This chemical is the tab that you use in order to help measure the range of chlorine in the water. They employ a type of chemical that helps to distinguish the levels of free, combined and total chlorine thanks to a simple test sample. It is suitable for both spectrophotometers or colorimeters. You are going to get a reading of anywhere between 0 and 10 ppm so you know what the chlorine level of your water is. These is an instrument grade type of product and it is going to have a shelf life of around three years so that you can easily keep some of them on hand in case you need to test the chlorine in the water.