A SENSITIVE KINETIC SPECTROPHOTOMETRIC DETERMINATION OF TRACES OF TUNGSTEN IN SOLUTION BASED ON ITS INHIBITORY EFFECT ON THE DECOLORIZATION REACTION OF POTASSIUM PERMANGANATE

The present work describes a rapid and sensitive method for the determination of ultramicro amounts of tungsten (VI) based on its inhibitory effect on the oxidation of 4hidroxycoumarine by potassium permanganate in the presence of hydrochloric acid. The sensitivity of the method is 20 ng/cm 3 . The probable relative error is -4.8-13 % for the W(VI) concentration range 250 to 20 ng/cm 3 , respectively. Kinetic equations for the investigated process which determine the order of the reactions regarding to each reaction parameter under certain experimental conditions were proposed, and they allowed quantification of the unknown concentrations W(VI) in solution. The detection and quantification limit of the method are 4.5 and 15.1 ng/cm 3 . The effects of certain foreign ions upon the reaction rate were determined for the assessing the selectivity of the method.


INTRODUCTION
Tungsten is a metal which occurs naturally in the lithosphere, usually in small concentrations and it is also found in oceans and sometimes in other groundwaters in trace amounts (Koutsospyros et al., 2006).This metal is biologically active and it can be found in a number of tungsten-containing enzymes (W-enzymes) (L'vov et al., 2002).
Tungsten usually can be found in the most stable oxidation state VI, but it also occurs in the oxidation states III, IV and V.It has the highest melting/boiling points among elements, a tensile strength at high temperature, resistance to corrosion and a high density and hardness.Because of these unique properties, tungsten has various applications in industry.It is used as a filament in incandescent lamps, as steels in magnetos, springs, contact points, spark plugs, and valves and for other products.(Koutsospyros et al., 2006).
In the past, tungsten was considered as less toxic element due to its insolubility and insufficient knowledge about environmental or toxicological effects (Koutsospyros et al., 2006), (Tajima, 2003).Recently, it has been reported that dissolution of metallic tungsten particles may cause adverse environmental effects such as soil acidification as well as direct and indirect toxic effects in plants, and other living organisms (Strigul a, 2010).Based on these studies, the environmental regulations of tungsten as a "nontoxic" and "environmentally inert" definitely need re-evaluation (Strigul a, 2010), (Strigul b et al., 2010).
The kinetic spectrophotometric methods are known as sensitive methods for element quantification (Muller et al., 1980).By studding red-ox reactions of permanganate with organic compounds (Petković et al., 2007), (Micić et al., 2006), it is noticed that tungsten has performed inhibitory effect on the oxidation of 4-hidroxycoumarine by KMnO 4 in the presence of hydrochloride acid (pH 1.2-2).The reaction was recorded spectrophotometrically, based on measuring the decrease in the absorbance of the solution.The aim of this work was to investigate the possibility of sensitive and selective tungsten determination by measuring the rate of change in absorbance of reaction system (4-hidroxycoumarine by KMnO 4 in the presence of hydrochloride acid and W(VI)) with time at 525 nm.

Apparatus
The investigated reaction rate was folowed by spectrophotometric method at Perkin-Elemer Lambda 15 spectrophotometer, with thermostated cylindrical cells, length 10 cm.All pH values were measured by the radiometer PHM 29Bb pH-meter and a combined glass-calomel electrode, GK2311C.Before the beginning of the reaction, all solutions were termostated in water-bath.The optimal wavelength of 525 nm, as a higher absorbance maximum of KMnO 4 , was applied for kinetic measurements.

Reagents
The KMnO 4 solution (0.02 mol/dm 3 ) was prepared from an ampoule produced by "Merck".The 4hidroxycoumarine 1 .10 -3 mol/dm 3 solution was made by direct weighing of the solid substance.The HCl solution (0.2 mol/dm 3 ) was prepared from 37% concentrated HCl solution.The basic W(VI) solution 1000 ng/cm 3 was prepared by dissolving 1.7942 g Na 2 WO 4 .2H 2 O (analytical grade) in deionized water with conductivity less than 0.5 μS.The exact concentration was determined volumetrically.Analytical grade reagents, deionized water and polyethylene vessels were used throughout.

