All I really need to know is that there must be 10.5 grams of sulfite in the liter of working solution + 100 ml of Solution A (pyro, bisulfite and KBr) + 100 ml of Carbonate (solution C). So, I'll just mix 100 ml of the 10.5% sulfite solution for each development session and voila! My sulfite will always be fresh. Thanks for the tip, Jorge.