Simultaneous Potentiometric Determination of Peracetic Acid and

10305064) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. M.I.A. thanks the .... 1959, 31, 1914. [ACS Full Text ACS Fu...
1 downloads 0 Views 87KB Size
Download PDF
Anal. Chem. 2003, 75, 2688-2693

Simultaneous Potentiometric Determination of Peracetic Acid and Hydrogen Peroxide Mohamed Ismail Awad, Tadato Oritani, and Takeo Ohsaka*

Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan

A rapid and highly selective potentiometric method for the simultaneous analysis of peracetic acid (PAA) and hydrogen peroxide (H2O2) has been proposed, for the first time, using glassy carbon (GC) as an indicator electrode and I2/I- potential buffer. On the basis of the large difference in the reaction rates of PAA and H2O2 with I-, which was confirmed using stopped-flow spectrophotometry, a transient potential response corresponding to the reactions of the two species with I- was observed. The response times were typically a few seconds and several minutes for PAA and H2O2, respectively. The effects of the concentrations of molybdate catalyst, H+, I2, and I- in the potential buffer on the selectivity as well as the sensitivity were examined. The potential response obtained using the GC indicator electrode was found to be Nernstian over a wide range of their concentrations (typically from micromolar to millimolar) with slopes of 30.5 and 29.5 mV for PAA and H2O2, respectively (in close agreement with the theoretical value, that is, 29.6 mV). O2 was found to have no substantial effect on the potential change at the GC electrode in the present potential buffer. Peracetic acid (PAA) has received increasing attention in view of its importance as a superior disinfectant agent, because it displays a wide spectrum of attack against microbes.1 In addition, PAA is characterized by its easy technical preparation and its environmental benefits (the reaction products are oxygen, water, and acetic acid).1 However, the coexistence of PAA with H2O2 is unavoidable for several reasons,2-5 for example, the synthesis of PAA from H2O2 and acetic acid and its continuous decomposition to H2O2. Thus, an analytical method for the analysis of PAA is required to achieve high selectivity and less cross-reaction with the coexistent H2O2. A rapid analytical technique is also desirable for analysis of PAA, since its diluted solutions are unstable. Both peroxides are strong oxidizing agents, and further H2O2 may also function as a reducing agent toward other strong oxidizing agents. Because of the similarities of the behavior of both species, the * Corresponding author: Tel: +81-45-924-5404. Fax: +81-45-924-5489. Email: [email protected]. (1) Swern D. E. Organic Peroxides; John Wiley & Sons: New York, 1970; Vol. 1, p 362. (2) Yuan, Z.; Ni, Y.; van Heiningen, A. R. P. Can. J. Chem. Eng. 1997, 75, 37. (3) Yuan, Z.; Ni, Y.; van Heiningen, A. R. P. Can. J. Chem. Eng. 1997, 75, 42. (4) Ball D. L.; Edwards, J. O. J. Am. Chem. Soc. 1994, 78, 133. (5) Koubek, E.; Hagget, M. L.; Battagalia, C. J.; Ibne-Rasa, K. M.; Pyun, H. Y.; Edwards, J. O. J. Am. Chem. Soc. 1963, 85, 2262.

