From this thread:
Originally Posted by JasonLionJack's says practically nothing about what is in stain solution #1. They do, however, give a solubility in water number which is half of the solubility of ascorbic acid. The dosage amounts are double the typical dosage for ascorbic acid. They also refer to it as non-acidic. Ascorbic acid is an acid, though a weak one.Originally Posted by waterbear
On the other hand the appearance and safety considerations are identical to ascorbic acid. If it isn't ascorbic acid it isn't obvious what else it could be. The solubility number could simply be wrong and everything else just marketing. Or maybe there is something in there in addition to ascorbic acid.