Quinhydrone Electrode Research Articles

Ionic equilibrium in mixtures of polar protophobic and protophilic non-hydrogen bond donor solvents: Acids, salts, and indicators in acetonitrile with 4 mass % dimethylsulfoxide

Acetonitrile (AN) and dimethylsulfoxide (DMSO) belong to the so-called polar non-hydrogen bond donor solvents, in accordance with Bordwell's classification. At that, according to Kolthoff, AN is a protophobic, and DMSO is an expressed protophilic solvent. This article presents a study of the acid-base and related equilibria in a binary AN – DMSO solvent system with mass ratio 96: 4, which corresponds to the molar fraction xDMSO = 0.021. Owing to the aforesaid property of DMSO, the utilization of this binary mixture makes it possible to neglect the interfering influence of water traces, which is difficult to overcome in pure AN. As result, in the solvent under study, the proton is solvated first of all by DMSO despite predominance of AN. The solvent composition was chosen basing on the pKa values of the picric acid, which were determined via UV–visible spectroscopy at different AN – DMSO ratios. The interfering influence of water traces was checked. An operational paH+∗ scale was developed using a quinhydrone electrode in a concentration cell with liquid junction, basing on a picrate buffer solution with paH+∗ = 3.27. The cation-anion association constants of a set of salts were determined using the conductance method, the pKa values of the buffer acids (benzoic, salicylic, 2,4- and 2,6-dinitrophenols) were calculated, and the constants KAHA−f of homoassociation (A− + HA ⇄ AHA−) were estimated. The response of the glass electrode to the acidity of the solutions was proved within the range of 2.5 ≤ paH+∗ ≤ 15.5 (slope = 59.69), the paH+∗ values of 16 picrates of organic bases were determined, and the pKas of 7 indicators, nitrophenols and sulfonephthaleins, were obtained spectroscopically in the solvent under study. The dissociation constants of salts, acids, and indicators in AN–DMSO (96:4) mixed solvents were compared with those in other non-hydrogen bond donor solvents. Basing on some reliable assumptions, the values of the Gibbs energy of the H+ transfer from the binary solvent to AN and DMSO were estimated as ΔGtro = +44 ± 3 and −22.6 ± 3 kJ mol−1, respectively.

Study of the pH function of a mechanically renewed graphite-quinhydrone indicator electrode

A composition of quinhydrone and spectral graphite powders and also an epoxy resin with polyethylene polyamine as a binder is studied for fabricating a solid indicator electrode, renewed by mechanically cutting a thin surface layer, intended for measurements of the pH of solutions. It is shown that after the renewal of the electrode surface, the dissolution of the components of quinhydrone occurs in the near-electrode layer and the electrode potential depends on the pH of the solution and the time of the contact of the electrode with the solution. The pH dependence of potentials (Et) at t = const in the pH range 2.0–7.0 is linear and close to the theoretical one. In the pH range 7.0–13.9, the results of measurements of Et by the proposed carbon-quinhydrone electrode, in contrast to the traditional quinhydrone electrode, are well reproducible and linearly depend on pH. This opens up the possibility of using the electrode for the determination of Et also in alkaline media.

Ionic equilibrium in mixtures of protophobic and protophilic polar non‐hydrogen bond donor solvents: acids, salts, and indicators in acetone containing 5 mol% DMSO

AbstractProperties of a protophobic polar non‐HBD solvent can be strongly modified by introduction of a small amount of a protophilic polar non‐HBD solvent. In this paper, acetone (AC) with 5 mol% additive of DMSO, a solvent with $\varepsilon _{\rm r}^{{\rm 25}} = 22$, was considered as a media for acid–base reactions. Conductance was used for determination of dissociation constants of a set of salts, hydrogen chloride, and picric acid. The last‐named was also studied by UV‐vis spectroscopy. The introduction of 5 mol% of DMSO results in suppressing, to some extent, the homoconjugation processes in AC media as well as of proton hydration by (possible) traces of water. The dissociation of salicylic acid and 2,4‐dinitrophenol was examined utilizing quinhydrone electrodes in a cell with liquid junction. The pKa values of buffer acids and ${\rm p}a_{{\rm H}^ + }^* $ values of buffer solutions were calculated by taking into account the incomplete dissociation of salts. The response of the glass electrode appeared to be satisfactory, which allowed the estimation of the pKa value of benzoic acid. The apparent ionization constants of 22 acid–base indicators in buffer mixtures and perchloric acid solutions were determined in (AC + 5 mol% DMSO) using the spectrophotometric procedure. Copyright © 2009 John Wiley & Sons, Ltd.

