Prediction of distribution of microspecies as function of the pH and its application for modeling of gallium complexing behaviour Jozsef Szegezdi +* , Alexander Fedin ** , Ferenc Csizmadia * + corresponding author: jszegezdi@chemaxon.com, Discovery Group * Chemaxon Ltd., 1031. Budapest, Záhony u. 7, Hungary ** 49005 Ukraine Dnepropetrovsk Pisargevsky str., 5, ISIC WESTA, Laboratory of clean water, prof. Fedin A.V. Abstract Polyprotic acids are ionized in aqueous solution in a wide pH range. The distribution of the ionized species is a multimodal function of the solution pH. Most of the physical-chemical model of a solution requires information about the pK a values of the molecules. In the present calculation a "trihydroxyfluorone" derivative, TOF, investigated in a complex formation reaction with Gallium ions. The metal elements in the XIII. group of the periodic system has significant rule both in medical