Responses of Metalloporphyrin-Based Ion-Selective Electrodes to pH

Responses of Metalloporphyrin-Based Ion-Selective Electrodesto pH

Takuya Inoue, Toshiyuki Baba, Akio Yuchi*

Graduate School of Engineering, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466-8555, Japan*e-mail: [email protected]

Received: July 26, 2010;&Accepted: September 20, 2010

AbstractThe ISEs based on [M(tpp)Cl] (M: Al, Ga, In, Mn, Fe; H2tpp: tetraphenylporphin) had pH responses across theirrespective pH ranges, which had some correlation with the pH ranges of the two-phase hydrolysis. Such pH respons-es are ascribed to the phase boundary potentials relating to the acid-base pairs of [M(tpp)(H2O)]+ and[M(tpp)(OH)] and/or [M2(tpp)2O]. The potential responses of the In and Fe complexes had the upper limitation topH of 90 % hydrolysis, whereas those of the Al and Ga complexes had the extension to at least pH 12, indicatingstable existence of [M(tpp)(H2O)]+ even in contact with strongly alkaline solutions.

Keywords: Ion-selective electrodes, pH response, Porphyrin complexes, Potentiometry, Two-phase reactions

DOI: 10.1002/elan.201000475

1 Introduction

Metalloporphyrins have been used extensively as the car-rier for the potentiometry of anions with ion-selectiveelectrodes (ISEs) and for their optical detection. The cen-tral metal ions exhibit a wide range of anion selectivities,depending on the “hard and soft acids and bases” princi-ple. For example, Al(III) complexes show a high selectivi-ty to F� [1–3], while In(III) complexes to Cl� [4–9]. Witha view to enhancing the selectivity, reducing the electricresistance, elongating the life time, and ensuring the theo-retical response, lipophilic anions were added at somemolar fractions to the complex as in other charged carri-ers [10, 11]. The addition, however, induces the formationof cationic dimers [12,13], which may interfere with relia-ble potentiometry. The merits and demerits of this reac-tion were discussed [14], and the countermeasure, such asthe use of picket fence-porphyrins and the immobilizationof the carrier onto a polymer network, have been pro-posed [12,15–17].

The response to pH is another issue to be considered.The ISEs based on metalloporphyrins, in the absence of asample anion, showed a linear potential response to pHover a wide range; (carrier, pH range, potential slope):([Mn(tpp)Cl], 3.5–10, 52) [18]; ([Sn(tpp)Cl2], 3.5–10, 40)[19]; ([Co(tpp)(NO2)], 4–10, 40) [20]; ([MoO(tpp)(OEt)],1.5–11, 57) [21]; ([In(oep)Cl], 2.5–4, 59) [4]; ([Cr(tpp)Cl],6–9.5, 49) [22]; (In(HEPEAC)Cl), 7.5–10, 46) [15]; ([TiO-(tpp)], 2–11, 44) [23]; ([VO(tpp)], 4–11, 48) [23]; ([Hf-(tpp)Cl2], 7–12, 52) [23]; ([Zr(tpp)(OH)2], 2–13, 43)[23,24, H2tpp: tetraphenylporphin, H2oep: octaethylpor-phin, HEPEAC: a porphyrin covalently attached to apolymer]. The pH adjustment of a sample solution is thus

usually essential for the potentiometric determination ofan anion, although the pH response is diminished in thepresence of the sample anion [25–30]. Such a pH re-sponse is ascribed to the acid-base equilibrium between apositively charged aqua complex formed by the dissocia-tion of an auxiliary ligand from the carrier and a neutralOH complex in the membrane phase [18,19] or otherwiseascribed to the direct replacement of an auxiliary ligandby OH� [21]. The presence of such an OH� carrier is,however, still under question [31].

The objective of this paper is to compare the pH re-sponses of a series of trivalent metal complexes, such asthose of Al(III), Ga(III), In(III), Mn(III), and Fe(III),under the same conditions and to correlate the pH re-sponses to their hydrolyzing tendencies studied by thetwo-phase reactions to confirm the pH response mecha-nism. The effects of lipophilic anions were also examined.

