Solutions of hydrochloric acid in formamide—IV. Thermodynamic properties from the study of cells using the quinhydrone electrode at 25°C

The emf of cells Ag AgCl, HCl(m), QH₂, Q Pt, Hg Hg₂Cl₂, HCl(m), QH₂, Q Pt and Pt H₂, HCl(m), QH₂, Q Pt have been measured at 25°C using formamide as solvent. The standard potential of the quinhydrone electrode obtained from the above cells is 0·7151 ± 0·0009, 0·7137 ± 0·0005 and 0·7140 ± 0·0002 V respectively. The calculated mean molal activity coefficients of HCl agree fairly well with earlier reported results.     

Solution of hydrochloric acid in formamide—V. Thermodynamic properties from EMF of the cell Pt,H2|HCl(m)|Hg2Cl2, Hg over a temperature range 5–55°C (278.15–328.15 K)

The emf of cells (A) Pt, H₂|HCl(m)|Hg₂Cl₂, Hg and (C). AgAgCl|HCl(m)|Hg₂Cl₂, Hg have been measured at intervals of 5°C over the temperature range 5–55°C (278.15–328.15 K) using formamide as the solvent. The molal standard potentials of Hg, Hg₂Cl₂ electrode, determined on the basis of emf data of cell (A), agree well with those derived from the emf of cell (C) using known values of E⁰_Ag-AgCl reported earlier. Based on emf data of these cells, the standard thermodynamic changes ΔG⁰, ΔH⁰ and ΔS⁰ for the cell reactions as well as the mean molal activity coefficients of HCl at rounded molalities have been determined.     

Thermodynamics of hydrochloric acid in tetrahydrofuran-water mixtures

From the emf measurements of the cell Pt, H₂ (g, 1 atm):HCl (m), X% tetrahydrofuran, Y% water: AgCl, Ag the standard potentials of AgAgCl electrode have been determined in eighteen tetrahydrofuran-water mixtures at nine different temperatures in the range 15–55°C. The temperature variation of the standard emf was utilized to calculate the standard thermodynamic functions for the cell reaction and the standard thermodynamic quantities for the transfer of one mole of HCl from water to tetrahydrofuran-water media. The primary medium effects of various solvents upon HCl were also calculated. The results have been interpreted in regard to the acid-base properties and the structure of the solvent.     

Application of multilinear regression analysis to the evaluation of the standard EMF of the hydrogen—silver/silver chloride cell in methanol + water mixtures over wide ranges of temperature and composition

The one-stage multilinear regression method previously applied to the analysis of emf data for aqueous electrochemical cells as a function of temperature and solute concentration has been extended to the analysis of emf data for the cell: Pt|H₂|HCl (m), CH₃OH(x), H₂O (1 ? x) | AgCl|Ag as a function of methanol mole fraction (x) as well as solute molality (m) and temperature. In this way discrepancies between various sets of data are revealed and the consistent set of data for the standard potential of the cell so obtained can be used for the evaluation of thermodynamic acid dissociation constants and standard reference pH values in this important, much studied mixed aqueous solvent system.     

Potential of the Pb,PbSO4/H2SO4 electrode at temperatures up to 240°C

Standard lead—lead sulphate electrode potential was determined over the temperature range 20–240°C from emf measurements of the Pb, PbSO₄H₂SO₄ (0.05M)K₂SO₄KClHCl(0.1M)/AgCl, Ag and Pb, PbSO₄H₂SO₄(m)K₂SO₄H₂SO₄(0.05M)PbSO₄, Pb cells where m = 0.005, 0.01, 0.1 and 0.5 M. To this effect lead—lead sulphate electrode potential was calculated using the temperature relationship of the standard silver—silver chloride electrode potential and activity coefficients of hydrochloric acid determined by Greeley et al. at temperatures up to 260°C. Diffusion potentials occurring at the phase boundaries in the cells under investigation were calculated using the Henderson's equation. Values of the standard lead—lead sulphate electrode potential were determined by extrapolation of the E°′ function to the zero ionic strength which was calculated using the second sulphuric acid dissociation constant determined by Lietzke et al. at temperatures up to 300°C. The standard electrode potential was described in the temperature range 20–240°C by the following relationship: E°_{Pb, PbSO4/SO^2?4}(V) = 0.040-0.00126T. A change in entropy ΔS° of the electrode reaction Pb + SO^2?₄ = PbSO₄ + 2e^? is constant in this temperature range and is ?243 JK^?1 mol^?1 (?1018 cal K^?1 mol^?1).     

