The stability constants of complexes formed by metal cations Ni2+, Co2+, Sr2+, Zr4+ with citric acid ligand in water and in APG10surfactant medium –water were determined by the pH titration method combined with using Hyss 2009simulation software.Under the experimental conditions, metal cations Ni2+, Co2+, Sr2+, Zr4+with citric acid form some species of complexes of [NiCit]-, [NiHCit], [NiCit2]4- and [NiHCit2]3- for Ni2+, [CoCit]-, [CoHCit] and [CoCit2]4- for Co2+,[SrCit]-, [SrHCit] and [SrCit2]4- for Sr2+, [ZrCit]+, [ZrCitH]2+ and [ZrCit2H]- for Zr4+ both in water and in APG10/water solution. The logβ values of the stability constants of the complexes are linearly dependent and decreases with increasing concentration of APG10.In 0.5% APG10 solution, the decrease in complex stability constant valueswere compared with the values in the aqueous environment as following: the decrease in the values from 4.367 times to 14.126 times for complexes of Ni2+ and citric acid; for complexes formed by, from 2.089 times to 5.012 times for Co2+ and citric acid; from 1.445 times to 3.098 times for the Sr2+ and citric acid and this value decreases from 2.884 times to 4.571 times for Zr4+ and citric acid.
