Measurement of chlorine in the field is relatively easy. The fact that chlorine can be easily detected and measured makes chlorine a favorite water sanitizer for those concerned with the safety of water supplies, public and private pools, and other sources or reservoirs. Chlorine concentrations in the range of 0.1 to 0.4 parts per million are usually maintained in municipal supplies, while ranges from 1 to 3 ppm are used in both pools and spas.
Three types of chlorine can be measured:
- Free chlorine – most effective as a disinfectant
- Combined chlorine – formed when free chlorine reacts with other compounds within the water.
- Total chlorine – the sum of free and combined chlorine.
Since free chlorine is the primary disinfectant, a means of measuring free chlorine is needed.
Chlorine Test Method
The method used for measuring chlorine residuals uses a chemical compound called DPD (N,N-Diethylparaphenylenediamine), either through the use of a chlorine comparator or photometric instrument (this provides the highest accuracy and precision for measuring chlorine residuals). This compound can be provided as a tablet or as a liquid. DPD reacts instantly to produce a red color in the presence of chlorine. The subsequent addition of potassium iodide evokes a rapid color response from the combined forms of chlorine (typically chloramines).
Test strips utilize a different indicator as a means of measuring chlorine..
A color comparator allows the individual testing for chlorine to compare the reacted sample with established color standards, thereby determining the concentration of chlorine in the sample. The tester simply places the reacted sample in the comparator, comparing the sample to the colors in the sidebar, thereby determining the concentration of the chlorine in the sample.
A colorimeter passes a white light beam through an optical filter which transmits only one particular color or band of wavelengths of light to the photodetector, where it is measured. The difference in the amount of colored light transmitted by a colorless sample (blank), and the amount of colored light transmitted by a colored sample, is a measurement of the amount of colored light absorbed by the sample. In most colorimetric tests, including chlorine, the amount of colored light absorbed is directly proportional to the concentration, and is reported by the meter.
Utilizing a different indicator, test strips are small strips, typically produced from plastic, with chemically impregnated pads on the end. These pads are designed to react with specific ions and produce a specific color change. Once a test strip is reacted, and a color is developed, the strip is then compared to a printed color chart. The printed color chart is specifically designed to represent color reactions at various concentrations. Matching the strip to the closest color match produces a concentration reading.