Comparison of Several Calibration Procedures for Glass Electrodes in Proton Concentration

Summary. Several reported procedures for calibrating glass electrodes in proton concentration are compared. Some recommendations for non-experts are also given. The examined procedures can be classified into two broad categories, namely: those based on direct potential difference measurements of solutions of known proton concentration and those that use one or several pH standards to calibrate the electrode and subsequently measure the pH of solutions containing known proton concentrations. With a single buffer, the two types of procedures lead to equivalent results. However, if two pH buffers are used, the slope of the calibration graph in proton concentration will differ from the real electrode slope to an extent proportional to the difference between the liquid junction potentials of the two buffers. Therefore, any other method is preferable under these circumstances.     

Micellar Solubilization of Biopolymers in Hydrocarbon Solvents III. Empirical Definition of an Acidity Scale in Reverse Micelles

This paper deals with the problem of defining, and measuring, the pH inside the water pool (which we define as pH_wp) of reverse micelles, i.e. micelles formed by surfactants dissolved in apolar solvents in the presence of minimal amounts of water. The conceptual and experimental difficulties are discussed, and it is argued that no absolute determination of pH_wp is possible, mostly because water in the water pools of reverse micelles is a new solvent, for which no standardization of acidity is available. The problem can be approached only on the basis of an empirical acidity scale. An empirical acidity scale for water pools in reverse micelles of bis (2-ethyl-hexyl) sodium sulfosuccinate (AOT) in isooctane has been defined by measuring the ³¹P-chemical shifts of phosphate buffers. The chemical shifts in bulk water were compared to those found in reverse micelles under the assumption that the pK of phosphate ion is the same in the two systems. It was found that in most cases there was little difference (less than 0.4 pH units) between pH_wp and the pH of the starting buffer in bulk water (which we define as pH_st). However, this difference between pH_wp and pH_st may become much larger in certain cases. The difference (pH_wp–pH_st) is measured under a variety of conditions, and this permits the determination of an operational acidity in the micelle water pools as a function of the pH_st with which the aqueous micelles are prepared. The significance of such data for interpreting the behaviour of enzymes confined in the micelles water pool is discussed. Based on the pH_wp scale, the apparent pK_a of phenol-red and 4-nitrophenol were determined in reverse micelles containing different buffers and different water content. The pK_a values obtained were rather sensitive to changes of both these factors, which was taken to signify that organic dies have only a very limited applicability to measure the acidity of the water pools of reverse micelles.     

A solid-state pH sensor based on WO3-modified vertically aligned multiwalled carbon nanotubes

In this communication, a novel solid-state pH sensor based on WO₃/MWNTs nanocomposite electrode will be reported. WO₃ nanoparticles were homogeneously coated on vertically aligned MWNTs by magnetron sputtering. Potentiometric pH response of the WO₃/MWNTs electrode in Britton–Robinson buffers revealed a linear working range from pH 2 to12 with a slope of about ?41 mV pH^?1 and a response time less than 90 s. The stability of the electrode remained over a month. Moreover, the WO₃/MWNTs electrode displayed excellent anti-interference property. Compared to conventional pH sensors, the pH sensor based on WO₃/MWNTs nanocomposite electrode also showed excellent reproducibility, high stability and superb selectivity.     

Characterization of a Silver Modified PVCAc Electrode and Its Application as a Ag(I)‐Selective Potentiometric Sensor

An all solid‐state Ag(I) ion‐selective electrode has been prepared by simply immersing a glassy carbon rod coated with PVCAc, which contained plasticizer and additive but no ionophore, into the AgNO₃ solution. The response of the electrode was linear with a Nernstian slope of 60.25 mV/decade within the concentration range from 1×10^?1 to 1×10^?5 M and with a detection limit of 4.25×10^?6 M. The stability as an effect of various cations was defined. The electrode is suitable for use in high acidic solutions (pH<1 to 7) and has successfully been applied for the determination of silver(I) concentrations in different samples.     

