Antioxidant properties of fullerenol-d Текст научной статьи по специальности «Химические науки»

Antioxidant properties of fullerenol-d

D. P. Tyurin1, F. S. Kolmogorov1, I. A. Cherepkova1, N. A. Charykov1'5, K.N. Semenov1'2, V. A. Keskinov1, N. M. Safyannikov5, Yu.V. Pukharenko3, D. G. Letenko3, T.A. Segeda4, Z. Shaimardanov4

1 St. Petersburg State Technological Institute (Technical University), Moskovsky prospect, 26, Saint Petersburg, 190013, Russia 2St. Petersburg State University, 7/9 Universitetskaya emb., Saint Petersburg, 199034, Russia 3St.Petersburg State University of Architecture and Civil Engineering (SPSUACE), 2nd Krasnoarmeiskaya St. 4, 190005, Saint Petersburg, Russia 4D. Serikbayev East Kazakhstan state technical university, A. K. Protozanov Street, 69, Ust-Kamenogorsk city, 070004, The Republic of Kazakhstan 5 St. Petersburg Electrotechnical University "LETI", ul. Professora Popova 5, 197376, Saint Petersburg, Russia

keskinov@mail.ru

PACS 61.48.+c DOI 10.17586/2220-8054-2018-9-6-798-810

Fullerenol-d C6o(OH)22—24 was synthesized by the method of direct heterogeneous oxidation of fullerene C60, dissolved in o-xylene, by NaOH, dissolved in water, in the presence of interphase catalyst [i — (C4Hg)4 N OH. Identification of fullerenol-d was provided by: C-H-N elemental analysis, High performance liquid phase chromatography, IR - and Electronic spectroscopy, Mass-spectrometry. The antioxidant properties of aqueous fullerenol-d solutions were investigated against free radicals, generated by hydrogen peroxide and molecular I2. Measurement of fullerenol antioxidant activity was based on the potentiometric titration of fullerenol solutions by hydrogen peroxide and molecular I2 solutions and vice versa with compact Pt as working electrode. As a comparison, the very popular and strong anti-oxidant - ascorbic acid was used. Pourbaix Diagrams (pH — Eh) for hydrogen-oxygen and iodine forms were constructed. Fullerenol-d is a weaker antioxidant than ascorbic acid, but in contrast, fullerenols-d molecules are able to undergo multiply reversible absorption-desorption of some free radicals.

Keywords: fullerenol, antioxidant properties, hydrogen peroxide, iodine, Pourbaix Diagrams, platinum electrode.

Received: 19 July 2018 Revised: 12 November 2018 Final revision: 21 November 2018

1. Introduction

This research continues the series of reports, concerning synthesis, identification and investigation of the properties of polyhydroxylated derivatives (also named as fullerenols) of light fullerenes C60 and C70 (see, for example [1-28]).

Antioxidant properties of fullerenols were previously investigated [7,9,17,42-50]. Several mechanisms for the antioxidant activity of fullerenol nanoparticles have been proposed [42]. The possible mechanism of the antioxidative activity of fullerenol C60(OH)24 is the radical-addition reaction of 2n(OH*) (here and further (R*) -is free radical with one free electron) radicals to the remaining olefinic double bonds of the fullerenol core to yield [C60 (OH)24](OH*)2n. The other proposed mechanism is the possibility of a hydroxyl radical to abstract a hydrogen from fullerenol, including the formation of a relatively stable fullerenol radical [C60(OH)23](O*) [46]. In addition, a hydroxyl radical may abstract one electron from fullerenol yielding the radical cation [C60(OH)24]+. One more proposed mechanism is that the polyanion nanoparticles have numerous free electron pairs from oxygen, distributed around the fullerenol molecules, and have a great capacity to form coordinative bonds with prooxidant metal ions [9]. The obtained result demonstrated that fullerenol decreased the reduction of cytochrome-C for 5 -40 rel. % [7]. The hypothetical mechanism of action of the polyanion fullerenol C60(OH)24 with the superoxide radical anion is presented in [50]. Some results suggest that C60(OH)32 • 8H2O scavenges (OH*) owing to the dehydrogenation of C60(OH)32 • 8H2O and is simultaneously oxidized to a stable fullerenol radical [47]. The antioxidant ability of C60(OH)32 • 8H2O was also confirmed in a beta-carotene bleaching assay [48]. The results suggest that fullerennols possess NO-scavenging activity in vivo [7]. The scavenger activity of fullerenol with a smaller or moderate number of hydroxyl groups with (OH*) radicals can be explained by addition to sp2 carbon atoms [46,49].

