Abstract: Anew method for determining the ionization constants of polyprotic acid by using Evolving Factor Analysis with chemical equlibrium on pH-spectrophotometric data is proposed. It is based on the fact that a species which first appears must be the first to disappear in a polyacid equlibrium system.The method was tested by the ionization constants of Alizarine Red Sand Chromazurol S.The criterion for correctly locating the regions of the existence of a given component is also discussed in detail. The results show that this method is stable and reliable and has a fast convergence speed. The obtained constants are consistent with those cited in references.Another advantage of this method is that it can provide the absorbance curve of every component simultaneously in the system and is suitable for a great deal of color complexing systems.

Key words: pH-spectrophotometry, Evolving Factor Analysis, Bilinearity, Ionization constants of polyprotic acid