Chemiluminescent flow sensor for H2O2 based on the decomposition of H2O2 catalyzed by cobalt(II)-ethanolamine complex immobilized on resin

Analytica Chimica Acta 426 (2001) 57–64

Chemiluminescent flow sensor for H2 O2 based on the decomposition of H2 O2 catalyzed by cobalt(II)-ethanolamine complex immobilized on resin Soichi Hanaoka, Jin-Ming Lin∗ , Masaaki Yamada Department of Applied Chemistry, Graduate School of Engineering, Tokyo Metropolitan University, 1-1 Minami-Ohsawa, Hachioji, Tokyo 192-0397, Japan Received 5 June 2000; received in revised form 21 August 2000; accepted 5 September 2000

Abstract The decomposition of H2 O2 catalyzed by transition metal ion has been investigated by chemiluminescence (CL). Using a heterogeneous catalyst, Co(II)-monoethanolamine complex immobilized on Dowex-50W resin, the rapid decomposition of H2 O2 was observed. A very weak CL appeared during the H2 O2 solution mixed with the immobilized resin by batch method. When appropriate amount of neutral luminol solution was added into the heterogeneous system, a bright CL emission was recorded. This heterogeneous CL system was developed as a H2 O2 flow sensor. Detection limit of H2 O2 using Dowex 50W-X4-Co-monoethanolamine as catalyst is 1 × 10−7 M (S/N = 3). The flow sensor is of long lifetime, highly selective and very stable in the neutral solution. The application of the proposed method to determine H2 O2 in rainwater samples without any special pre-treatment gives out a satisfactory result. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Chemiluminescence; Sensor; Hydrogen peroxide; Rainwater

1. Introduction The rapid decomposition of H2 O2 has practical application in many fields, e.g. the fuel cell, rocket propellants and bleaching as a source of oxygen. During H2 O2 decomposition, several active oxygen species, superoxide (O2 •− ), hydroxyl (• OH), HOO• radicals or singlet oxygen (1 O2 ), etc. were found as intermediates depending on the catalysis [1,2]. In some cases, the formation of free-radical intermediates was evidenced by ESR or other methods [3,4]. These free-radicals have been widely used in chemiluminescence (CL) [5,6]. But until now, to our knowledge, almost all ∗

Corresponding author. Tel.: +81-426-77-1111; fax: +81-426-77-2821. E-mail address: [email protected] (J.-M. Lin).

of luminol or lucigenin-H2 O2 -metal ion CL systems were carried out in basic solution (pH > 8.0), except a few special micro-heterogeneous systems, e.g. microemulsion [7] or micellar medium [8]. The reasons that luminol/H2 O2 system could not emit in neutral or weak acidic homogeneous solution could be explained by the fact that H2 O2 molecule is relatively stable to the metal ion or its complex in a low pH solution. Generally, the decomposition of H2 O2 started from the hydroperoxide ion. It is formed by the acid-base equilibrium of H2 O2 in basic solution [9]. H2 O2

pKa =11.7

↔

HO2 − + H+

(1)

The pKa of reaction (1) indicated that in basic solution it is beneficial to form hydroperoxide ion. But it is also well-known that the CL systems of luminol

0003-2670/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 3 - 2 6 7 0 ( 0 0 ) 0 1 1 8 1 - 8

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were influenced by a lot of substances in the basic solution. Therefore, many luminol CL methods were of high sensitivity but the selectivity of these could not be flattered. Due to the importance of luminol in CL research field, to develop a neutral or weak acidic medium luminol CL system in analysis, has high scientific value. However, because of the chemical characters of H2 O2 and luminol, it is difficult to establish a homogeneous luminol CL system in neutral or weak acidic solution even though many analysts have made a lot of efforts. Recently, we reported the CL determination of amino acid based on the decomposition of H2 O2 catalyzed by Cu(II)-amino acid complexes in a dilute carbonate aqueous solution [10]. The fact that the catalytic activity of Cu(II)-amino acid on H2 O2 decomposition was higher than that of Cu(II) ion was confirmed. But this CL reaction also took place in basic solution. Therefore, we tried to immobilize the Cu(II)-amino acid complex on a cation-exchange resin which may be used as the catalyst to decompose H2 O2 in neutral solution. Some papers reported that the transition metal complexes sorbed on Dowex-50W resin were of high catalytic activity to H2 O2 decomposition [11–13]. The decomposition of H2 O2 suggested that at the start of the reaction, the transition metal complex reacts with a molecule of H2 O2 to form the peroxo-metal complex. Then the peroxo-metal reacts with another molecule of H2 O2 to yield the active intermediate products. The kinetic investigations indicated that the transition metal complexes sorbed on Dowex-50W resin are very stable even after the decomposition reaction is complete [13]. Based on this observation, a simple, highly sensitive and selective, near zero-emission H2 O2 CL flow sensor was assembled.

