Formal Redox Potential Research Articles

One-electron redox reaction of di- tert-butyl nitroxide at platinum electrode in acetonitrile

The electrochemical oxidation of di- tert-butyl nitroxide (DTBN) at a platinum electrode in acetonitrile was examined. The cyclic voltammogram indicated an irreversible response during a normal time scale measurement, whereas chemically reversible voltammograms were obtained during a shorter time using a micro disk electrode with relatively fast sweep rates. The apparent formal redox potential and heterogeneous electron transfer rate constant were estimated to be 0.218 V (versus Fc +|Fc) and 0.035 ± 0.015 cm s −1 from the digital simulation analysis.

Electrochemistry of Unfolded Cytochrome c in Neutral and Acidic Urea Solutions

The present investigation reports the first experimental measurements of the reorganization energy of unfolded metalloprotein in urea solution. Horse heart cytochrome c (cyt c) has been found to undergo reversible one-electron transfer reactions at pH 2 in the presence of 9 M urea. In contrast, the protein is electrochemically inactive at pH 2 under low-ionic strength conditions in the absence of urea. Urea is shown to induce ligation changes at the heme iron and lead to practically complete loss of the alpha-helical content of the protein. Despite being unfolded, the electron-transfer (ET) kinetics of cyt c on a 2-mercaptoethanol-modified Ag(111) electrode remain unusually fast and diffusion controlled. Acid titration of ferric cyt c in 9 M urea down to pH 2 is accompanied by protonation of one of the axial ligands, water binding to the heme iron (pK(a) = 5.2), and a sudden protein collapse (pH < 4). The formal redox potential of the urea-unfolded six-coordinate His18-Fe(III)-H(2)O/five-coordinate His18-Fe(II) couple at pH 2 is estimated to be -0.083 V vs NHE, about 130 mV more positive than seen for bis-His-ligated urea-denatured cyt c at pH 7. The unusually fast ET kinetics are assigned to low reorganization energy of acid/urea-unfolded cyt c at pH 2 (0.41 +/- 0.01 eV), which is actually lower than that of the native cyt c at pH 7 (0.6 +/- 0.02 eV), but closer to that of native bis-His-ligated cyt b(5) (0.44 +/- 0.02 eV). The roles of electronic coupling and heme-flattening on the rate of heterogeneous ET reactions are discussed.

Direct Electrochemistry of Redox Proteins and Enzymes Promoted by Carbon Nanotubes

The redox protein and enzyme, such as hemoglobin (Hb), horseradish peroxidase(HRP) and glucose oxidase (GOx), was immobilized on the surface of the carbon nanotubemodified glassy carbon (CNT/GC) electrode, respectively. The cyclic voltammetric resultsindicated that the redox protein and enzyme underwent effective and stable direct electrontransfer reaction with a pair of nearly symmetrical redox peaks. The formal redox potential,E0’, was almost independent on the scan rates, the average value of E0’ for Hb, HRP andGOx was –0.343 ± 0.001, –0.319 ± 0.002 and –0.456 ± 0.0008 V (vs. SCE，pH 6.9),respectively. The dependence of E0’ on the pH solution indicated that the direct electrontransfer of Hb and HRP was a one-electron-transfer reaction process coupled with oneproton-transfer, while the GOx was a two-electron-transfer coupled with two-protontransfer.The apparent heterogeneous electron transfer rate constant (ks) was 1.25 ± 0.25,2.07 ± 0.69 and 1.74 ± 0.42 s-1 for Hb, HRP and GOx, respectively. The method presentedhere can be easily extended to immobilize other redox enzymes or proteins and obtain theirdirect electrochemistry.

