Two principles for measuring residual chlorine—DPD colorimetric method and ampere current method

Puzzle-solving class: Full analysis of the principles of two major technologies for residual chlorine measurement - DPD colorimetry and amperometric method

In the field of water quality disinfection, residual chlorine monitoring is a key link in ensuring water quality safety. The current mainstream measurement methods are mainly divided into two technical routes: DPD colorimetry and ampere current method. It should be noted that the terms "polarographic method" and "constant voltage method" often heard in the field of electrochemical measurement are essentially common names for ampere current method in different technical implementation methods. As a professional water quality analysis solution provider, YuanGan Technology also provides a series of products based on these two principles. This article will provide you with an in-depth analysis of the working principles, technical features and applicable scenarios of the two, helping you make the most appropriate choice based on your specific needs.

In the field of water quality disinfection, residual chlorine monitoring is a key link in ensuring water quality safety. The current mainstream measurement methods are mainly divided into two technical routes: DPD colorimetry and ampere current method. It should be noted that the terms "polarographic method" and "constant voltage method" often heard in the field of electrochemical measurement are essentially common names for ampere current method in different technical implementation methods. As a professional water quality analysis solution provider, YuanGan Technology also provides a series of products based on these two principles. This article will provide you with an in-depth analysis of the working principles, technical features and applicable scenarios of the two, helping you make the most appropriate choice based on your specific needs.

1. DPD colorimetry: the precise art of chemical color development

1. Core principles and key reagents

The full name of DPD method is N,N-diethyl-p-phenylenediamine method. Its core is a specific redox reaction: the residual chlorine in the water sample (mainly hypochlorous acid HClO) reacts with the DPD reagent to generate a stable purple-red quinone compound.

The chemical essence of this reaction is: DPD (colorless) + HClO → quinoid DPD (purple red).

The depth of the solution color is strictly proportional to the residual chlorine concentration. By measuring the absorbance at a wavelength of 515nm with a spectrophotometer, the residual chlorine content can be accurately calculated.

2. Technical implementation form and characteristics

There are two ways to implement the DPD method:

• Laboratory manual analysis: classic offline analysis method, the results are accurate and reliable, often used as the basis for arbitration
• Online automatic analyzer: Completes measurement through automatic sampling, dosing, mixing, and colorimetry, enabling continuous monitoring

Features of YuanGan Technology’s online DPD analyzer:

• Advantages: High measurement accuracy, strong anti-interference ability, in line with national standard methods, suitable for compliance testing required by regulations
• Notes: Reagents need to be replaced regularly, suitable for scenarios with professional maintenance teams and extremely high data accuracy requirements, such as accurate monitoring of incoming and outgoing water in large water plants.

2. Amperometric current method: intelligent monitoring of electrochemical sensing

1. Core principles and electrode system

The amperometric method is based on the principle of electrochemical sensing and measures the concentration by measuring the current signal generated by the reaction of residual chlorine molecules on the electrode surface. Its electrode systems all adopt a three-electrode system design to ensure measurement accuracy and stability:

Three-electrode system composition:

• Working electrode: the core electrode where electrochemical reactions occur, usually gold electrode or platinum electrode
• Counter electrode: forms a current loop, commonly used stainless steel electrodes or platinum electrodes
• Reference electrode: Provides a stable potential reference to ensure a constant potential of the working electrode

2. Product type and technical characteristics

YuanGan Technology offers two different types of amperometric sensors:

Film-coated sensor:

• Using selective breathable membrane technology, only hypochlorous acid molecules are allowed to pass through
• The three electrodes are all isolated from the sample, and material exchange occurs through the membrane.
• The diaphragm cap and electrolyte need to be replaced regularly to have stronger anti-interference ability.

Exposed sensor:

• A three-electrode system is used, in which the reference electrode has built-in protection and the working and counter electrodes are directly exposed to the water sample.

A three-electrode system is used, in which the reference electrode has built-in protection and the working and counter electrodes are directly exposed to the water sample.

• There are various combinations of electrode materials, including: Double platinum ring electrode (working electrode + counter electrode) Platinum ring + stainless steel electrode combination Platinum wire + stainless steel electrode combination, etc.

There are various combinations of electrode materials, including:

• Double platinum ring electrode (working electrode + counter electrode)
• Platinum ring + stainless steel electrode combination
• Platinum wire + stainless steel electrode combination, etc.
• No need for membrane head and electrolyte, simple maintenance and fast response speed

No need for membrane head and electrolyte, simple maintenance and fast response speed

• Suitable for different water conditions and application needs

Suitable for different water conditions and application needs

3. Core technology comparison and selection guide

In order to show the difference between the two more intuitively, we summarize as follows:

Selection suggestions:

• Scenarios for choosing DPD online analyzers: Accurate monitoring of incoming and outgoing water in large water plants, compliance testing required by regulations, and situations where complex water quality requires the highest accuracy
• Scenarios for choosing membrane-covered sensors: Municipal pipe network monitoring, sewage treatment effluent, industrial circulating water and other complex water quality continuous monitoring scenarios
• Scenarios for choosing exposed sensors: automatic dosing control of swimming pools, drinking water systems, unattended sites with relatively clean water quality

4. Technology complementarity and system integration

YuanGan Technology recommends users to establish a hierarchical monitoring system based on actual needs:

• The DPD method is used as a baseline method for regular calibration and compliance verification.
• Ampere current method provides continuous monitoring and automatic process control
• The two technologies form complementary verification to ensure the accuracy and reliability of the data.

Conclusion

DPD colorimetry and ampere current method each have their own advantages. As a professional manufacturer that masters both technologies, Chongqing YuanGan Technology can provide customers with a comprehensive range of residual chlorine monitoring solutions. Whether you need a DPD online analyzer with the highest accuracy, or a covered/exposed sensor suitable for long-term continuous monitoring, we can recommend the most appropriate technical solution based on your specific application scenarios, water quality conditions and management needs.

YuanGan Technology is committed to helping each customer build an efficient and reliable water quality safety monitoring system through a complete product portfolio and professional technical support.

—— Chongqing YuanGan Technology Co., Ltd., the water quality safety sensing expert around you