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Hangzhou Lohand Biological Co.,Ltd
Add: Zhejiang Hangzhou Jianggan 4F,Bldg 7,No.63,Jiuhuan Rd,Jiubao Town,Jianggan Dist
Tel: +86-571-85050999
Mob: +8613123950066
E-mail: info@yingaolab.com

Measurement of Chlorine Residual
May 22, 2018

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