Electrochemical redox reactions of uranium(VI) complexes with multidentate ligands in dimethyl sulfoxide

Abstract Electrochemical reactions of UO2(β-diketonato)2-dmso, UO2(trop)2dmso, UO2(sap)(dmso)2, and UO2(salen)dmso (dmso = dimethyl sulfoxide, β-diketonate = benzoylacetonate (ba), benzoyltrifluoroacetonate (btfa), and thenoyltrifluoroacetonate (ttfa), trop = tropolonate, sap = 2-salicylidenaminophenolate, and salen = N , N´-disalicylidenethylenediaminate) complexes in dmso solutions containing tetrabutylammonium perchlorate as a supporting electrolyte have been studied with cyclic voltammetry. These uranyl(VI) complexes were found to be reduced quasi -reversibly to U(V) species. The formal redox potentials (E°, vs . ferrocen/ferrocenium) for U(VI)/U(V) couples were determined to be -1.416 V for UO2(ba)2dmso, -1.073 V for UO2(btfa)2dmso, -1.082 V for UO2(ttfa)2dmso, -1.379 V for UO2(trop)2dmso, -1.500 V for UO2(sap)(dmso)2, and -1.602 V for UO2(salen)dmso. Furthermore, the resulting U(V) complexes, [UO2(btfa)2dmso]-, [UO2(ttfa)2dmso]-, and [UO2(salen)dmso]-, were found to be able to exist stably in dmso solutions.

The heme iron coordination of unfolded ferric and ferrous cytochrome c in neutral and acidic urea solutions. Spectroscopic and electrochemical studies

The heme iron coordination of unfolded ferric and ferrous cytochrome c in the presence of 7–9 M urea at different pH values has been probed by several spectroscopic techniques including magnetic and natural circular dichroism (CD), electrochemistry, UV–visible (UV–vis) absorption and resonance Raman (RR). In 7–9 M urea at neutral pH, ferric cytochrome c is found to be predominantly a low spin bis–His-ligated heme center. In acidic 9 M urea solutions the UV–vis and near-infrared (NIR) magnetic circular dichroism (MCD) measurements have for the first time revealed the formation of a high spin His/H 2O complex. The p K a for the neutral to acidic conversion is 5.2. In 9 M urea, ferrous cytochrome c is shown to retain its native ligation structure at pH 7. Formation of a five-coordinate high spin complex in equilibrium with the native form of ferrous cytochrome c takes place below the p K a 4.8. The formal redox potential of the His/H 2O complex of cytochrome c in 9 M urea at pH 3 was estimated to be −0.13 V, ca. 100 mV more positive than E°′ estimated for the bis–His complex of cytochrome c in urea solution at pH 7.

Direct electrochemistry of heme multicofactor-containing enzymes on alkanethiol-modified gold electrodes

Direct electrochemistry of heme multicofactor-containing enzymes, e.g., microbial theophylline oxidase (ThOx) and d-fructose dehydrogenase (FDH) from Gluconobacter industrius was studied on alkanethiol-modified gold electrodes and was compared with that of some previously studied complex heme enzymes, specifically, cellobiose dehydrogenase (CDH) and sulphite oxidase (SOx). The formal redox potentials for enzymes in direct electronic communication varied for ThOx from −112 to −101 mV (vs. Ag|AgCl), at pH 7.0, and for FDH from −158 to −89 mV, at pH 5.0 and pH 4.0, respectively, on differently charged alkanethiol layers. Direct and mediated by cytochrome c electrochemistry of FDH correlated with the existence of two active centres in the protein structure, i.e., the heme and the pyrroloquinoline quinone (PQQ) prosthetic groups. The effect of the alkanethiols of different polarity and charge on the surface properties of the gold electrodes necessary for adsorption and orientation of ThOx, FDH, CDH and SOx, favourable for the efficient electrode–enzyme electron transfer reaction, is discussed.

The Formal Redox Potential of the Yb(III, II) Couple at 0 °C in 3.22 Molal NaCl Medium

Following our previous investigations on aqueous solutions of hypooxidized and iperoxidized species, we managed, by lowering the temperature of the solutions to 0 degrees C, to obtain, by electrochemical methods, Yb(II) and Yb(III) mixtures, enough stable to determine by a potentiometric method the formal redox potential of the Yb(IlI, II) couple. Its value, in a large range of total Ytterbium concentration, is -1233 +/- 3 mV against the molal hydrogen electrode in the 3.22 m NaCl medium.

Selective activation of glycosyl donors utilising electrochemical techniques: a study of the thermodynamic oxidation potentials of a range of chalcoglycosides.

