In forming a chelate with a metal ion, a mixture of free EDTA (abbreviated Y 4 − Y4− ) and metal chelate (abbreviated MY n − 4 M
Yn−4 ) can buffer the free metal ion concentration at values near the dissociation constant of the metal chelate, just as a weak acid and a salt can buffer the hydrogen ion concentration at values near the acid dissociation constant. The equilibrium M n + + Y 4 − − ⇀ ↽ − MY n − 4 Mn++Y4−↽−−⇀MYn−4 is governed by the equation K ′ f = α Y 4 − ⋅ K f = [ MY n − 4 ] [ M n + ] [ EDTA ] Kf′=αY4−⋅Kf=[MYn−4][Mn+][EDTA] where K f Kf is the association constant of the metal and Y 4 − Y4− , α Y 4 − αY4− is the fraction of EDTA in the form Y 4 − Y4− , and [ EDTA ] [EDTA] is the total concentration of free (unbound) EDTA EDTA . K ′ f Kf′ is the conditional formation constant. How many grams of Na 2 EDTA ⋅ 2 H 2 O Na2EDTA⋅2H2O (FM 372.23 g/mol) should be added to 1.97 1.97 g of Ba ( NO 3 ) 2 Ba(NO3)2 (FM 261.35 g/mol 261.35 g/mol ) in a 500. mL volumetric flask to give a buffer with p Ba 2 + = 7.00 pBa2+=7.00 at pH 10.00? log K f logKf for Ba − EDTA Ba−EDTA is 7.88 7.88 and α Y 4 − αY4− at pH 10.00 is 0.30. mass Na 2 EDTA ⋅ 2 H 2 O.