2688 Analytical Chemistry, Vol. 75, No. 11, June 1, 2003

simultaneous analysis of both species is a challenging problem. The concept for the simultaneous analysis of the two species is based on the difference in their oxidizing power.6-16 For instance, the uncatalyzed reaction rate of PAA and I- is by 5 orders of magnitude larger than that of H2O2 with I-.17 Several techniques for detection of PAA are based on this concept. For instance, titration18 and spectrophotometric19 methods were suggested for the monitoring of the two species. Recently, an amperometric electroanalysis of PAA and H2O2 has been also reported by us.20-22 Those techniques encountered some operational disadvantages, such as time consumption, low sensitivity, and several steps for the analysis. In this paper, we present the use of a highly selective potentiometric method for the simultaneous analysis of PAA and H2O2 in their coexistence. The method is based on the detection of the transient change of the electrode potential for the I2/Iredox couple in the presence of an excess I-, that is, for the I3-/ I- redox couple that is caused by the composition change of the I3-/I- potential buffer as the result of the oxidation of I- by PAA and H2O2. The potential response for the reaction of PAA with Icould be achieved within a few seconds (1-3 s) while maintaining a high sensitivity and selectivity for the detection down to the micromolar range. A similar potentiometric technique has been also recently developed for the simultaneous flow injection analysis of oxychlorines, such as HClO, ClO2-, and ClO3- using an Fe3+/ Fe2+ potential buffer solution containing chloride.23 The simulta(6) Greenspan, F. P.; MacKellar, D. G. Anal. Chem. 1948, 20, 1061. (7) Di Furia, F.; Prato, M.; Quintly, U.; Salvagno, S.; Scorrano, G. Analyst 1984, 109, 985. (8) Di Furia, F.; Prato, M.; Scorrano, G.; Stivanello, M. Analyst 1988, 113, 793. (9) Frew, J. E.; Jones, P.; Scholes, G. Anal. Chim. Acta 1983, 155, 139. (10) Pinkernell, U.; Lu ¨ ke, H.-J.; Karst, U. Analyst 1997, 122, 567. (11) Saltzman, E.; Gilbert, N. Anal. Chem. 1959, 31, 1914. (12) Kru ¨ ssmann, H.; Bohnen, J. Tenside Surf. Deterg. 1994, 31, 229. (13) Pinkernell, U.; Effkemann, S.; Karst, U. Anal. Chem. 1997, 69, 3623. (14) Effkemann, S.; Pinkernell, U.; Neumu ¨ ller, R.; Schwan, F.; Engelhardt, H.; Karst, U. Anal. Chem. 1998, 70, 3857-3862. (15) Effkemann, S.; Brφdsgaard, S.; Mortensen, P.; Linde S. A.; Karst, U. J. Chromatogr., A 1999, 855, 551. (16) Pinkernell, U.; Karst, U.; Cammann, K. Anal. Chem. 1994, 66, 2599. (17) Awad, M. I.; Oritani, T.; Ohsaka, T. Inorg. Chim. Acta. 2003, 344, 253. (18) Sully, B. D.; Williams, P. L. Analyst 1962, 87, 653. (19) Davies, D. M.; Deary, M. E. Analyst 1988, 113, 1477. (20) Awad, M. I.; Harnoode, C.; Tokuda, K.; Ohsaka, T. Electrochemistry 2000, 68, 895. (21) Awad, M. I.; Harnoode, C.; Tokuda, K.; Ohsaka, T. Anal. Chem. 2001, 73, 1839. (22) Awad, M. I.; Harnoode, C.; Tokuda, K.; Ohsaka, T. Anal. Lett. 2001, 34, 1215. (23) Ohura, H.; Imato, T.; Yamasaki, S. Talanta 1999, 49, 1003. 10.1021/ac0204707 CCC: $25.00