solution was prepared by volumetric dilution of 30% (v/v) H2 O2 (Tokyo Kasei, Tokyo, Japan). The exact concentration of the stock solution was determined by titration. The concentration of the H2 O2 stock solution was checked daily with measuring the UV absorbance at 240 nm. Monoethanolamine (MEA), diethanolamine (DEA) and triethanolamine (TEA) were obtained from Nakalai Tesque Inc. (Kyoto, Japan). Luminol is the product of Tokyo Kasei, Tokyo. A 1 × 10−5 M luminol stock solution was prepared by dissolving 5.31 mg of luminol in 3 l water. This solution (pH = ca.6.0) can be stored at least 1 month. Dowex-50W resin in the hydrogen form was used as a strong acid cation exchanger. It is available as spherical beads of sulphonated styrene divinylbenzene copolymers in the hydrogen form from Dow Chemical (Midland, MI, USA). 2.2. Apparatus The schematic diagram of the flow system is shown in Fig. 1. It consists of a peristaltic pump (SJ1211, Atto, Tokyo), a CL detector (Lumiflow LF-800, Microtec NITI-ON, Funabashi, Japan) and a 90 ␮l loop injector placed close to the luminometer. The CL signals were recorded with a Shimadzu U-125MN recorder. The batch method for CL profile was carried out at a Lumincounter 1000 (Microtec NITI-ON, Funabashi). For pH measurements, a PHL-20, pH meter (DKK Corporation, Tokyo) was used. A Shimadzu

2. Experimental 2.1. Reagents All chemicals were of analytical-reagent grade and were used as received. Water was obtained from a Milli-Q purification system (Japan Millipore, Tokyo). Copper(II) sulfate, cobalt(II) sulfate, nickel(II) sulfate and other metal salts are the products of Kanto Chemical Co. Inc. (Tokyo, Japan). A 0.1 M H2 O2

Fig. 1. Schematic diagram of a cyclic flow injection chemiluminescent system. P: peristaltic pump, PMT: photomultiplier tube.

S. Hanaoka et al. / Analytica Chimica Acta 426 (2001) 57–64

UV-2200, UV-visible spectrophotometer (Shimadzu, Kyoto, Japan) was used. 2.3. Dowex-50W resin associated with Co(II)-MEA complex The resin was regenerated with 2 M HCl, thoroughly washed with H2 O and dried in air. The total weight capacity of the exchanger was determined by the batch method [14]. The resin was converted into the Co2+ form by equilibrating it with 1.0 M CoSO4 solution. The resin in the Co2+ form was collected and washed with H2 O until it was free from any excess of Co2+ ions. Then, the resin in the Co2+ form was immersed in an aqueous ethanol solution (50% v/v) for 1 day. The MEA solution of 0.1 M was added dropwise with continuous stirring until a slight excess of the ligand was detected. After equilibrium was attained, the resin was collected and washed with H2 O until it was free from any excess of the ligand. 2.4. Procedure A ca. 0.3 g of the dried resin was filled into a 3.0 cm length, 0.5 cm I.D glass tube which was placed at the front of photomultiplier tube (PMT) (Fig. 1). In order to protect the resin particles flow with the carrier, a small amount of glass wool was packed at the two sides of the glass tube. A sample of 90 ␮l H2 O2 standard solution or rainwater was injected into the carrier stream. Calibration curves for this method were obtained daily by using H2 O2 solutions prepared from the 0.1 M stock solution.