Mechano-elektrochemische Effekte,VI. Einfluß der Elektrodengeometrie auf Potentialverschiebungen bei der plastischen Deformation von Kupfer und Hinweise auf die Systemorganisationen

Summary Plastic deformation of a copper electrode in an aqueous 10^–4 M Cu(ClO₄)₂ solution leads to greater potential shifts, to smaller post strain effects and to greater relaxation rates as the ratio interfacial area: volume of copper is increased. It is suggested that its system organization is improved, as the interfacial area is increased. The system organization appears also to be improved as oxidic superstructures are developing at the interface, where the highest hierarchic levels of both electrode and electrolyte are in immediate contact. Further hints as to the system organization of the whole system Cu/CuSO₄ solution are obtained from the fact that atpH=2 andc _Cu + +=10^–1 mol/l the redox potential is independent of the pretreatment of the electrode and independent of the presence of oxidic superstructures. It is underlined that the passivity due to the presence of an oxidic superstructure is maintained only atpH >2, whereas at lowerpH-values copper is more readily dissolved. It is suggested that atpH=2 the system is in an optimal state of system organization (OSO), similar to those reported in the course of phase transformations, in extremely thin metal films and in solutions at the isokinetic temperature.

     

Electroreduction of phenol red,chlorophenol red and bromophenol red in buffer solutions at DME

The polarographic reduction of phenol red, chlorophenol red and bromophenol red has been investigated in buffered aqueous solutions and alcohol-water water mixtures over a widepH range (2–12). The results show that the three depolarizers are reduced through two irreversible one-electron waves of equal heights at allpH values. The two waves represent the reduction of the triphenylmethane carbon centre. The nature of the waves and the kinetics of the electrode reaction are discussed.

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Elektroreduktion von Phenolrot, Chlorphenolrot und Bromphenolrot an der Quecksilbertropfelektrode

Zusammenfassung Die polarographischen Reduktionen wurden in gepufferten wäßrigen Lösungen und in Alkohol—Wasser-Mischungen über einen weitenpH-Bereich (2–12) durchgeführt. Es zeigte sich, daß in allen drei Fällen die Reduktion über zwei irreversible Ein-Elektron-Wellen gleicher Höhen bei allenpH-Werten vor sich ging. Die Kinetik der Elektrodenreaktion wird diskutiert.

     

Polarographische Untersuchungen an Solochromviolett RS in Pufferlösungen in Abwesenheit und in Gegenwart oberflächenaktiver Stoffe (SAS)

The polarographic behaviour of solochrome violet RS is investigated in buffer solutions of varying pH in the absence and presence of surfactants (triton 100-X and dodecylbenzene-sulphonate). The reduction proceeds irreversibly along a single wave in acid solutions and two waves in alkaline ones. The electrode reaction corresponds to four electrons in media of pH<2.5 and pH>8.0; in media of pH 4 5.5 two electrons are consumed. In solution of pH 2.5 4 or 5.5–8.0, both reactions contribute in varying magnitudes. The addition ofSAS causes the inhibition of reduction beyond the 2-electrons stage in acid solutions; the polarograms comprise one wave in case ofDBS and two waves of equal height in presence of triton. In alkaline media the polarogram comprises three waves due to the splitting of the main reduction wave. The kinetic parameters of the electrode reaction are also determined.     

Flow Injection Potentiometric Determination of Coke Acidity and Acetic Acid Content in Vinegar Using an Antimony Electrode

Abstract

A flow injection potentiometric procedure is proposed for the determination of acidity of several samples using an antimony/ antimony oxide electrode. The ΔE/pH curve in phosphate buffer solutions in the pH range from 1.58 to 12.00 showed a linear sub-Nernstian behavior with an operational slope S =-42.1mV/pH_c and a conditional standard potential of 264.8mV. This flow potentiometric electrode responds linearly to low acidity variations of dilute acetic acid solutions from 3.16 to 100mM with a slope of 30.1mV/dec and analytical frequency of 23 results/h. Good reproducibility and an average standard deviation of ±0.5% was obtained. The application of this sensor for the determination of coke acidity and acetic acid in vinegar is reported.     

Challenges and recent progress in unraveling the intrinsic pH effect in electrocatalysis

The kinetics of many electrode reactions, especially those involving the consumption/production of H⁺/OH^?, show significant pH dependence. Systematic studies of the pH effect over a wide pH range can provide very useful information about their reaction mechanism(s) and help figure out the optimum reaction conditions. For fast electrode reactions in solutions of medium pH and low buffer capacity, correcting the effects induced from the shift of local pH near the electrode?electrolyte interface (pH^s) is a prerequisite for unraveling the intrinsic kinetics and its pH dependence. In this review, recent progress on how to estimate the pH^s, how to eliminate the effect induced by pH^s shift and how to deduce the pH dependent intrinsic reaction kinetics are summarized. Mechanistic and kinetic implication of pH effect on electrocatalytic processes will be discussed by taking formic acid/formate oxidation at Pt electrode as an example.     