Procedure
Inhibited reaction was followed spectrophotometrically by monitoring the change in absorbance at 525 nm.The initial concentrations of each of the reactants were varied in turn systematically (Muller et al., 1980), (Alekseev, 1973), while the initial concentrations of the other reactants were being kept constant.The same procedure as in previous works (Petković et al., 2007), (Micić et al., 2006) was used in experiments.The selected volumes of the reactants were put into a 20 cm 3 standard flask, in order: 4-hidroxycoumarine, HCl, inhibitor and water to make up the exact predetermined volume.The flask was kept in the thermostat for 10 min, and then the solution was filled up with KMnO 4 to the mark and vigorously shaken.The cell of the photometer was rinsed well and filled with the solution.The absorbance was measured every 15 s, for 36 min the time has been measured at the moment of KMnO 4 addition.The initial concentrations of the reagent solutions after dilution to 20 cm 3 were: 1 .10 -5 -4 .10 -5 mol/dm 3 4-hidroxycoumarine, 1.5 .10 -5 -4.5 .10 -5 mol/dm 3 KMnO 4 , 1.0 .10 -2 -5.62 .10 -2 mol/dm 3 H 3 O + ions; 10-250 ng/cm 3 W(VI).

RESULTS AND DISCUSSION
The determination of ultramicro amounts of tungsten(VI) is based on its inhibitory effect on the oxidation of 4-hidroxycoumarine by KMnO 4 in the presence of hydrochloric acid in strong acidic solution -pH less then 2. It is interesting that above the value of pH=2, W(VI) influences catalytically on the same reaction.Catalitic effect of W(VI) on mention reaction can used for determination of W(VI) in acetate buffer, but selectivity of the method is much lower than in proposed method with inhibitotory effect.On the basis of the results obtained by investigation of the indicatory reaction kinetics for the determination of W(VI) nanogram amounts, the kinetic method was elaborated for W(VI) trace determination within the range from 20 to 250 ng/cm 3 W(VI).Based on the shape of the curves obtained by mesuring absorbance for different concentartions of W(VI) depending of time (Fig. 1), the differencial time method for processing the kinetic data was selected (Muller et al., 1980).The maximum difference between non-catalytic and catalytic reactions occured at 195 s and this time is selected as optimal and fixed in further experiments.

Effect of pH
The effect of pH on non-inhibited and inhibited reaction rates is shown in Fig. 2. It appears that there is a complex relationship between pH and the reaction rate, i.e. a reaction order is variable with respect to hydrogen-ion concentration for the range of studied concentrations.For further work, pH =1.74 was selected for quantitative applications.

Effect of 4-hydroxyl coumarine concentration
The dependence of ΔAon the 4-hidroxycoumarine concentration is shown in Fig. 3.The maximum difference between the rates of inhibited and noninhibited reactions has been observed for 4hidroxycoumarine concentration of 2.75 .10 -5 mol/dm 3 .The inhibited reaction is the first-order in the full range of examined concentrations, while the basic noninhibited reaction is the first-order for the concentrations of 4-hidroxycoumarine less than 2.75 .10 -5 mol/dm 3 .Fig. 3. Dependence of the reaction rate on 4-hidroxycoumarine concentration.Initial conditions: 3.6 .10 -5 mol/dm 3 KMnO 4 ; pH = 1.74; 200 ng/cm 3 W(VI); temperature 25 ±0.1 C; 1-inhibited reaction; 2-non-inhibited reaction

Effect of potassium permanganate concentration
The dependence of the reaction rates on the concentration of KMnO 4 is shown in Fig. 4. It is obviously from Fig. 3 that the inhibited reaction is the first-order in the all range of presented concentrations, while the basic non-inhibited reaction is the first-order for the concentrations less than 3.6 .10 -5 mol/dm 3 .
where k 0 is the constant which is proportional to the constant rate of non-inhibited reaction.
Accuracy and precision are presented in Table 1.The probable relative error ranges from 13 to -4.8 % for W(VI) concentration from 20 to 250 ng/cm 3

Interference study
To access the selectivity of the method, influences of some foreign ions on the inhibited reaction rates were studied, at the constant W(VI) concentration of 200 ng/cm 3 .The method has relative good selectivity .The results are presented in Table 2.

Chemistry
Table 2. Influence of interfering ions for determination of W(VI) by proposed method * W(VI) concentration of 200 ng/cm 3 .

CONCLUSION
This work describes kinetic method for the determination of trace amounts of tungsten (VI), which is based on the inhibitory effect on the oxidation of 4hidroxycoumarine by KMnO 4 at pH=1.74.The optimal conditions for this kinetic spectrophotometric method are found and equation which allowed determination of unknown concentration of tungsten by measuring the absorbance after 195 s from the beginning of the reaction, at 525 nm, is evaluated.The interference study suggests a relative good selectivity, indicating possibility of successful determination in complex matrix of a real sample.Statistic calculations based on obtained results for five times repeated determinations under the same conditions show good accuracy and precision.

Table 1 .
Accuracy and precision of tungsten W(VI) determination