When you are using water for vaccine in humans or for veterinary purposes, the parameters and definitions of degree of purity are quite distinct and there is a need to validate the purification process for the intended use.

The APHA Standards are somewhat different from the norms established in pharmacopoeias

Then use the specific tests elicited in the pharmacopoeas (USP and EU). They only have to verified in your hands for your sample (which I assume is water).  Don't reinvent the wheel!