When testing for chlorine in drinking water you should test for free chlorine.
Testing for free chlorine in drinking water indicates the amount of chlorine that is available to disinfect and kill any harmful microorganisms.
Free chlorine is the active form of chlorine which effectively disinfects water by killing the bacteria, virus and other harmful microorganisms.
Free chlorine is the primary disinfectant in drinking water treatment and monitoring free chlorine levels helps ensure the water is safe for consumption.
And many regulatory agencies focus on free chlorine levels as a measure of water quality.
Combined chlorine is often in the form of chloramines and is formed when the chlorine reacts with ammonia.
Chloramines are less effective disinfectants than free chlorine.
And while total chlorine can be measured, the focus for water safety is on ensuring sufficient free chlorine levels.
Many test strips also measure both free and total chlorine although the most important reading is the free chlorine level.
Test kits are more accurate and can also be used to measure chlorine levels.