About the determination method of free chlorine

Chlorine is added to water, and after a certain period of contact, the general term for free chlorine and combined chlorine remaining in water. Residual chlorine can be divided into combined residual chlorine (referring to the compound of chlorine and ammonia in water, there are three kinds of NH2Cl, NHCl2 and NHCl3, NHCl2 is more stable and has good bactericidal effect), also called combined residual chlorine; free residual chlorine (referring to OC1+, HOCl, Cl2, etc. in water have fast sterilization speed and strong bactericidal power, but disappear quickly), also known as free residual chlorine; total residual chlorine is the sum of combined residual chlorine and free residual chlorine. Residual chlorine in tap water effluent refers to free residual chlorine. The residual chlorine standard is in the domestic drinking water of the pipe network that is chlorinated and disinfected. After chlorination and disinfection for 30 minutes, the content of free residual chlorine in the water should not be lower than 0.3mg/L; The content of free residual chlorine in the water at the end of the net should not be lower than 0.05mg/L; the standard value of free residual chlorine in artificial swimming pool water is 0.3~0.5mg/L; The content of free residual chlorine on the surface should be less than 0.3mg/L. Determination of residual chlorine in industrial circulating cooling water: 0.03mg/L～2.50mg/L. Determination method Residual chlorine refers to the chlorine remaining in the water after the water has been in contact with the chlorine-based disinfectant for a certain period of time. There are three forms of residual chlorine: 1. Total residual chlorine: including free residual chlorine and combined residual chlorine. 2. Free residual chlorine: including HOCl and OCl-, etc. 3. Compound residual chlorine: including NH2Cl, NHCl2, NCl3 and other chloramine compounds. The role of residual chlorine is to ensure continuous sterilization and prevent water from being re-polluted. However, if the amount of residual chlorine exceeds the standard, it may aggravate the taste and odor produced by phenol and other organic substances in the water, and may also generate harmful substances such as chloroform."San Zhi"Functional organochlorides. Determining the content and state of residual chlorine in water is extremely important for doing a good job in drinking water disinfection and ensuring the safety of water hygiene. There are many methods for measuring residual chlorine. At present, the following three determination methods are generally used: 1. Portable residual chlorine detector 1. Scope of application ⑴. This method is applicable to the determination of free residual chlorine, total residual chlorine and chemical properties of drinking water, source water, waste water and seawater respectively. Chlorine. ⑵. If the water sample is colored or turbid, it can be used as a blank to adjust to zero to offset its influence. ⑶. The highest detection concentration of this method is 4.5mg/l available chlorine. 2. Principle When the water sample does not contain iodide ions, the free available chlorine immediately reacts with the DPD reagent to produce a red color, and the addition of iodide ions acts as a catalyst to make the combined chlorine react with the reagent to develop color. Measure the absorbance respectively to obtain free chlorine and total chlorine, and subtract free chlorine from total chlorine to obtain combined chlorine. 3. Interfering effects ⑴. The presence of alkalinity greater than 250mg/l or acidity of 150mg/l in water, such as CaCO3, will inhibit all color development or the color will fade immediately. Neutralize the sample solution to pH 6-7 with 1N H2SO4 or 1N NaOH. ⑵. Monochloramine will gradually increase the free chlorine reading. Read within one minute, each 3.0mg/l of monochloramine will increase the free chlorine reading by 0.1mg/l. (3) Oxides of bromine, iodine, ozone and manganese and chromium will increase the free chlorine reading. ⑷. In order to reduce the influence of Mn4+ and Cr6+, adjust the pH value to 6-7 as above ⑴. Take 25ml water sample, add 3 drops of 30g/l KI solution, mix and wait for one minute. Add 3 drops of 5g/lNa2AsO3 and mix (according to 'Water and Wastewater Standard Test Method? The 15th Edition', 0.25% thioacetamide solution can be used instead of sodium arsenite, and 0.5ml 0.25% thioacetamide is added to every 100ml water sample) . If chromium is present, it reacts with DPD in both assays, readout. This reading is then subtracted from the chlorine reading obtained from the initial analysis. 2. Ortho-toluidine colorimetric method (OT method) 1. Scope of application ⑴. This method is applicable to the determination of total residual chlorine and free residual chlorine in drinking water and its source water. ⑵. The minimum detection concentration of this method is 0.01mg/l residual chlorine. 2. Principle In an acidic solution with a pH value less than 1.3, residual chlorine reacts with o-toluidine to form a yellow quinoid compound, which can be quantified visually by colorimetry, and a permanent solution prepared by potassium dichromate-potassium chromate solution can also be used. Visual color comparison was carried out with neutral residual chlorine standard solution. 3. Interference effect When the water contains suspended matter interference, it can be removed by centrifugation. The maximum allowable content of other interfering substances is as follows: high iron, 0.2mg/l; tetravalent manganese, 0.01mg/l; nitrite, 0.2mg/l. 3. On-line electrochemical residual chlorine analyzer 1. Scope of application This method is applicable to the determination of total residual chlorine, free chlorine, bromine chloride, bromine and iodine in drinking water, waste water, cooling water and other water. 2. Primary battery principle In a primary battery, changes in ion concentration can be detected by testing the current in it. The current in the bottle is proportional to the change in chloride ion concentration. The cathode in the primary battery is metallic gold. When hypochlorous acid (or hypochlorite ions) is present in the solution, a chemical reaction occurs at the negative electrode to produce chloride ions. HOCl + 2e -←→ Cl -+OH- The anode is metallic copper, and when the electrode reacts, the oxidation product remains on the anode. At this time, the abrasive machine (the cleaning ball is constantly stirred) will cooperate to remove the oxidation products on the metal surface. The current in a primary battery is greatly affected by changes in pH. The current in the bottle was stable at pH 4.0-4.5. Therefore, a pH buffer is used to stabilize the current. If CO2 is used as the buffer, the pH value can be adjusted to 5.5-6.0. When adding KI, the total free iodine generated by the reaction is equivalent to the total residual chlorine participating in the reaction, so as to determine the total residual chlorine. 3. Interference effects ⑴. Changes in temperature and pH will affect the accuracy of the instrument. Therefore, the thermal variable resistor is used to compensate the temperature change of the water sample and the pH buffer is added to adjust the pH value to overcome the influence of these two factors. ⑵. There is no indication that the presence of manganese, chromium, nitrite, etc. will have interfering effects.
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