Taylor R-0718-A-24 Silver Nitrate Reagent (10 mL sample, 1 drop = 200 ppm NaCl), .75 oz, Dropper Bottle. Taylor formulates its reagents to remain effective for at least one year, with only very few exceptions (molybdenum indicator in liquid form is one; after four months old it should be tested against a standard periodically). As a general precaution, replace all reagents more than one year old, or at the beginning of a new testing season.