The phototoxic antioxidant properties of fullerenols have also been reported [42,51,52]. Fullerenol C6o(OH)24 produces a mixture of reactive oxygen species under both visible and ultraviolet irradiation through two types of photochemical mechanisms [51], with the greatest rates of oxygen consumption at acidic pH (pH = 5). Evidence of both singlet oxygen (xO2) and superoxide radical ion production (O2)- was obtained and when compared to other known sensitizers of reactive oxygen; fullerenol C60(OH)24 produced higher quantities of active oxygen species at a rate at least two times that of other sensitizers [52]. Comparing phototoxicity toward HaCaT of (Y-CyD)2/C60 (c-cyclodextrin bicapped C60) and fullerenol, Zhao et al. concluded that fullerenol was less phototoxic [53].

2. Pourbaix diagram

A Pourbaix diagram (diagram of predominant forms, Eh — pH diagram) is one that illustrates thermodynam-ically stable forms of existence of elements (ions, molecules, atomic crystals and metals) in solutions at different values of hydrogen indicator - pH and redox electrode potential for compact Pt electrode - Eh [54,55]. These diagrams were proposed by Marcel Pourbaix [54]. For each element, you can build a separate Pourbaix diagram. Pourbaix diagrams for one element may vary depending on temperature, solvent and presence of ligands in solution. But, as a rule, Pourbaix diagrams are for aqueous solutions at the temperatures near 25 °C. Pourbaix diagrams are constructed on the basis of the Nernst equation and the standard redox potentials.

The Pourbaix diagram is constructed in coordinates of Eh (ordinate) - pH (abscissa). It reflects the species that are thermodynamically stable at a given pH value and the oxidation-reduction potential of the medium - Eh. At a lower potential, the corresponding form can be reduced to the underlying (if any), at a higher - oxidized to the overlying (if any). The boundaries between the existing species of a solution-solid or solution-gas usually depend on the concentration of dissolved forms; the boundaries between the existing species of dissolved forms, as a rule, do not depend on their concentration. Often, the Pourbaix diagram is applied to the boundary of the region of existence of water. The upper of them (Eh = 1.23 - 0.059 pH) corresponds to the release of oxygen (that is, at higher potentials it is possible to oxidize water to oxygen):

4H2O — 4e- =4H+ + O2 (pH < 7), (1.1)

4OH- — 4e- = 2H2O + O2 (pH > 7). (1.2)

The lower limit (Eh = —0.059pH) corresponds to the release of hydrogen (that is, at lower potentials it is possible to recover water to hydrogen):

2H+ + 2e- =H2 (pH < 7), (2.1)

2H2O + 2e- =H2 + 2OH- (pH > 7). (2.2)

The Pourbaix diagram [53, 54] is one of the most powerful means of predicting the direction of chemical reactions of compounds of this element. From this, it is possible to determine the conditions of most acid-base and redox reactions of compounds of this element without taking into account the interaction with different ions. It is possible to predict the processes of disproportionation and non-proportionally different forms, whether they can contribute hydrogen and oxygen. By comparing the Pourbaix diagrams for the two elements, it is possible to predict the redox reactions between their compounds.

3. Experimental section

3.1. Fullerenol-d synthesis and identification

For synthesis, the authors used fullerene C60, produced by ILIP Corporation (S-Petersburg, Russia) with the purity 99.5 mass. %. All other reactants had qualification "pure for the analysis".