3. Results and discussion 3.1. Preparation of the CL flow sensor The preparation of the transition metal complex immobilized resin was carried out with the following three main steps: (a) regeneration of Dowex-50W with 2 M HCl, (b) immobilizing Co2+ on the resin, (c) combination of the ligands to the Co2+ to form resin. Lastly, the complex immobilized resin particles were packed into a glass tube, which was used as a flow sensor. From these steps, the treatment of

59

Dowex-50W resin seems to be carried out such that HCl, Co2+ and ligand solutions flow through the glass tube packed with resin particles in orderly fashion. The latter preparing method is more convenient than the batch method. The CL intensities of 1×10−6 M H2 O2 solution were the same using these two different prepared resins, but the repeatability of the two methods were different. The reproducibility (n = 15) for the determination of 1 × 10−6 M H2 O2 was 2.9% for the batch method and 6.6% for the on-line method. These results may be due to the incomplete ion change with the on-line treatment. On the other hand, in order to investigate the mechanism of CL reaction, appropriate amount of transition metal complex immobilized resin is needed. Using batch method, the amount and the reproducibility could be ensured. The ratios of metal ion to ligand in the resin were also determined by titration and back-titration. The [cobalt]/[ligand] ratios of the complexes before and after 100 times CL measurements with 1 × 10−5 M luminol/1 × 10−5 M H2 O2 solution were the same. They were 1:2. The capacity and the moisture content of the resin were determined at the end of experiment and were found to be unchanged. Thus, the resin was not degraded during the decomposition of H2 O2 , which proves that the resin is stable under the present working conditions. 3.2. CL signals of batch and flow-injection methods The catalytic decompositions of H2 O2 with MEA-Co(II)-resin and Co(II)-MEA complex solution were compared by CL. The mixing of resin with high concentration of H2 O2 give out a weak CL which was enhanced by adding a low concentration of luminol solution. As shown in Fig. 2a, adding 100 ␮l of 1 × 10−4 M H2 O2 solution into 200 ␮l of 1×10−5 M luminol solution containing 10 mg of resin in Co(II)-MEA complex form, a strong CL emission was recorded. The CL signal is sharp at the beginning and then decrease relatively slowly. However, without resin or using Co(MEA)2 2+ solution (pH = ca.8.2) no CL was observed (Fig. 2b). It is a well-known fact that the CL of luminol/Co(II)/H2 O2 system take place in the basic aqueous solution but in neutral or acidic solution there was no CL emission [15]. However, using the resin as catalyst the lumino/H2 O2 CL reaction happened. In many research fields, e.g. biochemistry

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preliminary experiments, we could not get a satisfactory reproducibility for each injection without controlling the injection interval time. We noticed that the CL intensity was affected by the injection interval time. From Fig. 3, the injection interval time increased from 20 to 50 s, the CL intensity also increased. This phenomenon may be explained that the reactivity of the solid catalyst after CL reaction needs appropriate time. 3.3. Optimum conditions for the CL flow sensor

Fig. 2. Comparison of the luminol/H2 O2 CL signals with (MEA)2 Co-resin and Co(MEA)2 2+ solution as catalysts: (a) injection of 100 ␮l of 1 × 10−4 M H2 O2 into 200 ␮l of 1 × 10−5 M luminol solution containing 10 mg MEA-Co-resin, (b) injection of 100 ␮l of 1 × 10−4 M H2 O2 into 200 ␮l of 1 × 10−5 M luminol solution containing 1 × 10−4 M Co(MEA)2 2+ . Resin: Dowex 50W-X4.

and electrochemistry, the determination of H2 O2 in neutral or weak acidic condition is desired. The present H2 O2 decomposition system could be developed as a CL flow sensor for H2 O2 using the apparatus as shown in Fig. 1. Resin particles were packed in a glass tube, which was placed at the front of PMT. The carrier is a 1 × 10−5 M luminol unbuffered solution. From the valve, 90 ␮l of H2 O2 solution was injected into the carrier and the CL profile is shown in Fig. 3. During the

Fig. 3. Effect of the injection interval time on CL intensity. Conditions: the flow system is the same in Fig. 1. MEA-Co-Dowex-50W was used as catalyst. 1 × 10−5 M luminol solution at 2.0 ml/min flow rate. The injection volume of H2 O2 solution is 90 ␮l.