A series of six chalcoglycosides (phenyl-2,3,4,6-tetra-O-benzoyl-1-seleno-beta-D-glucopyranoside, phenyl-2,3,4,6-tetra-O-benzyl-1-seleno-beta-D-glucopyranoside, phenyl-2,3,4,6-tetra-O-benzyl-1-thio-beta-D-glucopyranoside, p-tolyl-2,3,4,6-O-benzoyl-1-thio-beta-D-glucopyranoside, p-tolyl-2,3,4,6-O-benzyl-1-thio-beta-D-glucopyranoside, and phenyl-2,3,4,6-O-benzyl-beta-D-glucopyranoside) are voltammetrically interrogated in dimethyl sulfoxide, so as to determine their formal (i.e. thermodynamic) redox potentials. The electrochemical oxidation of the chalcoglycoside is shown to follow an overall EC-type mechanism, in which the electro-generated cation radical undergoes an irreversible carbon-chalcogen bond rupture to produce the corresponding glycosyl cation, which may react further. The kinetics of the initial heterogeneous electron transfer process and subsequent irreversible homogeneous chemical degradation of the radical cation are reported, with values for the standard electrochemical rate constant k(0) in the order of 10(-2) cm s(-1) and the first order homogeneous rate constant, k(1), of the order of 10(3) s(-1). The formal oxidation potentials were found to vary according to the identity of the chalcogenide, such that OPh > SPh similar to STol > SePh.

Electrochemical and spectroelectrochemical properties of neptunium(VI) ions in nitric acid solution

Electrochemical behavior of NpO22+ on platinum (Pt) and glassy carbon (GC) electrodes have been studied in an aqueous 3M HNO3 solution using cyclic voltammetry and spectroelectrochemical method. The NpO22+ species was found to be reduced to Np(V) quasi-reversibly with the formal redox potential of +0.907 V (Pt) and +0.909 V (GC). The reduction process of NpO22+ in an aqueous 3M HNO3 solution was also followed spectroelectrochemically by using an optical transparent thin layer electrode cell. It was found that the absorption spectra measured at the applied potentials in the range of +1.05 to +0.65 V have isosbestic points at 1051 and 1124 nm and the evaluated electron stoichiometry is 0.93. These results indicate that the reduction product of NpO22+ is NpO2+. Further details for reaction mechanism of Np(VI) are discussed on the basis of digital simulation of the experimental cyclic voltammograms.

Cyclic Voltammetry Investigation of the Phenolic Content of Croatian Wines

Cyclic voltammograms taken to 500 mV in a model wine solution (12% ethanol, 0.033 M tartaric acid, pH 3.6) at a glassy carbon electrode were used to determine the concentrations of antioxidants, expressed in catechin equivalents (CE), present in three red and two white Croatian wines. The order of increasing total phenol content for wines obtained from the electrochemical <i>Q</i>₅₀₀ measure (the charge passed to 500 mV) was comparable to the one obtained by the conventional Folin-Ciocalteu procedure and points to the reliability of cyclic voltammetry in estimation of total phenols. The anodic oxidation current peak, <i>E</i>_p,a, observed around 400 mV (versus Ag/AgCl) was attributed to compounds with an <i>ortho</i>-dihydroxy-phenol group, while the peak detected at 650 mV in red wines was ascribed to malvidin anthocyanins, ferulic acid, and <i>t</i>-resveratrol. Red wines had formal redox potentials of <i>E</i>⁰⁹, about 30 mV more negative than the white wines and, therefore, should exhibit stronger antioxidative properties. Electrochemical parameters, derived from cyclic voltammograms, were used to perform the diagnostic tests regarding the reversibility of catechin and oxidation of wine phenolics. For catechin, a clear picture of the reversibility of the oxidation reaction was obtained from a linear relationship between the oxidation current peak, <i>I</i>_p,a, and catechin concentration, <i>c</i>. For wine phenolics, the difference between the oxidation peak, <i>E</i>_p,a, and the half peak potential, <i>E</i>_p/2, was used as the diagnostic criterion. The value, close to 59 mV, indicates a one-electron transfer process.

Electropolymerized cobalt macrocomplex-based films for thiols electro-oxidation: effect of the film formation conditions and the nature of the macrocyclic ligand

We report here on the study of the electrocatalytic behavior of adsorbed and thin electropolymerized cobalt porphyrin and phthalocyanine films towards thiols oxidation with a special emphasize on the effect of the anion used for preparing the films and the nature of the macrocyclic ligand on the catalytic activity. Our results show that there are no effects related to the anions used during the preparation of the films on the electrocatalytic activity of the complex coatings, as evaluated by the cyclic voltammetry investigation of various thiols of different sizes. In contrast, the nature of the ligand around the cobalt centre of the macrocyclic complex has a strong effect on the catalytic activity for the oxidation of 2-mercaptoethanol (2-ME). This clearly illustrates the fact that the Co II/Co I formal redox potential of the macrocycle needs to be located in a rather narrow window to obtain maximum activity.

