ANALYTICAL SCIENCES JUNE 1993, VOL. 9 351

Triiodide Ion-Selective Electrode Based on Manganese(III)- Tetraphenylporphine

Hirofumi SuzuKI*, Hiroshi NAKAGAWA*,Masaki MIFUNE**and Yutaka SAIT0*** *Pharmaceuticals Research Center, Kanebo, Ltd, Tomobuchi, Miyakojima, Osaka 534, Japan ** The Graduate School of Natural Sciences and Technology, Okayama University, Tsushima-Naka, Okayama 700, Japan ***Faculty of Pharmaceutical Sciences, Okayama University, Tsushima-Naka, Okayama 700, Japan

A new sensitive and selective electrode for triiodide ion has been developed. The electrode was constructed by incorpo- rating Mn(III)-tetraphenylporphine into a plasticized poly(vinyl chloride) membrane. The electrode shows super- Nernstian response over the range 10-5to 10-3M triiodide ion in the pH range of 2 to 9, with an anionic slope of 87 mV/ concentration decade. The electrode exhibits high specificity for triiodide ion over other anions (salicylate, benzoate, propionate, perchlorate, iodide, bromide, chloride, fluoride, nitrate and bicarbonate). The electrode has been applied to measure triiodide ion generated from iodine. The results have indicated that iodine can be determined by simple potentiometric measurements. A similar method may be applied to determine oxidizing agents, bromine and hydrogen peroxide, which oxidize iodide ion to iodine.

Keywords Ion-selective electrode, manganese(III)-tetraphenylporphine, triiodide ion, iodine, hydrogen peroxide, bromine