© 2003 American Chemical Society Published on Web 04/30/2003

neous determination of ClO3- or -ClO2- (or HClO) is based on the difference in pH-dependent reactivity of these oxychlorines with chloride, and their determination has been achieved by designing the complicated flow systems with three- or four-channel flows and combined with the cation-exchange column in the Fe(II) form.23 In contrast to the method used for the analysis of oxychlorines,23 the present procedure is simple and can be achieved without any change of the experimental parameters through the measurement. EXPERIMENTAL SECTION Safety Note: Caution!24 PAA solutions cause severe irritant to eyes, skin, and mucous membranes. Both PAA and H2O2 are strong oxidizing agents that are completely incompatible with easily oxidized substances and can form explosive mixtures with them; therefore, their concentrated solutions should be handled very carefully and should not be mixed with either reducing agents or organic substances, including solvents. Reagents. All solutions were prepared using deionized water (Milli-Q, Millipore, Japan), and all chemicals were of analytical grade. The H2O2 and PAA solutions of appropriate concentrations were prepared from their stock solutions (30% for H 2O2 and 39.4% for PAA). The concentration of the H2O2 solution was determined by volumetric titration using a standard solution of potassium permanganate. The concentration of PAA stock solution, which was obtained from Mitsubishi Gas Chemicals Co., was analyzed to be 5.5 M using the conventional method originally proposed by Greenspan and Mackellar.6 The concentration of H2O2 coexisting in the PAA stock solution was determined to be 1.7 M by the same method. Acetate buffer solutions (0.05 M) were used to prepare solutions of different pH’s. Ammonium heptamolybdate, (NH4)6Mo7O24‚4H2O (abbreviated as Mo(VI)) was purchased from Kanto Chemicals Co. Inc., Japan. Electrodes. Glassy carbon (GC, 3.0-mm diameter) electrodes were polished with aqueous slurries of successively finer alumina powder (down to 0.06 µm) and were sonicated for 10 min in Milli-Q water. Platinum and gold (1.6-mm diameter) electrodes were pretreated in the same manner as the GC electrode. In addition, gold electrodes were etched for 4 min in a 1:3:6 (in volume) solution of concentrated HNO3 + concentrated HCl + water. The Au electrodes were then electrochemically pretreated in 0.05 M H2SO4 solution by repeating the potential scan in the potential range of - 0.2 to 1.5 V versus Ag/AgCl (NaCl sat.) at 100 mV s-1 for 10 min or until the cyclic voltammetric characteristic for a clean Au electrode was obtained. Measurements. Cyclic voltammetry and the measurement of the electrode potential were performed at 25 ( 1 °C in a twocompartment three-electrode cell with a Au, Pt, or GC working (indicator) electrode, a Pt wire auxiliary electrode and a NaClsaturated Ag/AgCl reference electrode using a computercontrolled electrochemical analyzer (BAS 100B/W, ALS/CHI 604). For the stopped-flow measurements, an RA-401 stopped-flow spectrophotometer (Otsuka Electronic Co.) was used. One of the driving syringes of the stopped-flow unit was filled with I- solution, and the other one was filled with PAA solution. For each run, equal volumes of both solutions were mixed in the mixing (24) Cheremisinoff, N. P. In Handbook of Hazardous Chemical Properties; Butterworth-Heinemann: Woburn, MA, 2000.

Figure 1. Absorbance-time curves at λ ) 352 nm for I2 liberated from the oxidation of I- by (a) 0.22 mM H2O2 and (b) 0.27 mM PAA (containing 0.083 mM H2O2) in 0.05 M acetate buffer solution (pH 5.4) containing 10.0 mM KI. In the case of a, the solution contained 0.8 mM ammonium heptamolybdate.

chamber, and the change in the absorbance of I2, which can be formed by the oxidation of I- by PAA (and H2O2), with time was monitored at 352 nm8 with a minimum time interval of 5 ms after mixing. The reaction of I- and H2O2 was examined by observing the absorbance change of I2 at 352 nm using a V-550 UV/vis spectrophotometer (JASCO Co.). RESULTS AND DISCUSSION Spectrophotometric Measurements. It is well-known that the rate of oxidation of I- with PAA is much higher than that with H2O2.19 To confirm this in 0.05 M acetate buffer solutions used in this study, we measured the time-course of the absorbance at 352 nm of I2 liberated from the oxidation of I- by PAA or H2O2. A typical example of such measurements is shown in Figure 1, where the reactions of I- with PAA and H2O2 were followed by the use of the stopped-flow spectrophotometer and the conventional UV/vis spectrophotometer, respectively. In curve a, equal volumes of 0.44 mM H2O2 solution and 20 mM KI solution had been mixed in the presence of Mo(VI) catalyst. The leveling off of the absorbance, indicating the completeness of the reaction, was obtained after ∼120 s. In this case, the reaction was found not to be complete even after 30 min in the absence of Mo(VI) catalyst. In the case of the reaction of PAA and I- (curve b), the absorbance quickly increased after mixing and reached its constant value within 3 s. Here, it should be noted that the PAA solution used contained H2O2 (in this case, 0.083 mM), but the reaction of I- with H2O2 was essentially negligible in the time scale used for following the reaction of I- with PAA. These confirm the much faster rate of the oxidation of I- with PAA than with H2O2. This large difference in both rates will be utilized in the potentiometric analysis of these two species based on the measurement of the electrode potential of the I2/ I- redox couple in the coexistence of PAA and H2O2, as mentioned below. Electrochemical Measurements. The potential of an indicator electrode dipped in a potential buffer solution containing iodine and excess iodide can be represented by the Nernst equation of eq 1, that is, eq 2: Analytical Chemistry, Vol. 75, No. 11, June 1, 2003