Novel macrocyclic EuII complexes: fast water exchange related to an extreme M-O water distance.

Eu(II) complexes are potential candidates for pO(2)-responsive contrast agents in magnetic resonance imaging. In this regard, we have characterized two novel macrocyclic Eu(II) chelates, [Eu(II)(DOTA)(H(2)O)](2-) and [Eu(II)(TETA)](2-) (H(4)DOTA=1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid, H(4)TETA=1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid) in terms of redox and thermodynamic complex stability, proton relaxivity, water exchange, rotation and electron spin relaxation. Additionally, solid-state structures were determined for the Sr(II) analogues. They revealed no inner-sphere water in the TETA and one inner-sphere water molecule in the DOTA complex. This hydration pattern is retained in solution, as the (17)O chemical shifts and (1)H relaxation rates proved for the corresponding Eu(II) compounds. The thermodynamic complex stability, determined from the formal redox potential and by pH potentiometry, of [Eu(II)(DOTA)(H(2)O)](2-) (lg K(Eu(II))=16.75) is the highest among all known Eu(II) complexes, whereas the redox stabilities of both [Eu(II)(DOTA)(H(2)O)](2-) and [Eu(II)(TETA)](2-) are inferior to that of 18-membered macrocyclic Eu(II) chelates. Variable-temperature (17)O NMR, NMRD and EPR studies yielded the rates of water exchange, rotation and electron spin relaxation. Water exchange on [Eu(II)(DOTA)(H(2)O)](2-) is remarkably fast (k298(ex)=2.5 x 10(9) s(-1)). The near zero activation volume (DeltaV++ =+0.1+/-1.0 cm(3) mol(-1)), determined by variable-pressure (17)O NMR spectroscopy, points to an interchange mechanism. The fast water exchange can be related to the low charge density on Eu(II), to an unexpectedly long M-O(water) distance (2.85 A) and to the consequent interchange mechanism. Electron spin relaxation is considerably slower on [Eu(II)(DOTA)(H(2)O)](2-) than on the linear [Eu(II)(DTPA)(H(2)O)](3-) (H(5)DTPA=diethylenetriaminepentaacetic acid), and this difference is responsible for its 25 percent higher proton relaxivity (r(1)=4.32 mM(-1) s(-1) for [Eu(II)(DOTA)(H(2)O)](2-) versus 3.49 mM(-1) s(-1) for [Eu(II)(DTPA)(H(2)O)](3-); 20 MHz, 298 K).

Electrochemical and Spectroelectrochemical Studies on UO2(saloph)L (saloph = N,N‘-Disalicylidene-o-phenylenediaminate, L = Dimethyl Sulfoxide or N,N-Dimethylformamide)

To examine properties of pentavalent uranium, U(V), we have carried out electrochemical and spectroelectrochemical studies on UO(2)(saloph)L [saloph = N,N'-disalicylidene-o-phenylenediaminate, L = dimethyl sulfoxide (DMSO) or N,N-dimethylformamide (DMF)]. The electrochemical reactions of UO(2)(saloph)L complexes in L were found to occur quasireversibly. The reduction processes of UO(2)(saloph)L complexes were followed spectroelectrochemically by using an optical transparent thin layer electrode cell. It was found that the absorption spectra measured at the applied potentials from 0 to -1.650 V versus ferrocene/ferrocenium ion redox couple (Fc/Fc(+)) for UO(2)(saloph)DMSO in DMSO have clear isosbestic points and that the evaluated electron stoichiometry equals 1.08. These results indicate that the reduction product of UO(2)(saloph)DMSO is [U(V)O(2)(saloph)DMSO](-), which is considerably stable in DMSO. Furthermore, it was clarified that the absorption spectrum of the [U(V)O(2)(saloph)DMSO](-) complex has a very small molar absorptivity in the visible region and characteristic absorption bands due to the 5f(1) orbital at around 750 and 900 nm. For UO(2)(saloph)DMF in DMF, the clear isosbestic points were not observed in the similar spectral changes. It is proposed that the UO(2)(saloph)DMF complex is reduced to [U(V)O(2)(saloph)DMF](-) accompanied by the dissociation of DMF as a successive reaction. The formal redox potentials of UO(2)(saloph)L in L (E(0), vs Fc/Fc(+)) for U(VI)/U(V) couple were determined to be -1.550 V for L = DMSO and -1.626 V for L = DMF.

