What could cause free chlorine measurements to be higher than total chlorine?
Free chlorine measurements are higher than total chlorine
If free chlorine measurements are higher than total chlorine using a DPD method, check the alkalinity and pH of the sample. Samples with pH of 7.6 or above may have 250 ppm alkalinity or above. When adding the DPD reagent, a pH of 6.3 should be observed. Higher amounts of bicarbonate in the water can cause small bubbles (looks like turbidity) to form which will increase the free chlorine results. The readings will gradually decrease after 1 minute as the turbidity decreases. To account for this interference, sample pH should be adjusted to 6-7 before running the test.