2689

3I- ) I3- + 2e

(1)

E ) Eo - (2.303RT/2F) log{[I-]3/[ I3-]}

(2)

where E and Eo are the indicator electrode potential and the formal potential of the I3-/I- redox couple, respectively, [I3-] and [I-] and are the concentrations of I3- and I-, respectively, in the potential buffer solution. R, F, and T have their usual meanings. Equation 2 shows that the indicator electrode potential is quite sensitive to the changes in [I3-], [I-], and their ratio in the potential buffer solution. Upon the addition of the oxidant (PAA or H2O2) into the I3-/ I- potential buffer solution, the oxidation reaction of iodide by H2O2 and PAA takes place as follows:

H2O2 + 3I- + 2H+ f 2H2O + I3-

(3) -

+

CH3COOOH + 3I + 2H f CH3COOH + H2O + I3

(4)

Consequently, an increase in the concentration ratio of I3- and ([ I3-]/[I-]) will take place, leading to a change of the potential of the indicator electrode from E1 to a value E2, where E1 and E2 are the electrode potentials of the indicator electrode before and after the addition of the oxidant into the I3-/I- potential buffer solution, respectively, which contains the initial concentrations of I3- and I-, [ I3-]0 and [I-]0. Under the condition of [I-]0 . [I2]0, E2 can be expressed by the following Nernst equation, I-

E2 ) E0 - (2.303RT/2F) log{([I-]0 - 3[Ox])3/ ([I3-]0 + [Ox])} (5)

where [Ox] represents the concentration of the oxidant added. Thus, the difference in the electrode potential before and after the addition of the oxidant, ∆E, can be expressed as follows:

∆E ) (2.303RT/2F) log{(1 + [Ox]/[I3-]0)/ (1 - 3[Ox]/[I-]0)3} (6)

where the value of the Nernstian factor 2.303RT/2F is 29.6 mV at 25 °C. Using eq 6, the concentration of the oxidant added can be determined by knowing the change in the potential of the indicator electrode and the initial concentrations of I3- and I- in the potential buffer solution. Effect of pH. The rate of the reaction of H2O2 with I- has been known to be largely dependent on pH, whereas the reaction of PAA with I- is almost independent of pH (in the range of pH 3-6).13 So the effect of the solution pH on the potential change was examined. Figure 2 shows the change in the electrode potential on the addition of 0.91 mM H2O2 (curves a, c, and e) and 0.91 mM PAA + 0.28 mM H2O2 (curves b, d and f) to the potential buffer containing 0.050 M I- and 0.47 mM I3- at different pHs. In all cases, 0.08 mM Mo (VI) was added for accelerating the reaction of H2O2 and I-. Inspection of this figure reveals the following points. At pH 4.0, curve a, where only H2O2 was added 2690 Analytical Chemistry, Vol. 75, No. 11, June 1, 2003

Figure 2. Potential change of the GC indicator electrode by the addition of (a, c, e) 0.91 mM H2O2 and (b, d, f) 0.28 mM H2O2 + 0.91 mM PAA to 0.05 M acetate buffer solution containing 0.10 M I-, 0.47 mM I3-, and 0.08 mM Mo(VI) catalyst at pH ) 4.0 (A), 5.4 (B), and 6.5 (C).

