Potentiometeric studies have been carried out on 1:1 ML mononuclear complexes, where Ca (II), Fe (III)  and Zn (II) represent the essential metals whereas Pb (II), Cr (III, VI) and  Ni  (II)  are  the biotoxic metal  ions. The ligands selected are  the polyamino carboxylic acida(PAC)such  as  EDTA,  DTPA and   CDTA.  pH  titrations were  performed in  aqueous solutions at an  ionic strength of 0.l M  and  temperature 30°C. Formation of above mentioned metal-PAC complexes were  studied and compared. DTPA  was found comparatively a strong acid among the three PAC acids and suitable for complexation especially at the pH below 2.5 contrarily to CDT A and EDTA.