Omortag Budevsky

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

Abstract 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, p K SH = 18.40, agrees well with reported data. The p K 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.

Determination of acid-base constants in amphiprotic nonaqueous solvents by direct potentiometry

Abstract A direct potentiometric method for the determination of acid-base constants of protolytes in nonaqueous solvent is proposed. The method is based on the treatment of the data of a single potentiometric titration of a base or an acid in nonaqueous solvent. The titration is performed in a galvanic cell consisting of a glass and a silver-silver chloride electrode with liquid junction. Two alternatives are proposed by which at first, only the acid or only the base constant is directly determined. By the second alternative the acid, the base as well as the autoprotolysis constant are determined simultaneously from a single experiment. The method is checked with the determination of some acid-base constants in water as well as in some mixed water-alcoholic mixtures. The constants obtained coincide relatively well with the data of other investigators.

Acid-base equilibria in ethylene glycol—II: Autoprotolysis constants and acid-base properties of ethylene glycol and its mixtures

The acid-base behaviour of ethylene glycol and its mixtures with water (1, 5 and 10%), methanol, ethanol, isopropanol, nitromethane (each 10%) and ethyl methyl ketone-chloroform (5 + 5%) have been investigated by potentiometric titration, in a cell without liquid junction, equipped with a glass and a silver-silver chloride electrode. The autoprotolysis as well the protolysis constants of phthalic acid were determined for each mixture. The added solvents improve the properties of ethylene glycol, decreasing the viscosity without changing the acid-base behaviour of the ethylene glycol itself, which is favourable for the titration of weak bases. Water increases the basic, and nitromethane the acidic, properties of the mixture. Small quantities of water (ca. 1%) do not impair the titration conditions.

Acid—base equilibria in the mixed solvent 80% dimethyl sulfoxide—20% water : Determination of pK Values and Investigation of the Conditions for Titration of some Aromatic Carboxylic Acids and their Conjugated Bases

Abstract The acid—base properties of some aromatic carboxylic acids of zero charge type HA have been investigated in the mixed solvent, 80% DMSO—20% water, by means of potentiometric measurements with a glass electrode. The p K a values of the following aromatic carboxylic acids have been determined: benzoic, o -chlorobenzoic, p -chlorobenzoic, m -bromobenzoic, 2,4-dichlorobenzoic, 2,5-dichlorobenzoic, 2,6-dichlorobenzoic, 2,3,5-triiodobenzoic, 3,5-dinitrobenzoic, p -methylbenzoic, m -aminobenzoic, o -hydroxybenzoic 3,4-dihydroxybenzoic, 3,4,5-trihydroxybenzoic, o -methoxybenzoic, 2,3-dimethoxybenzoic, 5-amino-2-hydroxybenzoic, pyridine-2-carboxylic, thiophene-2-carboxylic, 1-naphthylacetic, diphenylglycolic. The mixed solvent investigated offers better titration conditions for the determination of these acids than water and some other non-aqueous solvents.

pH method for the interpretation of high-performance liquid chromatographic behaviour with non-aqueous polar mobile phases

Abstract A pH method for the interpretation of the chromatographic behaviour of ionogenic substances in both reversed-phase and ion-exchange high-performance liquid chromatography is proposed. The pH determination in a particluar polar mobile phase is easily performed by means of a simple potentiometric procedure. Equations are derived concerning the retention of ionogenic substances in the light of the proposed pH approach. An experimental check with two different water—organic mobile phases showed good agreement between theory and experiment. The method allows one to predict the sequence of appearance of the components in a chromatographic separation and even to change this sequence.

Potentiometric determination of autoprotolysis constants of solvents

A new method is proposed for determination of autoprotolysis constants by titration of a weak acid with strong base or strong acid with weak base, and applied to water and ethylene glycol (EG). The constants found were pK(w) = 13.90-13.97, and pK(EG) = 15.4 +/- 0.2.

NABTIT—a computer program for non-aqueous acid—base titration

A program NABTIT written in BASIC has been developed for the treatment of data (ml/mV) obtained from potentiometric acid-base titrations in non-aqueous solvents. No preliminary information on equilibrium constants is required for the input. The treatment of the data is based on known equations and uses least-squares procedures. The essence of the method is to determine the equivalence volume (V(e)) accurately, and to use the data acquired by adding titrant after V(e) for the pH*-calibration of the non-aqueous potentiometric cell. As a by-product or the calculations, the pK* value of the substance titrated is also obtained, and in some cases the autoprotolysis constant of the medium (pK*(s)). Good agreement between experiment and theory was found in the treatment of data obtained for water and methanol-water mixtures.

Investigation of an alkylamine modified and pH-controlled mobile phase for separation of pyrazolone derivatives by high-performance liquid chromatography.

The separation of pyrazolone derivatives with different acid-base properties and polarities has been investigated in view of recent concepts about the retention mechanisms in reversed-phase liquid chromatography. It is shown that both the protonated (BH+) and uncharged (B) bases undergo a dual mechanism of retention due to solvophobic and silanophilic interactions. The latter are easily suppressed by the addition of an alkylamine modifier (dibutylamine) to the mobile phase. An ion-pairing mechanism is found to be valid in the retention of negatively charged (A-) pyrazolone derivatives, since the dibutylamine modifier (BH+) serves as an appropriate counter ion. The behaviour of these solutes can be predicted from the pH dependence of their capacity factors, and a new simple method is proposed for calculating the theoretical sigmoidal curves. It is shown that the elution order of the compounds can be changed by control of both the pH value and the modifier concentration in the mobile phase.

Acid—base equilibria in the mixed solvent 80% dimethyl sulfoxide—20% water: Part 3. Determination of Acid—Base Indicator Constants

Abstract The acid—base constants of ten nitrophenol, sulfonphthalein and phthalein indicators in 80% dimethyl sulfoxide—20% water are reported. The indicators were selected so that their transitional intervals cover a large part of the pH-scale. The relationship between the constants in the mixed solvent and in water is linear: (p K HI ) SH = 1.48 (p K HI ) H 2 O — 1.68.

Acid-base equilibria in ethylene glycol—III selection of titration conditions in ethylene glycol medium, protolysis constants of alkaloids in ethylene glycol and its mixtures

Theoretical titration curves are used for the selection of appropriate conditions for the acid-base volumetric determination of weak bases in ethylene glycol medium. The theoretical curves for titration of some alkaloids are deduced graphically on the basis of the logarithmic concentration diagram. The acid-base constants used for the construction of the theoretical titration curves were determined by potentiometric titration in a cell without liquid junction, equipped with a glass and a silver-silver chloride electrode. It is shown that the alkaloids investigated can be determined accurately by visual or potentiometric titration. The same approach for the selection of titration conditions seems to be applicable to other non-aqueous amphiprotic solvents.