How to detect the residual chlorine content in water?

- Jul 13, 2020-

Detection methods: The most common methods for detecting residual chlorine are DPD, OTO, TMD and membrane methods. (Among them, OTO is relatively toxic and shows unstable data, and is gradually replaced by DPD method.)

Detection principle:

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

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

Advantages: The DPD determination method has the advantages of simplicity and rapidity, and is a widely used effective chlorine determination method.

Operation process: 1.) Take a clean colorimetric tube and add a special water sample to the 15mL scale of the tube. 2.) Pour the residual chlorine determination reagent into the colorimetric tube, and move it until it is completely dissolved. 3.) Lift the tube to a blank space about 2 cm away from the colorimetric card at the standard color scale white and visually compare the color. The same color scale as the solution in the tube is the residual chlorine content in the water (mg/L ).

residual chlorine


(2) TMD method: Under acidic conditions (pH ≤ 2), the reaction with residual chlorine mainly produces a yellow quinoid compound, that is, a yellow two-electron oxide-TMB diimine salt.

Advantages: Accurate and reliable data-use classic colorimetric analysis, multi-level test requirements, use colorimetric card for visual reading. The operation is simple and no other auxiliary consumables are needed. Convenient and fast-it only takes 0-1 minutes to test samples. Packed in dry sealed barrels, the shelf life is up to 18 months.

Operation process: 1. Throw off the excess liquid on the test paper; 2. Compare the color with the color block on the residual chlorine test paper packaging barrel; 3. The corresponding color is the residual chlorine concentration value of the liquid.

residual chlorine

(3) Membrane method: the electrolyte and the permeable membrane separate the electrolytic cell and the water sample. The permeable 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 prompt to indicate that the range exceeds the limit. 2.) With RS-485 communication interface (MODBUS protocol is partially compatible), the residual chlorine value corresponding to 4-20 mA current output can be set arbitrarily. Can automatically control dosing. 3.) Simple maintenance, time-saving and labor-saving testing, generally only need to change the electrolyte every 6 months. No other character intervention is required. 4.) It can be connected to a computer to facilitate remote real-time observation of disinfection tank data, and export observation data to EXCL files. No need to waste manpower to detect and record.

Collection of samples: Free chlorine and total chlorine are unstable. The samples should be measured on site as much as possible. If the samples cannot be measured on site, fixatives need to be added to the samples for preservation. The 1% NaOH solution (6.6) of the sampling volume 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 tightly, to prevent the water sample from contacting the air. If the sample is acidic, the amount of NaOH solution should be increased to ensure that the pH of the water sample is greater than 12. Sample storage: Water samples are transported in a refrigerated container, stored in the laboratory at 4C, protected from light, and measured within 5 days.

residual chlorine