There difference in the purity depends on if the Amidol is recrystallized or not. Recrystallizing removes most of the impurities.

There is a hair grade and a dye grade. Amidol has only a nitch use in photography, there is no 'photo' grade. Hair grade goes into hair coloring formulations. There are several threads discussing amidol at
http://www.michaelandpaula.com/AzoForum/default.asp