Establishment of a Hydrogen Scale in 80 Mass % Propylene Carbonate + p-Xylene Medium

Details of the standardization of the reference electrode Hg/HgCl2(s) versus the SHE by a potentiometric method using two acids (viz., perchloric and 2,5-dichlorobenzene-sulfonic acids) in 80 mass % propylene carbonate (PC) + p-xylene (PX), are presented. Using this reference electrode, the standard electrode potential of the quinhydrone electrode was determined in this medium. The reference electrode potential and the standard electrode potential of the quinhydrone electrode were found to be (0.277±0.003) and (0.760±0.003) V versus SHE in 80 mass % PC + PX at 25 °C, respectively. The voltammetric behavior of the quinhydrone system in this medium was investigated at a micro platinum electrode against a non-aqueous double-junction Ag/AgCl reference electrode. The standard electrode potential of the quinhydrone system was also calculated using voltammetric and chronocoulometric data that is comparable with the value obtained by potentiometry.

Thermal diffusion of hydrogen chloride in aqueous solutions of N,N-dimethylformamide and acetone

The initial and stationary temperature coefficients of voltage were measured at 298.15 K for symmetrical thermogalvanic cells with silver-chloride and quinhydrone electrodes in the HCl-H2O-S systems (S is N,N-dimethylformamide or acetone) to determine Soret coefficients, the standard entropies of thermal diffusion transfer of hydrogen chloride, and the standard entropies of H+ and Cl− ions moving in water mixtures with N,N-dimethylformamide and acetone. The Agar model was used to calculate the standard entropies of thermal diffusion transfer of ions and the standard partial molar entropies of the H+ and Cl− ions.

Entropy characteristic of solvation and thermal diffusion of hydrogen chloride in water-1-propanol solutions: A thermoelectrochemical determination

Thermogalvanic cells with silver chloride and quinhydrone electrodes in the HCl-H2O-1-PrOH system are studied experimentally. The results are used to determine standard entropies of thermal diffusion transport of hydrogen chloride, entropies of mobile H+ and Cl− ions, and Soret coefficients of the electrolyte at 298 K. Thermal diffusion entropies and partial molar entropies of said ions in water-1-propanol solutions are calculated within the Agar model. The results are interpreted with the application of basic concepts of the De Bethune theory concerning thermal diffusion transport in electrolytes.

Electrochemical determination of standard thermal diffusion characteristics for chlorides of hydrogen and potassium in water-ethanol solutions

Initial and steady-state temperature coefficients of the voltage generated by thermogalvanic cells comprising silver chloride and quinhydrone electrodes in the systems HCl-H2O-C2H5OH and KCl-H2O-C2H5OH are determined. Standard thermal diffusion Soret coefficients, the entropy of thermal diffusion transport of electrolytes, and the entropy of mobile ions H+, K+, and Cl− are calculated for 298 K. The effect the nature of ions and the solvent composition exert on the obtained thermal diffusion transport characteristics is considered in terms of the De Bethune theory.

Standard characteristics of thermodiffusion of hydrogen chloride in the water-2-propanol system at 298.15 K

The initial and stationary temperature voltage coefficients for symmetric thermogalvanic cells with silver/silver chloride and quinhydrone electrodes in the HCl-H2O-2-PrOH system are measured at 298.15 K. From the experimental data obtained, the standard entropies of thermodiffusional transfer of the electrolyte, the standard entropies of moving hydrogen and chlorine ions, and the Soret coefficients of HCl in aqueous solutions of 2-propanol are calculated. The influence of the composition of the mixed solvent and the nature of ions on the entropy characteristics is discussed.

