constant and varying Co(II) concentration as shown in the figure-4. In the second case the Co(II) concentration is kept constant and Cd(II) concentration is varied as shown in the figure-5. In order to obtain greater sensitivity, graphs are drawn between Cd(II) concentration and peak amplitude as well as Co(II) concentration and peak amplitude and are presented in the figures-6 and 7. Linear graphs are obtained in both the cases, hence using the second order derivative spectrophotometric method; we can determine Cd in presence of Co and vice-versa.
b. Effect of pH:
Cd(II) and Co(II) react with FFTSC reagent form an intense yellow and brown colored complexes. The colored solutions show maximum absorbances at 355.4 and 364.5 nm for Cd(II) and Co(II) respectively in the pH range 2-11 and these are shown in the figures-8 and 9. Further studies reveal that the maximum absorbance is observed at pH 9 for both Cd(II) and Co(II) and the results are reproducible at this pH. A solution of pH 9.0 is selected for further detailed investigations and simultaneous determination of both the metal ions. Moreover Cd(II) and Co(II) do not form stable complexes in acidic medium it may be due to hydrolysis of the reagent or the complex itself. In highly alkaline medium (>pH 11) slow turbidity develops which may be due to formation of hydroxides.
Table-1 Tolerance limit of foreign ions
|
Foreign ion |
Tolerance limit (μg/ml) |
Foreign ion |
Tolerance limit (μg/ml) |
|
Fluoride |
15.55 |
U(VI) |
90.8 |
|
Chloride |
14.77 |
Al(III) |
1.09 |
|
Iodide |
24.64 |
W(VI) |
6.95 |
|
Nitrate |
19.38 |
Mo(VI) |
28.2 |
|
Acetate |
30.45 |
Se(IV) |
125.45 |
|
Oxalate |
37.25 |
Pd(II) |
0.35 |
|
EDTA |
49 |
Cr (II) |
1.41 |
|
Thiosulphate |
21.13 |
Cu(II) |
0.81 |
|
|
Sn(II) |
4.84 |
|
|
Ni(II) |
0.71 |
||
|
Zr(IV) |
1.73 |
||
|
Sr(II) |
12.75 |
||
|
La(III) |
31.81 |
||
|
Ti(IV) |
7.76 |
||
|
Th(IV) |
64.01 |
||
|
Cr(VI) |
0.48 |
||
|
Mg(II) |
19.98 |
||
|
Fe(II) |
0.68 |
Tolerance limit of foreign ions in the determination of 2.698 μg/ml of Cd (II) and 0.943 μg/ml of Co(II). pH = 9.0 λmax = 364 nm
The effect of reagent concentration was studied by measuring the absorbance values at 355.4 and 364.5 nm of solution containing a fixed amount of Cd(II) and Co(II) by varying concentration of FFTSC. It was observed that 20-fold excess of FFTSC is sufficient for maximum color development with both the metal ions. However, the excess concentration of the reagent did not show any substantial change in absorbance.
c. Applicability of Beer`s law:
The effect of metal ion concentration on absorbance is studied from 0.540-5.396 µg/ml for Cd(II) and 0.118-1.886 µg/ml for Co(II). The concentration of reagent is kept constant at 3.6x10-4 M and the absorbance values are measured at 355.4 nm for Cd(II) and at 364.5 nm for Co(II) against a blank solution containing no metal ions and these are shown in the figures-10 and 11. Thus the method can be employed for the determination of Cd(II) in the range 0.540-5.396 µg/ml and Co(II) in the range 0.118-1.886 µg/ml. The molar absorptivity and Sandell`s sensitivity of Cd(II) and Co(II)-FFTSC complexes are 3.367x105 L/mol/cm, 3.886x105 L/mol/cm and 0.00297µg/cm2, 0.00257 µg/cm2 respectively.
The color reaction between Cd(II) and Co(II)-FFTSC is instantaneous at room temperature. The complex is stable for more than two hours and hence can be used for analytical applications.
d. Composition and stability of the complex:
The stoichiometry of Cd(II) and Co(II)-FFTSC complex was studied by Job`s method of continuous variation and also by the mole ratio method. Both the methods indicated the formation of a 1:2 complex between Cd(II) and FFTSC, 1:1 between Co(II) and FFTSC. The stability constants are calculated as 3.238x105 and 1.256x105 respectively.
e. Effect of diverse ions:
The effect of diverse ions in the determination of Cd(II) and Co(II) was examined under the optimum conditions. The extent of interference by various anions and cations was determined by measuring the absorbance of solutions containing a constant amount of Cd(II) and Co(II) and varying amounts of diverse ions, most of the ions did not interfere in the determination. The tolerance limits for various cations and anions are listed in the table-1.
Fig-1: Zero order spectrum of Cd (II)+Co(II) in presence of FFTSC.
[Cd (II)] = [Co (II)] = 2.4x10-5M;
[FFTSC] = 5.2x10-4 M; pH = 9.0
Fig-2:2nd order derivative spectrum of Cd(II)+Co(II) in presence of FFTSC.
[Cd(II)] = [Co(II)]=2.4x10-5 M; pH=9.0; [FFTSC] = 5.2x10-4 M;
a
Wavelength (nm)
Fig-3:2nd order derivative spectrum of Cd(II)+Co(II) in presence of FFTSC.
[Cd(II)] = [Co(II)]=2.4x10-5 M; pH=9.0; [FFTSC] = 5.2x10-4 M;
a) 0.5 ml of Cd(II) and Co(II) each
Fig-4:2nd order derivative spectrum of Cd(II)+Co(II) in presence of FFTSC.
Cd2+ concentration is kept constant by varying concentration of Co2+.
[Cd(II)] = [Co(II)]=2.4x10-5 M; pH=9.0; [FFTSC] = 5.2x10-4 M;