Autoprotolysis constants and standardization of the glass electrode in acetonitrile-water mixtures. Effect of solvent composition

Standard e.m.f.s for the cell GE/HCl/AgCl/Ag/Pt (GE=glass electrode) in acetonitrile-water mixtures containing 0–70% (w/w) of acetonitrile were obtained. Values of the autoprotolysis constant, K_ap, of these mixed solvents were also determined from e.m.f. measurements of the cell GE/KCl + KS/AgCl/Ag/Pt. The influence of variations in the solvent composition on pK_ap values was evaluated. Over the whole of the composition range studied the pK_ap values were linearly correlated with the mole fraction of acetonitrile and with the reciprocal of the relative permittivity of solvent mixtures. Linear relationships were also obtained for pK_ap values vs. the Kamlet-Taft π^★ polarizability/dipolarity parameter in the range 0–50% (w/w).     

Determination of pK a of benzoic acid- and p-aminobenzoic acid-modified platinum surfaces by electrochemical and contact angle measurements

Acidity constant values of benzoic acid (BA)-modified platinum electrode (Pt-BA) and p-aminobenzoic acid (pABA)-modified platinum electrode (Pt-NHBA) surfaces were determined using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and contact angle measurements (CAM). Diazonium tetrafluoroborate salt reduction and pABA oxidation reactions were used to prepare (Pt-BA) and (Pt-NHBA) surfaces, respectively. Both surfaces exhibited pH dependence with [Fe(CN)₆]^3?/4? redox probe solutions at different pH; this allowed us to estimate the surface pK _a values. Acidity constants for Pt-BA surface were found to be pK _a (3.09 ± 0.25), (4.89 ± 0.11), and (3.91 ± 0.54) by CV, EIS, and CAM techniques, respectively, while the values for Pt-NHBA surface were pK _a (3.16 ± 0.45), (4.24 ± 0.40), and (5.64 ± 0.12). The Pt-BA surface pK _a values were lower in CV and CAM measurements relative to the bulk solution of BA, while a higher value was observed in EIS for Pt-BA surface. The pK _a values determined for Pt-NHBA surface via both CV and EIS were lower than the bulk value; however, the result obtained from CAM was one unit higher than pK _a of bulk pABA.     

Analytical properties of the amidoxime group: X. Potentiometric and thermometric behaviour of 3,5-dimethyl-4-isoxazolylacetamidoxime

Dissociation equilibria of 3,5-dimethyl-4-isoxazolyl-acetamidoxime (DMIAA) were studied by means of potentiometric and thermometric techniques in ethanol-water media.Refined pK values (pK₁ = 4.05; pK₂ = 11.34) and neutralization enthalpy (ΔH₁ = −7.57 kcal mol⁻¹) were obtained using several computer programs (GRAN, MINIPOT, MINIQUAD and MINITERM).Titration errors and thermodynamic parameters of DMIAA are compared to the corresponding values obtained for other amidoximes studied in previous papers.     

Author index

Electromotive-force measurements on cells without liquid junction have been used to determine the pK_a values of 7 mono-, 6 di-, and 2 tricarboxylic acids in formamide at 9 temperatures from 5 to 45°C. From the pK_a values, the thermodynamic quantities ΔG⁰, ΔH⁰, and ΔS⁰ for the acids have been calculated in formamide at 25°C.     

Experimental evaluation of adsorption and electrode field effect on the constants of weak acid dissociation near the mercury electrode surface

Two methods are described for the experimental evaluation of the weak acid dissociation constants in the near electrode surface. One of them, based on the dependence of differential capacity of pH of the solution at constant potentials, gives the pK_A^s values for adsorbed acids; the second one uses the ψ₁-value dependence on pH in the presence of the cationic acids (as pyridinium ions) and comparison of the pH of the buffer solution of the weak acid under investigation with pK_A^s of the cationic acid and gives the pK_A values in the volume just near the electrode.     