Thermodynamic and voltammetric studies of salicylaldehyde-4-(2-pyridyl)-3-thiosemicarbazone

Summary UV spectra, dc polarograms, cyclic voltammetry and coulometry measurements of salicylaldehyde-4-(2-pyridyl)-3-thiosemicarbazone (H₂ SPT) were recorded inBritton-Robinson buffer solutions ofpH 2–12. The dissociation constants of H₂ SPT evaluated spectrophotometrically and potentiometrically in a 5% (v/v)DMF-water mixture are concordant. The dc-polarograms in solutions ofpH<8.5 exhibit a single 4-electron diffusion-controlled polarographic wave, whereas in solutions ofpH>8.5 two waves are observed. The cyclic voltammograms give a single cathodic peak in acidic and neutral solutions, whereas two cathodic peaks can be identified in alkaline ones. An electrode mechanism is proposed and discussed. The effect of temperature on the dissociation constants and the polarographic data was also investigated; the corresponding thermodynamic parameters were derived and are discussed.

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Thermodynamische und voltammetrische Untersuchungen an Salicylaldehyd-4-(2-pyridyl)-3-thiosemicarbazon

Zusammenfassung UV-Spektroskopische, dc-polarographische, voltammetrische und coulometrische Messungen an Salicylaldehyd-4-(2-pyridyl)-3-thiosemicarbazon (H₂ SPT) inBritton-Robinson-Pufferlösungen (pH 2–12) wurden durchgeführt. Die spektrophotometrisch und potentiometrisch ermittelten Dissoziationskonstanten von H₂ SPT in 5% (v/v)DMF/Wasser stimmen überein. In Lösungen mit einempH-Wert unter 8.5 zeigen die dc-Polarogramme eine einzelne diffusionskontrollierte polarographische Welle (4e^–), während in Lösungen mit einempH-Wert von über 8.5 zwei Wellen zu beobachten sind. Die cyclischen Voltammogramme weisen in sauren und neutralen Lösungen einen, in basischen Lösungen hingegen zwei kathodische Peaks auf. Ein Elektrodenmechanismus wird vorgeschlagen und diskutiert. Der Effekt der Temperatur auf die Dissoziationskonstanten und die polarographischen Ergebnisse wurde ebenfalls untersucht. Die entsprechenden thermodynamischen Größen wurden bestimmt und werden diskutiert.

     

Studies on bisazocompounds I

The polarographic behaviour of the title compound in buffer solution ofpH 2–21 is discussed. The two observed waves in alkaline media (pH9.5) are assigned to the formation of hydrazo and the bishydrazo derivatives, whereas in acid solutions the reduction leads to cleavage of both two azo groups. The nature of the waves is investigated and a general mechanism for the reduction process in alkaline and acid media is suggested. The values of the kinetic parameters for the electrode reactions are reported.

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Untersuchungen an Bisazo-Verbindungen I. Polarographisches Verhalten von 1-[4-(4-Natriumsulfonato-phenylazo)-3-natriumsulfonato-phenylazo]-2-naphthol in Lösungen mit verschiedenem pH an der tropfenden Quecksilber-Elektrode Polarographic behaviour of 1-[4-(4-sodiumsulfonato-phenylazo)-3-sodiumsulfonato-phenylazo]-2-naphthol in solutions of varyingpH at the dropping mercury electrode

Zusammenfassung Es wird das polarographische Verhalten der Titelverbindung in Pufferlösungen vompH 2–12 diskutiert. Die zwei im alkalischen Bereich (pH9,5) beobachteten Wellen werden der Bildung der Hydrazo- bzw. Bishydrazo-Derivate zugeordnet, im sauren Bereich führt die Reduktion zur Spaltung beider Azo-Gruppen. Die Natur der Wellen wird untersucht, und ein genereller Mechanismus für die Reduktion im alkalischen und sauren Bereich wird vorgeschlagen. Die kinetischen Parameter der Elektrodenreaktionen werden angegeben.