Studies in isodielectric media Part—I. Standard potentials of the silver—silver chloride electrode in dimethyl formamide + ethylene glycol mixtures at 25°C

Standard potentials (_sE^s) of the silver—silver chloride electrode have been determined at 25°C in some approximately isodielectric media containing ethylene glycol (EG) and 10, 30 and 50% by weight of N-N-dimethyl formamide (DMF) from emf measurements performed on the cell: H₂ (g, 1 atm)/HCl (m). solvent/AgClAg. From these values the activity coefficients (^s_sγ) of HCl at several concentration in each of the mixed solvents and the standard Gibbs energies of transfer (ΔG⁰_t) of HCl from EG to the mixed solvents have been computed. Comparison of the observed ΔG⁰_t (HCl)-composition profile with those for three other related solvent systems studied earlier, reveals that the observed profile results from the opposing effects of the larger proton-accepting and the pronounced anion-desolvating propensities of DMF and its glycolic mixtures compared to pure EG and that the increased stabilization of proton is superseded by the increased desolvation of Cl^? at higher proportions of DMF.     

Activity measurements in aqueous mixed electrolyte solutions—III. HClCaCl2 mixtures of constant total ionic strength

Emf measurements have been made in cells without liquid junction of the type H₂/HX (m₁), MX_n(m₂)/AgCl, Ag . . (I) containing mixtures of HCl and CaCl₂ in different proportions, but at constant total ionic strength (I = 2, 1, 0.5). The measurements have been carried out at 5°C intervals over the temperature range 5–35°C. AgAgCl electrodes prepared by two different methods (thermal and thermoelectrolytic) were used of which the thermoelectrolytic electrodes were found to give more consistent results. The results when extrapolated properly lead to E° values at the different temperatures which agree well with those reported in literature. The application of the Pitzer method leads to the calculation of the ionic interaction coefficients α₁₂ and α₂₁, at the different constant ionic strengths. The nature of variation of these α's with the total mixture ionic strength at different temperatures has been considered.     

Thermodynamic studies on liquid AgClKClLiCl solutions

The thermodynamic properties of ternary AgClKClLiCl solutions were determined at various values of

and a different AgCl concentrations, using the formation cell: Ag(s)|AgClKClLiCl(l)|Cl₂(g), graphite. Dilute AgCl solutions were investigated in detail in order to extrapolate in γ_Agcl to X_AgCl å 0. Experimental values of Δ$\text{G}$^E_AgCl were interpreted by the Pelton, Flengas method and compared with Krupkowski's formalism, based only on data from the constituent binaries.     

Thermodynamics of hydrochloric acid in 1-propanol-water mixtures from EMF measurements at different temperatures

Emf measurements of cells of the type Pt, H₂ (g, 1 atm) |HCl (m), 1-PrOH (X), H₂O (Y)| AgCl/Ag at 5–45°C and m 0·005–0·15 mol/kg have been used to derive the standard potentials of the cells by a theoretically justified polynomial curve-fitting technique, the mean activity coefficient of HCl (molal scale), the medium effect, the relative partial molal heat content and the thermodynamic function (mol-fraction scale) for the transfer of HCl from water to the respective 1-PrOH-H₂O media. E_m° is expressed as a function of temperature. The electrostatic part of the thermodynamic functions was computed according to Born's model and hence the chemical effect of the solvents on the transfer process has been evaluated. The significance of the results discussed with regard to the acid-base properties and the structural effects of the solvents with further additions of 1-PrOH.     

Thermodynamics of hydrochloric acid in dioxane—water mixtures

From the emf measurements of the cell H₂¦Cl¦AgCl¦Ag the standard potentials (IUPAC convention) of AgAgCl electrode have been determined in 10, 30, 40 and 60 wt% of dioxanewater media at 25, 30, 40 and 45°C, together with the activity coefficients of HCl in the same media. The relevant thermodynamic functions for hydrochloric acid in the same media were also calculated.     

Solvent effect on the dissociation of m- nitroanilinium ion in water-ethylene glycol media at 25°C

The dissociation constants (^sK)_BH⁺ of m-nitroanilinium ion, have been determined at 25°C in aqueous mixtures of 10, 30, 50, 70 and 90 wt.% of ethylene glycol from spectrophotometric measurements. The solvent effect on the dissociation behaviour of this acid: Δ⁰_t(BH⁺ ?B)_sys = 2.303 RT [p(_sK)^N_BH⁺ ?p(^wK)^N_BH⁺] in these media results in a characteristic minimum with change in the solvent composition. The solvent effect has been discussed in terms of the free energies of transfer (ΔG⁰_t) from water to the aqueous glycol solvents, of the uncharged base (B) evaluated from measured solubilities, of the hydrochloride of the base (BHCl) computed from the corresponding values for HCl, and also of the individual ions assessed from the previous knowledge of ΔG⁰_t(Cl^?). The corresponding data for the protonated Tris (hydroxymethyl) methylamine (Tris H⁺) and p-nitroanilinium ion in the same solvent mixtures obtained from the literature are also compared. Solvent effect on the dissociation of m-nitroanilinium ion has been found to be nearly the same as that for the p-nitroanilinium ion with similar chemical composition but different from that observed in the case of Tris H⁺ of different chemical nature.     

