How to detect residual chlorine in water ?

- Mar 06, 2020-

How to detect residual chlorine in water ?


Detection principle:


DPD residual chlorine detection kit (0.05-1mg / L), DPD residual chlorine analyzer LH-M900 (0-12mg / L)

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(1) DPD method: Free chlorine reacts directly with N, N-diethyl-1,4-phenylenediamine (DPD) to form a red compound.

Advantages: The DPD method has the advantages of simplicity and speed, and it is an effective chlorine method that is widely used.

Operating procedure: 1.) Take a clean colorimetric tube and add a special test water sample to the 15mL graduation line of the tube. 2.) Pour the residual chlorine measurement reagent into the colorimetric tube, and then move until it is completely dissolved. 3.) Lift the tube to a blank about 2 cm high from the color card. The color is visually compared to the standard color scale white. The color scale with the same color tone as the solution in the tube is the residual chlorine content in the water (mg / L). ).


Residual chlorine test paper (0-10mg / L)

residual chlorine

(2) TMD method: Under acidic conditions (pH ≤ 2), it mainly reacts with residual chlorine to form a yellow quinone compound, that is, a yellow dielectron oxide-TMB diimine salt.

Advantages: accurate and reliable data-classic colorimetric analysis, multi-level test requirements, and visual readings with colorimetric cards. Simple operation without additional auxiliary supplies. Convenient and quick-testing samples takes only 0-1 minutes. Packed in dry sealed barrels, the shelf life is up to 18 months.

Operation procedure: 1. Shake off excess liquid from the test strip; 2. Compare the color with the color block on the barrel of the residual chlorine test strip; 3. The corresponding color is the residual chlorine concentration of the liquid.


Online residual chlorine monitor LH-CL650 residual chlorine (0-20mg / L), hypochlorous acid (0-10mg / L), PH (0-14).

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(3) Membrane method: The electrolyte and the osmosis membrane separate the electrolytic cell from the water sample. The osmosis membrane can selectively allow ClO- to penetrate; there is a fixed potential difference between the two electrodes, and the generated current intensity can be converted into Residual chlorine concentration.

Advantages: 1.) 24-hour real-time monitoring, multiple parameters such as residual chlorine value, hypochlorous acid value, pH value, temperature, output current, etc. are displayed at the same time, intuitive and easy to read, and there is a range over limit prompt. 2.) With RS-485 communication interface (compatible with MODBUS protocol part), the residual chlorine value corresponding to 4-20 mA current output can be arbitrarily set. Dosing can be controlled automatically. 3.) Simple maintenance, saving time and effort in testing, and generally only need to change the electrolyte every 6 months. No other characters need to interfere. 4.) Can be connected to a computer to facilitate remote real-time observation of sterilization tank data, and observation data can be exported to EXCL files. No need to waste manpower to detect and record.

Collection of samples: Free chlorine and total chlorine are unstable. Samples should be measured on site as much as possible. If samples cannot be measured on site, fixative should be added to the samples for storage. The sampling volume of 1% NaOH solution (6.6) can be added to the brown glass bottle in advance, and the water sample is collected to fill the sampling bottle, immediately capped and sealed to prevent the water sample from contacting the air. If the sample is acidic, increase the amount of NaOH solution to ensure that the pH of the water sample is greater than 12. Sample storage: Water samples are transported in a refrigerator, stored in a laboratory at 4C and protected from light, and measured within 5 days.