Acid-base equilibria in ethylene glycol--I: definition of pH and determination of pk-values of acid-base indicators

A pH-scale in ethylene glycol is defined, based on potentiometric measurements for a cell without liquid junction: glass electrode || 0.1M (HCl + KCl), ethylene glycol | AgCl:Ag; and a potentiometric-spectrophotometric method for the determination of pK-value of acid-base indicators in ethylene glycol on the basis of this pH scale is proposed, in which the pH scale is established and the pK-value determined in a single titration at constant ionic strength. The following pK-values of indicators are reported: Cresol Red-3.00, Thymol Blue-3.39, Bromophenol Blue-6.49, Bromocresol Green-7.38, Bromocresol Purple- 9.00 and Cresol Red (2nd change)-11.07. A linear relationship was found between the pK-values of four sulphonphthalein indicators in ethylene glycol and in water according to the following equation (pK(HI)(c))(EG) = 2.3 + 1.1 (pK(HI)(c))(W). An attempt is made to explain this dependence, based on the idea of solute-solvent interaction of species of the same charge type.     

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.     

Polarographic and potentiometric determination of the stability constants of lead, cadmium and zinc complexes with dithiocarbaminoacetic acid

The stability constants of lead, cadmium and zinc dithio-carbaminoacetic acid (TCA) complexes have been determined. Lead and cadmium TCA complexes were studied by means of a polarographic method. A competitive polarographic method was used for the study of zinc TCA complexes. Zinc and cadmium TCA complexes were also studied potentiometrically with an amalgam electrode. The stability constants found were: lead, log beta(2) = 13.2; cadmium, log beta(1) = 5.4 (pot.), log beta(2) = 9.3 (pot.), log beta(3) = 13.7 (pot.), 13.6 (pol.); zinc, logbeta(1) = 3.4(pot.), <3.8(pol.), logbeta(2) = 6.6 (pot.), 6.4(pol.), logbeta(3) = 8.9 (pot.), 9.3 (pol.).     

Selective extraction of zinc dithizonate in the presence of dithiocarbaminoacetic acid as masking agent Quantitative treatment of an extraction equilibrium

Dithiocarbaminoacetic acid (TCA) is proposed as an effective masking agent in the dithizone extraction determination of zinc in the presence of lead, cadmium and cobalt(II). The extraction equilibrium is treated quantitatively by means of two approaches: the "conditional distribution coefficient" and the computer program HAL-TAFALL. Both methods give similar results and enable the most suitable conditions for the separation and determination of zinc in the presence of the other ions to be determined. A successful attempt is made to predict the maximum permissible quantity of interfering elements. The experimental check, performed under various conditions, is in satisfactory agreement with the theory.     

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.     

Dithiocarbaminoacetic acid as a masking agent in complexometry

Dithiocarbaminoacetic acid (TCA) forms very stable, water soluble complexes with a number of metal ions and is a suitable masking agent in complexometry. TCA masks from EDTA and complexometric indicators at pH 2-6 the following elements: bismuth(III), indium(III), thallium(III), cadmium(II), lead(II), mercury(II) and copper(II), thus making possible the complexometric determination of other elements in their presence.     

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.     

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

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: (pK_HI)_SH = 1.48 (pK_HI)_H2O— 1.68.