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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, C(0)mol/L) and a strong acid (HB, Cmol/L) solutions, assumes constant pH, independently on the volumes of the solutions mixed, provided that the relation C(0)=C+C(2)·10(pK(1)) is valid, where pK(1)=-log K(1), K(1) 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 pK(1) value. The new method is particularly useful for weak acids HX characterized by low pK(1) 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 pK(1) 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., (Copyright © 2010 Elsevier B.V. All rights reserved.)

Abstract: The isohydricity property, referred to systems composed of acids or bases, is presented. The related isohydricity conditions are formulated for different pairs of solutions. The pH titrations in the system of isohydric solutions are put in context with conductometric titrations. The differences between pK1 values obtained according to both methods are not too significant and the conductometric titration can be perceived, in some instances, as a reasonable alternative to the pH titration. [Copyright &y& Elsevier]

Abstract: The isohydricity (pH constancy) principle is referred to the pair of solutions: weak acid (HL, C 0 mol/L) and strong acid (HB, C mol/L) when mixed e.g., according to titrimetric mode. Such a case takes place if the relation C 0 = C + C 2 ×10pK 1 is valid, where pK 1 =−log K 1, K 1 – dissociation constant for a weak monoprotic acid HL. This principle, outlined and formulated in earlier paper (Michałowski et al., Talanta 82 (2010) 1965), is the basis for a sensitive method of pK 1 determination, confirmed for a series of weak acids in presence of basal electrolytes or in water+organic solvent (dimethyl sulphoxide, methanol, isopropanol) media. The results of titrations were elaborated according to principles of regression analysis, with use of least squares method. A new criterion for precision of the results obtained according to this method is formulated. The pK 1 values obtained are comparable with ones found in literature. [Copyright &y& Elsevier]