Paper strips to test for presence of Total (Free, or available, plus Bound) Chlorine. This is the standard test strip used for detection of chlorine in solution. This product measures levels usually required by Public Health Standards for sanitizing surfaces. Free Chlorine is that which is available to react with the test strip. Total Chlorine is that which is Free, plus that which is Bound (usually as Sodium Hypochlorite). An equilibrium exists between the Free (available) chlorine and the Bound chlorine until bound chlorine is exhausted, at which time a fresh solution is necessary. The active form of chlorine which acts as a microbicide is Hypochlorous acid (HOCl). Dip the strip into the solution under question, read results by comparison to label. This product is viable for two years from the date of Purchase when kept in cool dark storage.