The thermal diffusion characteristics of hydrogen and alkali metal chlorides in aqueous-methanolic solutions at 298.15 K

The paper presents the results of experimental studies of thermoelectrochemical systems comprising silver chloride and quinhydrone electrodes in aqueous-methanolic solutions of hydrogen, lithium, potassium, and cesium chlorides. These results and literature data are used to calculate the standard entropies of transfer of electrolytes, Soret thermal diffusion coefficients, and entropies of moving ions at 298.15 K. The influence of the nature of electrolytes and mixed solvent composition on the characteristics of thermal diffusion transfer in the systems studied is discussed.

Teaching pH Measurements with a Student-Assembled Combination Quinhydrone Electrode

A combination pH electrode that can be assembled by the student is described. It consists of a glass holder and two sensors in the form of rubber stoppers that contain quinhydrone and graphite. The combination electrode is suitable to teach potentiometric measurements, pH measurements, and the interplay of acid–base and redox equilibria. The electrode meets highest safety standards and overcomes the troubles associated with the use of the conventional quinhydrone electrode. Compared with commercial combination glass electrodes, the combination quinhydrone electrode has the didactic advantage that the sensor and the reference electrodes are both visible and not preassembled.

Re-Evaluation of Stoichiometric Dissociation Constants from Electrochemical Cell Data for Propionic and n-Butyric Acids at (0 to 60) °C and for Some Other Aliphatic Carboxylic Acids at (18 or 25) °C in Aqueous Sodium Chloride Solutions

Equations were determined for the calculation of stoichiometric (molality scale) dissociation constants Km of propionic and n-butyric acids in buffer solutions from their thermodynamic dissociation constants Ka and molalities of components in solution at temperatures from (0 to 60) °C. These equations are based on single-ion activity coefficients from Hückel-type equations. The required thermodynamic dissociation constants and propionate and n-butyrate parameters were determined from Harned cell data of Harned and Ehlers (1933) and Harned and Sutherland (1934). The parameters agreed closely with parameters for acetate species determined recently. The required interaction parameters between hydrogen ions and chloride ions were taken from recent HCl data reassessment, and between hydrogen ions and sodium ions and between sodium ions and chloride ions were taken from the recent acetate work. The new equations were used to calculate pH values of propionate buffers with equal molalities of propionic acid and sodium propionate of (0.01 or 0.1) mol kg-1 at temperatures from (0 to 60) °C. Values of p(mH), giving molality of protons directly, calculated from these equations are tabulated, and p(mH) values for buffer solutions with NaCl as major component and propionate species as minor components are also given. The Hückel equations were also applied to a re-evaluation of Ka for formic acid and activity parameters for formate species at 25 °C from Harned cell data of Prue and Read (1966) and the parameter values for species from acetic, formic, propionic (propanoic), n-butyric (n-butanoic), 2-methylpropanoic, n-pentanoic, 2,2-dimethylpropanoic, n-hexanoic, 4-methylpentanoic, and n-heptanoic acids at 18 °C from quinhydrone electrode data of Larsson and Adell (1931).

Problems of Electrochemical Controls of Oxidation-Reduction Systems in Aqueous-Organic and Nonaqueous Media. The rH Index with Establishment of the Relevant Scales and Standards

The state and problems of the electrochemical controls of oxidation-reduction systems in aqueous-organic and nonaqueous solvent media with particular emphasis on the rH index of the reducing power of such systems are discussed. In the frame of a generalized treatment of the rH-metric domain, the key definitions, the acquisition of primary rHS standards, the comparison of rH scales in different solvent media, and the recommended procedure of rHX determination are described. The most recent results here obtained in the studies of the behaviour of the quinhydrone electrode with respect to the hydrogen gas electrode, and of the key cell Pt|H2|H2O|HgO|Hg|Pt are reported, and the guidelines for further development are outlined.

