Qualifying the term Photo grade there's listings in books like LFA Mason, Photographic Chemistry, and elsewhere of the actual maximum levels of impurities in each chemical. I only have the BS Standards but they mirror the ASA/ANSI now ISO standards.

Purity is based on final use, food grade might be higher or lower than techniocal grade which is the industry norm, photograde is usually technical grade except for a few chemicals which may need to be purer and closer to analytival grade.

Ian