2 Experimental

2.1 Reagents

The aluminum-chloro complex, [Al(tpp)Cl], was preparedas described in literature [32]. Since the conversion to thehydroxo complex with methanol used as a solvent accord-ing to this literature was unsuccessful due to the contami-nation by [Al(tpp)(OCH3)], the two-phase reaction witha NaOH solution was adopted. The product was identi-fied as the dimeric complexes of [Al2(tpp)2O] by the ap-pearance of a characteristic IR band at 850 cm�1 assignedto v(Al�O�Al). As described in the text, this complexwas transformed into [Al(tpp)(OH)] according to thetotal concentration of the complex and water in organic

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solvents. The gallium- and indium-chloro complexes wereprepared as described in literature [33,34], and were simi-larly converted to [Ga(tpp)(OH)] and [In(tpp)(OH)] bythe two-phase reaction. The other complexes of [Mn-(tpp)Cl] (Aldrich) and [Fe(tpp)Cl] (Wako) and the lipo-philic anions, such as sodium tetrakis[3,5-bis(trifluorome-thyl)phenyl]borate (NaTFPB, Wako), sodium dodecyl sul-fate (NaDS, Wako), and sodium perfluorooctanesulfonate(NaPFOS, Wako), were used as received. As a membranesolvent, 2-nitrophenyl octyl ether (o-NPOE) was pre-pared by modifying the method in the literature, while di-octyl sebacate (DOS) was used as received. Chloroben-zene was purified by successively shaking with sulfuricacid, a sodium hydroxide solution, and water. Poly(vinylchloride) (PVC) was purified by pouring a tetrahydrofur-an solution into methanol.

2.2 Two-Phase Reactions

Chlorobenzene was used as an organic solvent for two-phase reactions by taking into account the solubilities andstabilities of complexes. Aliphatic chlorohydrocarbonshaving the higher solubilities were not suited, due to theirdechlorination by OH complexes as in the case of theNb(V) complex [35]. The chlorobenzene solution of eachcomplex was shaken with a series of solutions containingpH buffers, lipophilic anions, or acids. The chloride elutedinto the aqueous phase was determined by ion chroma-tography in the reactions with buffers, while a change inthe absorption spectrum of the chlorobenzene phase wasmonitored in the reactions with lipophilic anions or acids.

2.3 Potentiometry

The polymeric membrane consisted of 1 % of the com-plex, 33% of PVC, and 66 % of the membrane solvent byweight, with the addition of 0–100 mol% of the lipophilicanion. The electromotive force of the cell, Ag/AgCl jKCl-(satd.) j j test solution jmembrane j internal solution jAg/AgCl, was monitored by a potentiometer at 25�0.1 8C.The potential with a drift of less than 0.1 mV min�1 wasmeasured during the titration of the H2SO4-containingtest solution by a NaOH solution to pH 12. For assess-ment of the reversibility, the resulting solution was fur-ther titrated with a H2SO4 solution.

3 Results and Discussion

3.1 Hydrolysis of [M(tpp)Cl] by Two-Phase Reaction

The organic phase containing [M(tpp)Cl] at the total con-centration (Ccomp) of 10�5 mol L�1 level was shaken withvarious aqueous phases containing a series of buffers atdifferent pH values for 48 hours; Cl� dissociated from thecomplex and dissolved into the aqueous phase was deter-mined by ion chromatography. It took a fairly long timefor the equilibration. A portion of the Al complex wasprecipitated by this reaction, due to a limited solubility of

the resulting complex (10�5.3 mol L�1). The percent recov-eries of chloride plotted against pH of the aqueous phaseafter the equilibration are shown in Figure 1. Each sig-moidal curve was shifted to a higher pH range in theorder of Al<Fe<Ga< In<Mn. A simple hydrolysis re-action giving [M(tpp)(OH)]o, where the subscript �o� indi-cates the organic phase, is assumed to be,

½MðtppÞCl�o þH2OÐ ½MðtppÞðOHÞ�o þHþ þ Cl� ð1Þ

and the equilibrium constant was optimized to give aminimum error square sum on the percent recovery ofCl� . The equilibrium constants were>10�3 for Al, 10�6.4

for Fe, 10�7.4 for Ga, 10�11.7 for In, and 10�12.8 for Mn. Thecalculated curves using these constants reproduced theexperimental points.

