Chlorine is very unstable in aqueous solution, especially when the concentration is low, the content will decrease rapidly. Exposure to sunlight and other intense light or agitation accelerates the rate of chlorine reduction. Therefore, it cannot be stored after sampling, and the determination of chlorine must be started immediately, while avoiding light exposure and stirring the water sample. All operations in the determination process should avoid direct sunlight, preferably at the lowest possible temperature and soft light, and all colorimetric methods need to use color and turbidity blanks to compensate for the color and color of the raw water. The blank value must be determined especially when the turbidity and color are high. When using the o-toluidine visual colorimetric method to measure
residual chlorine, if the water sample is mixed evenly with the standard o-toluidine solution and then the color is compared immediately, the measured result is free residual chlorine. After high chroma, colorimetry is carried out, and the result obtained is the total residual chlorine. The total residual chlorine minus the free residual chlorine is the combined residual chlorine. When using the o-toluidine visual colorimetry method, if the amount of residual chlorine is large, it will produce orange yellow; if the water sample is too high in alkalinity and the amount of residual chlorine is small, it will produce light green or light blue. At this time, an additional 1mL o-toluidine standard solution can be added to produce a normal light yellow color. The measurement of residual chlorine electrodes can be carried out by photoelectric colorimetry or electrochemical sensing. After understanding the measurement principles of different brands of residual chlorine electrodes, you can apply them according to their characteristics. The following is the personal experience of using general residual chlorine electrodes as follows: a. Optical colorimetry: Principle: The dpd indicator is pumped in by a peristaltic pump to react with water to develop color, and the absorptiometer judges the residual chlorine content according to the color intensity. Features: The measured value is different from the pH value of water, and the humidity will affect the interpretation value. The dpd indicator must be used for calibration. The measurement environment is not synchronized with the pH of the water. The bactericidal ability of the water is affected by the pH and combined chlorine, and the actual bactericidal strength of the water may be low. b. Film-covered electrode: Principle: The electrode is immersed in the electrolyte cavity, and the electrolyte cavity contacts 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. The 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. c. Membraneless electrode: principle: residual chlorine is a strong oxidant, and its oxidation-reduction potential (orp) is exponentially related to the residual chlorine content in the solution, and the oxidation-reduction potential is converted to the residual chlorine content through a program. Features: Convert residual chlorine value with oxidation-reduction potential (orp). The zero point and slope must be calibrated with the dpd indicator. The measurement environment is synchronized with the pH and combined chlorine of the water. The measured value shows the converted residual chlorine value of the actual bactericidal strength of the water. The actual residual chlorine concentration of the water may be too high.
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