Thermodynamics of HCl in water—ethylene carbonate mixtures

The emf of the cell: Pt; H₂ (1 atm.)/HCl (m), Ethylene carbonate (x), H₂O (100?x)/AgCl, Ag is measured at 5° interval in the temperature range 25 to 45°C. The standard potential of the silver—silver chloride electrode, E^o_m, is determined in each solvent mixture covering the range 0–78% EC. The temperature coefficient is calculated using the method of least squares. The mean activity coefficient of HCl, γ_±, is determined for molalities between 0.01 and 0.05 m in each case. The primary, secondary and total medium effects are calculated. From the variation of the mean activity coefficient with temperature, the relative partial molal heat content $\text{L}$₂ of HCl is computed. $\text{L}$₂ passes through a minimum at 30% for 0.05 m HCl at all temperatures. The effect of temperature on $\text{L}$₂ permits the evaluation of the corresponding relative partial molal specific heat $\text{J}$₂.     

Standard potentials of the silver/silver-iodide electrode in aqueous mixtures of ethylene glycol at different temperatures and the thermodynamics of transfer of hydrogen halides from water to glycolic media

The standard potentials (E°) of the silver/silver-iodide electrode in aqueous mixtures of ethylene glycol (containing 10, 30, 50, 70 and 90 wt-% glycol) have been determined from emf measurements of the cell Pt, H₂(g, 1 atm)/HOAc(m₁), NaOAc(m₂), Kl(m₃), solvent/AgI/Ag at nine temperatures ranging from 5 to 45°C. These E° values have been utilized to compute free energy (Δt_t°), entropy (ΔS_t°) and enthalpy (ΔH_t°) changes accompanying the transfer of HI from water to each of the solvents. These values as well as those for HCl and HBr and obtained earlier have also been utilized to evaluate ΔS_t°(i) for individual ions by a method of ‘simultaneous extrapolation,’ which in turn furnished the values of ΔH_t°(i) for these ions. These quantities and also the ‘chemical’ contributions for the halide ions as obtained by subtracting the ‘electrostatic’ contribution computed with the Born equation, have been examined in the light of ion—solvent interactions as well as the structural changes of the solvents. The observed results conform with what is expected from the competitive effects of the preferential solvating capacities of water towards halide ions and that of other solvents towards hydrogen ions, and also of the effects arising from the structural changes of the solvents that are likely to occur in the over-all transfer process.     

Solutions of benzoic acid in formamide—I: The activity coefficients of the undissociated part of benzoic acid in potassium benzoate solutions at 25°C

The emf of the buffered cell Pt. H₂ |;HBz(m₂)|; Hg₂Bz₂|; Hg, has been measured at 25°C using formamide as the solvent. The standard potential of the HG, Hg₂Bz₂ electrode is 0·2545 ± 0·0005 V in good agreement with the value, 0·2541 ± 0·0003 V, obtained from the study of the cell Pt, H₂ HBz(m) Hg₂Bz₂, Hg. From the standard potential of the Hg, Hg₂Bz₂ electrode, the standard free energy charge, ΔG° for the cell reaction in formamide medium has been calculated. In addition, the activity coefficients of the undissociated benzoic acid in potassium benzoate solutions have been evaluated for 25°C.     

Solutions of benzoic acid in formamide—II. thermodynamic properties from electromotive force measurements

The emf of the cell Pt, H₂, HBz(m), Hg₂Bz₂, Hg in formamide has been measured at 5°C intervals over the temperature range 5–45°C. The values of the standard potentials of the mercury—mercurous benzoate electrode can be fitted in the form of equation, obtained by using the least squares method:

The standard thermodynamic quantities (ΔG°, ΔH°, and ΔS°) for the cell reaction have been evaluated for different temperatures.     

Thermodynamic studies of sodium bromide in 2-propanol—water mixtures (10, 30 and 50 wt.%) from electromotive force measurements

The emf of the cell: Pt|Na(Hg)|NaBr(m), w wt.% 2-propanol—water mixture|AgBr, Ag has been measured with w = 10, 30 and 50 at temperatures of 293.15, 303.15 and 313.15 K in the molality range of NaBr from 0.04 to 0.18 mol kg^?1. From these data the standard emf of the cell has been obtained and by utilizing data from the literature for the E⁰ of Ag/AgBr electrode the standard potential of sodium amalgam electrode has been determined. Mean activity coefficients of NaBr and thermodynamic functions ΔG⁰_t, ΔH⁰_t and ΔS⁰_t for transfer of NaBr from water to 2-propanol—water mixtures have also been calculated. Thermodynamic quantities of transfer have been compared to those for the HBr electrolyte.     