to the potential buffer, the steady state of the potential was attained within 40 s. The potential change in this case equals 15.0 mV, similar to the value expected from eq 6, that is, 15.0 mV. In curve b, where both PAA and H2O2 were added, the constant potential was attained without a considerable difference in the response time for the reactions of the two species and I-. Thus, the potential change for the reaction of each species with I- was very difficult to determine,and we can determine only the total change in the potential corresponding to the reaction of both oxidants (PAA and H2O2) and I-. Its value was found to equal 17.5 mV, which is in a very close agreement with the expected one, that is, 17.8 mV. As pH is increased, the reaction of H2O2 and I- becomes slower.25 For example, the response time for the reaction of H2O2 and I- at pH 5.4 is ∼2 times longer than that at pH 4.0 (compare curves a and c). However, as shown in curve d (where the two species are added), the selective response for the two species is still difficult to determine under these conditions. Only the total response for the both species can be determined. Again, the obtained potential change is in very close agreement with that expected from eq 6. The potential change in this case is similar to the previous case, that is, at pH 4.0, as is expected from the independency of the potential change on pH (see eq 6). In a trial to further increase the selectivity, the potential response was measured at higher pH, that is, pH 6.5 (curves e and f). As expected, the selectivity was increased as a result of the decreased rate of the reaction of H2O2 and I-. Consequently, we could, though roughly, observe the respective potential changes corresponding to the reactions of PAA and H2O2 with I-. The potential change from its beginning after the addition of the PAA + H2O2 solution to the inflection point (shown by an asterisk, *) of the potential-time curve (i.e., region I) corresponds to the reaction of PAA and I-, whereas the potential change from the inflection point to the steady state (region II) is due to the reaction of H2O2 and I-. Here, it should be noted that the discrimination of region I from region II of the response curve (25) Copper, C. L.; Koubek, E. Inorg. Chim Acta 1999, 288, 229.

Figure 3. Potential change of the GC indicator electrode by the addition of (a, c) 0.91 mM H2O2 and (b, d) 0.28 mM H2O2 + 0.91 mM PAA to 0.05 M acetate buffer solution (pH 5.4) containing (A) 0.050 or (B) 0.010 M I- and 0.47 mM I3-.

shown in Figure 2C seems to be still arbitrary, but the inflection point can be more clearly determined by choosing appropriately the initial concentrations of I- and I2 in the I2/I- potential buffer (vide infra). The potential changes estimated for PAA and H2O2 were found to be in fair agreement with those expected from eq 6. For H2O2 (curve e), the experimental and the theoretical values are 16.0 and 15.0 mV, respectively. In curve f, the potential changes for PAA and H2O2 equal 13.5 and 5.5 mV, respectively, while the theoretical values are 15.0 and 6.3 for PAA and H2O2, respectively. Thus, it became clear that with increasing pH of the solution (in this case, from 5.4 to 6.5), the selectivity of the present method is increased as expected from the different pH dependence of the reactions of PAA and H2O2 with I-.13 On the other hand, the reaction of H2O2 and I- was very slow and needed more than 10 min for completion. Thus, a pH of 5.4 was selected as the optimum one from the viewpoint of response time, whereas the selectivity will be further improved by examining the effect of I- concentration, as mentioned below. Effect of Iodide Concentration. Figure 3 shows the potential change on the addition of PAA, H2O2, or both to the potential buffers of pH 5.4 containing different initial concentrations of I-. As shown from Figure 3 and its comparison with Figure 2B, the inflection point of the potential change corresponding to the end of the reaction of PAA and I- becomes more clear as the concentration of I- is decreased because of the decrease in the rate of the reaction of H2O2 and I-.25-29 That is, it was found that the selectivity is more improved at the lower concentration of I-. In these cases, the potential changes observed for PAA and H2O2 were also found to be comparable with those expected from eq 6. It is worthwhile to note that, as is clear from this figure, the sensitivity highly increases with the decrease in I- concentration (as expected from eq 6). For example, the potential change on the addition of 0.91 mM PAA to 50 mM KI (curve b) equals 14.0 mV, but that resulting from the addition of the same concentration (26) Karunakaran, C.; Muthukumaran, B. Transition Met. Chem 1995, 20, 463. (27) Z-Lun, F.; S-Kun, X. Anal. Chim. Acta 1983, 145, 151. (28) Hadjiioanou, T. P. Anal. Chim. Acta 1975, 36, 17. (29) Garcia, F.; Gomez-Lara, P. Rev. Soc. Quim. Mex. 1969, 13, 222A.