3.2 Dissolution States of OH Complexes inChlorobenzene

The effects of the total concentration of [M(tpp)(OH)]on the absorption spectrum were studied in water-saturat-ed chlorobenzene using a series of cells with lengths of 1to 100 mm. The spectra of the Ga and In complexes wereindependent of Ccomp between 10�6 and 10�4 mol L�1. Incontrast, a shift of the absorption maximum in the Soretband to a longer wavelength with a decrease in Ccomp wasfound for Al (416 to 420 nm, Figure 2), Mn (471.5 to478.5 nm) and Fe (410 to 418 nm) complexes. The meas-urable concentration range for the Al complex was ex-tremely limited both by the lower saturated concentrationin chlorobenzene and by the higher adsorptivity to avessel than the other complexes. The wavelength of theabsorption maximum in the Soret band plotted againstlog Ccomp are shown in the insertion of Figure 2. In analo-gy with the Zr complex [24], this change was ascribed tothe monomer-dimer equilibrium as given by Equation 2.

2½MðtppÞðOHÞ�o Ð ½M2ðtppÞ2O�o þH2O ð2Þ

Fig. 1. Hydrolysis of [M(tpp)Cl] by two-phase reactions. Organ-ic phase: chlorobenzene. Ccomp =10�5 mol L�1. M: ~, Al; *, Fe;^, Ga; &, In; &, Mn. Solid lines were calculated using the con-stants obtained.

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Responses of Metalloporphyrin-Based ISEs to pH

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The relative stability of the dimer to the monomer washigher in the order of Fe>Al,Mn�Ga,In, although theadsorption of the complex onto the vessel interfered withthe rigorous equilibrium analysis. The two-phase hydroly-sis equilibrium given by Equation 1 is affected by the di-merization equilibrium with regard to the Fe, Al and Mncomplexes. The constants obtained above are apparent orconditional in this sense but will be used without any cor-rection to facilitate the comparison of these metal com-plexes.

The effects of the membrane solvents (DOS and o-NPOE) having the hydrogen bond-accepting ability onthe spectrum of [M(tpp)(OH)] and/or [M2(tpp)2O] wereexamined. The respective absorption maxima of the mon-omeric Ga and In complexes at 10�5 mol L�1 level wereslightly shifted to shorter wavelengths by the addition ofeither of these solvents at 7 �10�4 mol L�1. Among the di-merizable complexes, the Mn and Fe complexes gave neg-ligibly small spectral changes, while the Al complex gavea large shift to a longer wavelength (Figure 2, up to423 nm). This was ascribed to the reverse reaction ofEquation 2, which is induced by the stabilization of themonomeric species by the hydrogen bond between theOH coordinating to the Al center and the carbonyl O ofthe membrane solvent. Since the actual concentration ofthe carrier in the solvent within the PVC membrane wasas high as>10�2 mol L�1, the dimeric structure was ex-pected to be kept for the Mn and Fe complexes in the ab-sence of anions.

3.3 Two-Phase Reactions of [M(tpp)(OH)] and/or[M2(tpp)2O] with Lipophilic Anions

The chlorobenzene solution of [M(tpp)(OH)] and/or[M2(tpp)2O] (M: Al, Ga, In) was shaken with various

aqueous solutions containing varying concentrations oflipophilic anions, such as NaTFPB, NaDS, and NaPFOS.The total concentration of the complex was kept at10�5 mol L�1 level for the Ga and In complexes and at 4 �10�6 mol L�1 for the Al complex. Both [Al(tpp)(OH)]and [Al2(tpp)2O] coexist at this concentration. Thechanges in the visible absorption spectra of the organicphases containing the Ga complex and the Al complexare shown in Figure 3. Those of the preliminary study onthe In complex had been described elsewhere [36].