Melting parameters of poly(glycolic acid)

Equilibrium melting temperature T_m⁰, heat of fusion ΔH_f, and entropy of fusion ΔS_f of poly(glycolic acid) (PGA) was determined by using Clapeyron-Clausius equation. Equilibrium melting temperature T_m⁰ was 504.6 K which was determined by Hoffman-Weeks plots. The pressure dependence of T_m⁰ was determined by high pressure DTA up to 150 MPa. Volume change ΔV_f at melting was determined by using dilatometer. Heat of fusion in PGA was 183.2 (J g⁻¹), which is very close to the value reported by Chujo et al. who determined it by using T_m depression in copolymer with poly(lactic acid). ΔS_f of PGA was 0.363 (J g⁻¹ K⁻¹), which is about twice that of PLA, and the reason was discussed on the basis of the elastic modulus below T_m.     

DC and AC voltammetry of a free-base porphyrin adsorbed onto basal-plane graphite under acidic conditions: An example of a close to ideal reversible two-electron surface-confined redox process at sub-monolayer coverages

The free-base porphyrin, 5,10,15,20-tetrakis(1-methyl-4-pyridyl)-21H,23H-porphine (H₂TMPyP), adsorbs onto a basal-plane graphite electrode. Under DC cyclic voltammetric conditions, the fully protonated dication, [H₄TMPyP(0)]²⁺, undergoes an apparently close to ideal surface-confined two-electron reduction to the neutral [H₄TMPyP(-II)] species when the supporting electrolyte consists of aqueous 1 M HCl and 1 M NaCl and coverages are sub-monolayer. The reversible potential calculated from the average of the oxidation and reduction peak potentials is 0.138 ± 0.002 V (vs Ag/AgCl, 3 M NaCl) whilst their separation ΔE_p, approaches 0 mV at slow scan rates, as expected theoretically for an ideal surface-confined electron transfer process. Comparisons of simulated and experimental data imply that the increase in ΔE_p observed at scan rates above 10 V s⁻¹ is consistent with uncompensated Ohmic IR_u drop effects, and not limitations imposed by electron transfer kinetics. Analysis of fundamental and higher harmonic components derived from large-amplitude sine-wave AC voltammetry is consistent with a very fast electron transfer rate constant, k⁰, in excess of 10⁶ s⁻¹ for the overall two-electron process. However, careful comparison with AC theory highlights minor levels of non-ideality not attributable to purely capacitative background or uncompensated resistance effects. These are particularly evident when greater than monolayer surface coverages are employed. It is likely that subtle contributions from heterogeneity in the adsorbed layer and complexities in the reaction mechanism are present in this close to ideal surface-confined process, but they are more readily detected under conditions of large-amplitude Fourier transformed AC cyclic voltammetry than with the conventionally used DC cyclic format.     

Thermodynamics of the silver—silver chromate electrode in water + 10, + 20, + 30 and + 40 mass percent of dioxane at different temperatures

The emf of the cell: Ag, AgCl, NaCl(m)?Na₂CrO₄(m/2), Ag₂CrO₄, Ag in water + 10, + 20, + 30, and + 40 mass percent of dioxane has been measured at 5°C intervals over the temperature range 15–45°C. The values of the standard potentials of the silver—silver chromate electrode have been determined in these mixed solvent media at these temperatures. The standard thermodynamic quantities (ΔG°, ΔH°, and ΔS°) for the cell reaction in water + dioxane mixtures have been evaluated at these temperatures. The results are discussed in terms of the preferential solvation of the ions.     

XPS investigation of surface oxidation layers on a platinum electrode in alkaline solution

A thick oxidation layer on a platinum electrode has been grown in 1 N NaOH at 3 V vs Ag/AgCl reference electrode. After transferring the Pt electrode into an ultrahigh vacuum chamber the surface layer was analysed by X-ray photoelectron spectroscopy. Pt4f_5/2 amd O1s electron binding energies of 74.3, 77.6 and 530.9 eV respectively, as well as the broad peak shape of the O1s signal and the oxygen-to-platinum intensity ratio of 3.08 point towards a platinum—oxyhydroxide PtO(OH)₂. This formula is in good agreement with cyclic voltammetry curves, measured for the same electrode, that revealed two cathodic reduction peaks for oxygen surface coverages equivalent to more than two hydrogen monolayers. These two peaks were assigned to PtOH and PtO. XPS analysis at elevated temperatures showed that the thick (5 nm) oxidation layer decomposes at 400 K to a mixture of several oxides and hydroxides of Pt⁴⁺ and Pt²⁺ and Pt metal with a ratio of O-to-Pt of 1. This mixture further gradually decomposes to only a monolayer of oxygen at 770–870 K. Sodium cations were found to be present in trace amounts in the adlayer and to strongly shift the O1s binding energy to lower values.