ЖУРНАЛ АНАЛИТИЧЕСКОМ ХИМИИ, 2008, том 63, № 1, с. 32-35
^=ОРИГИНАЛЬНЫЕ СТАТЬИ =
УДК 543
SIMULTANEOUS SECOND DERIVATIVE SPECTROPHOTOMETRIC DETERMINATION OF COBALT AND VANADIUM USING 2-HYDROXY-3-METHOXY BENZALDEHYDE THIOSEMICARBAZONE
© 2008 r. A. Praveen Kumar, P. Raveendra Reddy, V. Krishna Reddy
Department of Chemistry, Sri Krishnadevaraya University Anantapur - 515 003 (A.P.), India Received 15.09.2006; in final form 12.01.2007
2-Hydroxy-3-methoxy benzaldehyde thiosemicarbazone reacts with cobalt(II) and vanadium(V) at pH 6.0 forming respectively brown and green coloured complexes in aqueous dimethyl formamide. The second derivative spectrum of Co(II) complex shows zero amplitude at 434.5 nm and considerably large amplitude at 409.5 nm, while V(V) complex shows sufficient amplitude at 434.5 nm and zero amplitude at 409.5 nm. Further, the derivative amplitudes obey Beer's law at 409.5 and 434.5 nm for Co(II) and V(V) in the range 0.0593.535 and 0.051-4.074 |g/mL, respectively. This enables the simultaneous determination of Co(II) and V(V) without separation. A large number of foreign ions do not interfere in the present method. The method is applied to the simultaneous determination of Co(II) and V(V) in synthetic mixtures and alloy steel samples.
The direct spectrophotometry determination of metal ions in multicomponent systems is often complicated due to interferences from the matrix and spectral overlapping. The interferences have been treated in many ways, such as solving two simultaneous equations [1] or using absor-bance ratios at certain wavelengths [2-4]. However, on the application of these methods, the presence of spectral interferences or spectral overlap would certainly lead to erroneous results [5]. Other approaches aimed at solving this problem have been employed, including pH induced differential spectrophotometry [6], least squares [7] and orthogonal function [5, 7, 8] methods.
Derivative spectrophotometry is a useful means of resolving two overlapping spectra and eliminating matrix interferences in the assay of two-component mixtures using the zero-crossing technique [9-11]. In the absence of a zero-crossing point, two simultaneous equations can be solved to determine the components in such a mixture [12]. The latter method is based on criteria for selecting the optimum working wavelength [2]. In addition, the component being determined should make a reasonable contribution to the total derivative reading of the mixture at the selected wavelength. Derivative spectrophotomet-ric analysis of two-component mixtures is also carried out without the need to solve simultaneous equations. The compensation method [13] is also used for the purpose. This is a non-mathematical method for the detection and elimination of unwanted absorption during photometric analysis.
Thiosemicarbazones [14-16] are one of the important class of reagents widely employed for the spectro-photometric determination of metal ions. In the present paper 2-hydroxy-3-methoxy benzaldehyde thiosemicarbazone (HmbAtSC) is used as a spectrophotometry
reagent for the simultaneous second derivative determination of cobalt(II) and vanadium(V). HMBATSC reacts with cobalt(II) forming brown coloured complex and vanadium(V) forms green coloured complex with regent at pH 6.0. The second derivative spectrum of Co(II) complex shows zero amplitude at 434.5nm and considerably large amplitude at 409.5 nm, on the other hand, V(V) complex exhibits sufficient amplitude at 434.5 nm and zero amplitude at 409.5 nm. Further, the derivative amplitudes obey Beer's law at 409.5 nm and 434.5 nm for Co(II) and V(V), respectively. Hence, a simultaneous second derivative spectrophotometric method is employed for determination of Co(II) and V(V) without solving simultaneous equations.
EXPERIMENTAL
The absorbance and pH measurements were made on a Shimadzu UV-visible spectrophotometer (Model UV-160A) fitted with 1cm Quartz cells and Phillips digital pH meter (model L1 613), respectively. Second order derivative spectra were recorded with a scan speed of nearly 2400 nm/min, slit width of 1 nm with nine degrees of freedom, in the required wavelength range (nm). The derivative amplitudes were measured at required wavelengths and plotted against amount of cobalt(II) or vanadium(V) to obtain the calibration plot.
Reagents. Preparation of the reagent:
The reagent (HMBATSC) is prepared by the Sah and Daniels [17] procedure. 11.25g of 2-hydroxy-3-methoxy benzaldehyde (I) and 4.55g of thiosemicarba-zide (II) are dissolved in sufficient volume of methanol and the mixture is refluxed for 60 minutes. The con-
tents are allowed to cool and the product is separated by filtration. A crude sample (yield 80%) is obtained (C9HnO2SN3). The resultant product is recrystallised
twice from hot methanol. Pure light yellowish green crystals of HMBATSC (III) (m.p. 220-222°C) are obtained.
h-c=o
oh + h2n-nh-c-nh2
reflux
s
och3
(I)
(II)
h-c=n-nh-c-nh2
■ II 2
oh s
och3 (III)
A 0.01M solution of HMBATSC in dimethyl forma-mide (DMF) was employed in the present studies.
Cobalt(II) solution. Stock solution of Co(II) (1 x x 10-2 M) was prepared by dissolving appropriate amount of Co(NO3)2 ■ 6H2O in doubly distilled water containing a few drops of concentrated hNo3 in a 100 mL volumetric flask and standardized gravimetrically [18].