On the reaction of [In(tpp)(OH)] with NaTFPB, boththe 2 : 1 and 1 :1 ion pairs were stoichiometrically formedin a stepwise manner even at an unfavorable pH level of6:

2½MðtppÞðOHÞ�o þNaþ þ TFPB� Ðð½M2ðtppÞ2ðOHÞ�,TFPBÞo þNaþ þOH�

ð3Þ

ð½M2ðtppÞ2ðOHÞ�,TFPBÞo þNaþ þ TFPB� þ 2H2OÐ2ð½MðtppÞðH2OÞ�,TFPBÞo þNaþ þOH�

ð4Þ

In contrast, only the 1 :1 ion pair was stoichiometricallyformed with NaDS or NaPFOS at the same pH level of6:

½MðtppÞðOHÞ�o þNaþ þ ðDS=PFOSÞ� Ðð½MðtppÞðH2OÞ�,ðDS=PFOSÞÞo þNaþ þOH�

ð5Þ

An unusual reaction of metathesis found for the two-phase reaction with sodium tetraphenylborate is of inter-est from the view of synthetic organic chemistry but isbeyond the scope of this research [36].

The reactions of the Ga complex with NaTFPB weresubstantially the same as those of the In complex at pH 6(Figure 3a). The absorption maximum at 422 nm wasonce shifted to a shorter wavelength of 403.5 nm andthen back to 418.5 nm. The reactions with NaDS andNaPFOS gave only the 1 :1 ion pair with a spectral shiftfrom 422 to 419.5 nm, while a pH decrease from 6 to 3was essential for the stoichiometric reaction (Figure 3b).

The reactions of the Al complex with NaTFPB wereslower than the Ga and In complexes. Only the 1 : 1 ionpair was stoichiometrically formed after 12 hours with aspectral shift from 418.5 to 421 nm (Figure 3c). Since anappreciable portion of the Al complex was the dimericspecies, a spectral shift in an apparently reverse directionto those of other metal complexes was observed. After 24hours, however, partial formation of the 2 : 1 ion pair wassuggested by an increase in absorption at around 405 nm(Figure 3d). The relatively low stability of the 2 :1 ionpair was ingeniously utilized for the potentiometric andoptical detections of F� [1–3]. The reactions with NaDSand with NaPFOS giving the 1 : 1 ion pairs were not quan-titative even at pH 3.

Fig. 2. Changes in absorption spectra of [Al(tpp)(OH)] and/or[Al2(tpp)2O] by the total concentration (solid lines, from 10�5.6 to10�5.3 mol L�1) and by the addition of DOS (dotted lines, 7� 10�4

and 2.1 �10�3 mol L�1) and changes of the absorption maximumwavelength of [M(tpp)(OH)] and/or [M2(tpp)2O] by the totalconcentration in insertion. M: ~, Al; *, Fe; ^, Ga; &, In (left or-dinate); &, Mn (right ordinate).

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In summary, the ion-pair formation reaction was lessfavorable for the complex of a metal center having thehigher affinity to OH� ions, e.g. the Al complex. Lipo-philic anions with long alkyl chains did not form 2 :1 ionpair but formed 1 : 1 ion pair.

3.4 Two-Phase Reactions of [M(tpp)(OH)] with Acids

The two-phase reactions of [In(tpp)(OH)] and [Ga-(tpp)(OH)] with various concentrations of four differentacids having distinct characteristics (HClO4, HNO3, HCl,CH3COOH) were examined as in the cases of Zr(IV) andSn(IV) complexes [24,37]. The changes of the molar ab-sorption coefficient at the respective characteristic wave-lengths plotted against the logarithmic concentration ofan acid are shown for In-HClO4, HNO3, and Ga-HClO4

in Figure 4. In the reaction with HClO4, for example, theabsorption maximum shifted from 428 to 423 nm at theconcentration of 10�5 to 10�1 mol L�1 with no observationof a single isosbestic point. A slight increase in absorptionat 411.5 nm at a HClO4 concentration of 10�3 mol L�1 in-dicated partial formation of a cationic dimer [In2-(tpp)2(OH)]+ [11,12, 36]:

2½InðtppÞðOHÞ�o þHþ þ ClO4� Ð

ð½In2ðtppÞ2ðOHÞ�þ,ClO4�Þo

ð6Þ

There were further smaller spectral changes for HX:HNO3 (Figure 4b), HCl and CH3COOH. Only a roughmeasure was obtained for the following reaction (106 forHClO4, 107 for HNO3, and 1011 for HCl):

