Which DPD tablets do I use to determine Chlorine in Water

There are several different types of DPD tablets available for chlorine testing, depending on what you wish to determine. Free chlorine – this is a measure of any active chlorine in solution, and is measured using DPD No.1 Total chlorine – this is a measurement of all the active and inactive chlorine compounds in water. It is measured by either using DPD No.4, or by a combination of DPD No.1 + DPD No.3 test tablets (add DPD No. 3 to a sample which has been tested for Free Chlorine). Combined chlorine – this is formed by the reaction between hypochlorous acid and ammonia / nitrogenous based compounds. It is measured by the difference between free chlorine and total chlorine tests. i.e. Total chlorine = DPD No 1. + DPD No 3 tests. In addition, there are 2 different grades of DPD tablets available, depending on the method of analysis used. Rapid grade tablets are designed for use with comparators, whereas Instrument grade tablets are designed for use with photometers. Rapid grade tablets do not dissolve fully, thus they will give erroneous readings if used in a photometer.