Effect of pH on Metal-Complex Equilibria

Metal complexing agents (Lewis bases) are also Bronsted bases in many cases and will, therefore, be affected by changes in pH. For example, NH3, CN, en, trien, and the EDTA anion will accept protons and hence the fraction of their total concentrations, which are available for metal complexa-tion, vary with the pH. 

This will pose no particular difficulty we will simply employ the strategy described in Chapter 4. The concentration of the complexing species, [L], is the product of its analytical concentration. Cl, and the fraction, Ul, corresponding to the ratio of [L] to Cl. 

Let us illustrate this approach on the Cu - NH3 system. In the example above, it was assumed that the pH was sufficiently high so that aj4jj3 = 1. In general, however, we must write [NH3] = aj4H3 CJ4JJ3. This is illustrated below. 

Describe the solution composition for 0.005 M Cu, 0.010 M NH3 with the solution pH as 9.0. We again may recognize that this solution can be described as 0.005 M Cu (NH3)2 so that the Pointer Equation 5-5 applies. The composition differs in that [NH3] for each Cu-containing species is replaced by 0.010 where 

The conditional combined formation constants would then be 

---