Question

In forming a chelate with a metal ion, a mixture of free EDTA (abbreviated Y4−) and metal chelate (abbreviated MYn−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

Mn++Y4−−⇀↽−MYn−4

is governed by the equation

K′f=αY4−*Kf=[MYn−4]/[Mn+][EDTA]

here Kf is the association constant of the metal and Y4−, αY4− is the fraction of EDTA in the form Y4−, and [EDTA] is the total concentration of free (unbound) EDTA. K′f is the conditional formation constant.

How many grams of Na2EDTA⋅2H2O (FM 372.23 g/mol) should be added to 1.55 g of Sr(NO3)2⋅4H2O (FM 283.69 g/mol) in a 500. mL volumetric flask to give a buffer with pSr2+=8.00 at pH 10.00? logKf for Sr−EDTA is 8.72 and αY4− at pH 10.00 is 0.30.

Answer 1