Fundamentals of Chlorine Measurement

When chlorine is added to drinking water, it proceeds through a series of reactions described below.

Chlorine Addition Flow Chart

When chlorine is added to water, some of the chlorine reacts first with inorganic and organic materials and metals in the water and is not available for disinfection (this is called the chlorine demand of the water). After the chlorine demand is met, the remaining chlorine is called total chlorine. Total chlorine is further divided into: 1) combined chlorine, which is the amount of chlorine that has reacted with inorganic (nitrates, etc.) and organic nitrogen-containing molecules (urea, etc.) to make weak disinfectants that are unavailable for disinfection and, 2) Free chlorine, which is the chlorine that is left over and is available to inactivate disease-causing organisms; it is a measure of the potability of the water. Thus, total chlorine equals the sum of the combined chlorine and free chlorine measurements.

For example, if using completely clean water with no contaminants, the chlorine demand will be zero, and since there will be no inorganic or organic material present, no combined chlorine will be present. Thus, the free chlorine concentration will be equal to the concentration of chlorine initially added. In natural waters, especially surface water supplies such as rivers, organic material will exert a chlorine demand, and inorganic compounds like nitrates will form combined chlorine. Thus, the free chlorine concentration will be less than the concentration of chlorine initially added (Free chlorine = Total chlorine measurement – Combined chlorine measurement).

Why Do We Test Free Chlorine in Drinking Water?

The SWS Program recommends testing free chlorine in two circumstances:

  • To conduct dosage testing in project areas prior to the start of a program.
  • To monitor and evaluate projects for chlorination compliance by testing stored water in households.

The goal of dosage testing is to determine how much chlorine (sodium hypochlorite solution) to add to water that will be used for drinking to maintain free chlorine in the water for the average time of storage of water in the household (typically 4-24 hours). This goal differs from the goal of infrastructure-based (piped) water treatment systems, whose aim is effective disinfection at the endpoints (i.e., water taps) of the system: defined by the WHO (1993) as: "a residual concentration of free chlorine of greater than or equal to 0.5 mg/L (0.5 ppm or parts per million) after at least 30 minutes contact time at pH less than 8.0." This definition is only appropriate when users drink water directly from the flowing tap. A free chlorine level of 0.5 mg/L of free chlorine will be enough residual to maintain the quality of water through the distribution network, but is most likely not adequate to maintain the quality of the water when this water is stored in the home in a bucket or jerry can for 24 hours.

Thus, the SWS Program recommends for dosage testing that:

  • At 30 minutes after the addition of sodium hypochlorite there should be no more than 2.0 mg/L of free chlorine present (this ensures the water does not have an unpleasant taste or odor).
  • At 24 hours after the addition of sodium hypochlorite to containers that are used by families to store water there should be a minimum of 0.2 mg/L of free chlorine present (this ensures microbiologically safe water).

The SWS Program methodology leads to free chlorine levels that are significantly lower than the WHO guideline value for free chlorine in drinking water, which is 0.5 mg/L value. The SWS Program recommends testing free chlorine in homes of SWS users for evaluation of whether or not users are using the system and if they are using it correctly. Households can be visited and 'spot checked' to determine if, and how much, free chlorine is present in their drinking water. This approach is very useful for program monitoring because the presence of free chlorine in stored water obtained from an unchlorinated source is an objective measure that people are using the hypochlorite solution.

Methods to Test Free Chlorine in the Field in Developing Countries

There are three main methods to test free chlorine residual in drinking water in the field in developing countries: 1) Pool test kits, 2) Color-wheel test kits, and 3) Digital colorimeters. All three methods depend on a color change to identify the presence of chlorine, and a measurement of the intensity of that color to determine how much chlorine is present.

Pool Test Kits

The first option for testing uses a liquid chemical OTO (orthotolidine) that causes a color change to yellow in the presence of total chlorine. You simply fill a tube with water, add 1-5 drops of the solution, and look for the color change. These kits are sold in many stores as a way to test the concentration of total chlorine in swimming pool water. This method does not measure free chlorine.

Benefits of the pool test kits:

  • Low cost
  • Very easy to use

Drawbacks of the pool test kits:

  • Degradation of the OTO solution that causes inaccurate readings over time
  • Generally not reliable quantitative results
  • Lack of calibration and standardization

Color-wheel Test Kit

Color wheel test kits use a powder or tablet chemical DPD (N,N diethyl-p-phenylene diamine) that causes a color change to pink in the presence of chlorine. The color wheels are simpler and less expensive than digital meters because to measure the intensity of the color change, the field worker uses a color wheel to visually match the color to a numerical free or total chlorine reading. The test kit can be used to measure free chlorine and/or total chlorine (using different chemicals in the kit), with a range of 0 – 3.5 mg/L, equivalent to 0 – 3.5 ppm (parts per million).

Benefits of the color-wheel test kits:

  • Accurate readings if used properly
  • Low cost

Drawbacks of the color-wheel test kits:

  • Potential for user error
  • Lack of calibration and standardization

Digital Colorimeters

Digital colorimeters are the most accurate way to measure free chlorine and/or total chlorine residual in the field in developing countries. These colorimeters use the following method: 1) addition of DPD tablets or powder into a vial of sample water that causes a color change to pink; and, 2) insertion of the vial into a meter that reads the intensity of the color change by emitting a wavelength of light and automatically determining and displaying the color intensity (the free and/or total chlorine residual) digitally. The range of the meter is 0 - 4 mg/L, equivalent to 0 - 4 ppm (parts per million).

Benefits of the digital colorimeters:

  • Highly accurate readings
  • Fast results

Drawbacks of the digital colorimeters:

  • Expense (higher cost than other methods)
  • Necessity of calibration with standards

Reagents Free Chlorine Residual Measurement

The revolutionary ChloroSense® boasts unique sensor technology - developed specifically for measuring free and total chlorine. Designed for field analysis, the instrument is robust, waterproof and battery powered and has the following key features:

  • Simple and rapid measurement process – fill the sample vessel, insert the sensor then close the lid - in less than one minute results for free and total chlorine are available
  • No reagents and no glassware – ideal for food processing applications

Aldo Zaffalon
Aldo Zaffalon

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