Fullerenol-d C60(OH)22-24 was synthesized by the method of direct heterogeneous oxidation of fullerene C60, dissolved in o-xylene, by NaOH, dissolved in water, in the presence of interphase catalyst [t — (C4H9)4 N] OH (see, for example [27-29,42]). The following synthesis reactions were realized:

C60|(o-xylene) + (22 — 24)NaOH(water) + [t — (C4H9)4 N] OH(interphase) ^ C60 (ONa)22-24 (water) + (11 — 12)H2;

C60 (ONa)22-24 + (22 — 24)HCl ^ C60 (OH)22-24 + (22 — 24)NaCl(CH3OH — solution).

Reaction was provided for 7 days; the aqueous phase was separated from the organic phase; product (fullerenol-d) was salting-out by methanol and then purified by triple recrystallization (methanol-water) [27-29,42]. Identification of fullerenol-d was provided by: C-H-N element analysis, High performance liquid phase chromatography

(HPLC) - device chromatograph "Shimadzu Europa GmbH", IR - and Electronic spectroscopy, devices: IRAffinity-1S and UV-1280, Mass-spectrometry, device - Bruker MS [27-29]. In particular, with the help of mass-spectrum we determined the number of hydroxyl groups (22-24), purity was determined by HPLC (98.5 mass %).

3.2. Antioxidant properties investigation

To investigate antioxidant properties of ascorbic acid and fullerenol (against oxidants: hydrogen per-oxide and iodine) we determined redox potential Eh at fixed value of hydrogen indicator pH= 4.77, which was set by acetate buffer solution CH3COOH/CH3COONa (molar relation 1/1). We used electrochemical cells, containing two electrodes: Pt (compact) is working electrode; Hg, Hg2Cl2/KCl (1 mole/dm3) - reference normal calomel electrode with constant potential and investigated solution in acetate buffer solution. Device pH-meter "ATC pH 200" was used as a Voltmeter during potential-metric titration of anti-oxidants by oxidants and vice-versa.

Titration of comparative "standard" antioxidant - ascorbic acid by hydrogen peroxide and molecular I2 and vice versa.

3.3. Hydrogen peroxide titration

It is well known, that in an aqueous solution of comparative "standard" antioxidant - ascorbic acid, may realize oxidation-reduction equilibrium between ascorbic acid (reduced form) and dehydroascorbic acid (oxidized form). It is only the first step of oxidation [56,57] by hydrogen peroxide (eq. 3.1) or I2 in KI water solution (eq. 3.2):

(3.1)

Ascorbic a

We used following electrochemical cells:

Hg, Hg2Cl2/KCl (1 mole/dm3) /investigated solution/Pt (compact),

(3.2)

(4)

where: Pt (compact) is working electrode; Hg, Hg2Cl2/KCl(1 mole/dm3) - reference normal calomel electrode with constant potential E0 = 0.281V [58]. Investigated solution in had two compositions:

ascorbic acid + dehydroascorbic acid + H2O2 + O2 + H+ (acetate buffer); /ascorbic acid + dehydroascorbic acid +12 + I- (KI) + H+ (acetate buffer).

(5.1)

(5.2)

The integral and differential curves of the titration of H2O2 by ascorbic acid and vice-versa ascorbic acid by H2O2 are represented in Fig. 1(a-d).

One can see absolutely equivalent quantities of ascorbic acid and hydrogen peroxide, according to eq. 3.1 and I2, according to eq. 3.2.

One can also see that upper plateau in two curves E1 « 0.32V (Fig. 1(a,c)) corresponds to the following electrode semi-reaction:

O2 +2H+- +2e ^ H2O2(Pt), (6.1)

upper plateau in two curves E2 « 0.34V (Fig. 2(a,c)) corresponds to the following electrode semi-reaction:

I2 (liquid)+ 2e ^ 2I- (Pt), (6.2)

bottom plateau in four curves E3 « 0.03V (Figs. 1(a,c), 2(a,c)) corresponds to the same electrode semi-reaction:

(c) (d)