In order to establish optimal conditions for the determination of H2 O2 , the CL intensity emitted from the CL flow sensor was measured as a function of carrier solution, metal ion, ligand, resin and the flow rate. The concentration of luminol carrier solution was examined. The higher the concentration of luminol, the stronger CL was observed. But the dissolution of luminol in water is very low. The highest concentration of luminol in warm water (ca.45◦ C) is 1 × 10−4 M. This solution was transparent and no precipitation was observed. In order to keep the concentration of luminol at a constant level, a luminol solution of 1 × 10−5 M was used. The pH value of this solution is ca. 6.0, it is stable enough to keep the solution for at least 1 month. Therefore, as shown in Fig. 1, a cyclic flow injection CL system was possibly developed to determine H2 O2 using the low concentration luminol as carrier, which could save the reagent and decrease the environment pollution from the waste solution. Instead of luminol solution, another CL reagent, lucigenin solution was also a CL enhancement agent, effective for H2 O2 determination. Comparing to a 1 × 10−5 M luminol solution, using a 1 × 10−4 M lucigenin solution, the CL intensity of 1 × 10−5 M H2 O2 was near the same as that using luminol solution. Other fluorescent organic compounds often used as CL enhancement agent, such as, brillian sulfoflavine, rhodamine B, eosin Y etc. were tested. There was no evident CL enhancement by these compounds. Therefore, a 1×10−5 M luminol solution was used in this work. The effects of six different transition metal ions on CL were compared. As shown in Table 1, the resin treated with Co2+ has the highest catalytic activity. Using the resin treated with Fe3+ or Fe2+ as catalyst, there was no evident CL emission even though the decomposition of H2 O2 over Fe2+ in aqueous solution was well-known (Fenton reaction) [16]. In

S. Hanaoka et al. / Analytica Chimica Acta 426 (2001) 57–64 Table 1 Effect of metal ions on CL determination of H2 O2 Dowex 50W-X4

S/N

Metal ion

Ligand

1 × 10–4 M H2 O2

1 × 10−5 M H2 O2

MEA MEA MEA MEA MEA MEA

NDa 520 200 154 5 6 5

ND 55 7 6 ND ND ND

Non Co(II) Cu(II) Ni(II) Mn(II) Fe(III) Fe(II) a

No detection.

homogeneous luminol CL system, Co2+ , Cu2+ , Ni2+ and Fe2+ ions were often used as catalyst. All of them have high catalytic activity and have been widely used in CL analysis. But when Cu2+ and Ni2+ ions were immobilized on the resin, their catalytic activity on the decomposition of H2 O2 was much lower than that using Co2+ ion. The reason of this phenomenon is not clear even though all of them are transition metal ions. The kinetic study of the H2 O2 decomposition was also carried out with changing the metal ion on the resin. The rate constant of the decomposition reaction was in the order Co(II) > Cu(II) > Ni(II) > Fe(II). This order of the rate constant is the same as the order of CL intensity (see Table 1). These results indicated that the faster decomposition of H2 O2 corresponds to a higher CL intensity. The ligand combined with the metal ion is also an important factor in the catalysis. The selection of

61

ligand was based on two requirements. One is that the ligand can donate lone pair, and another is that the ligand can form five or six-ring structure with the transition metal ion. Table 2 showed two type of ligands, ethanolamines and ethylenediamines. We found that although MEA and ethylenediamine were only different in the –OH and –NH2 groups. Both oxygen and nitrogen atoms in the groups can donate the lone pair to Co2+ ion. The oxygen atom of –OH group contributed two lone pairs to Co2+ ion, which made the Co2+ ion easier to be oxidized by H2 O2 or the oxygen dissolved in carrier. It was interesting to find that using triethylendiamine (DABCO) as ligand a relative strong CL was observed. This result was exceeding our expectations. It could be considered that the DABCO and Co(II) formed complex on the surface of resin but however, the combination is not clear. The CL intensity with Co(II)-ethanolamine complexes decreased in the following order: MEA > DEA > TEA. The base strength of the three amines decreases in the same order. This result means that the greater the base strength of amine, the greater its catalytic effect on the H2 O2 decomposition. On the other hand, comparing the [cobalt]/[ligand] ratio before and after the resin was used for the H2 O2 decomposition reaction, the molar ratios were the same, which means that the complexes immobilized on the resin were stable. The ethanolamines were not only strongly affected on the CL intensity but also strongly sorbed by Dowex-50W resin in the Co2+ form to form stable complexes. Based on these results, MEA was selected as ligand in this work.

Table 2 Effect of ligands on CL determination of H2 O2 a Ligand

MEA DEA TEA Ethylenediamine Diethylenetriamine

Structure

HOCH2 CH2 NH2 (HOCH2 CH2 )2 NH (HOCH2 CH2 )3 N NH2 CH2 CH2 NH2 (NH2 CH2 CH2 )NH

DABCOd a d

Sensor phase: Dowex 50W-X4-Co-Ligand. Triethylendiamine.

