Overview and application fields of residual chlorine detector
Chlorine detector application field Chlorine detector is usually used in food and beverage, medical and health, CDC, tap water supply, secondary water supply, swimming pool, breeding and other industries. Overview of Chlorine On-line Detector On-line PH meter: knowledge about pH measurement of pure water It is difficult to obtain a satisfactory result for the measurement of pH value of pure water, especially in the laboratory with a conventional composite electrode. This is especially true for open measurements. The performance is often: slow response, drift of indication, poor repeatability, low accuracy, etc. 1. Reasons: 1. In order to ensure the pH zero potential of the composite electrode, the salt bridge must use high-concentration Kcl, and at the same time, in order to prevent the Ag/AgCl coating from being dissolved by high-concentration Kcl, powdery Kcl must be added to the salt bridge AgCl, to saturate the salt bridge solution with AgCl. However, according to the above item 1, due to the reduction of Kcl concentration in the salt bridge solution, the AgCl originally dissolved in it is supersaturated and precipitated, thereby blocking the liquid junction. 2. Since the ion concentration in pure water is very low, and the reference electrode salt bridge solution selects Kcl with a high concentration of 3mol/L, the concentration difference between them is relatively large, which is very different from its situation in ordinary solutions. In pure water, the penetration rate of the salt bridge solution will be increased, and the loss of the salt bridge will be promoted, thereby accelerating the reduction of the concentration of K+ and CL-. It causes the change and instability of the liquid junction potential, and the potential of the Ag/AgCl reference electrode itself depends on the concentration of CL-. When the concentration of CL- changes, the potential of the reference electrode itself will also change accordingly. This makes the indication drift. This is especially true for composite electrodes that cannot be replenished with internal reference solution. 3. Pure water is easy to be polluted. If it is measured in an open beaker, it is easily affected by the absorption of CO2, and the pH value will continue to drop. The relevant international standards stipulate that the measurement must be carried out in a special device in a closed environment. But it is difficult to implement in general laboratory. 4. The PH value reflects the activity of H+, (H+) rather than the concentration [H+] of H+, and the relationship is (H+)=f×[H+]. F is the activity coefficient of H+. It is determined by the total concentration of all ions in the solution and not only by the concentration of the measured ions. In theoretically pure water, the activity coefficient f is equal to 1, but as long as other ions exist, the activity coefficient will change, and the pH value will also change. That is, the pH value is affected by the total ion concentration in the solution. If the total ion concentration changes, the pH value will change. Since the liquid junction of the composite electrode is very close to the pH-sensitive glass bulb, the salt bridge solution leaked from the liquid junction first gathers around the sensitive bulb, changing the total ion concentration near it. From the above reasons, the measured value Only the pH value near the sensitive bulb has been changed, which cannot reflect its true pH value. Although this situation can be changed by stirring or shaking the beaker, practice has proved that the test value will be different if the stirring speed is different, and at the same time stirring or shaking will accelerate the dissolution of CO2, so it is not advisable. 5. Since the internal resistance of the glass resistor is very high, the higher the internal resistance, the thicker the glass film will be, and the asymmetric potential will increase, and the inertia of the electrode will also increase accordingly, and the generation of electromotive force will be slower. At the same time, since pure water is a non-buffering liquid, which has completely different properties from the standard buffer solution, if an electrode that performs well in the standard buffer solution is moved to non-buffering pure water at once, the establishment time of the electrode potential is undoubtedly will become sluggish. 2. Overcoming and improving methods: In summary, it is difficult to test the PH value of pure water, mainly due to the structural performance of the electrode and the instability of the junction potential of the electrode liquid caused by this time, the non-buffering performance of pure water and the CO2 in the air. and other factors. To this end, the following methods can be used to overcome and improve. 1. Using a pH electrode suitable for pure water should be the first consideration. At present, some domestic manufacturers have developed some electrodes suitable for or specially used for the measurement of the pH value of pure water. 2. When using a separate electrode plate, that is, a separate pH glass electrode and reference electrode for measurement, keep the reference electrode as far away from the glass electrode as possible to improve the phenomenon analyzed in item 4 above. 3. Increase the sampling volume of the pure water to be tested, and possibly reduce the contact surface with the air. At the same time, do not stir and shake to reduce the absorption of CO2. 4. Add a neutral salt (such as Kcl) to the water sample to be tested as an ionic strength regulator to change the total ionic strength in the solution, increase the conductivity, and make the measurement fast and stable. This method stipulates in the national standard GB/T6P04.3-93: 'In order to reduce the influence of liquid junction potential and quickly achieve stability when measuring water samples, add a drop of neutral 0.1moL/LKCL solution to every 50ml water sample.' Although this method Changing the ionic strength in the water sample caused its PH value to change to a certain extent, but experiments have proved that this change only changes about 0.01PH in value, which is completely acceptable. However, when using this method, it must be noted that the added Kcl solution should not contain any alkaline or acidic impurities. Therefore, the Kcl reagent should be of high purity, and the water quality of the prepared solution should also be high-purity neutral water. 5 The general use time of the electrode is half a year to one year, and the time of industrial pH electrodes with poor use conditions may be shorter.