Fig. 1. (a,b) - Titration of 30 cm3 of ascorbic acid solution (C = 0.05 mole/dm3) by the hydrogen peroxide solution (C = 0.025 mole/dm3) in 30 cm3 of acetic buffer CH3COOH/CH3COONa (1/1) - integral (a) and differential (b) curves. (c,d) - Titration of 30 cm3 of hydrogen peroxide solution (C = 0.025 mole/dm3) by ascorbic acid solution (C = 0.05 mole/dm3) in 20 cm3 of acetic buffer CH3COOH/CH3COONa (1/1) - integral (c) and differential (d) curves. E is potential of Pt electrode relative normal calomel electrode

(6.3)

The full oxidation-reduction reaction is given by reaction (3.1). It is the difference between semi-reactions (6.1) and (6.3). Full oxidation-reduction reaction is given by reaction (3.2): I is the difference between semi-reactions (6.2) and (6.3).

Differences of electromotive forces in plateaus in Figs. 1(a,c) and 2(a,c) are, correspondingly:

AE

3.1

0.32 - 0.03 ^ 0.29V; AE

3.2

0.34- 0.03 ^ 0.31V.

(7)

Fig. 2. (a,b) - Titration of 25 cm3 of ascorbic acid solution (C = 0.025 mole/dm3) by I2 (in KI) solution (C = 0.05 mole/dm3) in 20 cm3 of acetic buffer CH3COOH/CH3COONa (1/1) - integral (a) and differential (b) curves. (c,d) - Titration of 30 cm3 of I2 (in KI) solution (C = 0.05 mole/dm3) by ascorbic acid solution (C = 0.025 mole/dm3) in 20 cm3 of acetic buffer CH3COOH/CH3COONa (1/1) - integral (c) and differential (d) curves. E is potential of Pt electrode relative normal calomel electrode

So, change of Gibbs potential of the oxidation ascorbic acid reactions (3.1) and (3.2), correspondingly in forward directions is:

AG3.1 « -56.0 kJ/mole; AG3.2 « -59.8 kJ/mole, (8)

and equilibrium constant (Keq) of both these reactions (at normal conditions) are sufficiently large: ln [Keq] = 22 - 24 rel .un.; Keq-3.1 = 3.6 • 109, Keq -3.2 = 2.6 • 1010, and both reactions are practically irreversible. It is also proved by the form of titration curves Figs. 1(a-d) and 2(a-d).

3.4. Titration of fullerenol-d by hydrogen peroxide and molecular I2 and vice versa

To investigate antioxidant properties of fullerenol-d acid we determined redox potential Ehat fixed value of hydrogen indicator pH = 4.77, which was set by acetate buffer solution CH3COOH/CH3COONa (molar relation

1/1). We used following electrochemical cells (see earlier):

Hg, Hg2Cl2/KCl (1 mole/dm3) /investigated solution/Pt (compact), (9)

where: Pt(compact) is working electrode; Hg, Hg2Cl2/KCl - reference normal calomel electrode with constant potential E0 = 0.281V [58]. Investigated solution in had two compositions:

/fullerenol-d + oxy - fullerenol-d + H2O2 + O2 + H+ (acetate buffer) /; (9.1)

/fullerenol-d + oxy - fullerenol-d +12 + I- (KI) + H+ (acetate buffer) /. (9.2)

The integral and differential curves of the titration of H2O2 by fullerenol-d and vice-versa fullerenol-d by H2O2 are represented in Fig. 3(a-d).

■ 1 1 1 ■ 1 ■ 1 ■ 1 ■ 1 1 1 ■ 1 —1—1—1—1—,—1—1—1—,—i—1—1—1—1—,—1

0 U if w it Igl p> Iff ils a 'M ' >® .№> wf ; noo bo t4o> " -tsa

Volume fullerenol-d solution V (cm3) Volume fullerenol-tt solution V (cm3)

(c) (d)

Fig. 3. (a,b) - Titration of 25 cm3 fullerenol-d solution (C = 0.005 mole/dm3) by the hydrogen peroxide solution (C = 0.020 mole/dm3) in 20 cm3 of acetic buffer CH3COOH/CH3COONa (1/1) - integral (a) and differential (b) curves. (c,d) - Titration of 25 cm3 hydrogen peroxide solution (C = 0.020 mole/dm3) by fullerenol-d solution (C = 0.005 mole/dm3) in 20 cm3 of acetic buffer CH3COOH/CH3COON (1/1) - integral (c) and differential (d) curves. E is potential of Pt electrode relative normal calomel electrode

The integral and differential curves of the titration of I2 (in KI) by fullerenol-d and vice-versa fullerenol-d by I2 (in KI) are represented in Fig. 4(a-d).