½InðtppÞðOHÞ�o þHþ þX� Ð ½InðtppÞX�oor ð½InðtppÞðH2OÞ�þ,X�Þo

ð7Þ

The constant of the reaction with HCl is in reasonableagreement with the constant of the reverse reaction givenby Equation 1. Not only electrostatic but coordinative in-teractions contribute to the stabilities of the complexeswith NO3

� and especially with Cl� .The reactions of [Ga(tpp)(OH)] with HClO4 (Fig-

ure 4c) corresponding to Equations 6 and 7 were ob-served at the higher concentrations than those of [In(tpp)(OH)], due to the stronger bonding of OH� to theGa center.

3.5 pH Responses of ISEs Based on [M(tpp)Cl]

The pH responses of the ISEs based on [M(tpp)Cl] usingDOS are shown in Figure 5a. The complexes of Group IIImetal ions showed sub-Nernstian pH responses over awide range. (M, membrane solvent, pH range, potentialslope/mV decade�1): (Al, DOS, 1.5–12, �44.6); (Al, o-NPOE, 1.5–12, �43.5); (Ga, DOS, 4–12, �45.1); (Ga, o-NPOE, 5–12, �42.1); (In, DOS, 6–11, �53.6); (In, o-NPOE, 6–12, �49.6). The dynamic ranges with DOS werecomparable to or wider than those with o-NPOE, where-as the potential slopes with DOS were steeper than thosewith o-NPOE. The Mn complex with DOS gave a re-sponse at pH>9; the behavior was appreciably differentfrom those reported previously using dibutyl sebacate asa membrane solvent [18]. In contrast, the ISEs based onthe Fe complex showed a pH response at pH<5.

Fig. 3. Changes in absorption spectra on the reactions of [Ga(tpp)(OH)] and [Al(tpp)(OH)] and/or [Al2(tpp)2O] with NaTFPB andNaDS. Complex: (a,b) [Ga(tpp)(OH)]; (c,d) [Al(tpp)(OH)] and/or [Al2(tpp)2O]. Lipophilic anion: (a,c,d) NaTFPB; (b) NaDS. Reac-tion time: (a,b,c) 12 h; (d) 24h. pH: (a,c,d) 6; (b) 3. l/nm for a molar ratio plot: (a,b) 421.5; (c) 416.5; (d) 418.5.




The pH-responsive liquid membrane ISEs based on lip-ophilic amines and lipophilic anions had been developedin early 1980s [38]. The correlation of the dynamic pHrange to the basicity of the carrier was found [39], andlater the theoretical formulation was thoroughly de-scribed [40]. The phase boundary potential is expressedby Equation 8:

E ¼ E0 þ 0:059 logð½C�m½Hþ�=½CHþ�mÞ, ð8Þ

where C denotes a neutral carrier and the subscript m de-notes a membrane phase. The ratio of [C]m/[CH+]m wascontrolled by the incorporation of a lipophilic anion, theamount of which was theoretically optimized at 50 mol%to the chemical amount of a carrier. Deviation from thelinear pH response was observed in a lower pH regiondue to the co-extraction of proton and an anion, and wasobserved more markedly in a higher pH region due tothe ion exchange by metal ions. In the absence of a lipo-philic anion, on the other hand, the potential had a sub-

Nernstian slope and was leveled on both acidic and alka-line sides by the interfacial reactions giving changes incompositions of both the membrane boundary([C]m/[CH+]m) and the solution boundary ([H+]) [41]. Ina high pH region, for example, [CH+]m decreases in pro-portional to [H+], so that the second term of Equation 8and thereby E is constant.

