Free Chlorine & Total Chlorine Determination

Chlorine is a chemical that is widely used to disinfect water before it is discharged into the distribution system. When chlorine is introduced into the water, it reacts with any iron, manganese or hydrogen sulfide that may be present. To ensure that the water is adequately treated throughout the distribution system, an excess of chlorine is usually added. However, excess chlorine in water can react with other harmless organic substances in drinking water to form Trihalomethanes (THMs), such as chloroform. THMs have been shown to be potentially carcinogenic, so total chlorine is carefully monitored in chlorinated water systems to avoid adverse health risks. Total chlorine is divided into combined chlorine and free chlorine.

Figure 1. Total Chlorine Test Flow

Wet Chemical Analysis Methods for Free Chlorine & Total Chlorine Determination

At BOC Sciences, we use the standard method of ISO 7393-2:2017 for the determination of free and total chlorine in water, which is readily applicable for laboratory and field testing. The method is developed based on measuring absorbance, the red DPD color complex in a photometer, or measuring color intensity by visually comparing the color to a calibrated standard ratio. Our method is suitable for measuring drinking water and other environmental water.

Figure 2. Total and Free Chlorine in Water Colorimetric test

DPD Colorimetric Method

The DPD colorimetric method has become the most widely used method for the determination of free and total chlorine in water and wastewater. Our chemists has used the DPD (N,N-diethyl-p-phenylenediamine) method to detect residual chlorine. The direct reaction of monochloramine and dichloramine with DPD at near neutral pH is slow. To quantify these chloride species, we used tests with a slightly acidic solvent in the presence of iodide ions, in the presence of Iodide reacts with chloramine to form triiodide ions. The dichloramine content can also be determined by adding an excess of potassium iodide.

DPD amine is oxidized by chlorine to two oxidation products:

• At a near neutral pH, the major oxidation product is a semi-quinoid cationic compound called Würster dye
• When DPD reacts with small amounts of chlorine at near neutral pH, DPD can be further oxidized to a relatively unstable, colorless imine compound. This is due to the fact that higher oxidant levels can cause a significant 'fading' of the colored solution

Figure 3. How to Test Free Chlorine (Wallender, L. 2022)

DPD Titration Method

The DPD titration method is based on the same chemical principle as the DPD colorimetric method - i.e., DPD is oxidized to a red species by chlorine (or iodine in the case of chloramines). It is then titrated to a colorless endpoint with an iron reducing agent (ferrous titrant). The visual estimation of the titration end point is imprecise compared to the measurement of color obtained by using a colorimeter or spectrophotometer.

Iodide Titration Method

We generally use the starch-iodide titration method for total chlorine testing. The titration is usually done between pH 3-4 of the sample, and the iodometric titration using a starch indicator should be done at a sample temperature below 20°C. The end point of the titration is the disappearance of the blue starch-iodide complex. In the iodide titration process, a known amount of thiosulfate is first added to exceed the amount of chlorine in the sample. The amount of unreacted thiosulfate is titrated with a standard iodine solution. Finally, the total chlorine can be calculated based on the thiosulfate equivalent in the sample.

Figure 4. Total chlorine level.

Amperometric Titration Method

At BOC Sciences, amperometric method is used to determine free chlorine, we titrate chlorine at pH 7 with a standard reducing agent (e.g. thiosulfate or phenylarsenic). During the titration, the chlorine is reduced at the cathode to chloride (Cl^-) in reaction with phenylarsine oxide (PAO). As long as the oxidant (free chlorine) is present in the sample being titrated, a current flows through the cell. When all the oxidant has been reacted, the rate of change of the current is zero, marking the end point of the titration. The amount of PAO used at the end of the titration is proportional to the concentration of chlorine in the sample. For the determination of chloramine, the pH is lowered to 4 and potassium iodide is added to convert the chloramine to an equivalent amount of triiodide ions.