Triiodide ion is frequently used as a standard solution to titrate the reducing agents (e.g., arsenic(III), ascorbic Experimental acid, formaldehyde and glucose) and is used as a target ion to determine oxidizing agents (e.g., hydrogen Reagents peroxide, bromine and chlorine). For the determina- Methyltris(tetradecyl)ammonium chloride (MTTDA- tion of triiodide ion, titrimetry with thiosulfate solution, Cl), methyltris(dodecyl)ammonium chloride (MTDA- polarography and spectrophotometry are frequently Cl), tetraphenylporphine (TPP) and 2-fluoro-2'-nitro- used. However, most of these methods are either time diphenyl ether (FNDPE) were obtained from Dojindo consuming or require expensive instruments or both. Laboratories (Kumamoto, Japan). PVC (high relative On the other hand, numerous ion-selective electrodes molecular weight) was purchased from Aldrich (Mil- have been developed and applied to a wide area in waukee, WI, USA). The porphyrin compounds, man- analytical chemistry.12 Ion-selective electrodes offer ganese(III)-tetraphenylporphine (Mn-TPP), copper(II)- advantages of simplicity, rapidity and relatively low cost tetraphenylporphine (Cu-TPP), iron(III)-tetraphenyl- over other established analytical methods. So far, no porphine (Fe-TPP), cobalt(III)-octaethylporphine (Co- triiodide ion-selective electrode has been reported. It is OEP), manganese(III)-phthalocyanine (Mn-PC), man- worth noting that plastic membrane electrodes con- ganese(III)-protoporphine (Mn-PP) and manganese- taining metalloporphyrin have been used for anion- (III)-tetrakis(p-carboxyphenyl)porphine (Mn-TCPP) selective electrodes.3-10 We have focused our interest on were prepared according to the methods described in the these plastic membrane electrodes containing metal- literature. 11,12 Iodine and bromine standard solutions loporphyrin and found that the plastic membrane (0.1 M) for volumetric analysis (the Pharmacopoeia of electrode containing Mn(III)-tetraphenylporphine was Japan) were purchased from Wako Pure Chemicals sensitive and selective for triiodide ion. (Osaka, Japan). Peroxidase from horseradish (POD) In this paper, we report on the triiodide ion-selective was purchased from Sigma (St. Louis, MO, USA). All electrode based on metalloporphyrin. The electrode is of the other chemicals used were of analytical reagent also applicable to the determination of oxidizing agents grade. that can produce iodine from a reaction with iodide anion. Electrode preparation The solvent polymeric membranes were prepared 352 ANALYTICAL SCIENCES JUNE 1993, VOL. 9 according to the method described previously13 using a solutions of a constant level of interference (10-1 M) and liquid membrane electrode kit (Denki Kagaku Keiki). several concentrations of triiodide ion from 103 to The membrane composition of the metalloporphyrin 10-5 M. based electrode was typically 5% (w/ w) metalloporphy- rin, 68% (w/ w) FNDPE and 27% (w/ w) PVC. The Applicationsfor oxidizing agents membrane composition of the quaternary ammonium Hydrogen peroxide. Aliquots (5 ml) of hydrogen per- based electrode was 1% (w/ w) quaternary ammonium- oxide solution (10-6 to 10-3M) were added to 20 ml of chloride, 66% (w/ w) FNDPE and 33% (w/ w) PVC. 10-2M KI solution (pH 5.6) and 1 ml of POD solution These compounds were dissolved in tetrahydrofuran (10 unit/ ml). This mixture was placed at 25°C for (THF). A PTFE membrane filter was dipped in the 15 min. Then the e.m.f. values of the resulting sample THE solution and placed on top of the electrode kit. solutions were measured by the triiodide ion-selective Several drops of the mixture were applied to the PTFE electrode with the reference electrode. membrane and the membrane was air dried at room Bromine. Aliquots (10 ml) of bromine solution (10-6 to temperature. The internal reference solution of the 10-3M) was added to 50 ml of water, 2 ml of 36% electrode was 10-2M LiCI solution. The electrode was hydrochloric acid and 2 ml of 1 M KI solution (pH 5.6). preconditioned by soaking overnight in a 10-4 M tri- This mixture was placed at 25° C for 5 min. Then, the iodide ion solution of acetate buffer before use and then e.m.f. values of the resulting sample solutions were stored in the same solution. measured by the triiodide ion-selective electrode with the reference electrode. E.mf measurements Cell assemblies of the following type were used: Ag- AgCI 110-2 M LiCI solution II PVC membrane I test Results and Discussion solution (0.1 M acetate buffer, pH 5.6) I Ag-AgCI double junction reference electrode. All measurements Selection of electroactive agent were made at 25±1°C with continuous stirring of the As electroactive agents for the triiodide ion-selective solutions at a constant rate by using a magnetic stirrer. electrode, we tested metalloporphyrins3-6, quaternary ammonium compounds15-19and metallophthalocyanine. Apparatus The membranes of these electrodes consisted of PVC and All potentiometric measurements were carried out FNDPE as a plastic matrix and a plasticizer, respectively. using an expandable ion analyzer (Orion, Cambridge, The e.m.f. responses of these electrodes