S/N 1 × 10−4 M H2 O2

5 × 10−5 M H2 O2

540 230 121 13 4

300 125 28 6 2

61

90

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Table 3 Effect of the degree of crosslinkage and the mesh size of resin on the CL intensity Dowex-50 X

CL intensity (S/N)

DVB%

Exchange Mesh size capacity (meq/ml)

5×10−5 M H2 O2

Blank

8

1.7

50–100 100–200 200–400

195 223 272

18 37 54

4

1.1

50–100 100–200 200–400

247 297 345

20 29 41

2

0.6

50–100 100–200 200–400

349 401 452

35 49 58

From Table 3 we can find that the CL intensity decreased with increasing the degree of resin crosslinkage. The greater the crosslinkage degree of Dowex-50W resin, the higher the exchange capacity of the resin, and the more Co(II)-MEA complexes absorbed on the resin. Generally, the CL intensity is stronger with the resin of the higher exchange capacity. But here, the result is contradictory. It could be explained that the greater the degree of resin crosslinkage, the smaller the H2 O2 concentration in the resin and the fewer reactive species produced. This phenomenon was called the “salting-out effect” [17]. Another parameter of resin is the mesh size. When the mesh size of the resin particles changed from 50–100 to 200–400, the CL intensity increased. This result arises from the increase of the effective surface area with the decrease of the mesh size of the resin particles. But we also found that the lifetime of the CL sensor depended on the exchange capacity of the resin. The higher the exchange capacity of the resin the longer the lifetime of the sensor because the more Co(II)-MEA complexes were sorbed to the resin. By using resin of 8%DVB the lifetime of the sensor is ca. two-fold longer than the resin of 2%DVB, even though, the CL intensity was twice smaller. Considering the sensitivity of H2 O2 determination and the lifetime of the sensor, a resin of 4% DVB and 100–200-mesh size is the most suitable for the following experiments. The flow rate of luminol solution was also examined. Similarly to the usual flow injection analysis,

the low flow rate caused a broad CL signal. Too high flow rate of luminol solution shortens the lifetime of the sensor, which may be due to that the complex was lowly exchanged by the proton ion. On the other hand, as shown in Fig. 3, the CL intensity was also affected by the injection interval time. The fast flow rate was unnecessary. A flow rate in the range 1.5–2.5 ml/min is suitable and a 2.0 ml/min was used. 3.4. Calibration curve and detection limit Under the selected conditions given above, the CL signals were sharp and stable and the noise was low. The ratio of CL signal to noise (S/N) versus the concentration of H2 O2 (X) was linear in the range 2 × 10−7 –2×10−5 M with a detection limit of 1×10−7 M. The regression equation was Y (S/N ) = 2.46 + 5.53 × 106 X and the correlation coefficient was 0.9990 (n = 12). The determination of H2 O2 could be performed in 1 min. The sample frequency was ca. 60 times/h. The relative standard deviation was 3.0% for 5 × 10−6 M H2 O2 (n = 13). These results indicated that the precision is good enough for the determination of H2 O2 at low concentrations. The successfully prepared sensor could be used at least a week for 1000 measurements of H2 O2 at 10−5 –10−7 M level. 3.5. Interference study From the preliminary experiments, it was found that most of the usual weakly oxidant or reducer, such as NO2 − , NO3 − , SO3 2− , S2− , Fe2+ and C2 O4 2− at 1 × 10−5 M level have no CL signal when they were injected into the flow line separately. The influences of other foreign species were investigated by analyzing a standard solution of 5 × 10−6 M H2 O2 , to which increasing amounts of interfering species were added. The tolerable limit of a foreign species was taken as a relative error not >5%. The tolerable ratio of foreign ions to 5 × 10−7 M H2 O2 was 1000 K+ , Na+ , Ca2+ , Mg2+ , Mn2+ , Co2+ , Zn2+ , Ni2+ , Cl− , HCO3 − , CO3 2− , SO4 2− , H2 PO4 − and NH4 + , 100 for Br− , I− , Al3+ , Cr3+ , Fe3+ and urea, 50 for Pb2+ , Sn2+ , Cu2+ and SO3 2− , and 10 for tryptophan and Fe2+ . The interference of OCl− ion was serious. Instead of H2 O2 solution, injection of a 90 ␮l of 10−6 M NaOCl solution into the flow system, a CL intensity of 60 (S/N)

S. Hanaoka et al. / Analytica Chimica Acta 426 (2001) 57–64

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Table 4 Results of analysis of H2 O2 in rainwater samplesa Sample