Spectkophotometric determination of the dissociation constants of n,n'-bis(3-hydroxypropyl)dithiooxamide and n,n'-bis(2-hydroxypropyl)dithiooxamide

The dissociation constants of N,N'-bis(3-hydroxypropyl)dithiooxamide and N,N'-bis(2-hydroxypropyl)dithiooxamide were determined by a spectrophotometric method using a weighted least squares technique for the calculations. For N,N'-bis(3-hydroxypropyl) dithiooxamide a thermodynamic constant pK₁^T of 11.37 was found. At ionic strength μ = 1, pK₁ = 11.27 and pK₂ = 14.29. For N,N'-bis-(2-hydroxypropyl) dithiooxamide, these values were respectively: pK₁^T = 11.11; pK₁ = 10.99 and pK₂ = 13.75.     

Dissociation and solubility variation versus pO2- of scheelite CaWO4 in molten NaCl−KCl (At 1000 K)

The conditional solubility of scheelite CaWO₄ in molten NaCl?KCl (1:1) has been studied either in oxobasic or oxoacidic media. In the former case it is increased by formation of sparingly soluble CaO according to:Ca²⁺ + O²CaO(s), pK_s^CaO = 10800/T ? 5.8 (molality scale) In the later case, WO₄^2- behaves as an oxobase according to the following equilibria:WO₄^2?WO₃ (s) + O^2? pK₀ = 10.0 at 1000 K3 WO₄^2?W3O₁₀^2? pK₃ = 12.7 at 1000KThe latter equilibrium favours an increase in the W^VI solubility when pO^2- is increased, for instance by injection of HCl. The whole set of results has been summarized by a conditional solubility diagram of scheelite versus pO^2-.     

The acidity of superacidic media derived from fluorosulfuric acid and trifluoromethanesulfonic acid

The acidity level of superacidic media derived from fluorosulfuric acid and trifluoromethanesulfonic acid (TFMS) was determined by using the potentiometric indicators tetrachloroquinone and tetrafluoroquinone. A voltammetric and potentiometric study of these indicators was conducted to evaluate their pH indicator range, from the strongest acid media (SbF₅ solution) to the strongest base media (H₂O or KF solution). The R₀ function in 1 M strong base medium was found to be R₀ = ?17.2 (fluorosulfuric acid) and R₀ = ?16 (TFMS). The autoprotolysis constant (pK_i) was evaluated in both solvents; pK_i = 6.1 (fluorosulfuric acid) and pK_i = 6 (TFMS).     

Acid—base equilibria in the mixed solvent 80% dimethyl sulfoxide—20% water : Part I. definition of ph scale and determination of pk values of aliphatic monocarboxylic acids

The acid—base equilibria in the mixed solvent, 80% dimethyl sulfoxide—water, have been investigated by potentiometric measurements with a glass—silver—silver chloride electrode couple. The response of the glass electrode is quick and reproducible over the whole pH scale. The autoprotolysis constant (itK_SH) of the mixed solvent has been determined; the value obtained, pK_SH = 18.40, agrees well with reported data. The pK_a values of the following aliphatic monocarboxylic acids are reported: formic, acetic, propionic, n-butyric, iso-butyric, n-valeric, iso-valeric, capric, pentadecanoic, stearic, monochloroacetic. The mixed solvent offers better titration conditions than water for the determination of these acids.     

A new sensor for thermometric titrations

A new thermometric sensor, which is a transistor (OC71), has been introduced to follow thermometric titrations successfully to clear end points. The sensor was suitable in both normal and differential modes of titration. It is possible to titrate down to 1.32 μmol of HCl and 26.4 μmol of H₃BO₃in a final 20 ml solution with accuracy and precision of 1%, 2.2% and 1.4%, 2.2%, respectively. The sensor, in association with a pH glass electrode, was used for the determination of pK values of some well established weak acids such as, acetic acid (4.77), phosphoric acid (pK₁ = 2.18, pK₂ = 7.20 and pK₃ = 12.32) as well as for a very weak acid of uncertain pK values H₃BO₃ (pK₁ = 9.20, pK₂ = 12.7 and pK₃ = 13.80). The sensor was also examined for kinetic catalytic determination of iron(III) in water, milk and pharmaceuticals.     

