Use 15 to 20 grams citric acid per liter of stop bath. Citric acid is commonly sold in supermarkets or bakery supply outlets. Bags of 25 kilograms or so can be purchased at very low unit cost from large scale chemical suppliers. These terms mean the contents are approved for use in Kosher cooking. 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.