Indian Journal of ChemistryVol. 26A, June 1987, pp. 536-537

Spectrophotometric Determination ofThorium (IV) Using Semi-xylenolOrange & Calculation of Reaction

Kinetic Parameters

M A H HAFEZ* &M E M EMAMFaculty of Science, Mansoura University, Mansoura, Egypt

Received 7 April 1986; revised 24 July 1986; accepted28 October 1986

Semi-xylenol orange (SXO) is proposed as a spectrophotomet-ric reagent for the determination of sub-micro amounts of thor-ium (IV). It forms red coloured 1:1 complex with thorium (IV) ina medium of urotropine buffer (pH = 5.8). The formation ofTh(IV)-SXO complex follows first order kinetics.

Sato et al,' reported a spectrophotometric methodfor the determination of zirconium (IV) using xyle-nol orange (XO) and semi-xylenol orange (SXO) asthe reagents I. Pijpers and coworkers/ described achelatometric titration method for determination ofsub-micro amounts of thorium (IV) with photomet-ric end-point detection at pH 2·-5 using perchloratebuffer. In the present study the efficiency of SXO asa spectrophotometric reagent for the determinationof sub-micro amount of Th(IV) has been investigat-ed. The interference due to cations and anions andorganic acids has also-been studied. The kinetic par-ameters for the complex formation have been calcu-lated and the best conditions for the maximum co-lour development are suggested.

All the salts used were of BDH grade. Stock solu-tion (0.01 mol dm - 3) ofTh(IV) was prepared by dis-solving Th(NO,)4' 4H20 in doubly distilled watercontaining a few drops of cone nitric acid (BDH) andstandardized by photometric titration". Urotropine-nitric acid buffers [ionic strength = 0.2 to 1.0 (KN03)

and pH = 5.5-5.8] were prepared as described in li-terature3

A Unicam SP IHOO spectrophotometer (England)provided with an ultra-thermostat (accuracy± 0.1 "C) was used for absorbance measurements. Adigital Tacussel electronic pH meter (France) with aglass-Sf.E assembly was used.

Kinetic studyA series of five solutions having[Th(IV)] =

2 x 10 - 4 mol dm -:1 and different [SXO] =4 x 10 - ~to 1 x 10 4 mol drn -3 were preapred. The ionicstrength (1= 0.2), pH (= 5.H) and the temperature(= 2H ± 0.1 "C) were kept constant during kinetic

S1fi

studies. The absorbance (x) of the reaction mixtureswas measured as a function of time (t); t = 0 at themoment of mixing of the reactants. It was found thatabsorbance (x) increased with time and reached amaximum constant value (a). The order ofthe reac-tion was determined by the graphical method byplotting log (a-x) against time (t). A linear plot indi-cated that the reaction followed first order kinetics.In the case of a curved plot between (a- x) versus time(t ) the order of the reaction was calculated using Eq.(1) (differential method)

log (a-XI Idt) - log (a-XI Idt)n= () () ... (1)log a-xI - log a-xl

where, (a-x/dt) and (a-x/dt) are the slopes of anytwo tangents on the curve at the corresponding twocoordinates (a-xI) and (a-x2) respectively. At Th(IV):SXO ratios of 1:2,1:3,1:4 and 1:5, both the graphi-cal and the differential methods indicate that thereaction follows first order kinetics. The reaction be-tween, SXO and Th(IV) may be expressed in ac-cordance with Eq, (2)SXO+Th-Th-SXO ... (2)

The reaction rate is given by Eq. (3)Rate = klSXO] ° [Th] ... (3)where (k) is the specific rate constant of the reaction.

15,---------------------,

10

•.uCo.0

s.0-c

6

>.. = 500nm

>.. = 530>.. = 510 '

2 41M] x lO-~mol dm-3

Fig .. I - Spect ruphotorm-rr ir calibration curves for Th( IV) us-Ill/,! SXO 14 x 10 'mol dm ') indicator in urotropine buffer at

pH S.X, /= 0.2 (KNO,!

Table 1 - Spectrophotometric Determination of Th(IV)in Three Different Series in the Absence of Other

Cations(en 2x 10-5,1 x 10-5, 4x 10-5 mol dm-3 respectively, Csxo4 x 10-5 mol dm - '; urotropine buffer pH = 5.8, /= 1.0 (KN03);

temp.=25°C±O.1; selected wavelength=510 nm; pathlength = 10 mm)

Th(IV) (~gl25 ml) Error

Taken116.02 (4.64 pprn)58.oI (2.32 ppm)23.20 (0.93 ppm)

Found117.7656.8524.30

1.502.504.74

From Eq. (3) it is clear that the reaction kinetic is firstorder with respect to [metal] and zero one with re-spect to [ligand]. This was proved by several experi-mental data in which the reaction order was inde-pendent of the variation of [ligand] but depended on-ly on the [metal].

Energy of activation and the frequency factorwere determined from the Arrhenius plot (tempera-ture range: 298-323K) and the values are: E,= 10.62kcal mol" ' andA=4.61 x 107

•

Spectrophotometric determination of Th( IV):Stability and stoichiometry

The stoichiometry of the complex, as determinedby standard methods (Jobs, mol ratio and st~a~ghtline) was found to be 1:1. The complex exhibitedmaximum absorption at 510 nm. The stoichiometrywas confirmed by the appearance of isobestic pointat 467 nm in the mol ratio method at pH = 5.8 and1=0.2 using different ratios ofTh(IV): SXO.

Spectrophotometric determination of Th(IV) ionsAs mentioned before (SXO) forms mainly a red

1:1 chelate with Th(IV). Figure 1 shows the spectro-

NOTES

photometric calibration curves at different wave-lengths and 1= 0.2, The plots at three wavelengthsare linear, but the molar absorptivity at each wave-length is different. The molar absorptivities are3.5 x 104, 2,9 X 104 and 2.4 x 104 dm ' mol-1 at 510,530 and 500 nm respectively. Thorium (IV) can bedetermined easily in the concentration range of0-2.8 x 10-5 mol dm-3 (0-6.5 ppm) where Beer'slaw is obeyed in the absence of other interfering ions(see Table 1).

Interference from other ionsThe influence of different cations was investigated

in the determination of 1.2 x 10- 5 mol dm - 3 Th(IV)in 25 ml solution at pH 5.8 ± 0.1 and Amax 510 nm. Itwas found that Ca(II), Sr(II), Mg(II), Mn(II) andBi(III) interfered when present in 100-fold excessover Th(IV). Pb(II), Cu(II), Ni(II), Co(II), Zn(II),Cd(II) and heavy lanthanides interfered seriouslyeven when their concentrations were equal to that ofTh(IV). Iron(III), Cr(lII) and Al(III) interfered slight-ly at concentration equal to that of Th(IV). Anionsand organic acids like HPO~-, CDTA, EDTA, citricacid when present in 100-fold excess over that ofTh(IV), were able to completely mask the metal.Cl-, 1-, NO) and F- did not interfere even whenpresent in 100-fold excess. However, at this concen-tration the interference due to SO~- was slight. Tar-taric and oxalic acids interfered seriously.

References1 Sato H, Yokoyama Y & Momoki K, Anal Chim Acta, 99

(1978) 167.2 Pijpers C J C, Denop-Weever L G, den Boef G & Van der

Linden W E, Microchim Acta, (1976) 667.3 Britton H T S, Hydrogen ions, Vol. 1 (Chapman and Hall, Lon-

don) 1955, pp 365.4 Laidler K J, Chemical kinetics (McGraw-Hili, London) 1965,

pp 211.

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