(c)

(d)

Fig. 4. (a,b) - Titration of 25 cm3 fullerenol-d solution (C = 0.005 mole/dm3) by I2 (in KI) solution (C = 0.020 mole/dm3) in 20 cm3 of acetic buffer CH3COOH/CH3COONa (1/1) -integral (a) and differential (c) curves. (c,d) - Titration of 25 cm3 of I2 (in KI) solution (C = 0.020 mole/dm3) by fullerenol-d solution (C = 0.005 mole/dm3) in 20 cm3 of acetic buffer CH3COOH/CH3COONa (1/1) - integral (c) and differential (d) curves. E is potential of Pt electrode relative normal calomel electrode

One can see nearly equivalent quantities of fullerenol-d and hydrogen peroxide and I2 (KI), according to eq. (10):

Ceo (OH)20-22 O2 + 2H+ + 2e ^ Ceo (OH)22-24. (10)

— Upper plateau Ei « 0.32V (Fig. 3(a)) and initial part of curve (Fig.3(c)) corresponds to the electrode semi-reaction (6.1): O2 +2H+' + 2e ^ H2O2 (Pt) as earlier.

— Upper plateau E2 « 0.34V (Fig. 4(a)) and initial part of curve (Fig. 4(c)) corresponds to electrode semi-reaction (6.2): I2 (liquid) + 2e ^ 2I- (Pt).

— Bottom plateau in four curves E3 « 0.23V (Figs. 3(a,c), 4(a,c) corresponds to the same electrode semi-reaction (10).

Full oxidation-reduction reactions in electrochemical cell are:

Ceo(OH)22-24 + O2 = Ceo(OH)2o-22O2 + H2O2, (11.1)

Ceo(OH)22-24 +12 = Ceo(OH)2o-22O2 + 2HI. (11.2)

Reaction (11.1) is the difference between semi-reaction (6.1) and (10), reaction (11.2) is the difference between semi-reaction (6.2) and (10).

Differences of electromotive forces in plateaus in Figs. 3(a,c) and 4(a,c) are, correspondingly:

AE3.1 « 0.32 - 0.23 « 0.09V; AE3.2 « 0.34 - 0.23 « 0.11V. (12)

So, change of Gibbs potential of the oxidation fullerenol-d reactions (11.1) and (11.2), correspondingly in forward directions is:

AG3.1 « -17.3 kJ/mole; AG3.2 « -21.2 kJ/mole, (13)

and equilibrium constant (Keq) of both these reactions (at normal conditions) are not so large (as in the case of ascorbic acid): ln [Keq] = 7.0 - 8.6 rel.un.; Keq-3.1 = 1.1 • 103, Keq-3.2 = 5.4 • 103 rel.un. and both reactions are more or less reversible. It is also proved by the form of titration curves Figs. 3(a-d) and 4(a-d). One can see, that integral titration curves (3.1, 3.3, 4.1, 4.3) do not have sigmoid character. Differential curves (3.2, 3.4, 4.2, 4.4) meanwhile sometimes (with I2 (KI) as oxidant) keep extremal character, but considerably less expressed than in the case of ascorbic acid. The values of the derivatives modules /dE/dV (V/cm3) / in the case of ascorbic acid are considerably higher than in the case of fullerenol-d (compare extremal /dE/dV/ values: 0.007, 0.018, 0.12, 0.35 V/cm3 (Figs. 1(b,d), 2(b,d)) in the first case and /dE/dV/ values: absence, absence, 0.02, absence V/cm (Figs. 3(b,d), 4(b,d)). So, we can state, that:

— Fullerenol-d is more week antioxidant in the comparison with ascorbic acid, at least, in relation to free radicals, generated by hydrogen peroxide and iodine.

