Question

The equilibrium constant for a titration reaction, Ktitr’n , should generally be larger than 1.0 x 106 in order for the titration to be feasible. Assuming that no auxiliary ligand (L) is present, is the direct titration of Zn2+ with EDTA feasible at pH = 3.00? At pH = 7.00? The absolute (unconditional) formation constant of Zn2+ with EDTA is 3.2 x 1016 . At pH = 3.00, αY 4- = 2.1 x 10-11 ; at pH = 7.00, αY 4- = 3.8 x 10-4 .

Answer 1

We know that , for a titration the conditional formation constant, K_f^' = K_f x α_y-4

_{Given that,}

K_f = 3.2 x 10¹⁶

at pH = 3.00 _, α_y-4 = 2.1 x 10^-11

therefore, K_f^' = 2.1 x 10^-11 x 3.2 x 10¹⁶

= 6.72 x 10⁵  

K_f^' at pH =3.00 is less than 1.0 x 10⁶, hence titration is not feasible at pH = 3.00

at pH = 7.00 _, α_y-4 = 3.8 x 10^-4

therefore, K_f^' = 3.8 x 10^-4 x 3.2 x 10¹⁶

= 1.22 x 10¹³  

K_f^' at pH =7.00 is much greater than 1.0 x 10⁶, hence titration is feasible at pH = 7.00