Testing for water hardness using EDTA as a titrant:
Suggest a reason why the buffer is good for only a week. The buffer contains disodiumEDTAdihydrate, MgSO47H20 and distilled water. NH4Cl and NH4OH is also added then diluted.
I know that after a week, the buffer will be bad and no longer be possible to achieve a sharp endpoint, but I don't understand the reason behind it.