Quote Originally Posted by tkamiya View Post
For all the ingredients, only Sodium Sulfite and Sodium Carbonate has hydrate specification and nothing else do. Why?
Not every compound can bind water when it crystallizes, and Metol, Hydroquinone and Potassium Bromide generally don't, therefore it is not necessary to specify "anhydrous" for them.

Note that crystal water is only part of the story. KSCN has no crystal water yet it is so hygroscopic that it will literally go into solution in the water it grabs out of ambient air. To my best knowledge, neither Metol, Hydroquinone, nor Potassium Bromide have this problem.