what is the unit of purity of a chemical sustance?

If you are referring to chemical purity by grade, I suggest you research the following terms:

ACS Grade (American Chemical Society) - highest level of purity

Reagent grade - Almost as stringent as ACS

USP Grade - Equivalent to ACS for many drugs

NF Grade (National Formulary) - Equivalent to ACS for many drugs

Laboratory Grade

Purified Grade

Technical Grade