Figure 4. Potential change of the GC indicator electrode by the addition of 0.91 mM PAA + 0.28 mM H2O2 to 0.05 M acetate buffer solution (pH 5.4) containing 0.010 M KI and 0.47 mM I3-. In case a, Mo(VI) catalyst was previously added to the acetate buffer solution, whereas in case b, it was added at the point shown by an asterisk (*) (in both cases, its concentration was 0.08 mM).

of PAA to 10.0 mM KI (curve d) equals 25 mV. Thus, we can achieve a suitable response time while maintaining a high selectivity. Effect of Mo(VI) Catalyst. A suitable method for analysis of PAA should achieve high selectivity and less cross-reaction toward the coexistent H2O2. The reaction rate of H2O2 and I- can be increased either with the addition of ammonium heptamolybdate (Mo(VI)) as catalyst or by decreasing the solution pH. In the absence of Mo(VI), the rate of the reaction of PAA and I- is 5 orders of magnitude higher than that of H2O2 and I-, but in its presence, the rate of the former reaction is only 2 orders of magnitude higher than that of the latter one.17 Thus, the effect of the addition of Mo(VI) catalyst on the potential-time curve was examined. The results are given in Figure 4. In the case of curve a, Mo(VI) was previously added to 0.05 M acetate buffer solution containing 0.010 M KI and 0.47 mM I3- before the addition of the PAA + H2O2 mixture, and in curve b, Mo(VI) was added for accelerating the reaction of H2O2 and I- after the reaction of PAA and I- was completed. We can see the clear inflection point and at the same time that the potential changes corresponding to PAA and H2O2 are comparable in both cases. Of course, the reaction of H2O2 and I3- was accelerated in the presence of Mo(VI) (compare Figures 4 and 3B). Thus, it is possible to simultaneously analyze PAA and H2O2 in their coexistence by measuring the potential change of the indicator electrode in the potential buffer solution containing the known concentrations of I- and I3- in the presence of Mo(VI). Response Time. Response time is an important parameter in analytical tools. Generally, a good analytical technique is characterized by its fast time response. However, in the case of onestep simultaneous analysis of, for example, two species, it is preferable that the response times for the two species are different enough to achieve high selectivity. The present method is based on monitoring the potential change due to the reactions of PAA and H2O2 with I- in the I3-/I- potential buffer. So, the response time is considered to depend on the reaction rates of both species and I-. The reaction rate of H2O2 and I- is highly affected by pH as well as by the concentration of Mo(VI) as catalyst. As pH is increased, the response time for the reaction of H2O2 and Iincreases (Figure 2). Similarly, the response time increases by adding Mo(VI) to the potential buffer solution (Figure 4). Analytical Chemistry, Vol. 75, No. 11, June 1, 2003

2691

Figure 5. Potential change of the GC indicator electrode (3.0 mm diameter) on the successive addition of PAA + H2O2 mixture to 40.0 mL of 0.05 M acetate buffer (pH 5.4) containing 10.0 mM KI, 0.3 mM I3-, and 0.08 mM Mo (VI) under an atmosphere of (a) O2 and (b) N2. Concentrations of PAA and H2O2: (1) 0.091 and 0.028, (2) 0.183 and 0.056, (3) 0.275 and 0.085, (4) 0.366 and 0.113, (5) 0.458 and 0.142, (6) 0.55 and 0.170, (7) 0.641 and 0.198, (8) 0.733 and 0.227, (9) 0.825 and 0.255, (10) 0.916 and 0.284, (11) 1.0 and 0.30, and (12) 1.10 and 0.34 mM.