Vanadium(V) solution. A 1 x 102 M solution of ammonium meta vanadate (NH4VO3) was prepared by dissolving 0.1170 g of the salt in hot distilled water and standardized titrimetrically [19].
The working solutions were prepared daily by diluting the stock solution to an appropriate volume. All other chemicals used were of analytical grade.
Buffer solutions. The buffer solutions were prepared by mixing 1 M hydrochloric acid and 1 M sodium acetate (pH 1.0-3.0), and 0.2 M acetic acid and 0.2 M sodium acetate (pH 3.5-7.0). The pH of these solutions was checked with a pH meter.
Preparation of alloy steel sample solution. 0.10.5 g of steel sample was dissolved completely in minimum amount of aqua regia by slow heating on sand bath and then heated to fumes of oxides of nitrogen. After cooling, 5-10 mL of H2O:H2SO4 mixture (1:1) was added and evaporated to dryness. Sulphuric acid treatment was repeated three times to remove all nitric acid. The residue was dissolved in 20 mL of distilled water and filtered. The filtrate was made up to 100 mL in a volumetric flask with distilled water. The sample solution was appropriately diluted to obtain the concentration in the required range.
RESULTS AND DISCUSSION
HMBATSC reacts with cobalt(II) and forms brown coloured complex. Vanadium(V) forms green coloured complex at pH 6.0. The second order derivative spectra of Co(II) - HMBATSC and V(V) - HMBATSC were re-coded for the solutions containing 1.2 mL of Co(II) or V(V) (1 x 10-4 M), 5 mL of buffer solution (pH 6.0) and 0.5 mL of HMBATSC (1 x 10-2 M) in a total volume of 10 mL. The typical curves are shown in figure. From the figure it was clear that the second order derivative spectrum of Co(II) complex with HMBATSC showed zero amplitude at 434.5 nm and considerably large amplitude at 409.5 nm. On the other hand, V(V) complex with HM-
BATSC showed sufficient amplitude at 434.5 nm and zero amplitude at 409.5 nm. Further, the derivative amplitudes obeyed Beer's law at 409.5 and 434.5 nm for Co(II) and V(V) respectively. Hence, Co(II) and V(V) can be determined simultaneously in a mixture without separation by measuring the second order derivative amplitudes at 409.5 and 434.5 nm, respectively.
Verification of Beer's law validity. Individual calibration plots were constructed by plotting the second derivative amplitudes measured at 409.5 nm against the amount of Co(II) and those measured at 434.5 nm against the amount of V(V). The plots revealed that Beer's law was obeyed in the range 0.059-3.535 |g/mL of Co(II) and 0.051-4.074 |g/mL of V(V).
Simultaneous determination of Co(II) and V(V).
Variable amounts of Co(II) (0.2357 to 1.1785 |g/mL) and V(V) (0.2037 to 1.0185 |g/mL) were treated with the required volume of the reagent at pH 6.0 in total volume of 10mL and the second derivative spectra were recorded in the range from 390 to 525 nm. The derivative amplitudes were measured at 409.5 and 434.5 nm and the amounts of Co(II) and V(V) present in the mixture
Amplitude 0.4
0.1
0 -0.1
-0.2 -0.3
390 420 450 480 510
Wavelength, nm
Second derivative spectra of: a - V(V) - HMBATSC system Vs reagent blank; b - Co(II) - HMBATSC system Vs reagent blank; [V(V)] = [Co(II)] = 1.2 x 10-5 M. pH = 6.0.
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Table 1. Simultaneous second order derivative determination of Co(II) and V(V)
Amount taken, |g/mL Amount found*, | g/mL Relative standard deviation, %
Co(II) V(V) Co(II) V(V) Co(II) V(V)
0.2357 0.2037 0.236 0.204 0.89 1.17
0.2357 0.4047 0.235 0.404 0.88 0.68
0.2357 0.6111 0.236 0.612 0.88 0.38
0.2357 0.8148 0.235 0.814 1.08 0.29
0.2357 1.0185 0.236 1.019 0.98 0.27
0.2357 0.2037 0.236 0.204 0.99 1.16
0.4714 0.2037 0.472 0.203 0.39 1.65
0.7071 0.2037 0.706 0.204 0.33 1.25
0.9428 0.2037 0.944 0.204 0.24 1.53
1.1785 0.2037 1.179 0.204 0.23 0.94
* Average of five determinations.
Table 2. Tolerance limits of diverse ions
Tolerance limit (|g/mL)
Diverse ion In the presence of 1.108 |g/mL of V(V) In the presence of 1.18 |g/mL of Co(II) Diverse ion In the presence of 1.108 |g/mL of V(V) In the presence of 1.18 |g/mL of Co(II)
Ascorbic acid 3500 3720 Pb(II) 2050 2000
Tartrate 2800 3520 W(VI) 1660 1800
Citrate 2400 2540 U(VI) 1500 1500
EDTA 2000 2430 Zr(IV) 1000 1100
Thiourea 2000 1900 Cd(II) 900 1100
Formate 1800 1800 Li(I) 800 1000
Urea 1800 1800 Th(IV) 640 900
Bromate 1600 1600 Na(I) 600 900
Oxalate 1600 1500 Te(IV) 500 800
Bromide 800 1440 K(I) 420 750
Phosphate 1500 1360 Cu(II) 400 500
Nitrate 1400 1280 Al(III) 300 500
Chloride 1200 1200 Zn(II) 260 450
Sulphate 1100 1120 Pt
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