1994, VOL. 10. Spectrophotometric. Determination of Trace Amounts of Iron by Its Catalytic Effect on the Chlorpromazine-Hydrogen. Peroxide Reaction. Takashi.
Spectrophotometric Determination of Trace Amounts by Its Catalytic Effect on the Chlorpromazine-Hydrogen Peroxide Reaction
Department of Chemistry, Faculty of Science, Kagoshima University, Kagoshima 890, Japan
A catalytic photometric method for the determination of trace amounts of iron is proposed. In the presence of iron (Fe" and Fe11), chlorpromazine is oxidized by hydrogen peroxide in a hydrochloric acid solution to form a red intermediate, which is further oxidized to a colorless compound. The reaction is followed by measuring an increase in the absorbance at 525 nm; the absorbance/time curve increased with the increase in the reaction time, and reaches a maximum value. Since the slope of the linear range of absorbance/time curves increases with the increase in the iron concentration, tan a [=d(abs.)/d(sec.)] is used as a parameter for the iron determination. Under the optimum experimental conditions (0.011 M chlorpromazine, 0.43 M hydrochloric acid, 0.12 M hydrogen peroxide, 30° C), iron can be determined in the range 5 - 200 µg 1-'. The relative standard deviations are 0.8, 2.3 and 6.6% for 80, 40 and 10 µg 1-', respectively. Under the reaction conditions, WV',52032-,103-, I-, NO2- and BrO3- showed interference at concentrations of 1 mg l-'. These ions showed no interference at concentrations one order lower. The procedure has been applied to the determination of iron in tap and natural fresh water samples. Keywords
A number of catalytic methods based on different indicator reactions have been proposed for iron determination.1-' Some of them were applied to the determination of iron in water samples.3'6 However, the detection limit of these methods was in the sub-mg 1-1 order, and the determination of iron in water samples at µg 1-1levels was difficult. Kawashima et al. reported the N (p-methoxyphenyl)-N',N'-dimethyl p-phenylenediamine(MDP)-H2O2 system, which allowed the determination of µg L1 level of iron.' But the determinable range of the method, 2 --14 µg 1-1 iron, was not wide enough. In an acidic medium, chlorpromazine hydrochloride (CPH) can be oxidized to a red intermediate by the action of several oxidizing agents. As the authors have reported, when sulfuric acid and hydrogen peroxide were used, this reaction was catalyzed by iodide.8 In the present study, we found that iron catalyzed the color formation reaction and that the catalytic effect of iodide decreased in hydrochloric acid solution. Thus, a kinetic spectrophotometric method for the determination of iron based on its catalytic effect on the CPH-hydrogen peroxide reaction was developed. The resulting method is highly sensitive and reproducible: as low as 5 µg 1-1of iron can be determined with reasonable reproducibility. This method has been successfully applied to the determination of iron in tap and natural water samples.