     

白杨素在汞电极上的电化学行为及其消除超氧阴离子自由基能力研究

The electrochemical behavior of chrysin in pH 2.0-9.0 Britton-Robinson （B-R） buffer solutions was studied by the means of linear sweep voltammetry and cyclic voltammetry at a static mercury drop electrode. In different pH range of B-R buffer solutions, chrysin could cause four reduction waves. In pH 2.0-5.8 B-R buffer solutions, wave P1 yielded by chrysin is a one-electron reduction wave, and wave P1 caused by further reduction of the products of wave P1 in pH〈3.0 B-R buffer solution is also a one-electron reduction wave. But in 3.0〈pH〈5.8 B-R buffer solution wave P1 was overlapped by the hydrogen wave. Between pH 5.8 and 9.0, chrysin could yield two reduction waves P2 and P3- The former is an irreversible adsorptive wave of ionized chrysin involving one electron and the latter is also an irreversible adsorptive wave of reduction intermediate radical of chrysin involving one electron and one proton. And a linear relationship between ip3 and the concentration of chrysin can be established from 1.0×10^-6 to 4.0×10^-5 mol·L^-1 （r=0.9924） with the detection limit of 5×10^-7 mol·L^-1. In addition, the antioxidant ability of chrysin was investigated by linear sweep voltammetry （LSV）. The determination result of IC50 of chrysin showed that chrysin is a good antioxidant.     

o-phthalic acid as a standard substance for calibrations of hydrogen ion concentrations with a glass electrode

o-Phthalic acid is proposed as a standard substance for buffer solutions of known hydrogen ion concentration (I ? 0.2 M KCl, p[H⁺] = 3.0–5.4, 25°C). Its crystallinity, purity and slightly wide buffer range afford advantages over acetic acid. Empirical relationships between measured pH (pH_m) and calculated [H⁺] were derived for sequences of buffer solutions at several ionic strengths: pH_m - Mp[H⁺] + C. These calibration lines were parallel and of unit slope as required by theory. A table of p[H⁺] values for o-phthalic acid buffer solutions at I = 0.1 M (KCl) is presented and the method of calculation of p[H⁺] values for a buffer series generated by additions of potassium hydroxide is outlined.     

The electrode responses of a tungsten bronze electrode differ in potentiometry and voltammetry and give access to the individual contributions of electron and proton transfer

A tungsten wire covered with Na_0.75WO₃ acts in potentiometry as a reversible pH electrode having a pH dependent open-circuit potential E_ocp with nernstian slope. The mid-peak potential E_mp of cyclic voltammograms also depends on pH. At low pH (e.g., pH 2) and slow scan rates (e.g., 2 mV s^–1) the voltammetric response is almost completely reversible. At higher pH and faster scan rates, the voltammetric systems exhibit features of increasing irreversibility. Under the conditions of reversibility, the E_ocp and E_mp differ significantly. E_ocp is determined by the proton transfer at the electrode surface; whereas E_mp is determined by the electron transfer equilibrium tungsten(VI)/tungsten(V) and the proton transfer at the electrode surface. The difference between E_ocp and E_mp provides the individual thermodynamic contributions of electron and proton transfer to the overall pH dependent redox electrode. This is the first time that both contributions can be separated for an insertion electrochemical system (thin surface layer). It is also shown for the first time that the mechanism of an ion-sensitive electrode can differ in potentiometry and voltammetry.     

Potentiometric and Multinuclear Magnetic Resonance Study of the Solution Equilibria Between Aluminium(III) Ion and L-Aspartic Acid

