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.