1. The problem statement, all variables and given/known data The concentration of Ca2+ in hard water can be determined by titration with a reagent X which forms a complex with Ca2+ giving a change in colour. Since the colour change is sensitive to pH, the titration has to be carried out in an alkaline buffer. Th structure of X is similar to that of an amino acid. The way X acts as a ligand can be understood by considering how aminoethanoic acid forms co-ordinate bonds with a calcium ion. Explain why the complex between aminoethanoic acid and aqueous calcium ions is more stable at pH 10 than at pH 4. 2. Relevant equations 3. The attempt at a solution I really have no idea how pH would affect the dative bonding.