Determination of stoichiometric dissociation constants of lactic acid in aqueous salt solutions at 291.15 and at 298.15 K

Equations were determined for the calculation of the stoichiometric (molality scale) dissociation constant, K m, of lactic acid in aqueous salt solutions at 291.15 and 298.15 K from the thermodynamic dissociation constant, K a, of this acid and from the ionic strength, I m, of the solution. The salt alone determines mostly the ionic strength of the solutions considered in this study, and the equations for K m were based on the single-ion activity coefficient equations of the Hückel type. New data measured by potentiometric titrations in a glass electrode cell at 298.15 K and the literature data obtained by Larsson and Adell with quinhydrone (qh) electrode cells at 291.15 K were used in the estimation of the parameters for the Hückel equations (HE) of lactate ions in NaCl and KCl solutions. The Harned cell data measured at 298.15 K by Nims and Smith were used to obtain the activity parameters for lactate ions in dilute LiCl, BaCl 2 and SrCl 2 solutions. The conductance data measured at 298.15 K by Martin and Tartar were used to determine the thermodynamic value of the dissociation constant of lactic acid. By means of the calculation method suggested in this study, K m can be obtained almost within experimental error at least up to I m of about 0.5 mol kg −1 for lactic acid in NaCl and KCl solutions at the two temperatures considered. The K m values obtained by this method were compared to those obtained by the calculation methods presented recently in the literature for a general treatment of thermodynamics of weak acids in NaCl and KCl solutions.

Coulometric titrations of bases in propylene carbonate and γ-butyrolactone using hydroquinone as the depolarizer and a quinhydrone indicator electrode

The application of hydroquinone for the coulometric generation of hydrogen ions in propylene carbonate (PC) and ?-butyrolactone (GBL) is described. The current-potential curves recorded for this depolarizer, titrated bases, indicator and the solvents used showed that the investigated depolarizer is oxidized at lower potentials than the oxidation potentials of other components in the solution. The hydrogen ions generated by the oxidation of hydroquinone were used for the titration of organic bases (triethylamine, n-butylamine, pyridine, quinoline, aniline, N,N'-diphenylguanidine, piperidine, and 2,2?-bi?pyridine) in PC and GBL with with visual (Crystal Violet as indicator) and potentiometric end-point detection using a quinhydrone electrode as the indicator electrode. The quinhydrone added to the to be analyzed solution served both as a source of hydrogen ions and, together with the immersed platinum electrode, as a quinhydrone electrode. The relative error of the determination of the bases was about 1%.

Determination of stoichiometric dissociation constants of benzoic acid in aqueous sodium or potassium chloride solutions at 25°C

Equations were determined for the calculation of the stoichiometric (molality scale) dissociation constant K m of benzoic acid in dilute aqueous NaCl and KCl solutions at 25°C from the thermodynamic dissociation constant K a of this acid and from the ionic strength I m of the solution. The salt alone determines mostly the ionic strength of the solutions considered in this study and the equations for K m were based on the single-ion activity coefficient equations of the Hiickel type. The existing literature data obtained by conductance measurements and by electromotive force (EMF) measurements on Harned cells were first used to revise the thermodynamic value of the dissociation constant of benzoic acid. A value of K a = (6.326 ± 0.005) x 10 -5 was obtained from the most precise conductivity set [Brockman and Kilpatrick] and this value is supported within their precisions by the less precise conductivity set of Dippy and Williams and by the EMF data set measured by Jones and Parton with quinhydrone electrodes. The new data measured by potentiometric titrations in a glass electrode cell were then used for the estimation of the parameters of the Huckel equations of benzoate ions. The resulting parameters were also tested with the existing literature data measured by cells with and without a liquid junction. The Huckel parameters suggested here are close to those determined previously for anions resulting from aromatic and aliphatic carboxylic acids. By means of the calculation method based on the Huckel equations, K m can be obtained almost within experimental error at least up to I m of about 0.5 mol-kg -1 for benzoic acid in NaCl and KCl solutions.

The mechanism of bicarbonate reabsorption in the proximal and distal tubules of the kidney. 1965.

