Chlorine is used as part of the water treatment process.

The chemical carries out several functions when added to untreated water.

There are 5 measures required when referring to chlorine in water:

Added chlorine: this is the total amount of chlorine dosed into untreated water
Chlorine demand, this is the amount of chlorine required to react with and neutralise any metals, compound and organic material in the water.
Total chlorine: this is the chlorine left over after reaction to the impurities in the water, this value is the sum of points 4 & 5
Combined chlorine: some of the chlorine will react and combine with the nitrogen in the water, this is also not available disinfection
Residual chlorine: this is the amount of chlorine remaining which is available for disinfection

Step 1: Testing

There are a number of chemical tests to determine the chlorine residual in drinking (or pool) water. This amount diminishes over time as the chlorine reacts and is no longer available to disinfect the water.

Based on WHO data, 30 mins after treatment water should be at ph8 with a chlorine residual of no more that 2mg/l. This is adequate to allow water to be stored for 24-48 hours and remain safe to drink but still not have a bad taste or odour.

There are strip tests, liquids, and colorimetric tests to determine the amount of chlorine residual.

The test I will describe is a colorimetric test using the DPD method.

This method uses a diethyl-p-phenylene diamine (DPD) powder, DPD reacts with the residual chlorine, turning it a shade of pink. The pinker the water, the more chlorine present.

The colorimeter then carries out a colorimetric analysis of the sample and returns a value of chlorine mg/l.

Step 2: Setup & Zero

The system I used here is a Hach Pocket Colorimeter II Chlorine analyzer. This unit is also fully lab calibrated as the company I work for works on the public water system.

Start by running the tap for 2-3 mins in order to push any standing water from the pipes.

Flush the sample vial with water to remove any remnant from previous tests.

Fill the vial to the 10ml marker line (this is important as more or less would effect the concentration of the reagent)

Place the sample in the test unit and cover. The cover prevents ambient light from skewing the value in the test.

Using the 0 button, the sample is analysied to set the standard for water clear of reagent. This is more important if the water is not 100% clear and establishes a baseline.

Step 3: Test

The sample is removed from the meter and opened.

The DPD is added, the lid replaced and the sample shaken to mix.

You will notice the sample change colour if there is chlorine present. I hope you can see from the image that my sample did change colour.

The vial is placed back in the meter and covered. The green button runs the analysis and shows a result.

Step 4: Results

I carried out 2 tests

Water from the running cold mains feed to my house
The other from the stored cold water tank

The inbound water had a residual of 0.28mg/l, this is quite low as I live in an urban area and the test was carried out at approx. 6pm when water demand would be high. I would have expected the residual to be higher but if the input water to the treatment plant was poor or it sat for some time in a reservoir or water tower.

The second test showed a residual of 0.07, this shows that as the water sat in the storage tank in my attic, there were elements in the water for the chlorine to attack, keeping the water potable.

Step 5: Conclusions

Though we are told (in Ireland anyway) not to drink the water from the storage tank (bathroom taps), the residual chlorine in my water tells me I would be safe to drink this water.

If, however, I wanted to store some water in say a bottle or glass by my bed overnight I would be better to fill from the main tap in the kitchen as the amount of chlorine in this water would keep the contaminants in the water at safe levels for longer than the tank water.

Also, I am reassured that the water does not contain too much chlorine as this is poisonous.