Added (×10−6 M)

Found (×10−6 M)

1

0 2 5 0 2 5 0 1

4.27 6.21 9.41 2.56 4.38 7.39 2.70 3.76

2

3

Recovery (%)

H2 O2 in sample (␮g/l)

Ref. method [18] (␮g/l)

145 (± 5.1%)

151

87 (± 2.3%)

80

92 (± 3.0%)

–

97 103 91 97 106

a Each sample was analyzed five times. The results are the averages. Sample 1, 2 and 3 were collected at the campus of Tokyo Metropolitan University on 12 January 2000 from 1:00–2:00 pm and 7:00–8:00 pm and 20 April 2000 from 11:00–14:00 pm, respectively.

was obtained. The decompositions of H2 O2 and OCl− with resin as catalyst are different. They produced different active species. The detail CL reaction of OCl− is continuously studied in our laboratory. The effect of OCl− on our target analyte, rainwater, could be neglected because H2 O2 and OCl− can not exist at the same solution. Another influent factor, the oxygen dissolved in water could not be overlooked. When we injected the water saturated with oxygen into the flow system, a CL signal corresponding to 10−6 M H2 O2 level was recorded. In order to eliminate the determination error from the dissolved oxygen in water, the water saturated with nitrogen and kept for 2 days was used to prepare the H2 O2 standard solution or diluting the sample solution. These data showed that the selectivity of the present method is high enough for the determination of H2 O2 in rainwater samples without any special pre-treatment. 3.6. Analysis of real samples The proposed method was applied to determine the trace amount of H2 O2 in rainwater samples. The results of several standard addition experiments on samples are shown in Table 4. Considering the instability of H2 O2 at low concentration, the analysis should be done immediately after collecting the samples. The results showed that the recoveries for the samples are 91–106%, which are good enough for practical use. In this work, on addition of a series of standard solution to the sample, all samples gave straight standard addition plots with the slopes very similar to that of the calibration graph. The relative standard deviation was 2–7% (n = 5) for each sample.

On the other hand, we also tried to use other methods, e.g. spectrophotometric or fluorometric method to determine the concentration of H2 O2 in rainwater. But the concentration of H2 O2 in rainwater was too low to be detected well. The luminol/Co(II)/H2 O2 /OH− CL system was also used as a comparison method. The sensitivity of this CL system for H2 O2 is high enough, but it suffers from the interference ions seriously, especially the transition metal ions. The CL system, KIO4 /CO3 2− [18] without luminol reagent was used as a comparison method for the determination of H2 O2 . Both of these two methods have given out the satisfactory results.

4. Conclusion This is the first report to use resin treated with transition metal complex as catalyst in the CL analysis. The immobilization of cobalt complex on the resin not only saves the reagent but also improves the catalytic activity for the decomposition of H2 O2 . A flow through H2 O2 CL sensor was developed based on the H2 O2 /resin/luminol heterogeneous CL system. The carrier luminol solution is an unbuffered solution which makes the present method with a highly selective and sensitive to the determination of H2 O2 . References [1] D.T. Saeyer, Oxygen Chemistry, Oxford University Press, New York, 1991. [2] Chemistry of Active Oxygen Species, Kikan Kagaku Sosetsu, The Chemical Society of Japan, Tokyo, 1990.

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[10] S. Hanaoka, J.-M. Lin, M. Yamada, Anal. Chim. Acta 409 (2000) 65. [11] M.Y. El-Sheikh, A.M. Habib, F.M. Ashmawy, A.H. Gemeay, A.B. Zaki, J. Mol. Catal. 55 (1989) 396. [12] M.Y. El-Sheikh, A.M. Habib, A.H. Gemeay, A.B. Zaki, J. Mol. Catal. 77 (1992) 15. [13] I.A. Salem, J. Mol. Catal. 80 (1993) 11. [14] M. Seno, M. Abe, T. Suzuki, Ion Exchange—Advanced Separation Technique and Principles, Kotansya, Tokyo, 1991. [15] K.-D. Gundermann, F. McCapra, Chemiluminescence in Organic Chemistry, Springer, Berlin, Germany,1987, pp. 77–106. [16] C. Walling, ACC. Chem. Res. 8 (1975) 125. [17] F. Helfferich, Ion Exchange, McGraw-Hill, New York, 1962. [18] J.-M. Lin, M. Yamada, Anal. Chem. 71 (1999) 1760.