Electrochemical characterization of lignin peroxidase from the white-rot basidiomycete Phanerochaete chrysosporium

Electrochemical analysis of lignin peroxidase (LiP) was performed using a pyrolytic graphite electrode coated with peroxidase-embedded tributylmethyl phosphonium chloride membrane. The formal redox potential of ferric/ferrous couples of LiP was −126 mV (versus SHE), which was comparable with that of manganese peroxidase (MnP) and horseradish peroxidase (HRP). Yet, only LiP is capable of oxidizing non-phenolic substrates with a high redox potential. Since with decreasing pH, the redox potential increased, an incredibly low pH optimum of LiP as peroxidase at 3.0 or lower was proposed as the clue to explain LiP mechanisms. A low pH might be the key for LiP to possess a high redox potential. The p K a values for the distal His in peroxidases were calculated using redox data and the Nernst equation, to be 5.8 for LiP, 4.7 for MnP, and 3.8 for HRP. A high p K a value of the distal His might be crucial for LiP compound II to uptake a proton from the solvent. As a result, LiP is able to complete its catalytic cycle during the oxidation of non-proton-donating substrates. In compensation, LiP has diminished its reactivity toward hydrogen peroxide.

Electrochemical Studies on [U(dmso)9]4+ and [UO2(dmso)5]2+ (dmso = dimethyl sulfoxide) Complexes in DMSO

Electrochemical studies on [U(dmso)9]4+ and [UO2(dmso)5]2+ complexes in DMSO containing tetrabutylammonium perchlorate(TBAP) as supporting electrolyte have been carried out by using cyclic voltammetry(CV). A three electrode system was utilized, i.e., Pt working, Ag/Ag+ in TBAP/CH3CN reference, and Pt auxiliary electrodes. A ferrocene/ferrocenium (Fc/Fc+) couple was used as a reference redox system. Measurements of CV were carried out in the range of -0.025 to -2.025 V and -0.025 to -1.175 V for [U(dmso)9]4+ and [UO2(dmso)5]2+ complexes, respectively It was found that the [UO2(dmso)5]2+ complex is reduced quasi-reversibly to U(V) at around -1.0 V, and that the [U(dmso)9]4+ complex is reduced quasi-reversibly to U(III) at around -0.93 V and the resulting U(III) species is further reduced to U(0) irreversibly at around -1.7 V. The formal redox potentials(vs. Fc/Fc+) for U(VI)/U(V) and U(IV)/U(III) couples were determined to be -0.970 and -0.901 V, respectively Furthermore, in order to examine whether the [U(dmso)9]4+ complex is oxidized to U(V), the cyclic voltammogram was measured in the range of -0.025 to +0.975 V at the scan rate of 0.05V/s. Any anodic and cathodic peaks were not observed within the potential window. This indicates that the [U(dmso)9]4+ complex is not oxidized to [Uv(dmso)9]5+. These results suggest that the U(V) species in non-aqueous solvents can be present more stably as yl-type(UO2+) than as U5+ type.

A study on Cu 2Co 2SOD and Co 2Co 2SOD by voltammetry and thin-layer spectroelectrochemistry

The electrochemistry of Co 2Co 2SOD and Cu 2Co 2SOD on a pyrolytic graphite electrode (PGE) without using mediators was investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The reversible and quasi-reversible voltammetric responses were observed for Co 2Co 2SOD and Cu 2Co 2SOD, respectively. Their formal redox potentials and electron numbers involved in electrode reactions were obtained, and are in agreement with those by spectroelectrochemistry (SEC).

