It's sold as "water purification drops" preferably in glass bottles as a two-part system. One part should be a 22.4% solution of NaCIO² (sodium chlorite) in water and the other is an activator, either HCI 4% (hydrochloric acid) or citric acid. The two parts are mixed 1:1 immediately before use to make chlorine dioxide. Four ounce bottles of each part last me years, and that's using it for myself, husband, two kids, and 3 cats. I have purchased it from a Seventh Day Adventist health food store and also on ebay for friends and family.