ANALYST. APRIL 1989. VOL. 114 517 SHORT PAPERS p-Anisaldehyde Thiosemicarbazone: a Reagent for the Polarographic Determination of Platinum(l1) Rasappan Palaniappan and Vedachalam Revathy Department of Analytical Chemistry, University of Madras, Guindy Campus, Madras 600 025, India p-Anisaldehyde thiosemicarbazone (PATSC) is proposed as a reagent for the rapid, relatively sensitive and selective determination of platinum(1l) in the approximate range 0.05-8.0 mM by d.c. polarography. This reagent quantitatively formed a yellow complex with platinum(l1) in 0.1-0.25 M HCI medium, which yielded a reversible, diffusion-controlled, two-electron reduction wave. Various polarographic parameters for the quantitative determination have been studied and the reliability of the method has been checked by statistical analysis.The senstivity of the method was increased approximately ten-fold by employing differential-pulse polarog ra p hy. Keywords : p -A nisalde h yde thiosem ica rbazo n e; pla tin urn (11); pola rograp h y Thiosemicarbazones have aroused special interest in the fields of medicinal and analytical chemistry. 1-3 These compounds act as complexing agents for various metal ions. Recently, several analytical applications of thiosemicarbazones in com- plexometric, gravimetric, spectrophotometric, fluorimetric, potentiometric and polarographic studies of many metal ions, especially the platinum group elements, have been reported.49 This work was undertaken with the objective of evolving a new method of analysing very dilute solutions of platinum as only a few sensitive methods10." have been reported.The proposed method was found to be highly selective, as the analysis was carried out at relatively low pH; interferences from other ions were, therefore, found to be a minimum. Experimental Analytical-reagent grade chemicals and doubly distilled water were used. Anhydrous PtC1, (Arora Metthey, India) was used as the source compound. Each 100-ml stock solution (ca. 0.01 hi) contained 4 ml of 4 M hydrochloric acid and was standardised as described in the literature.12 The stock solution (ca. 0 . 0 1 ~ ) was diluted further to the relevant concentration of platinum(I1) as required. For the study of interferences, other noble metal solutions such as palladium(II), rhodium(III), ruthenium(II1) and iridium(II1) in 2~ HC1 were freshly prepared as stock solutions from analytical-reagent grade salts PdCl2, RhC13.3H20, RuC13.3H20 and IrC13.3H20 (Arora Metthey) respectively. The solution of palladium( TI) was standardised gravimetric- ally by precipitation with dimethylglyoxime.13 The rho- dium( 111) solution was also standardised gravimetrically by precipitation as the sulphide, followed by ignition to the oxide and then reduction to the metal in the presence of hydrogen.13 The iridium(II1) solution was standardised using 2-mercap- tobenzothiazole and the ruthenium(II1) solution by spectro- photometry with thionalide. 12313 Osmium tetraoxide (Merck), weighed carefully in an ampoule, was dissolved in 0 . 5 ~ NaOH. neutralised with acid and diluted to a known volume, before being standardised by titrimetry.13 y-Anisaldehyde thiosemicarbazone (PATSC, m.p. 175 "C) was prepared according to Sah and Daniels.14 For polarographic work, a 50-fold excess of concentration of ligand over metal was used to ensure complete complexation. Potassium chloride (0.2 M) was used as supporting electrolyte. All d.c. polarographic experiments were carried out at a dropping-mercury electrode (DME) referenced to a saturated calomel electrode (SCE) using a Model CL-25D (Elico, India) automatic instrument. Polarograms were recorded at 25 k 0.5"C and purified nitrogen was used for de-aeration. The capillary characteristics in 0.2 M KC1 (open circuit) were as follows: mass flow of Hg, m = 2.72 mg s-1; drop time, t = 2.89 s at a mercury column height of 40 cm; m t = 2.3260 mg s-f.Differential-pulse polarograms were recorded using a Model CL-90 (Elico) instrument. The electrode system used was a DME working electrode, an SCE reference electrode and a platinum counter electrode. The pH was measured using an Elico digital pH meter (Model LI-120). Results and Discussion Polarogram of Platinum(I1) in the Presence of PATSC In highly acidic medium ( 0 . 6 ~ HCl), Pt" and PATSC in the presence of 0 . 2 ~ KC1 produced a wave with E+ = 0.006 V versus SCE. Simultaneously a black deposit was observed on the pool of mercury. This deposit may be due to incomplete complexation between Pt" and PATSC. By virtue of the positions of Hg and Pt in the e.m.f. series the chemical reaction (1) preceded the electrode reaction (2).15.16 The oxidised Hg22+ underwent cathodic reduction at Et = +0.006 V versus SCE.. . (1) Pt" + 2 Hg -+ PtO (black deposit) + Hg22+ . . . . (2) 0.25 0.75 1.25 - E N versus SCE Fig. 1. Polarograms of the Pt" - PATSC system. A, 50 mM PATSC in 0.2 M KCl + 0.2 M HCl; B. 1.4 mM Pt" + 50 mM PATSC + 0.2 M KCl + 0.6 M HCl; C, as B but in 0.4 M HCl; D, as B but in 0.2 M HCI518 ANALYST, APRIL 1989, VOL. 114 When the total acid concentration in the solution exceeded 0.25 M, the reduction wave for mercury(1) appeared resulting in an apparent two-wave pattern (reduction of Hg22+ and Pt" complex) (Fig. 1). Characteristics of the