Iron Catalyzed Chlorpromazine-Hydrogen

Transcription

1 ANALYTICAL SCIENCES APRIL 1996, VOL A Kinetic Study of the Peroxide Reaction and Iron Catalyzed Chlorpromazine-Hydrogen Its Analytical Implications TakaShi TOMIYASU, Hayao SAKAMOTO and Norinobu YONEHARA Department of Chemistry, Faculty of Science, Kagoshima University, Korimoto, Kagoshima 890, Japan The iron(ii) catalyzed oxidation reaction of chlorpromazine with hydrogen peroxide in hydrochloric acid solution was studied kinetically by an initial-rate method. All reactions were run at constant ionic strength 0.44 mol 1-1 and at 30 C and followed by measuring the absorbance at 525 nm of the red free radical formed. According to results of a kinetic study of the iron(ii) catalyzed reaction, a mechanism is proposed which leads to the following rate equation: Rocat= [Fe(II)]o[CP][H+][H2O2]/(2.OX 10-2[CP][H+]+2.2X 10-4[H2O2]+1.8X 10-3[H2O2][H+]+2.0[CP][H+][H2O2]). The optimal condition for iron determination by initial rate procedure was predicted using this rate equation and analytical implications of the mechanistic study are discussed. Keywords Catalyzed reaction mechanism, spectrophotometry, iron, chlorpromazine, hydrogen peroxide Because of the growing need for highly sensitive analytical methods, the catalytic kinetic method has become an attractive procedure, as the trace determination can be achieved without having to use expensive or special equipment. Many indicator reactions for the catalytic kinetic method have been reported, but most of the studies have been mainly directed at the development of better analytical procedures and their practical applications and not much work has been done on a complete understanding of the reaction based on mechanistic studies. However, the interdependence of reaction variables in kinetic methods is commonly rather complicated, so that a large number of experimental runs is required in order to optimize in detail analytical conditions; this experimental work load may be a disadvantage in developing analytical procedures. The rate equation, which accurately describes the quantitative dependence of the reaction rate upon the catalyst and all reactants, permits an accurate prediction of the behavior of the reaction over a wide range of reaction conditions. Hence mechanistic studies are valuable in optimizing the experimental conditions and in extending the analytical utility of the reactions. We have reported some catalytic methods based on the oxidation of chlorpromazine (2-chloro-l0-(3-dimethylaminopropyl)phenothiazine: C17H19C1N2S) by hydrogen peroxide as an indicator reaction in acidic media. The oxidation of chlorpromazine (CP) by hydrogen peroxide proceeds by two independent and parallel reactions, one of which proceeds through the formation of the red free radical, which is slowly oxidized to a colorless sulfoxide by an excess of the oxidant, and the other is the reaction directly to form the colorless product, which is catalyzed by traces of tungsten(vi). The red color formation is catalyzed by traces of iodide and iron. These catalytic effects were utilized for the trace determinations of these ions. 14 Moreover, the mechanisms of the tungsten(vi)- catalyzed4 and the iodide-catalyzed5 reactions and of their uncatalyzed reactions5 were also studied and their analytical implications were discussed. In this work, kinetics of the iron-catalyzed reaction have been studied for giving a better understanding of the reaction process and elucidating the mechanism. The proposed mechanism, which satisfies the experimental observations, was utilized to derive a rate equation which describes in detail the quantitative behavior of the reaction throughout the range of conditions studied and the utility of this rate equation was discussed for selecting the optimal condition for the trace determination of iron by the initial-rate procedure. Experimental Apparatus and reagents A Japan Spectroscopic Co. (JASCO) Ubest-35 Spectrophotometer was used with a thermostated cell holder (30±0.1 C) coupled with a JASCO PTL3965 plotter. The temperature was controlled with a Shibata Science Instrument Co. control unit (CU-85) circulating thermostat bath. For the measurement of absorbance,) cm glass cells were used. The reaction was initiated by the injection of a hydrogen peroxide solution using a Gilson Pipetman (Model P-200). For mixing, a remote-controlled magnetic Acrobat stirrer (MS Instrument, Osaka, Japan) was installed at the side of the cell holder in the spectrophotometer. Pure water was prepared by purifying distilled water

