Which test is commonly used to measure free chlorine residual?

Which test is commonly used to measure free chlorine residual?

• A. DPD colorimetric test (or online free chlorine analyzer).
• B. pH meter.
• C. Turbidity reader.
• D. Conductivity meter.

Answer: (A)

Free chlorine residual is measured using a colorimetric reaction that produces a color proportional to the amount of chlorine present. The DPD reagent turns pink in the presence of free chlorine, and the color intensity (or a corresponding reading on a color comparator or analyzer) indicates the chlorine concentration left in the water. This method is standard because it gives a quick, direct measure of the disinfectant remaining after treatment. Many systems also use an online free chlorine analyzer that implements the same principle or a compatible sensor to provide continuous readings. Other common instruments don’t measure chlorine residual: a pH meter gauges acidity or basicity, not disinfectant levels; a turbidity reader assesses cloudiness caused by suspended particles; and a conductivity meter measures how well water conducts electricity, reflecting dissolved ions rather than chlorine.

Free chlorine residual is measured using a colorimetric reaction that produces a color proportional to the amount of chlorine present. The DPD reagent turns pink in the presence of free chlorine, and the color intensity (or a corresponding reading on a color comparator or analyzer) indicates the chlorine concentration left in the water. This method is standard because it gives a quick, direct measure of the disinfectant remaining after treatment. Many systems also use an online free chlorine analyzer that implements the same principle or a compatible sensor to provide continuous readings.

Other common instruments don’t measure chlorine residual: a pH meter gauges acidity or basicity, not disinfectant levels; a turbidity reader assesses cloudiness caused by suspended particles; and a conductivity meter measures how well water conducts electricity, reflecting dissolved ions rather than chlorine.