However, the reaction rate of PAA and I- is actually independent of both factors.13 Thus, in the present method, we could relatively easily achieve the different response times for both species, that is, typically a few seconds for PAA and several minutes for H2O2, as mentioned above. Analytical Application. Figure 5 shows the typical potential response of the GC indicator electrode on the successive addition of the PAA + H2O2 mixtures to 0.05 M acetate buffer solution (pH 5.4) containing 10.0 mM KI, 0.3 mM I3-, and 0.08 mM Mo(VI) under an atmosphere of N2 and O2. The potential change was found to increase when continuously increasing the concentrations of PAA + H2O2. In this case, there was no appreciable difference in the potential change in the presence and absence of O2, although O2 may interfere with the analysis of both species, especially at low pHs, as shown by the following reaction.30

4I- + O2 + 4H+ ) 2I2 + 2H2O

(9)

The plots of ∆E vs log{(1 + [Ox]/[I3-]0)/(1 - 3[Ox]/[I-]0)3} for PAA and H2O2 are shown in Figure 6. Good straight lines passing through the origin with slopes of 30.5 ( 1.0 and 29.5 ( 0.9 mV and with correlation coefficients higher than 0.999 were obtained for PAA and H2O2, respectively. The values of these slopes are in an excellent agreement with the theoretical value (29.6 mV at 25 °C) and at the same time, both plots can be regarded to fall on the common straight line. The similar linear plots for the data under O2 atmosphere were also obtained. The slopes were found to be 29.5 ( 1.0 and 30.2 ( 0.9 mV, with correlation coefficients higher than 0.998, for PAA and H2O2, respectively. Similar data were obtained using Au and Pt indicator electrodes (data are not shown). The slopes of the linear plots of ∆E vs log{(1 + [Ox]/ (30) Douglas, A. S.; Donald, M. W. In Fundamentals of Analytical Chemistry, 4th ed.; Sundars Golden Sunburst Series; Holt-Sundars: Tokyo, Japan, 1982; Chapter 15, p 375.

2692 Analytical Chemistry, Vol. 75, No. 11, June 1, 2003

Figure 6. Plots of ∆E vs log{(1 + [Ox]/[I3-]0)/(1 - 3[Ox]/[I-]0)3} for PAA (a) and H2O2 (b). The data were taken from Figure 5. Table 1. Slopes of the Plots of ∆E vs log{(1 + [Ox]/ [I3-]0})/(1 - 3[Ox]/[I-]0)3} at Different Indicator Electrodes under N2 or O2 Atmospherea

PAA H2O2

GCE, mV

Pt, mV

Au, mV

30.5 ( 1.0b 29.5 ( 1.0c 29.5 ( 0.9b 30.2 ( 0.9c

29.5 ( 0.8b 31.9 ( 1.1c 27.9 ( 1.3b 35.5 ( 0.9c

31.9 ( 1.4b 32.8 ( 1.2c 33.0 ( 0.9b 34.1 ( 1.2c

a The measurements were carried out by successively adding the PAA + H2O2 mixtures in the concentration ranges of PAA from 6.8 × 10-6 to 1.2 × 10-3 M and of H2O2 from 2.1 × 10-6 to 1.3 × 10-3 M to 0.05 M acetate buffer solution (pH 5.4) containing 11.5 mM KI and 1.0 mM I2. The diameter of GCE, Pt, and Au indicator electrodes were 3.0, 1.6, and 1.6 mm, respectively. b Under N2 atmosphere. c Under O2 atmosphere.

