What is the difference between the tests for Free and Total Chlorine?

DPD reacts with free chlorine. In a test for total chlorine, potassium iodide is added to the sample. Combined chlorine oxidizes the iodide to generate iodine. DPD then reacts with the iodine to form a pink color in direct relation to the combined chlorine level. To determine the level of combined chlorine, subtract free chlorine results from total chlorine results.