I have purchased Potassium Carbonate in a 50 pound bag marked Kosher. Acetic acid is a funny thing. Dilute it to the 1.3 to 2 per cent range, and it is photographic stop bath. People find it irritating to smell. Canadians put it on their chips and get funny looks in restaurants when they travel in the United States.
Higher temperature demands more care to protect the surface of the print.
25 8x10 prints, or equivalent surface area of other prints, per liter. Adding more sodium sulfite helps maximize storage life, but will not counteract the effects of use. It is not necessary to measure them out with a balance or scale each time fix is needed. A much simpler method is to measure them out once, and put them in a suitable container which can be marked to indicate the level or depth of chemical required for the weight needed. Or the number of tablespoons of the chemical required can be counted and recorded.