Apparatus and reagents A Japan Spectroscopic Co. Ubest-35 spectrophotometer was used with a thermostated cell holder (30±0.1°C) coupled with a Japan spectroscopic Co. PTL3965 plotter. The temperature was controlled with a Shibata Science Instrument Co. control unit (CU-85) circulating thermostat bath. For the reaction, l cm glass cells were used. The reaction was initiated by the injection of a hydrogen peroxide solution from a Gilson Pipetman (Model P-200). For mixing, a remote-controlled magnetic Acrobat stirrer (MS Instrument, Osaka, Japan) was installed at the side of cell holder in the spectrophotometer. Pure water was prepared by purifying distilled water with a Millipore Milli-Q SP system just before use. Reagent-grade chemicals were used throughout. A CPH solution (0.15 M) was prepared by dissolving 5.328 g of chlorpromazine hydrochloride in water and diluting the mixture to 100 ml with more water. A hydrogen peroxide solution (1.2 M) was prepared by diluting 6.0 ml of a commercial 31% solution to 50 ml with water. The concentration of hydrogen peroxide was checked by permanganate titration. An iron(II) stock solution (1000 mg 1-1)was prepared by dissolving 1.404 g of ammonium iron(II) sulfate in 200 ml of 0.1 M hydrochloric acid. Working solutions
were prepared by diluting this solution with water. An iron(III) stock solution (1000 mg 1-1)was prepared by dissolving 1.727 g of ammonium iron(III) sulfate in 200 ml of 0.1 M hydrochloric acid. Working solutions were prepared by diluting this solution with water. A hydrochloric acid (5.7 M) solution was prepared by diluting concentrated hydrochloric acid with water. Recommended procedure To 10.0 ml of sample solution in a glass-stoppered tube, 1.0 ml of 5.7 M hydrochloric acid and 1.0 ml of 0.15 M CPH solution were added and this combination was thoroughly mixed. It was then kept at 30° C in a water bath for 15 min. Then an 1.8 ml aliquot was 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 1.2 M hydrogen peroxide solution (30° C). The increase in absorbance of the red intermediate at 525 nm was recorded against a purewater reference and the tan a [=d(abs.)/d(sec.)] of the linear range of reaction rate curves was used as a parameter for iron determination.
Oxidation of CPH by hydrogen peroxide and the accelerating effect of iron on the color formation CPH is oxidized by hydrogen peroxide in a hydrochloric acid solution to form a red free radical, which is further oxidized to a colorless compound. The color formation reaction is accelerated by tiny amounts of iron. The reaction can be followed by measuring the increase in absorbance of the red intermediate at 525 nm. The absorbance increases with the increase in the reaction time, and reaches a maximum value. The required time for the absorbance to reach the maximum after adding the hydrogen peroxide solution decreased with increas-
ing the iron concentration. After the absorbance maintained the maximum value for a few minutes, the value decreases rather slowly. Although the maximum value increased with increasing iron concentration, it is not adequate as a parameter for iron determination, because of the long reaction time and high blank value. Thirty seconds after the reaction start, the absorbance/ time curves became linear and the lineality was kept for a few minutes. The slopes of the linear range increased with the increase in the iron concentration. The difference of the slopes between catalyzed and uncatalyzed reaction was larger, because the color formation reaction on the uncatalyzed reaction progressed slowly. The slope, tan a [=d(abs.)/d(sec.)], was used as a parameter for the iron determination. Effects of the reaction variables The influence of temperature on the tan a was studied in the 20 - 45° C range under conditions explained in the recommended procedure. Although higher sensitivity was obtained at higher temperature, the reproducibility of the value was poor in the higher temperature range, as the reaction rate became higher. A temperature of 30° C was chosen. Figure 1(a) shows the effect of CPH concentration on the tan a. The value for the blank increased with increasing CPH concentration over the investigated concentration range. The sensitivity of this method increased with the increase in the CPH concentration up to 0.01 M and, over the higher concentration range, the sensitivity remained constant. A CPH concentration of 0.011 M was chosen, since it gave the highest sensitivity and lower blank. The influence of hydrogen peroxide concentration was examined in the range of 0.04 0.20 M. As can be seen in Fig. 1(b), the tan a of the uncatalyzed reaction remained constant in this concentration range. The maximum tan a on the catalyzed reaction was obtained for concentrations higher than 0.10 M. A hydrogen peroxide concentration of 0.12 M
Fig. 1 Effect of the experimental variables [blank (/), 100 µg 1-1iron(II) (A), 200 µg 1-1iron(II) (•)]: (a) CPH concentration; (b) hydrogen peroxide concentration; (c) hydrochloric acid concentration. Conditions as in recommended procedure (0.011 M CPH, 0.12 M hydrogen peroxide, 0.43 M hydrochloric acid, 30° C), except for the variable indicated on the abscissa.
was chosen. The reaction rate of both catalyzed and uncatalyzed reactions increased with increasing hydrochloric acid concentration. However, the difference in the tan a in the presence of iron and in its absence did not change significantly in the higher hydrochloric acid concentration range (Fig. 1(c)). A hydrochloric acid concentration of 0.43 M was selected by considering sensitivity and lower blank values as criteria.
