I just ran out of the NOW Foods Vitamin C I've using ever since I started making my own developer last year. The replacement is of The Vitamin Shoppe's house brand, which is also supposed to be pure. It's a finer powder, which means it mixes faster, but I've noticed it's weaker than what I used before. I need 40% more by weight to make the film developer as active as with NOW Foods Vitamin C. What does this mean? Is the VS powder getting old, or is it just slightly oxidised from the start because it's a finer powder? Anyone else noticed differences between brands? I mix the vitamin in just before development, so there should be no issues of impurities that would cause instability in stored solutions.