A study of some problems in determining the stoicheiometric proton dissociation constants of complexes by potentiometric titrations using a glass electrode

Possible errors in the measurement of acid dissociation constants by potentiometric titration techniques have been considered, with particular references to nitrilotriacetic acid (NTA) and ethylenediaminetetraacetic acid (EDTA). Unknown junction potentials can arise when pH measurements are carried out using a glass electrode with saturated calomel reference electrode which have been previously calibrated with a standard buffer solution. The magnitude of the influence of these unknown potentials has been demonstrated and an experimental procedure recommended which gives meaningful results.The precision of calculated acid dissociation constants will also be influenced by the presence of cationic species (e.g. H₄L⁴), the total acid strength, the absolute values of the constants and the value accepted for the auto-dissociation constant of water (K_w). All these factors have been considered quantitatively and their effect on metal complex formation constants, calculated from these acid dissociation constants, noted. The proton dissociation constant of the cationic species of NTA (i.e. H₄L⁺) has been found to have the value of pK₀=0.80 at 20° and μ=0.10 M.     

A new sensitive method of dissociation constants determination based on the isohydric solutions principle

The paper provides a new formulation and analytical proposals based on the isohydric solutions concept. It is particularly stated that a mixture formed, according to titrimetric mode, from a weak acid (HX, C₀ mol/L) and a strong acid (HB, C mol/L) solutions, assumes constant pH, independently on the volumes of the solutions mixed, provided that the relation C₀ = C + C² · 10^pK₁ is valid, where pK₁ = −log K₁, K₁ the dissociation constant for HX. The generalized formulation, referred to the isohydric solutions thus obtained, was extended also to more complex acid-base systems. Particularly in the (HX, HB) system, the titration occurs at constant ionic strength (I) value, not resulting from presence of a basal electrolyte. This very advantageous conjunction of the properties provides, among others, a new, very sensitive method for verification of pK₁ value. The new method is particularly useful for weak acids HX characterized by low pK₁ values. The method was tested experimentally on four acid-base systems (HX, HB), in aqueous and mixed-solvent media and compared with the literature data. Some useful (linear and hyperbolic) correlations were stated and applied for validation of pK₁ values. Finally, some practical applications of analytical interest of the isohydricity (pH constancy) principle as one formulated in this paper were enumerated, proving the usefulness of such a property which has its remote roots in the Arrhenius concept.     

4-carboxy-1,2-cyclohexanedionedioxime complexes of nickel(ii) in alkaline media

The reactions of 4-carboxy-1,2-cyclohexanedionedioxime and nickel(II) were studied in alkaline media. Spectrophotometric studies indicate the presence of a 1:1 complex ion, NiD^-. Magnetic susceptibility measurements on a series of solutions of varying ratios of vic-dioxime and nickel(II) showed that the 1:1 complex ion was diamagnetic and that two paramagnetic complexes, probably NiD₂^4- and NiD₃^7-, are present in solution. The stability constants for the three complexes were calculated from spectrophotometric and magnetic susceptibility data. The log K values were found to be log K₁ = 28.74 ± 0.60, log K₂ = 0.76 ± 0.15, and log K₃ = 3.67 ± 0.73, respectively.     

Solubility of oxides in the KBr-BaBr2 eutectic melt (0.495: 0.505) at 973 K

Solubility products of MnO (pK _s,MnO = 6.32 ± 0.1), NiO (pK _s,NiO = 8.06 ± 0.2), and PbO (pK _s,PbO = 4.04 ± 0.2) in KBr-BaBr₂ (0.495: 0.505) melt at 973 K were determined by potentiometric titration using a Pt(O₂)|ZrO₂(Y₂O₃) membrane oxygen electrode. A significant increase in oxide solubilities compared to those of 2CsBr · KBr melt is attributed to the enhancement of acidic properties of cations in the series Cs⁺, K⁺-Ba²⁺, ensuring a greater degree of binding of oxide ions that results from dissociation of the oxide under study to the cationic framework of the melt. The solubilities of the investigated oxides in chloride and bromide melts with similar cationic compositions are virtually identical.     