Kinetic study of the color formation reaction The authors have reported that, in the iodide catalyzed CPH-hydrogen peroxide reaction, CPH was not only consumed by the color formation reaction, but also consumed simultaneously by a reaction without any coloration competing with the former ones The same behavior was observed in this iron-catalyzed reaction. Thus, a kinetic investigation of the color formation reaction was carried out by the initial-rate method, in which the initial slopes of the reaction curves (d (abs.) / dt=Ro) are manually determined and then used as a measure of the initial reaction rate. Under various concentrations of each reactant (CPH, hydrogen peroxide and hydrochloric acid), Ro was plotted against a series of standard solutions of iron(II). The plot was linear in the investigated iron(II) concentration range (0 - 2.7X106 M in final reaction solution). Throughout the experiment the ionic strength, 0.43 M, was maintained constant by addition of sodium chloride. An apparent rate constant, kapp,which is the slope of the Ro vs. iron(II) plot, was plotted against each reactant concentration. Figure 2 shows that in the lower concentration range of these reactants, kapp increased with increasing reactant concentration. However, in the higher concentration range, kappbecame independent of these reactants' concentration. At higher concentration of reactants including the optimum conditions for determination of iron, the catalyzed reaction was zero order with respect to each reactant and resulted in the
Fig. 3 Calibration graphs: iron(II) (~) and iron(III) (A) by recommended procedure.
following rate law: Ro = kappLFeHI+ a. Here "a" is the rate of an uncatalyzed reaction. From an Arrhenius plot in the 20 - 45° C temperature range, the activation energy for a catalyzed reaction was calculated to be 61.8 kJ moll. Calibration graphs and reproducibility A series of standard solutions of iron was treated as in the recommended procedure (Fig. 3). The relative standard deviations for 10 replicate determinations of 80, 40, 10 sg 1-1 of iron(II) were 0.8, 2.3 and 6.6%, respectively; it was 17.7% even at a concentration as low as 5 µg 1-1,which was the lowest concentration that could
Fig. 2 Effect of the experimental variables for apparent rate constant, kapp:(a) CPH concentration; (b) hydrogen peroxide concentration; (c) hydrochloric acid concentration. Conditions as in recommended procedure, except for the variable indicated on the abscissa and the maintenance of ionic strength (0.43 M) by addition of sodium chloride.
Table 1 Effect of interfering ions on the determination of 90 µg 1-1of iron(II)
showed no interference, at least at the concentrations (mg 1-1)shown in parentheses: Na+ and Cl- (2000); Cat, K+, SO42- (1000); C032- (500); Mgt, Mn2+ (250); Pb2+, Lit, Cd2+, Bat, Ce3+, Nit, Sr2+, Asv, As", NH4+, Br, CH3000, 0103 , C104 (100); Zn2+,Cot, Sn4+(50); Ce4+, Sn2+, Cr", SbI", PO43- (10); Cue, Ag+, MovI, V03- (5). The interfering ions are listed in Table 1. Ions which are known to participate in a redox reaction as either oxidizing or reducing agents cause serious interference. These ions showed no interference at concentrations one order lower than those indicated in Table 1. Determination of iron in tap and natural fresh water samples In order to test the reliability of the present method, we applied it to the determination of iron in tap and natural fresh water samples. The determinations were made by using samples diluted at different times. The method was also checked by adding a known amount of iron to the samples. The results are shown in Table 2. The values corrected for dilution showed good agreement, and good recoveries of added iron were obtained, ranging from 97 to 104% (mean 100%).
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be determined. Effect offoreign ions The effect of various ions on 90 µg 1-1 iron(II) was examined.
the determination The following
(Received May 9, 1994) (Accepted July 4, 1994)