Formulary's Fixer Test Solution is used to tell when a fixer bath has become exhausted by accumulating excess silver ions. Unlike developing solutions, which discolor when they are exhausted, fixer baths show no outward appearance when they are spent. Therefore it is necessary to either keep an accurate count of the number of prints or rolls of film fixed or test the fixer solution periodically to determine when it has become exhausted. Fixer Test Solution provides a simple chemical test that allows you to determine when the fixer solution is exhausted and must be replaced. The chemicals in this kit are used to prepare the test solution which has a shelf life of one year.