Reactivities of organic phase biosensors: 6. Square-wave and differential pulse studies of genetically engineered cytochrome P450 cam (CYP101) bioelectrodes in selected solvents

Cytochrome P450 cam (CYP101) bioelectrodes suitable for application in organic phases were prepared from genetically engineered CYP101 and vesicular dispersions of didodecyldimethylammonium bromide. The amperometric biosensor system was characterised under anaerobic conditions by cyclic and square-wave voltammetric methods. Cyclic- and square-wave-voltammetry studies showed that the biosensors exhibited direct reversible electron transfer between the haem iron atom and the glassy carbon electrode surface. The formal redox potential estimated for the electrode in acetonitrile was −380 mV/Ag–AgCl. The formal potential shifted anodically as the organic phase biosensor responded irreversibly to substrate (camphor) under anaerobic and aerobic conditions in acetonitrile. Differential pulse analysis of the reactivities of the CYP101 enzyme electrode confirmed the square-wave voltammetry result, which showed that the binding of substrate decreased the redox potential necessary for initiating the monooxygenation reaction of cytochrome P450 cam.

Voltammetric monitoring of laccase-catalysed mediated reactions

Six different compounds capable of mediating laccase-catalysed reactions have been tested by cyclic voltammetry. They exhibited quasi-reversible electrodic behaviour with formal redox potentials ranging from 150 to 800 mV ( E 0′ vs. SCE). The immersion of a laccase-coated glassy carbon electrode (GCE) in mediator solutions generated large cathodic catalytic currents easily recorded by cyclic voltammetry at low-potential scan rates. This current showed two well-defined pH profiles, which correlated with the variation of the mediator redox potentials at the pH range tested. The relevant effect of temperature on the activity of laccase has been assessed here. Likewise, it was shown that the current record varied with the substrate concentration. This trend fitted Michaelis–Menten kinetics, which allowed us to give an estimation of the affinity of the fungal laccase for the different mediators.

Donnan permselectivity in layer-by-layer self-assembled redox polyelectrolye thin films.

Redox polyelectrolyte multilayers have been assembled with use of the layer-by-layer (LBL) deposition technique with cationic poly(allylamine) modified with Os(bpy)(2)ClPyCHO (PAH-Os) and anionic poly(styrene)sulfonate (PSS) or poly(vinyl)sulfonate (PVS). Different behavior has been observed in the formal redox potential of the Os(II)/Os(III) couple in the polymer film with cyclic voltammetry depending on the charge of the outermost layer and the electrolyte concentration and pH. The electrochemical quartz crystal microbalance (EQCM) has been used to monitor the exchange of ions and solvent with the external electrolyte during redox switching. At low ionic strength Donnan permselectivity of anions or cations is apparent and the nature of the ion exclusion from the film is determined by the charge of the topmost layer and solution pH. At high electrolyte concentration Donnan breakdown is observed and the osmium redox potential approaches the value for the redox couple in solution. Exchange of anions and water with the external electrolyte under permselective conditions and salt and water under Donnan breakdown have been observed upon oxidation of the film at low pH for the PAH-Os terminating layer. Moreover, at high pH values and with PVS as the terminating layer EQCM mass measurements have shown that cation release was masked by water exchange.

Ca2+–Fe3+–D-gluconate-complexes in alkaline solution. Complex stabilities and electrochemical properties

The complexation of Fe3+ and Ca2+ ions by D-gluconate (DGL) in alkaline solutions was studied by potentiometric titration. Stability constants for the complexes were determined by mathematical treatment of the data with the computer program BEST. Depending on the concentration of ligand, complexes of Fe3+ ∶ DGL stoichiometry 1 ∶ 1, 1 ∶ 2 and 2 ∶ 2 were identified. In the presence of Ca2+ ions mixed binuclear complexes are formed with the Ca2+ ∶ Fe3+ ∶ DGL stoichiometries 1 ∶ 1 ∶ 1 and 1 ∶ 1 ∶ 2. The complex system was studied with cyclic voltammetry and with constant current electrolysis experiments. The uptake of Ca2+ into the FeIIIDGL complexes results in a distinct change of the electrochemical properties of the iron(II/III)-redox couple. A shift of the formal redox potential towards more negative values is observed. The Ca2+ ∶ Fe3+ ∶ DGL complexes can be regarded as models for an iron(II/III)-redox system whose electrochemical behaviour is determined by the concentration of an electrochemically inactive ion.