Role of pH and Auxiliary Ligands in Complexometric Titrations

The titration curve developed in Example 9.1 was hypothetical. In practice, complexometric titrations are conducted at pH values at which the titrant is protonated to some extent. Often, an auxiliary complexing agent such as ammonia is present so that there are various metal-containing species in addition to just the simple hydrated metal ions. Calculations of such titration curves can be readily accomplished within the framework of the material presented in Chapter 5, particularly that dealing with p, the conditional constant. In the treatment that follows, side reactions of the titration ligand and metal ion will be considered using the familiar Ul and 

What changes are needed to modify the approach taken in Example 9.1 to deal with a Zn-EDTA titration if it is conducted in the presence of 0.1 M NH3 and at a pH of 9.00  

As shown in Chapter 8, selected individual points in complexometric titration curves can be calculated without recourse to the titration curve equation by (a) performing the reaction stoichiometry at the desired point, (b) recognizing the type of equilibrium calculation represented by the 

The development of visual indicators for use in complexometric titrations even in very dilute solutions has provided a stimulus for the increasing popularity of titrimetric determinations of metal ions. Such indicators behave in an exactly analogous manner as the acid-base indicators. 

One of the widely used indicators for calcium is Eriochrome Black T which is a sulfonated, o,o -dihydroxy azo dye that can be represented by the formula H2ln. The dissociation of the two protons can be represented by the equations  

---