— Fullerenol-d, in the contrast with ascorbic acid, is capable to the reversible absorption of free radicals, other words fullerenol-d molecules are able to sorb free radical and then (after change of ox-red potential - Eh or hydrogen indicator - pH) are able to desorb these free radicals and recover. Such process can easily materialize at transition of modified fullerenes from the mouth to the stomach then to the intestines.

— As a consequence fullerenols-d molecules are able to multiply reversible absorption-desorption of some free radicals.

3.5. Pourbaix diagrams hydrogen-oxygen and iodine forms

We calculated Pourbaix Diagrams for hydrogen-oxygen and iodine forms, based on data from Tables 1, 2 [58]. Pourbaix Diagrams for hydroxy species and ascorbic acid are represented in Fig. 5 and for iodine forms and ascorbic acid - in Fig. 6. Green spots symbolize our experimental conditions, red curves - Pt electrode potential formation reactions in our experiment, moving along the spot occurs because the concentrations of oxidized and reduced species are changed in the titration processes.

4. Conclusions

The antioxidant properties of aqueous fullerenol-d solutions were investigated against free radicals, generated by hydrogen peroxide and molecular I2 (KI). Measurement of fullerenol antioxidant activity was based on poten-tiometric titration of fullerenol solutions by hydrogen peroxide and molecular I2 (KI) solutions and vice versa with compact Pt as working electrode. Ascorbic acid, a common and strong antioxidant - was utilized as a comparative agent. Pourbaix Diagrams (pH - Eh) for hydrogen-oxygen and iodine forms were constructed. Fullerenol-d is a weaker antioxidant in comparison to ascorbic acid, but in contrast, fullerenols-d molecules are able to undergo multiply reversible absorption-desorption of some free radicals.

Acknowledgements

This work was supported by Russian Foundation of Basic Research - RFBR (Projects No. 16-08-01206, 18-08-00143) and Russian Found of the Support of Small Business (Project No. 24357).

Table 1. Oxygen-hydrogen oxidation-reduction reactions (water solutions, T = 298 K)

Reaction (field number of oxidized form// field number of reduced form) in Pourbaix diagrams (Fig. 5) Nernst Equation Standard ElectroMotive Force -E0 (V)

O': +2H+ + 2e ^ H2O(Pt) I/II E = E0 + RT/2F ln[ao:a[ + /aH2o ] = E0 - 0.059pH + 0.295 lg[ao:/aH2O] 2.422

O3 + 2H+ + 2e ^ O2 +H2 O(Pt) III//IV E = E0 + RT/2F ln[ao3 a[+ /a^o P02 ] = E0 - 0.059pH + 0.295 lg[ao3/a^2oPo2] 2.070

OH' + e ^ OH-(Pt) V//VI E = E0 + RT/F ln[aon/«ofl- ] = E0 + 0.059(14 - pH) + 0.059 lg aoH = E0 + 0.826 - 0.059pH + 0.059 lg aoH 2.020

H2 O2 + 2H+ + 2e ^ 2H2O(Pt) 2 VII/II E = E0 + RT/2F ln[aH2o2 aH+ /aH2o ] = E0 - 0.059pH + 0.0295 lg[aH2 o2 aH+ /aH2o] 1.776

O3+H2O+2e ^ O2 + 2OH-(Pt) VIII//IX E = E0 + RT/2F ln[ao3 aH2o^h- Po2 ] = E0 + 0.059(14 - pH) + 0.0295 lg[ao3 aH2o/Po2 ] = E0 + 0.826 - 0.059pH + 0.295 lg[ao3/a[2oPo2] 1.240

O2 + 4H+ + 4e ^ 2H2O(Pt) X/II E = E0 + RT/4F ln[Po2 aH+ /aH2o ] = E0 - 0.059pH + 0.01475 lg[Po2 /aH2o] 1.229