[I3-]0)/(1 - 3[Ox]/[I-]0)3} obtained at GC, Pt and Au electrodes under the atmosphere of N2 and O2 are summarized in Table 1, in which the concentrations of PAA and H2O2 were varied in the ranges from 6.8 × 10-6 M to 3.6 × 10-3 M and from 2.1 × 10-6 to 1.1 × 10-3 M, respectively. This table demonstrates that the slopes obtained using different indicator electrodes are close to the theoretical value of 29.6 mV with no considerable difference between the values obtained in the presence and the absence of O2. Thus, we can directly calculate the concentrations of the two species from the measured potential change using eq 6, except for the case in which I- is not in excess relative to the liberated I2. In this case, the liberated I2 was found to deposit on the electrode surface, and the potential response was found not to follow eq 6. The rectilinear range in the ∆E vs log{(1 + [Ox]/[I3-]0)/(1 - 3[Ox]/[I-]0)3} plots for the two species was found to increase with increasing concentration of I-. The concentration of I- in the sample solution should be high enough to dissolve I2 resulting from the reactions of PAA and H2O2 with I-. For example, when the potential buffer contained 0.1 M KI, the rectilinear range was found to increase up to ∼7.3 and 2.2 mM for PAA and H2O2, respectively (data are not shown). The linear ranges (typically micromolar to millimolar) obtained in the plots for PAA and H2O2

Table 2. Comparison between Potentiometric and Titration Methods potentiometrica

titrationb

PAA, mM

H2O2, mM

PAA, mM

H2O2, mM

0.84 (0.018) 1.35 (0.025) 1.63 (0.013)

0.29 (0.011) 0.46 (0.027) 0.51 (0.015)

0.80 ( 0.20 1.31 ( 0.20 1.60 ( 0.20

0.22 ( 0.30 0.39 ( 0.30 0.48 ( 0.30

a For each solution, the measurements were repeated four times, and the respective standard deviation (SD) is given in parentheses. b Concentrations of PAA and H O were determined volumetrically 2 2 according to ref 6.

are largely different from those obtained by their amperometric electroanalysis,20-22 for example, 0.36-110 and 0.11-34 mM for PAA and H2O2, respectively. Both the present potentiometric and the amperometric techniques are very useful in the simultaneous electroanalysis of PAA and H2O2. Thus, we can suitably choose either, depending on the concentrations of these analytes. The accuracy of the present method was tested by comparing several assays obtained by this method with the data obtained using the titration method proposed by Greenspan and MacKellar.6 The results are represented in Table 2. The t values of the Student’s test 31 were found to be >3.2 in all cases, demonstrating that because the t value for 95% probability level is 3.18 (n ) 4), the obtained results are highly significant. (31) Vogel, A. Vogel’s Textbook of Quantitative Inorganic Analysis, including Elementary Instrumental Analysis, 4th ed.; Longman Inc.: New York, 1978; Chapter 4.

CONCLUSIONS Using the potentiometric method, we have succeeded in achieving a highly selective simultaneous analysis of PAA and H2O2 in their coexistence. In the present method, the potential change of GC, Pt, and Au electrodes was measured when PAA, H2O2, or both were added to the potential buffer, which consisted of the appropriate concentrations of I2 and I-. The simultaneous analysis of PAA and H2O2 is based on the fact that the reaction of PAA and I- is much faster than that of H2O2 and I-. The sensitivity, selectivity, and response time were found to largely depend on the solution pH, the concentrations of I3- and I-, and the catalyst (Mo(VI)). The pH of 5.4 was used as the optimum pH. The optimum concentration ratio of I- to I2 in the potential buffer was selected depending on the concentration range of the two oxidizing agents to be analyzed. The linear plot theoretically expected from eq 6,which is common to PAA and H2O2, was obtained over a wide range of their concentrations (typically from micromolar to millimolar). ACKNOWLEDGMENT The present work was financially supported by Grants-in-Aid for Scientific Research on Priority Area (no. 417), Scientific Research (no. 12875164), and Scientific Research (A) (no. 10305064) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. M.I.A. thanks the Government of Japan for the Monbu-Kagakusho fellowship. Received for review July 22, 2002. Accepted March 4, 2003. AC0204707

Analytical Chemistry, Vol. 75, No. 11, June 1, 2003

2693