edta–metal complexes are less stable at low pH values. How does this affect the procedure for the titration?

A buffer solution of pH 10 is added to the titration flask to ensure that a high pH level is maintained.

Why are edta complexes less stable at low pH?

At low pH the anion of edta is protonated so less of it can form a complex with metal ions. The equilibrium between the anion and the complex moves towards the anion so the complex becomes less stable.

What can you say about the relative K_stab values of the complex formed between calcium ions and Eriochrome Black T and the complex formed by the calcium ions with edta?

K _stab for the complex formed by calcium ions with edta must be higher than K_stab for the complex formed with Eriochrome Black T because the edta replaces the indicator as the ligand.

What would a student need to do next in order to find an accurate value for the concentration of calcium ions in solution?

The student would need to repeat the titration until concordant titres were achieved.

Why might the concentration of calcium ions determined in this way not be accurate even if the student carries out the titration very carefully?

Other ions in solution such as magnesium ions will also react with edta, so the titre will be higher than that expected just for calcium ions.