The mechanism of HCO3- reabsorption in proximal and distal tubules was examined in rats undergoing NaHCO3 diuresis. The steady-state intratubular pH was measured with pH-sensitive glass microelectrodes and compared with the equilibrium pH calculated from the HCO3- concentration of the tubular fluid (measured with quinhydrone electrodes) and plasma Pco2. In the proximal tubule the intratubular pH and the equilibrium pH were identical, indicating no accumulation of excess H2CO3. After inhibition of carbonic anhydrase, however, intratubular pH was significantly lower (0.85 pH U) than the equilibrium pH. It was concluded that HCO3- reabsorption in the proximal tubule was mediated by H+ secretion, but that carbonic anhydrase located in the luminal membrane of the cell prevented H2CO3 from accumulating in the tubular fluid. In the distal tubule the intratubular pH was 0.85 U lower than the equilibrium pH. This difference could be obliterated by an intravenous injection of carbonic anhydrase. It was concluded that HCO3- reabsorption in this segment was also accomplished by H+ secretion. The accumulation of excess H2CO3 in the tubular fluid indicated that, in contrast to the proximal tubule, carbonic anhydrase was not located in the luminal membrane of distal tubular cells.

Effect of Some Urea Derivatives on Measurement of pH with Glass Electrodes

Abstract In solutions containing some urea or thiourea derivatives and alkali metal hydroxides at pH 6 to 10, the pH value measured with glass electrodes changes with time. The degree of change varies between 0.1 and 1.0 pH units and depends on the structure of the urea derivative, its initial concentration, and the glass electrode used, but not on the cation of the alkali metal hydroxide added. In contrast, pH measured in the same solution with a quinhydrone electrode or the UV spectra remain time-independent. It is proposed that urea derivatives are adsorbed in the gel at the surface of the glass electrode and there affect the ion-exchange process.

A solid composite pH sensor based on quinhydrone

AbstractA solid composite pH sensor can be made from quinhydrone, solid paraffin and surface‐modified graphite powder. It exhibits an almost theoretical slope of the potential–pH dependence (−57.7 mV at 25°C), a formal potential E0′ = 0.6922V (vs. NHE), low standard deviations for repeated measurements (0.04 pH units), fast potential response (15s) and a working range up to pH 9.5. The sensor can be used in moderately concentrated mineral acids and, unlike the conventional quinhydrone electrode, it is also applicable for measurements in milk. The sensor shows the same salt effect and sensitivity towards strong oxidants and reductants as the conventional quinhydrone electrode. The surface modified graphite was prepared by boiling graphite powder with concentrated nitric acid. By voltammetry it was shown that the surface of the modified graphite contains surface‐confined redox centers similar to those in quinhydrone, which can catalyze the electron transfer of the quinhydrone on the surface of the sensor.

Some conductance and potentiometric studies in 20 mass% propylene carbonate + ethylene carbonate: application of hydrogen and quinhydrone electrodes

The behaviour of methanesulfonic and 2,5-dichlorobenzenesulfonic acids in a 20 mass% propylene carbonate (PC) +ethylene carbonate (EC) mixture has been investigated by means of conductance and potentiometric measurements at 25°C. The conductance behaviour of acids in 20 mass% PC+EC is explained on the basis of simple dissociation (HA⇌H+ +A−) and homoconjugation (HA+A− ⇌HA−2) equilibria. The results of the measurements have been subsequently correlated with potentiometric studies involving the cell: Pt|H2(1 atm), HA in 20 mass% PC+EC∭HgCl2(s)|Hg ref. in 20 mass% PC+EC. In addition, the Hg | HgCl2(s) reference electrode has been used to determine the standard potential of the reaction: quinone+ 2H+ + 2e− ⇌hydroquinone vs. SHE in 20 mass% PC+EC. It was found to be 0.3516 ± 0.0023 V.

Development of quinhydrone electrode in chloroform. Application to the determination of equilibrium constants of acid—base reactions and potentiometric titration of some amine drugs

The optimum conditions for the functioning of quinhydrone electrode as acid indicator in chloroform was determined by voltammetric and potentiometric studies. This electrode was used for the determination of equilibrium constants of acid—base reactions and the potentiometric determination of the end point of acidimetric titrations of some amine drugs such as phenothiazinic derivatives in pharmaceutical preparations, after extraction into chloroform. The relative standard deviations of 20 mg of aliphatic amines, piperidine and piperazine substituted phenothiazines were 0.5, 0.7 and 1 % respectively.