to triiodide ion MA, USA, Model EA920) and potential signals were are summarized in Table 1. These electrodes responded plotted on a strip-chart recorder. An Orion Ag-AgCI well to triiodide ion. In the TPP based electrodes, the double junction reference electrode (Model 90-02) slope is affected by the central metal of TPP. The Mn- containing 10% potassium nitrate in the outer com- TPP based electrode exhibited the largest slope. partment as the reference was used. Measurements of Porphyrine species also influenced somehow the pH and temperature were performed with a glass-calomel response of the electrode. When TPP was used, the electrode (Horiba, Kyoto, Japan, Model 6026) and an slope was the largest and the linear response range was Orion automatic temperature compensator, respectively. the widest. Quaternary ammonium compounds were An Orion iodide ion-selective electrode (Model 94-53) inferior to the Mn-TPP electrode in terms of slope and/ with the Onion reference electrode (Model 90-02) was or linear range. Therefore, we selected Mn-TPP as the used. electroactive agent for the triiodide ion-selective electrode. We checked the effect of concentration of Standards Mn-TPP in the membrane to the slope. As shown in Standard 0.1 M iodine contains 12.7 g of iodine, 40 g Fig. 1, the response slope increased with increasing of potassium iodide (KI) and 1 ml of 36% hydrochloric concentration of Mn-TPP in the membrane up to 1 % acid in 1000 ml of water. Standard 10-3M triiodide ion (w/w). Therefore, we selected the following membrane solution was prepared by diluting 1 ml of 0.1 M iodine compositions for the triiodide ion-selective electrode: 5% standard solution with 23 ml of 0.1 M acetate buffer (w/ w) Mn-TPP, 68% (w/ w) FNDPE and 27% (w/ w) (pH 5.6) and adding 10-2M KI solution (0.1 M acetate PVC. buffer, pH 5.6) to make 50 ml. Standard solutions of The Mn-TPP based electrode and some electrodes in 10-' M to 10-4M triiodide ion for calibrations of the Table 1 showed super-Nernstian response (> -59 mV) electrode were prepared by diluting 10-3M triiodide ion toward triiodide ion. In the case of the salicylate- solution with 10-2 M KI solution (0.1 M, acetate buffer, selective electrode based on Sn-TPP10, an increase in pH 5.6). concentration of metalloporphynin in membranes caused larger slopes and super-Nernstian behavior was observed Selectivity coefficients at concentrations above 1% (w/w). As shown in Fig. 1, The selectivity coefficients were evaluated by the even at concentrations below 1% (w/ w), the Mn-TPP mixed solution method according to IUPAC recom- based electrode exhibited super-Nernstian response to mendations.14 The potential was measured with triiodide ion. On the other hand, the potentiometric ANALYTICAL SCIENCES JUNE 1993, VOL. 9 353 response of the Mn-TPP based electrode to the other extent of masking the contribution of the anionic anions (C104- and I-) did not exhibit super-Nernstian impurities in the PVC membranes. The calibration response (slope= -49 and -43 mV/ concentration intercept, C, may be described as a function of membrane decade, respectively). parameters21, e.g., There is no clear-cut explanation for the super- Nernstian response. However, one possible mechanism c= S log [KextI a (0) for the super-Nernstian response of Mn-TPP based thiocyanate ion-selective electrode is the failure in co-ion where S is the electrode slope, Kextis the ion extraction repulsion, which was introduced by negatively charged constant for triiodide ion and a(0) is the membrane impurities in PVC.5 Similarly, in the case of the concentration of ion-exchanger. The differences of the triiodide ion-selective electrode, the membrane might not intercepts would be results of the differences of S, Kext be ideally permselective to anions due to the fixed anionic and a(0) of each electrode. sites in PVC.20 As described later, triiodide ion is ex- The response mechanism of the Mn-TPP based pected to strongly associate with the Mn-TPP in view of electrode is presumed that Mn(III) in the center of the the high selectivity of the Mn-TPP based electrode porphyrin acts as an anion carrier similarly to other toward triiodide ion. Therefore, the Mn-TPP could not metalloporphyrines based electrodes.3-lo completely mask the contribution of the anionic impurities in the PVC matrix and the Mn-TPP based Calibration curve electrode would show the super-Nernstian response. Typical calibration curves (Fig. 2) are constructed on The calibration slopes and intercepts of the electrodes semi-logarithmic graph paper by plotting e.m.f.s versus in Table 1 are different. The differences of the associ- concentration of triiodide ion for the Mn-TPP based ation strength between triiodide ion and the ion- electrode and the iodide ion-selective electrode. The exchangers in the membranes would lead to the Mn-TPP based electrode exhibited a linear response in differences of the slopes, e.g. due to the differences of the the concentration range of 10-5 to 10-3 M triiodide ion, while the iodide ion-selective electrode did not respond to triiodide ion. The response time of the Mn-TPP based

