Examples of Conditional Constants Calculations

Exercise 1 Calculate the conditional constant of the nickel-EDTA complex in a solution buffered with a mixture of ammonia (O.lmol/L) and ammonium ions (0.1 mol/L). The measured pH is 9.24 (pK value of ammonium ion). The stability constants are pi = = 10 P3NH3 iq6.77 7.96  

Exercise 2 Calculate the conditional constant of the Cu -EDTA complex at pH = 13 in the absence of any buffer and of any other complexant. (We shall not take the acidic character of the complex or the formation of a binuclear complex into account.) We give K = 10 and Pi° = 10.  

Replacing [NH3] by its above expression as a function of [H+] permits us to draw aM as a function of pH (Fig. 26.2). The figure also mentions the change in aM with pH when no ammonia is added. In this case, at high pH values, aM increases only because of the formation of the hydroxo complex [Mg(OH)+]. The coefficient ay is given by the expression given in Exercise 1. In the pH range where the buffer is efficient, 

Evidently, K is a complicated function of pH. At pH = 9, for example, the conditional constant is about 200 times weaker than the thermodynamic one. 

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