Effectivity of solvents-A new approach in non-aqueous acid-base titrimetry.
A new approach for selection of a suitable solvent system as a medium for non-aqueous acid-base titration is proposed. The essence of the approach is the development of a new criterion called "effectivity". The latter is based on consequences of the Brønsted and Izmailov acid-base theories and represents a quantitative measure for improving or worsening the titration conditions of acids and bases in non-aqueous solvents as compared with water. The "effectivity" E is given by the relation E = DeltapK(s) - DeltapK(s) where DeltapK(s) is the difference between the logarithmic values of the autoprotolysis constants of water and the solvent in question, and DeltapK is the so-called medium effect. The latter is a constant value which shows that acids and bases with the same charge alter their strength to the same extent when transferred from water into a non-aqueous solvent. The medium effect is calculated by statistical treatment of a great number of acid-base constants determined experimentally both in water and the non-aqueous solvent in question. The effectivity of the solvents most often used in non-aqueous acid-base titrimetry, determined by this approach, shows that in many cases these solvents offer significant advantages over water, but drawbacks are also observed. Some limitations of the approach are discussed. Special attention is paid to dimethylsulphoxide and its mixtures with water, which prove to be highly effective media for the acid-base titration of many substances.
Budevsky
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Academy of Medicine, Institute of Pharmacology and Pharmacy, Ekz. Josif Str. No. 15, Sofia-1000, Bulgaria.