Stability Constants of Some Divalent Complexes of m‐ and p‐Phenylenediamine Tetraacetic Acid

Abstract

The dissociatio2 constants of m-, and Pphenylenediaminetetraacetic acid (m-, and $-, PDTA) have been determined potentiometrlcally at room temperature (about 25'C) in an ionic strength oi 0.10 M K N 0 3 . The stability cmstatns of their metal complexes have also been determined by UV spectroscopic, potentiometric, and polarographic methods. The formula of the metal complexes by spectroscopic and polarographic methods are both in good agreement to k e 1 : l .Cu>Ni>Zn>CO>Cd>Mg,Which is in accordance with Irving-William stability order.The order of the stability of the metal compIexes is: I. In t'roductionAlthough nz-, and 9-PDTA had long been used as chromatographic exchange r e~i n s c~'~) t o separate like-charge ions by their weak complexing p3wer; very little information of their chelating behavior has h e n reported. In this study, their formula and stability with Cu, Ni, Zn, Co, Cd, and Mg, Ca, Ba, Sr, were determined. The calculations were treated with a systematic algebratic rn~thodC;'~) a n d carried out by IBM 1620 with a computer programmi n g F Calculations(1) Potentiometric method. (A) Determination of dissociation constants. From the equilibrium equations of four stepwise dissociations, mass balance and electroneutrality relationships, we can derive the equations: --*Present address: Chung Shan Institute of Science and Technology. MY Cu-($-PDTA) Ni-(p-PDTA) Co-(#-PDTA> Zn-(#-PDTA) Cd-WPDTA) Mg-(pPDTA) Ca-CpPDTA) S r-(PPDTA) Ba-(pPDTA) Cu-(m-PDTA) Ni-(m-PDTA) Zn-( m-PDTA) CO-( m-PDTA) Cd-( m-PDTA) Mg-( m-PDTA) Ca-( m-PDTA) Sr-( m-PDTA) Ba-( m-PDTA) These are the average value Of m'cllti-determinations.