Solution equilibria between aluminium(III) ion and L-aspartic acid were studied by potentiometric, ²⁷Al, ¹³C, and ¹H NMR measurements. Glass electrode equilibrium potentiometric studies were performed on solutions with ligand to metal concentration ratios 1:1, 3:1, and 5:1 with the total metal concentration ranging from 0.5 to 5.0 mmol/dm³ in 0.1 mol/dm³ LiCl ionic medium, at 298 K. The pH of the solutions was varied from ca. 2.0 to 5.0. The non-linear least squares treatment of the data performed with the aid of the Hyperquad program, indicated the formation of the following complexes with the respective stability constants log β_p,q,r given in parenthesis (p, q, r are stoichiometric indices for metal, ligand, and proton, respectively): Al(HAsp)²⁺ (log β_1,1,1 = 11.90 ± 0.02); Al(Asp)⁺ (log β_1,1,0 = 7.90 ± 0.03); Al(OH)Asp⁰ (log β_1,1,−1 = 3.32 ± 0.04); Al(OH)₂Asp⁻ (log β_1,1−2 = −1.74 ± 0.08), and Al₂(OH) Asp³⁺ (log β_2,1,−1 = 6.30 ± 0.04). ²⁷Al NMR spectra of Al³⁺ + aspartic acid solutions (pH 3.85) indicate that sharp symmetric resonance at δ∼10 ppm can be assigned to (1, 1, 0) complex. This resonance increases in intensity and slightly broadens upon further increasing the pH. In Al(Asp)⁺ complex the aspartate is bound tridentately to aluminum. The ¹H and ¹³C NMR spectra of aluminium + aspartic acid solutions at pH 2.5 and 3.0 indicate that β-methylene group undergoes the most pronounced changes upon coordination of aluminum as well as α-carboxylate group in ¹³C NMR spectrum. Thus, in Al(HAsp)²⁺ which is the main complex in this pH interval the aspartic acid acts as a bidentate ligand with –COO⁻ and –NH₂ donors closing a five-membered ring.     

Potentiometric and Multinuclear Magnetic Resonance Study of the Solution Equilibria Between Aluminium(III) Ion and L-Aspartic Acid

Summary. Solution equilibria between aluminium(III) ion and L-aspartic acid were studied by potentiometric, ²⁷Al, ¹³C, and ¹H NMR measurements. Glass electrode equilibrium potentiometric studies were performed on solutions with ligand to metal concentration ratios 1:1, 3:1, and 5:1 with the total metal concentration ranging from 0.5 to 5.0 mmol/dm³ in 0.1 mol/dm³ LiCl ionic medium, at 298 K. The pH of the solutions was varied from ca. 2.0 to 5.0. The non-linear least squares treatment of the data performed with the aid of the Hyperquad program, indicated the formation of the following complexes with the respective stability constants log β_p,q,r given in parenthesis (p, q, r are stoichiometric indices for metal, ligand, and proton, respectively): Al(HAsp)²⁺ (log β_1,1,1 = 11.90 ± 0.02); Al(Asp)⁺ (log β_1,1,0 = 7.90 ± 0.03); Al(OH)Asp⁰ (log β_1,1,−1 = 3.32 ± 0.04); Al(OH)₂Asp⁻ (log β_1,1−2 = −1.74 ± 0.08), and Al₂(OH) Asp³⁺ (log β_2,1,−1 = 6.30 ± 0.04). ²⁷Al NMR spectra of Al³⁺ + aspartic acid solutions (pH 3.85) indicate that sharp symmetric resonance at δ∼10 ppm can be assigned to (1, 1, 0) complex. This resonance increases in intensity and slightly broadens upon further increasing the pH. In Al(Asp)⁺ complex the aspartate is bound tridentately to aluminum. The ¹H and ¹³C NMR spectra of aluminium + aspartic acid solutions at pH 2.5 and 3.0 indicate that β-methylene group undergoes the most pronounced changes upon coordination of aluminum as well as α-carboxylate group in ¹³C NMR spectrum. Thus, in Al(HAsp)²⁺ which is the main complex in this pH interval the aspartic acid acts as a bidentate ligand with –COO⁻ and –NH₂ donors closing a five-membered ring.     

Two-phase buffer systems containing a sparingly soluble diprotic acid as the solid phase

Two-phase buffer systems consisting of a sparingly soluble diprotic acid and its saturated aqueous solution (buffered phase) are investigated. The equations for the dependence of buffer capacity onpH and for evaluation of dilution effects in two-phase buffers of this type are derived. Theoretical considerations are experimentally verified on four buffer systems containing 1,2-, 1,3-, 1,4-benzenedicarboxylic and octanedioic acid. The main characteristic of the buffers of this type is a very high buffer capacity within a narrowpH range.

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Zweiphasen-Puffersysteme mit einer schwerlöslichen diprotischen Säure als feste Phase

Zusammenfassung Zweiphasen-Puffersysteme, bestehend aus einer schwerlöslichen diprotischen Säure und ihrer gesättigten wäßrigen Lösung (gepufferte Phase), wurden untersucht. Die Gleichungen für die Abhängigkeit der Pufferkapazität vompH und für die Berechnung der Verdünnungseffekte in derartigen Zweiphasen-Puffersystemen wurden aufgestellt. Dabei wurden die theoretischen Betrachtungen an vier Puffersystemen, die Benzol-1,2-, -1,3- und-1,4-dikarbonsäuren bzw. Oktandisäure enthielten, experimentell geprüft. Die wichtigste Eigenschaft solcher Puffer ist ihre sehr hohe Pufferkapazität innerhalb eines engenpH-Bereiches.

