When chlorine gas is in water at a pH value of 0, chlorine exists mainly in the form of chlorine gas in water. As the pH increases, chlorine gas reacts with water to form hypochlorous acid (especially prominent in the pH range 2-6). At higher pH, hypochlorous acid is neutralized to form its corresponding salt ion, the hypochlorite ion. When the pH value is about 9, the neutralization is complete, and there is basically no free chlorine in the water. Residual chlorine measurement method: 1. Principle of DPD colorimetric method: Under the condition of PH6.2~6.5, ClO2 first reacts with DPD to form a red compound, but the amount only reaches its total available chlorine content (equivalent to reducing ClO2 to one-fifth of the chlorite ion). If the water sample is acidified in the presence of iodide, chlorite and chlorate also react, and when neutralized by the addition of bicarbonate, the resulting color corresponds to the total available chlorine content of ClO2 . The interference of free chlorine can be suppressed by adding glycine, based on the fact that glycine can immediately convert free chlorine into chloroaminoacetic acid, but it has no effect on ClO2. Features: unattended operation, stable measurement value, user can choose the measurement cycle, need to replace the reagent. For example: American HFCLX (0-10mg/L recommended), American HACHCL17 (0-5mg/L) Application: drinking water, sewage, waste water and industrial circulating cooling water, etc. 2. Principle of film-covered electrode method: electrode is immersed in electrolyte chamber In , the electrolyte chamber is in contact with water through a porous hydrophilic membrane. The hypochlorous acid diffuses into the electrolyte cavity through the porous hydrophilic membrane, and forms a current on the electrode surface. The magnitude of the current depends on the speed at which the hypochlorous acid diffuses into the electrolyte cavity, and the diffusion rate is proportional to the concentration of residual chlorine
in the solution. Measure the magnitude of the current The concentration of residual chlorine in the solution can be determined. Features: No reagent required. There will be drift when used in the place containing interface active agent, and the membrane pores will be blocked by lipids, and the diaphragm and electrolyte need to be cleaned and replaced regularly. DPD indicator must be used for calibration. The measurement environment is synchronized with the pH of the water, and the combination of chlorine interferes with the bactericidal ability of the water, and the actual bactericidal strength of the water may be low. For example: Germany E+HCCM223/253+CCS140+CCA250 Application: drinking water, swimming pool water, industrial water, etc., measuring range 0-20mg/L3, constant voltage electrode method (membrane-free electrode method) principle: between measurement and reference electrode To maintain a stable potential, different measured components will produce different current intensities under this potential. It consists of two platinum electrodes and a reference electrode to form a micro current measurement system. At the measuring electrode, chlorine molecules or hypochlorite are consumed, and the resulting current intensity is related to the concentration of residual chlorine in the water. Features: In the range of pH 6-8, the measured signal decreases with the increase of pH value, so it is recommended to use this method for residual chlorine under the condition of pH ≤ 8. If the pH of the water is constant, then the pH effect can be compensated by field calibration. Real-time online detection, fast response, no need for regular replacement of reagents, electrolytes or membranes, simple maintenance, and low purchase cost. Example: UK PRIMAinnoCon6500CL+innoSens710+PA-711
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