2 244 ANALYTICAL SCIENCES APRIL 1996, VOL. 12 with a Millipore Milli-Q SP system just before use. Reagent-grade chemicals were used throughout. A CP solution (0.15 mol 1-') was prepared by dissolving g of chlorpromazine hydrochloride in water and diluting to 100 ml with water. A hydrogen peroxide solution was prepared by diluting a commercial 31% solution with water. The concentration of hydrogen peroxide was checked by permanganate titration. An iron(ii) stock solution (1000 mg L1) was prepared by dissolving g of ammonium iron(ii) sulfate in 200 ml of 0.1 mol 1-' hydrochloric acid. Working solutions were prepared by diluting this solution with water. A hydrochloric acid (6.1 mol 1-1) solution was prepared by distilling 1:1 hydrochloric acid solution. Procedure To a standard iron(ii) solution in a glass-stoppered tube, 0.15 mol 1-' CP solution and 6.1 mol 1-' hydrochloric acid solution were added. The final volume were adjusted to 12 ml with water, where the ionic strength was maintained at 0.49 mol 1-' by the use of sodium chloride. This solution was kept at 30 C in a water bath; a 1.8 ml aliquot was then taken into a 1 cm glass cell. The cell was placed in the holder at 30 C and the contained solution was magnetically stirred. The reaction was initiated by the injection of 0.20 ml of a hydrogen peroxide solution (30 C); where the ionic strength was 0.44 mol 1-1. The increase in absorbance of the red free radical at 525 nm was recorded against a pure-water reference. Fig. 1 Absorbance/time curves for the CP/hydrogen peroxide reaction. Concentrations of iron (X106 mol 1-'): (I) 0, (II) 0.80, (III) Conditions: mol 1-' CP, 0.43 mol 1-' hydrochloric acid and mol 1-' hydrogen peroxide, 30 C, ionic strength 0.44 moll'. Ro: d(mol 1-')/ A (s) at reaction time zero was used as a measure of the initial reaction rate. Results and Discussion Kinetics of the iron-catalyzed red free radical formation reaction between chlorpromazine and hydrogen peroxide Chlorpromazine is oxidized by hydrogen peroxide in an acidic media to form a red free radical, which is further oxidized to acolorless sulfoxide. Since CP was not only consumed by the red free radical formation reaction, but also consumed simultaneously by a reaction without any coloration competing with the former one', the kinetic investigation of the color formation reaction was carried out by the initial-rate method. The initial rate, Ro[=d(mol 1-1)/d(s)], was obtained by dividing the rate evaluated from the absorbance/time curves[d(abs.)/ A(s)] (Fig.l) by 3.0X1041 mol-' cm' of the molar absorption coefficient at 525 nm of the red compound.5 All concentrations given in the figures are the initial analytical concentrations in the reaction mixture at the initiation of reaction. In order to determine the dependence of the Ro upon iron concentration, a series of experiments were performed in which the iron concentration was varied while CP, hydrogen ion and hydrogen peroxide concentrations were held constant. The plots of Ro versus the total analytical concentration of iron([fe(ii)]o) were linear in the investigated range of 0-1.3X 10-6 mol 1-' iron (Fig. 2). It was established Fig. 2 Dependence of initial reaction rate Ro upon iron(ii) concentration. Concentrations of hydrogen peroxide (X 10-' mol 1-'): (1) 0.12, (~) 0.24, (0) 0.36, (p)1.2. Other conditions as in Fig. 1. that the iron catalyzed reaction was first order with respect to concentration of iron: Ro = kapp [Fe(II)]o + a (1) where kapp is the apparent rate constant and "a" is the intercept on ordinate which corresponds to the rate of an uncatalyzed reaction. The variation in kapp with hydrogen peroxide, hydrogen ion and CP concentrations is shown in Fig. 3 ( ). As can be seen in Fig. 4(a), a straight line obtained by plotting 1 /kapp versus 1 /[H2O2]

3 ANALYTICAL SCIENCES APRIL 1996, VOL Fig. 3 Effect of the experimental variables for apparent rate constant, kapp[(s) experimental values and (0) values calculated by inserting the experimental conditions into Eq. (11)']: (a) hydrogen peroxide concentration; (b) hydrogen ion concentration; (c) CP concentration. Conditions as in Fig. 1, except for the variable indicated on abscissa. Fig. 4 Dependence of apparent rate constant kapp upon hydrogen peroxide (a) and the dependences of its intercept on ordinate upon hydrogen ion (b) and CP (c). Conditions: as in Fig. 1, except for the variable indicated on abscissa. shows that the relation is given by kapp-[h2o2]/ (b+a[h2o2]), where "b" is the slope of the line and "a "is the intercept on ordinate. The dependences of "b" and "a" upon hydrogen ion concentration were examined under constant CP concentration. The slope, "b", did not change significantly in the investigated hydrogen ion concentration range of mol 1-1. The plots of "a" versus 1/[H+] were linear (Fig. 4(b)) and the relationship between "a" and hydrogen ion concentration was given by ac/[h]+d, where "c" is the slope of the line and "d" is the intercept on ordinate. Thus the initial rate of catalyzed reaction, Rocat - RO - a, is given by: Rocat = [H+][H2O2][Fe(II)]o/(b[H+] + c[h202] + d[h+][h2o2]) (2) The dependences of "b" and "a" upon CP concentration were also examined under constant hydrogen ion concentration, "b" did not change significantly over the investigated CP concentration range of mol l1. The linearity of plots of "a" versus 1 /[CP], as shown in Fig. 4(c), leads to the following equation: Rocat = [CP][H2O2][Fe(II)]o/(b[CP] + e[h202] + f [CP][H2O2]) (3)