The pH ranges for the potential responses in Figure 5aand for the two-phase hydrolyses in Figure 1 are summar-ized in Table 1. Although the correlation is not as good asthat found for the ISEs based on lipophilic amines [39],appreciably overlapping pH ranges are found for the Fe,In, and Mn complexes. This indicates that the pH re-sponse of the additives-free ISE is ascribed to the acid/base pair: C= [M(tpp)(OH)] and/or [M2(tpp)2O] andCH+ = [M(tpp)(OH2)]+ in Equation 8. The cationic spe-cies of [M(tpp)(OH2)]+ may come from the dissociationof Cl� from [M(tpp)Cl] or from the protonation of[M(tpp)(OH)] and may form an ion-pair with Cl� fromthe carrier and the inner solution or with HSO4

� used forpH adjustment. The dissociation of Cl� was expected forthe Al, Ga, and Fe complexes due to the lower affinitiesto Cl� and oxophilic properties of the central metal ions,and actually there found pH responses in acidic media. Incontrast, [In(tpp)Cl] and [Mn(tpp)Cl] were stable andhad no pH responses in acidic media, but hydrolyzed torespectively give [In(tpp)(OH)] at pH>6 and [Mn-(tpp)(OH)] at pH>7 and had pH responses. The dynamicranges for [Fe(tpp)Cl] and [In(tpp)Cl] had the upper limi-tation to pH corresponding to 90 % hydrolysis;[M(tpp)(OH2)]+ decreased in proportion to [H+] at highpH as in the cases of lipophilic amines [41].

In contrast to these complexes, the dynamic ranges for[Al(tpp)Cl] and [Ga(tpp)Cl] were extended to at leastpH 12 in an alkaline medium. Similar extension was ob-served for [MIVO(tpp)] (M: Ti, V) [23], [MIV(tpp)(OH)2](M: Zr, Hf) [23,24], [MV

2(tpp)2O3] (M: Nb, Ta), and[MoO(tpp)(OEt)] [21]. Such behaviors inevitably requirethe presence of a constant amount of [M(tpp)(OH2)]+ inthe membrane even in contact with a strongly alkaline so-lution and their participation in the membrane potentialgiven by Equation 8. Although [M(tpp)(OH2)]+ may in-teract with lipophilic anions, for example, sulfonate deriv-ative and sulfate ester of organic compounds, as impuri-ties in PVC (10�5 M level against 10�2 M level of the com-plex) to be stabilized [10,42], the interaction is not sostrong to survive in such highly alkaline media, as shownin the measurement results for DS� and PFOS� in thisstudy. Such stabilization was not observed for [In(tpp)-(OH2)]+ having the less acidity. The reason for the stable

Fig. 4. Changes of the molar absorption coefficient by the reac-tions of [In(tpp)(OH)] with HClO4 (a) and with HNO3 (b) andof [Ga(tpp)(OH)] with HClO4 (c). *: characteristic for[M(tpp)(OH)] (428.0 nm for (a) and (b) and 421.5 for (c)); ^:characteristic for [M(tpp)X] (423.0 for (a), 425.5 for (b), and416.5 for (c)); &: characteristic for [M2(tpp)2(OH)]+ ,X� (411.5for (a) and 404.0 for (c)).

Table 1. pH range for two-phase hydrolysis of [M(tpp)Cl] anddynamic pH range of ISE.

M Al Fe Ga In Mn

Hydrolysis pH range �2 0–4 1–5 6–10 7–11Dynamic pH range <2–12< <2–5 4–12< 6–11 9–12




presence of the conjugate acid form of [M(tpp)(OH2)]+

(M: Al, Ga) in the membrane is not clear at this stage.

3.6 pH Responses of ISEs Based on [M(tpp)(OH)] and/or [M2(tpp)2O] and the Effects of other Anions andMembrane Solvents

The pH responses of the ISEs based on [M(tpp)(OH)]and/or [M2(tpp)2O] using DOS are shown in Figure 5b.The potential-pH diagram of [Fe(tpp)(OH)] and/or [Fe2-(tpp)2O] was substantially the same as that of [Fe-(tpp)Cl], while the dynamic pH range with [M(tpp)(OH)](M: In, Ga and Mn) was appreciably shifted to a lowerregion than that with [M(tpp)Cl]. The reason for the shiftis explained below for the In complexes as an example.When [In(tpp)(OH)] in DOS of the low polarity was incontact with 10�2 mol L�1 of a H2SO4 solution, a portionof the complex was converted to the ion pair of [In(tpp)-(OH2)],HSO4 as those found for other acids in Figure 4.At pH>7, on the other hand, the ion pair was completelyconverted to [In(tpp)(OH)] and the least extractable spe-cies of SO4