     

Polarographic behaviour of 2,2′-dipyridyl-2-pyridylhydrazone (DPPH) and pyridine-2-aldehyde-2-quinolylhydrazone (PAQH) at the dropping mercury electrode

The polarographic behaviour of 2,2-dipyridyl-2-pyridylhydrazone (DPPH) and pyridine-2-aldehyde-2-quinolylhydrazone (PAQH) atdme were studied in aqueousBritton-Robinson buffers containing 50% ethanol. The polarograms consist of one irreversible wave in the acidic and alkaline medium. Two electrons are consumed in the splitting of the N-N bond. The effect ofpH on the limiting current andE as well as the reduction mechanism are discussed and compared with similar compounds. The values of the kinetic parameters for the electrode reaction at differentpH values are also computed from the polarographic results.

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Das polarographische Verhalten von 2,2-Dipyridyl-2-pyridylhydrazon (DPPH) und Pyridin-2-aldehyd-2-chinolylhydrazon (PAQH) an der tropfenden Quecksilberelektrode

Zusammenfassung Das polarographische Verhalten der TitelverbindungenDPPH undPAQH wurde in wäßrigenBritton-Robinson-Puffern mit 50% Ethanolgehalt an der tropfenden Quecksilberlelektrode untersucht. Die Polarogramme zeigen eine irreversible Welle im basischen und alkalischen Medium. Zwei Elektronen werden bei der Spaltung der N-N-Bindung verbraucht. DerpH-Effekt auf den Grenzstrom undE _1/2 und auch auf den Reduktionsmechanismus wird diskutiert und mit ähnlichen Verbindungen verglichen. Die kinetischen Parameter für die Elektrodenreaktion werden für verschiedenepH-Werte aus den polarographischen Daten berechnet.

     

Determination of Hydrogen Single Ion Activity Coefficients in Aqueous HCl Solutions at 25°C

The single ion activity coefficients of hydrogen and chloride ions in aqueous HCl solutions have been estimated at 25°C at concentrations up to 1 mol-kg^–1, using potentiometric measurements with ion-selective electrodes and appropriate calibration procedures. Two methods are described for an internal calibration of the electrodes in the extended Debye–Hückel concentration range. The results are compared to the conventional pH calibration with external buffer solutions. Since the latter calibration method does not account for the liquid junction potential E _J which arises at the reference electrode, the resulting activity coefficients are quite different in HCl solutions of higher concentration. These differences between internal and external calibration decrease significantly, when a correction for E _J is introduced into the conventional pH calibration. Hence, in solutions of higher ionic strength the accuracy of the conventional pH electrode calibration using buffer solutions is very limited, when exact H⁺ activities are required. The consistency of the results indicates that the liquid junction potentials in the examined systems calculated by the Henderson/Bates approximation are of reasonable precision.     

Determination of pH value of isoelectric solution and identification of passive iron surface phases using immersion method

Using the method of phase modeling, the pH values of solutions corresponding to the uncharged surface of passive iron and ferric oxide γ-Fe₂O₃ (pH₀) are compared. According to the theory of connected places, the charge of metal oxide surface is determined by the adsorption or desorption of hydrogen ions leading to a change in the potential drop at the oxide/solution interface. Preliminarily passivated iron electrode was washed with twice-distilled water and placed into 0.5 M NaNO₃ solution with various pH values; the variation in the potential (ΔE) with time was studied. The pH₀ value for passive electrode under the open-circuit conditions was determined by the dependence of ΔE on the pH value (pH₀ 6.2 ± 0.1). The pH₀ value was close to that for γ-Fe₂O₃ (pH₀ 6.2), which was determined by the method of potentiometrical titration of oxide suspension in the nitrate solution. The introduction of surface-active ions Ba²⁺ and Cl^? changes the charge of passive iron surface: Ba²⁺ ions increase the electrode potential, while Cl^? ions decrease it. Comparing the pH₀ values for passive electrode and metal oxides, one can identify the composition of passive electrode surface.