Standard potential of the Ag-AgCl electrode and acidity constant of glycine at constant molality of NaCl in glucose-water mixed solvents from 278.15 to 318.15 K

Standard electrode potentials E° of Ag-AgC1 electrode in molality scale and acidityconstants of glyeine pK_1° at constant molality of NaCl (1.0 mol·kg~(-1)) in 5 and 15 mass%glucose-water mixed solvents over a range of temperatures from 278.15 to 318.15 K weredetermined from precise emf measurements.The dependence of acidity constant on temperatureis given as a function of the thermodynamic temperature T by an empirical equation, pK_1°=A_1(K/T)-A_2+A_3(T/K).The corresponding thermodynamic quantities of the first dissociationprocess of glycine were calculated and the effects of both tho solvent and the salt on themwere also discussed.     

Ligand effects in the substitution chemistry of cis-bis(piperidine)tetracarbonylmolybdenum(O). A molybdenum-95 NMR study

Molybdenum-95 NMR chemical shifts are reported for a series of Mo(O) compounds of the type Mo(CO)₄(pip)_2−nL_n(n = 1,2; L = substituted pyridine ligands). The σ(⁹⁵Mo) values correlate well with the pK_a values for the substituted pyridines; for the n = 1 series, σ (⁹⁵Mo) ranges from − 1053 ppm (pK_a = 1.86 for 4-CN) to − 1120 ppm (pK_a = 9.61 for 4-NMe₂). The effects of solvent polarity and some in situ reactivity studies are described and the nature of the MoL bond compared to that with piperidine and some other ligands is discussed.     

Protonation and sodium ion-pairing of the sulfite ion in concentrated aqueous electrolyte solutions

A protocol has been developed for the reliable titration of aqueous sulfite solutions which minimizes photodecomposition effects. This procedure has been used to measure the protonation constants of the sulfite ion in aqueous solution by glass electrode potentiometry at 25.0‡C and ionic strengths (I) from 0.1 to 5.0M in NaCI media and atI = 1.0M in KC1 and Me₄NCl media. These measurements provided evidence of weak but significant ionpairing between SO2/3 -and Na⁺ with a formation constant of logK _Na = 0.431 in 1.0M Me₄NCl. This was in very good agreement with the value logK _Na = 0.410 measured directly by Na⁺ ion-selective electrode potentiometry. Evidence is also presented for an extremely weak association of K⁺ and SO ₂ ³ -with logK _k = 0.22 in 1.0M Me₄NCl.     

Acid-base strengths in 1,2-dichloroethane

The pK_a value of hydriodic acid in 1,2-dichloroethane was determined from conductivity measurements. A glass electrode was calibrated for dichloroethane in the potentiometric titration of hydriodic acid with tetramethylguanidine. From potentiometric titrations, the pK_a values in dichloroethane of hydrobromic acid, hydrochloric acid, picric acid and some sulfonphthaleins as well as some protonated nitrogen bases were determined. In the curves of the titrations of the carboxylic acids and the hydrogen halides with TMG, evidence was found for the formation of the complex B(HX)₂.     

Low-Temperature Heat Capacities of Terbium Molybdate Phosphates M 2 I Tb(MoO4)(PO4) (MI = Na or K)

The heat capacities of Na₂Tb(MoO₄)(PO₄) and K₂Tb(MoO₄)(PO₄) were measured by adiabatic calorimetry at low temperatures (6.34–333.74 and 7.20–341.17 K, respectively). Smoothed thermal-capacities values were used to calculate the entropy, enthalpy increments, and reduced Gibbs energy. The respective values at 298.15 K are as follows: for Na₂Tb(MoO₄)(PO₄), C _p ⁰ (298.15 K) = 240.1 ± 0.2 J/(K mol), ⁰ (298.15 K) = 307.4 ± 0.4 J/(K mol), H ⁰(298.15 K) ? H ⁰(0) = 44.95 ± 0.03 kJ/mol, and Φ⁰(298.15 K) = 156.6 ± 0.5 J/(K mol); and for K₂Tb(MoO₄)(PO₄): C _p ⁰ (298.15 K) = 245.1 ± 0.1 J/(K mol), S ⁰(298.15 K) = 322.9 ± 0.1 J/(K mol), H ⁰(298.15 K) ? H ⁰(0) = 46.58 ± 0.02 kJ/mol, and Φ⁰(298.15 K) = 166.6 ± 0.2 J/(K mol). The noncooperative magnetic component of the heat capacity was estimated.