2�. In a pH range of 2–7, where both [In-(tpp)(OH)] and [In(tpp)(OH2)],HSO4 coexisted, a pH re-sponse was observed. This was confirmed by two otherevidences. The potential-pH diagram of the ISE based on[In(tpp)Cl] showed some hysteresis, when pH of the solu-tion was once increased to 12 by adding NaOH and wasthen decreased by adding H2SO4 (Figure 6). The back-curve from the alkaline side was substantially the same asthat of [In(tpp)(OH)] (Figure 5b). When pH was de-creased by adding HNO3, the hysteresis was appreciablydiminished.

As for o-NPOE, in contrast, the pH response of ISEbased on [In(tpp)(OH)] was substantially the same asthose of [In(tpp)Cl] in DOS and in o-NPOE (Figure 5a).Non-pH response at pH<5 is ascribed to quantitativeformation of [In(tpp)(OH2)],HSO4 in the highly polar o-NPOE. At pH>6, however, hydrolysis to [In(tpp)(OH)]yielded the pH response.

3.7 Effects of Lipophilic Anions on pH Responses ofISEs

The effects of lipophilic anions on the pH responses ofthe ISEs are shown in Figure 7 for the In complex. Theaddition of 100 mol% NaTFPB diminished the pH re-sponses irrespective of the central metal ion and of themembrane solvent, while that of 100 mol% NaDS gaveonly a slight shift in potential (not shown). In the pres-ence of 50 % NaTFPB, rather complicated pH responsecurves were obtained. This composition apparently corre-sponded to the formation of [M2(tpp)2OH],TFPB, whichslightly disproportionated to yield [M(tpp)],TFPB and[M(tpp)(OH)] in the membrane.

½M2ðtppÞ2OH�,TFPBÐ ½MðtppÞ�,TFPBþ ½MðtppÞðOHÞ�ð9Þ

Both these species were in acid-base equilibria with theparent 2 :1 ion pair to give a complicated curve. This wasnot overcome even in the presence of 25% NaTFPB.

Fig. 5. pH responses of ISEs based on [M(tpp)Cl] (a) and [M(tpp)(OH)] and/or [M2(tpp)2O] (b). M: ~, Al; ^, Ga; &, In; *, Fe; &,Mn. Membrane solvent: DOS.

Fig. 6. Reversibility of pH responses. Carrier: [In(tpp)Cl]. (&)pH increase by NaOH; (&, .) pH decrease by H2SO4 and byHNO3, respectively.




4 Conclusions

The two-phase reactions of [M(tpp)Cl] with various buf-fers, lipophilic anions, and acids were studied in relationto possible changes in the compositions of the carriers ofthe ISEs based on these complexes. A large differencewas observed in the hydrolyzing tendency of [M(tpp)Cl].The ISEs based on [M(tpp)Cl] showed pH responses overrespective pH ranges, which had some correlation tothose of the hydrolysis by the two-phase reaction. The re-sponse is, as a first approximation, ascribed to the phaseboundary potential relating to the acid-base pair of[M(tpp)(H2O)]+ and [M(tpp)(OH)] and/or [M2(tpp)2O].The potential responses of the In and Fe complexes hadthe upper limitation to pH of 90% hydrolysis, whereasthose of the Al and Ga complexes had the extension to atleast pH 12. Similar extension had been reported for[MIVO(tpp)] (M: Ti, V), [MIV(tpp)(OH)2] (M: Zr, Hf),[MV

2(tpp)2O3] (M: Nb, Ta), and [MoO(tpp)(OEt)]. Thesefindings indicate the stable presence of cationic specieslike [MIII(tpp)(H2O)]+ (M: Al, Ga), [MIV(tpp)(OH)]+ ,and [MV(tpp)O]+ , in the membrane even in contact witha strongly alkaline solution, which awaits for the scientificelucidation.

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Fig. 7. Effects of NaTFPB on pH response of ISE based on [In(tpp)Cl]. Percent molar ratio of NaTFPB to [In(tpp)Cl]: &, 0; ^,25; ~, 50; *, 100.




Responses of Metalloporphyrin-Based Ion-Selective Electrodes to pH

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