4 246 ANALYTICAL SCIENCES APRIL 1996, VOL. 12 where "e" is the slope of the line and "f" is the intercept on ordinate. Mechanism of the iron-catalyzed red free radical formation reaction between chlorpromazine and hydrogen peroxide The kinetic behaviors of CP, hydrogen ion and hydrogen peroxide shown by Eqs. (2) and (3) suggest the formation of a complex of iron with these reactants. A mechanism which is consistent with the rate measurements described above is I CP + H+ ~ (CP-H)+ (rapid, Kl) (4) Fe(II) + (CP-H)+ (Fe-CP-H)3+ (rapid, K2) (5) (Fe-CP-H)3+ + H2O2 (Fe-CP-H-H2O2)3+ (rapid, K3) (6) II Fe(II) + H2O2 (Fe-H2O2)2+ (rapid, K4) (7) followed H+ + (Fe-H2O2)2+ --~ (Fe-H2O2-H)3+ (Fe-H2O2-H)3+ + CP by (rapid, K5) (8) :! (Fe-CP-H-H2O2)3+ (rapid, K6) (9) (Fe-CP-H-H2O2)3+ -> P (rate determining; k) (10) where P may be an intermediate converting rapidly to the red product and Fe(II) through several steps. The formation of the (Fe-CP-H-H2O2)3+ proceeds in two parallel pathways, shown by Eqs. (4), (5), (6) and Eqs. (7), (8), (9). The rate equation implied by this sequence, d[product]/dt=k[(fe-cp-h-h202)3] combined with the expression for total analytical concentration of iron; from leads [Fe(II)]o = [Fe(II)] + [(Fe-CP-H)3+] + [(Fe-H2O2)2+] which + [(Fe-H2O2-H)3+] + [(Fe-CP-H-H2O2)3+] [Fe(II)] = [Fe(II)]o/{1 + K1K2[CP][H+] to + K4[H202] + K4K5[H2O2][H+] + (K1K2K3+ K4K5K6)[CP][H+][H2O2]} Rocat = [Fe(II)]o[CP][H+][H2O2]/(P+ Q[CP][H+] + S[H202] + T[H2O2][H+] + U[CP][H+][H2O2]) (11) where F-1 /k(kik2k3+k4k5k6), Q=K1K2/k(K1K2K3+ K4K5K6), SK4/k(K1K2K3+K4K5K6), T=K4K5/k(K1K2K3+ K4K5K6), U=1/k. If we assume that under experimental conditions ICQ [CP][H+]+S[H2O2] +T[H2O2] [H+]+ U[CP] [H+][H2O2], Eq. (1l) becomes Rocat _ [Fe(II)]o[CP][H+][H2O2]/(Q[CP][H+] + S[H202] + T[H2O2][H+] + U[CP][H+][H2O2]) (11)' At constant [CP], Eq. (11)' is of the same form as the experimentally observed Eq. (2) with b= Q c = S/[CP] (12) (13) d= T/[CP] + U (14) At constant [H+], Eq. (11)' is of the same form as the experimentally observed Eq. (3) with e = S/[H+] + T f=u (15) (16) Values for the Q, S, T and U calculated from the experimental values of "b, c, d, e" and "f" are 2.0X102 mol 1-1 s, 2.2X104 mo121-2 s, 1.8X103 mol 1-1 s and 2.0 s, respectively. In order to check the validity of the proposed reaction mechanism, the kapp values calculated by inserting these values together with each experimental reaction condition into Eq. (11)' were compared with the experimental values. As shown in Fig. 3(a), (b) and (c), the plots of calculated kapp values marked (0) fell just on the plots of the experimental values marked (s). Rocat for 1.34X10.6 mol L1 and 6.7X10-' mol 1-1 iron(ii) were also calculated according to Eq. (11)'. The calculated Rocat values are shown as solid lines in Fig. 5(a) - (c). The plots of the experimental Rocat values marked (0) and (0) fell just on the lines. Since the calculated kapp and Rocat values agreed closely with the experimental values over the investigated reactants concentration range, the proposed mechanism is considered to be favorable. The estimation of the optimum condition for the iron determination by the initial rate method The initial rate (Ro) is one of the parameters for the kinetic determination and has its own advantages.6 The dependence of Ro upon iron concentration was given by Ro=kapp[Fe(II)]o+a (Eq. (1)) with kapp = [CP][Hi[H2O2]/(2.0 X 10-2[CP][H+] X 10-4[H2O2] X 10-3[H2O2][H+] + 2.0[CP][H+][H2O2]) (at 30 C, I=0.44 mol 1-1) (17)