Polarographic Wave A single well defined wave was obtained at Et = -0.430 V versus SCE in the acid (HC1) concentration range 0.25-0.1 M (Fig.1). When the concentration of HCl decreased from 0.6 to 0.2 M the wave shifted to more negative potentials indicating complex formation of higher stability. The limiting current value remained almost constant within 0.25-0.1 M HC1 and thereafter progressively decreased. The reduction wave was found to be diffusion controlled [constant id/V%-; h , mercury column height (cm)] and reversible [plots of log(ilid - i) versus EDME were linear with a slope = 0.029 k 0.002 V] involving a two-electron transfer electrode process. Exhaustive con- trolled potential electrolysis in the limiting current region of the wave (-0.600 V versus SCE) at 0.13 M HCl gave a value of 2 for n. the number of electrons involved in the electrode reaction, providing further support to the two-electron electrode process.The electrode reaction rate constant ( k f " ) was evaluated at optimum experimental conditions and found to be 1.06 x 10-2 s-1; the diffusion current constant and the diffusion coefficient were calculated as 3.382 and 7.66 X 10-6 cm2 s-1, respectively. Analytical Applications and Sensitivity of the Method The polarographic curves were found to be analytically useful in the HCI concentration range 0.1-0.25 M for accurately determining Pt" in solutions in the range 8-0.05 mM (error <1%); even a 0.025 mM solution could be determined with an error of ca. 3%. The interferences of diverse ions were also studied. Of the various cations and anions tested individually in the determination of 1 mM of platinum(II), no interference was observed in the presence of a 200-, 50-, 25-,lo- or 5-fold excess of ethylenediaminetetraacetate (EDTA), zinc( II), barium(I1) , cadmium(II), molybdenum(VI), tungsten(VI), fluoride , acetate, oxalate , chloride or sulphate; chromium(III), aluminium(III), nickel(II), manganese(II), cobalt(I1) or thiocyanate; osmium(VIII), iron(II,III), bromide or thiosulphate; ruthenium(II1); and rhodium(II1) or iridium(III), respectively.The method was found to be selective; only a few ions, i.e., palladium(II), copper(I1) and iodide interfered seriously even to the same level of concentra- tion of platinum(I1). The major interfering ions such as copper(I1) and palladium(I1) could be tolerated by adding EDTA (masking agent) and selectively precipitating17 before determination. I -I- 0.00 0.40 0.80 1.20 - E N versus SCE Fig.2. Typical differential-pulse polarograms of the Pt" - PA'TSC system. A, 50 mM PATSC in 0.2 M KCI + 0.2 M HCl; B, 0.5 mM Pt" + 50 mM PATSC + 0.2 M KCI + 0.6 M HCI; C, as B but in 0.4 M HCI; D, as B but in 0 . 2 ~ HCl Statistical Treatment of the Proposed Method The precision of the proposed method was evaluated by determining the same amount of platinum(I1) (2 mM) in six different samples. The average was found to be 1.994 mM and the corresponding standard deviation was 0.060. From the observations of linearity the value of the correlation coeffi- cient was 0.9994; this implied that the best linear association between the measured signals and the true (theoretical) concentrations was observed in the proposed method. Differential-pulse Polarographic (DPP) Determination of Platinum( 11) The polarograms were recorded under the following condi- tions: drop time, 1 s; pulse amplitude, 25 mV; sweep rate, 5 mV s-1.When platinum(I1) with PATSC was subjected to DPP analysis, at the concentration of HCl (0.25-0.1 M) applied for d.c. polarographic analysis, a single, symmetric, reversible, diffusion-controlled peak with high reproducibility was observed. In highly acidic medium, an apparent two-peak pattern was observed, the former being due to the reduction of mercury(1) and the latter to the reduction of the platinum(I1) - PATSC complex (Fig. 2). The peak current (&) - concentration relationship was found to be linear for 0.005-3.5 mM of platinum(I1). Linear calibration plots were obtained in two concentration ranges: 0.005-0.1 and 0.1-3.5 m M .The limit of determination was 0.005 mM. The average sensitivities were 24.58 and 22.88 pA mM-1 for 0.005-0.1 and 0.1-3.5 mM of platinum(I1) respectively. The standard deviations and coefficients of variation were 0.29,0.27 and 0.47, 2.16 for 0.06 and 2.0 mM of platinum( 11), respectively. Correlation coefficients of 0.9924 and 0.9894 were observed for 0.005-0.1 and 0.1-3.5 mM of platinum(II), respectively. As the proposed method proved to be sensitive, rapid and free from interferences from many ions (selective), it can be applied to the determination of platinum in real samples such as dental alloys, catalysts, electrical standard resistors or thermocouplesll~lx-2(~ by using either the d.c. or DPP mode. The authors thank Professor Agnes Paul, University of Madras, for providing necessary facilities.1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. References Petering, H. G., Buskirk, H. J . , and Underwood, C E., Cuncer Res., 1964, 64, 367. Dwyer, F. P., and Haro, R. T., Prog. Exp. Tumour Res., 1969, 12, 102. Singh. A. K., Katyal, M., and Lal, K . , Ra.cayan Savrzrksha, 1986, 2, 41. Gandhi, M. H., and Lahiri, S . A., Acta Crenc. 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