In complexometric titration (EDTA) for calcium, what must the pH be adjusted to prevent?

In complexometric titration with EDTA, adjusting the pH is crucial to ensure that the metal ions being titrated, particularly calcium and magnesium, form stable complexes with EDTA. If the pH is too low, both calcium and magnesium could potentially interfere with the titration due to their solubility and the nature of the complexes formed at varying pH levels.

At a low pH, magnesium ions can dominate the titration due to their higher affinity for the EDTA complex. As a result, they may bind with EDTA preferentially, which can lead to an inaccurate measurement of calcium concentration. By raising the pH, typically to around 10, it promotes the formation of calcium-EDTA complexes while also diminishing magnesium's ability to interfere due to its lower stability at higher pH levels. This adjustment ensures more selective and accurate titration of calcium, allowing for a clearer distinction between the two metals and reducing the interference from magnesium.

Therefore, preventing interference by magnesium is a key reason for adjusting the pH during a titration for calcium using EDTA.