O2 (g) + 2H+(Pt)+2e ^ H2O2 (g) XI//XII E = E0 + RT/2Fln [Po2/a (H +)2 /P^o2 E0 - 0.059pH + 0.0295lg 2Po2/Xh2o2KH2o21 0.839

Ceo (OH)20^22O2 + 2H+ + 2e ^ Ceo (OH)22^24 XIII//XIV E = E0 - 0.059pH+ 0.0295 lg (a0xy-fullerenol-d/afullerenol-d) 0.797

O2 + 2+' +2e ^ H2O2 (l) (Pt) XV/XVI E = E0 + RT/2F ln[Po2 aH+ /a^o2 ] = E0 - 0.059pH + 0.0295 lg[Po2 /a[2o2] 0.682

dehydroascorbic(acid) +2H++ 2e ^ ascorbic(acid) XVII//XVIII E = E0 - 0.059pH+ °.°295 lg (adehydroascorbic(acid)/aascorbic(acid)) 0.613

1/2O2 + 2H2O* + 2e ^ 2OH-(Pt) XIX/VI E = E0 + RT/2F ln^fa^o/a2oH- ] = E0 +0.059(14 - pH) + 0.0295 lg[Pl/2aH2o] = E0 + 0.826 - 0.059pH + 0.0295 lg[PlfaH2o] 0.401

H+- + e ^ 1 /2H2 (Pt) XX//XXI E = E0 + RT/F ln[aH+ /PH/2] = E0 - 0.059pH - 0.0295 lg[PH2] 0.000

2H2O+2e ^ H' + 2OH-(Pt) XXII//XXIII E = E0 + RT/2F ln[aH2o/a[ aooH- ] = E0 + 0.059(14 - pH) +0.0295lg[a[ o/aH• ] = E0 + 0.826 - 0.059pH + 0.0295 lg[aH2o/aH ] -0.828

Where: KH2O2, X(H2O2) - Henry constant and molar fraction of H2O2 in liquid phase, (l) and (g) - liquid and gaseous phase states of component; ai,pi - activity and partial pressure (atm.) of i-th component.

Table 2. Iodine oxidation-reduction reactions (water solutions, T = 298 K)

Reaction (field number of oxidized form// field number of reduced form) in Pourbaix diagrams (Fig. 6) Nernst Equation Standard ElectroMotive Force - E0 (V)

IO- + 6H+ + 5e ^ 1/2I2 + 3H2O(Pt) I//II E = E0 + RT/5F ln[aío-a6H + /a^a^a ] = E0 + 0.0118 lg[aja-/a}2/2aHa] - 0.0708pH 1.195

Ceo (OH)20^22O2 +2H+ + 2e ^ Ceo (OH^^4 III/IV E = E0 - 0.059pH+ 0.0295lg ( aoxy-fullerenol-d /afUllerenol-d ) 0.797

1/2I2(/)- + e ^ I-(Pt) IV/V E = E0 + RT/F ln[aj1/2/aj- ] = E0 + 0.059 lg[a1/2/a/- ] 0.628

dehydroascorbic(acid) +2H++ 2e ^ ascorbic(acid) VI/VII E = E0 - 0.059pH+ 0.0295 lg (adehydroascorbic(acid)/aascorbic(acid)) 0.613

1/2I2(cr)' + e ^ I-(Pt) VIII/V E = E0 + RT/F ln[1/aj- ] = E0 - 0.059 lg a¡- 0.536

I-' + 2e ^ 3I-(Pt) IX/V E = E0 + RT/2F ln[a7- /af - ] = E0 + 0.0295 lg[a7- / af - ] 0.536

Fig. 5. Pourbaix Diagrams for hydrogen-oxygen forms and ascorbic acid (green spot symbolizes our experimental conditions, red curves - Pt electrode potential formation reactions in our experiment, moving along the spot occurs because oxidized and reduced forms concentrations are changed in the titration)

Fig. 6. Pourbaix Diagrams for iodine forms and ascorbic acid (green spot symbolizes our experimental conditions, red curves - Pt electrode potential formation reactions in our experiment, moving along the spot occurs because oxidized and reduced forms concentrations are changed in the titration)

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