5 ANALYTICAL SCIENCES APRIL 1996, VOL Fig. 5 Effect of the experimental variables for initial reaction rate, Rocat: (a) hydrogen peroxide concentration; (b) CP concentration; (c) hydrogen ion concentration. The plots marked A and 0 are the experimental Rocat[6.7X 10-' mol 1-' iron(ii) (A),1.34X 10-6 mol 1-' iron(ii)(o)]. Conditions as in Fig. 1, except for the variable indicated on abscissa. Solid lines show the calculated initial reaction rate according to Eq. (11)' for (A) 6.7X10-' mol l-' iron(ii), (B) 1.34X10-6 mol 1-' iron(il) catalyzed reaction. Fig. 6 Variation in the calculated kapp value according to Eq. (17) by changing the concentrations of two reagents indicated on abscissas for a given concentration of another one. The concentration of CP was kept at mol 1-' (a), hydrogen ion at 0.44 mol 1-' (b) and hydrogen peroxide at 0.12 mol 1-' (c), respectively. where kapp is essentially constant under a given condition, because the [CP], [H2O2] and [H+] remain nearly constant at the early stage of the reaction. Hence, it is clear from Eq. (1) that use of the initial rate as a parameter gives the linear calibration graph with the slope of the kapp value and thus the larger the kapp value, the higher the sensitivity. This enables the sensitivity of the catalytic procedure to be optimized for the three variables CP, hydrogen peroxide and hydrogen ion concentrations on the basis of the rate equation derived. Figure 6 represents the slanting surfaces formed with the calculated plots using Eq. (17) of the dependence of kapp upon the concentrations of two reagents indicated on the abscissas for a given concentration of another one and contour lines are drawn at intervals of 0.02 s-1 of kapp. In each surface, (a), (b) and (c) in Fig. 6, it is observed that its slope decreases gradually to become nearly horizontal as both the reagent concentrations increase. Sets of conditions falling within this horizontal region are of the optimal conditions, because of its higher sensitivity and less influence of the reagent concentrations on the sensitivity. Thus, the range of conditions which should be investigated for the optimization can be narrowed down by this prediction. Although from this result the higher reagent concentrations are apparently suitable for realizing the higher sensitivity, the condition should be

6 248 ANALYTICAL SCIENCES APRIL 1996, VOL. 12 selected by considering the blank value which increases also with an increase of the reagent concentrations; too high reagent concentrations are not appropriate. As the reaction proceeds, the rate of this iron-catalyzed reaction does not fall off normally and during the early stage of the reaction a linear range on the reaction rate curve was observed under conditions of higher reagent concentrations. In the previous work3, the slope of this linear portion was conveniently used as the parameter for the iron-catalyzed determination. Since this linear portion is still in the early stage, its slope may be roughly comparable to the initial rates. The dependence of the slope on the reaction variables also showed the similar trends to these of the calculated results discussed above. The mechanistic study is useful in optimizing an analytical condition of catalytic method, especially when its reaction has a complicated kinetic dependence on reaction variables. References 1. T. Tomiyasu, H. Sakamoto and N. Yonehara, Anal. Sci., 8, 293 (1992). 2. T. Tomiyasu, H. Sakamoto and N. Yonehara, Anal. Sci., 10, 293 (1994). 3. T. Tomiyasu, H. Sakamoto and N. Yonehara, Anal. Sci., 10, 761 (1994). 4. T. Tomiyasu, Anal. Chim. Acta, 312, 179 (1995). 5. T. Tomiyasu, H. Sakamoto and N. Yonehara, Anal. Chim. Acta, in press. 6. H. A. Mottola, "Kinetic Aspects of Analytical Chemistry", pp , John Wiley & Sons, New York, (Received November 16, 1995) (Accepted January 12, 1996)