Table 1 Response characteristics for various PVC: FNDPE membrane-based triiodide ion-selective electrodes

Fig. 2 Calibration curves for triiodide ion using the Mn-TPP a. Average value and standard deviation of value for multiple based electrode (0) and the iodide ion-selective electrode (o). calibration.

Table 2 Selectivity coefficients for the triiodide ion-selective electrode

Fig. 1 Effect of the Mn-TPP concentration in the membrane on the slope of calibration curve. . ,

354 ANALYTICAL SCIENCES JUNE 1993, VOL. 9

The calibration lines for hydrogen peroxide and bromine by the above methods were straight between 10-5 - 10.3 M (slope= -69.0 and -59.0 mV/ decade, respec- tively). In the same manner, this method may be able to determine residual chlorine and ascorbic acid.

In conclusion, an electrode which is sensitive and highly selective for triiodide ion has been developed. The potentiometric method by use of triiodide ion- electrode has the advantages of simplicity, rapidity and wide adaptability to determination of reducing and Fig. 3 Effect of pH on the potential of the Mn-TPP based electrode for 10.4 M triiodide ion. oxidizing agents.

References electrode to reach a value of ±1 mV from the final equilibrium potential in this concentration range was 1. J. Janata, Anal. Chem., 64,196R (1992). short, viz., within 60 s. The linear range did not change 2. J. Koryta, Anal. Chim. Acta, 233, 1(1990). and the slope stayed stable within ±1.3 mV/decade over 3. D. Ammann, M. Huser, B. Krautler, B. Rusterholtz, P. a period of 2 months. Schulthess, B. Lindemann, E. Halder and W. Simon, Helv. Chim. Acta, 69, 849 (1986). 4. A. Hodinar and A. Jyo, Anal. Chem., 61,1169 (1989). Selectivity 5. A. Jyo and H. Egawa, Anal. Sci., 8, 823 (1992). The potentiometric anion selectivity coefficients were 6. H. Abe and E. Kokufuta, Bull. Chem. Soc. Jpn., 63,1360 determined by the mixed solution method. The Mn- (1990). TPP based electrode showed higher selectivity toward 7. S. Daunert, S. Wallace, A. Floride and L. G. Bachas, triiodide ion than the other anions, as shown in Table 2. Anal. Chem., 63, 1676 (1991). This is probably because triiodide ion is more 8. N. A. Chaniotakis, A. M. Chasser and M. E. Meyerhoff, hydrophobic and/ or associates with Mn-TPP more Anal. Chem., 60, 188 (1988). strongly than the other anions. 9. S. C. Ma, N. A. Chaniotakis and M. E. Meyerhoff, Anal. Chem., 60, 2293 (1988). Influence of pH 10. N. A. Chaniotakis, S. B. Park and M. E. Meyerhoff, Anal. The influence of pH on the Mn-TPP electrode re- Chem., 61, 566 (1989). sponse indicated an almost constant potential in the pH 11. T. Yonetani and K. Asaka, J. Biol. Chem., 244, 4580 range of 2 to 9 (Fig. 3). The significant increase of the (1986). 12. R. F. Pasternack, L. Francesconi, D. Raff and E. Spiro, potential observed above pH 9 may be due to the Inorg. Chem., 12, 2606 (1973). decrease of triiodide ion by the reaction of iodine with 13. H. Suzuki, K. Akimoto, H. Nakagawa and I. Sugimoto, hydroxide ion according to the following equations:22 J. Pharm. Sci., 78, 62 (1989). 14. IUPAC Analytical Chemistry Division, Commission on I2 + 2OH--- IO- + I- + H2O, Analytical Nomenclature, Pure Appl. Chem., 48, 127 (1976). 3I0---~ 103- + 21-. 15. C. J. Coetzee and H. Freiser, Anal. Chem., 41, 1128 (1969). Analytical application to oxidizing agents 16. H. Hara, S. Okazaki and T. Fujinaga, Anal. Chim. Acta, The potentiometric method by use of the triiodide ion 121, 119 (1980). 17. A. Mitsana-Papazoglou, E. P. Diamandis and T. P. Had- selective electrode was applied to the determination of jiioannou, Anal. Chim. Acta,159, 393 (1984). oxidizing agents. A potassium iodide solution is added 18. V. V. Cosofret and R. P. Buck, J. Pharm. Biomed. Anal., 4, to the sample containing oxidizing agents. The 45 (1986). electrode senses the amount of triiodide ion produced 19. G. N. Valsami, P. E. Macheras and M. A. Koupparis, when potassium iodide reacts with the oxidizing agent. Analyst [London], 114, 387 (1989). In the case of hydrogen peroxide and bromine, this 20. A. van den Berg, P. D, van der Wal, M. Skowronska- method can be applied according to the equations: Ptasinska, E. J. R. Sudholter, D. N. Reinhoudt and P. Bergveld, Anal. Chem., 59, 2827 (1987). H202 + 21- + 2H+ P 12+ 2H20, 21. G. J. Moody, R. K. Owusu and J. D. R. Thomas, Analyst [London], 112, 1347 (1987). I2 + I-~ 13, 22. N. Iritani and T. Ouno (eds.), "Kaisetsu Yakuhin Teiryou Bunseki (Quantitative analysis of drugs, in Japanese)" , 3rd ed., p. 295, Nankodo, Tokyo, 1977. Br2 + 2K1( 12 + 2KBr, (Received November 26, 1992) 12+I( - 13. (Accepted March 12, 1993)