E-mail: michael@shboqu.com   |   Expert in Water Quality Measurement and Water Treatment Project

how is residual chlorine measured

by:BOQU     2023-04-25

How Is Residual Chlorine Measured?

Water treatment facilities often use chlorine to disinfect water and kill harmful organisms that may be present. However, it is important to ensure that the concentration of residual chlorine in the water is maintained at appropriate levels to prevent under- or over-chlorination. So how is residual chlorine measured?

In this article, we will discuss the different methods used to measure residual chlorine and their advantages and disadvantages.

What is residual chlorine?

Residual chlorine refers to the concentration of free chlorine that remains in the water after the disinfection process. The amount of residual chlorine required varies depending on the type of water being treated and the level of disinfection required.

Why is residual chlorine important?

Residual chlorine is important for several reasons. Firstly, it acts as a disinfectant, killing any harmful organisms that may be present in the water, ensuring that it is safe to drink. Secondly, residual chlorine helps to maintain the quality of the water by preventing the growth of bacteria and other microorganisms. Finally, it can help to prevent the formation of harmful by-products, such as trihalomethanes (THMs), which can form when chlorine reacts with organic matter.

Measuring residual chlorine

There are several methods used to measure residual chlorine in water. These methods can be divided into two categories: colorimetric and electrochemical.

Colorimetric methods

Colorimetric methods involve adding a chemical reagent to the water being tested that reacts with the residual chlorine to produce a color change. The intensity of the color produced is then measured using a colorimeter or spectrophotometer.

There are several colorimetric methods used to measure residual chlorine, including the DPD (N,N-diethyl-p-phenylenediamine) method, the DPD-FAS (ferrous ammonium sulfate) method, and the DPD-iodometric method.

Advantages of colorimetric methods:

- Colorimetric methods are simple and easy to use, requiring minimal technical knowledge or skill.

- They are relatively inexpensive, making them accessible to small water treatment facilities and laboratories.

- They are highly accurate when used correctly.

Disadvantages of colorimetric methods:

- The accuracy of the results can be affected by the presence of interfering substances, such as iron, copper, or manganese, which can produce false readings.

- The reagents used in colorimetric methods can be hazardous if mishandled or disposed of improperly.

- Sample preparation can be time-consuming and requires careful attention to detail.

Electrochemical methods

Electrochemical methods use an electrode to measure the concentration of residual chlorine in the water. The electrode measures the electrical potential difference between the chlorine in the water and a reference electrode, which is proportional to the concentration of chlorine.

The most common electrochemical method used to measure residual chlorine is amperometric titration. This method involves passing a small current through the water being tested, which causes the residual chlorine to oxidize the electrode. The amount of current needed to oxidize the electrode is then measured and is proportional to the concentration of chlorine.

Advantages of electrochemical methods:

- Electrochemical methods are highly accurate and precise, producing consistent results.

- They are not affected by interfering substances, making them suitable for use in a wide range of water types and conditions.

- They are fast and require minimal sample preparation.

Disadvantages of electrochemical methods:

- Electrochemical methods can be more expensive than colorimetric methods, making them less accessible to small facilities and laboratories.

- They can be more complex to use, requiring more technical knowledge and skill.

- The electrodes used in electrochemical methods can be fragile and require careful handling and storage.


Measuring residual chlorine is an important part of water treatment, ensuring that the water is safe to drink and of high quality. There are several methods used to measure residual chlorine, including colorimetric and electrochemical methods, each with their advantages and disadvantages. By understanding these methods, water treatment facilities and laboratories can choose the most suitable method for their needs, ensuring that the water they produce is of the highest quality.

As we have known for quite some time, the success of BOQU in the future will depend greatly on our ability to strike a balance between valuable human insight and interaction with technology.
Serving others for customers a better life with water analyzer for employees respect and opportunity.
This is crucial when you need to maintain innovative information in water analyzer.
water analyzer is attracting a great positive feedback from the customers. And many of our clients are fully satisfied with it.
Custom message
Chat Online 编辑模式下无法使用
Leave Your Message inputting...