Key points for water quality testing operations in sewage treatment plants part eight

43. What are the precautions for using glass electrodes?

⑴The zero-potential pH value of the glass electrode must be within the range of the positioning regulator of the matching acidimeter, and it must not be used in non-aqueous solutions. When the glass electrode is used for the first time or is reused after being left unused for a long time, the glass bulb should be soaked in distilled water for more than 24 hours to form a good hydration layer. Before use, carefully check whether the electrode is in good condition, the glass bulb should be free of cracks and spots, and the internal reference electrode should be soaked in the filling fluid.

⑵ If there are bubbles in the internal filling solution, gently shake the electrode to let the bubbles overflow, so that there is good contact between the internal reference electrode and the solution. To avoid damage to the glass bulb, after rinsing with water, you can use filter paper to carefully absorb the water attached to the electrode, and do not wipe it with force. When installed, the glass bulb of the glass electrode is slightly higher than the reference electrode.

⑶After measuring water samples containing oil or emulsified substances, clean the electrode with detergent and water in time. If the electrode is scaled by inorganic salts, soak the electrode in (1+9) hydrochloric acid. After the scale is dissolved, rinse it thoroughly with water and place it in distilled water for later use. If the above treatment effect is not satisfactory, you can use acetone or ether (absolute ethanol cannot be used) to clean it, then treat it according to the above method, and then soak the electrode in distilled water overnight before use.

⑷ If it still doesn't work, you can also soak it in chromic acid cleaning solution for a few minutes. Chromic acid is effective in removing adsorbed substances on the outer surface of glass, but it has the disadvantage of dehydration. Electrodes treated with chromic acid must be soaked in water overnight before they can be used for measurement. As a last resort, the electrode can also be soaked in 5% HF solution for 20 to 30 seconds or in ammonium hydrogen fluoride (NH4HF2) solution for 1 minute for moderate corrosion treatment. After soaking, rinse it fully with water immediately, and then immerse it in water for later use. . After such severe treatment, the life of the electrode will be affected, so these two cleaning methods can only be used as an alternative to disposal.

44. What are the principles and precautions for using calomel electrode?

⑴The calomel electrode consists of three parts: metallic mercury, mercury chloride (calomel) and potassium chloride salt bridge. The chloride ions in the electrode come from potassium chloride solution. When the concentration of potassium chloride solution is constant, the electrode potential is constant at a certain temperature, regardless of the pH value of the water. The potassium chloride solution inside the electrode penetrates through the salt bridge (ceramic sand core), causing the original battery to conduct.

⑵ When in use, the rubber stopper of the nozzle on the side of the electrode and the rubber cap on the lower end must be removed so that the salt bridge solution can maintain a certain flow rate and leakage by gravity and maintain access to the solution to be measured. When the electrode is not in use, the rubber stopper and rubber cap should be put in place to prevent evaporation and leakage. Calomel electrodes that have not been used for a long time should be filled with potassium chloride solution and placed in the electrode box for storage.

⑶ There should be no bubbles in the potassium chloride solution in the electrode to prevent short circuit; a few potassium chloride crystals should be retained in the solution to ensure the saturation of the potassium chloride solution. However, there should not be too many potassium chloride crystals, otherwise it may block the path to the solution being measured, resulting in irregular readings. At the same time, attention should also be paid to eliminating air bubbles on the surface of the calomel electrode or at the contact point between the salt bridge and the water. Otherwise, it may also cause the measurement circuit to break and the reading to be unreadable or unstable.

⑷During measurement, the liquid level of the potassium chloride solution in the calomel electrode must be higher than the liquid level of the measured solution to prevent the measured liquid from diffusing into the electrode and affecting the potential of the calomel electrode. The inward diffusion of chlorides, sulfides, complexing agents, silver salts, potassium perchlorate and other components contained in the water will affect the potential of the calomel electrode.

⑸When the temperature fluctuates greatly, the potential change of the calomel electrode has hysteresis, that is, the temperature changes quickly, the electrode potential changes slowly, and it takes a long time for the electrode potential to reach equilibrium. Therefore, try to avoid large changes in temperature when measuring. .

⑹ Pay attention to prevent the calomel electrode ceramic sand core from being blocked. Pay special attention to timely cleaning after measuring turbid solutions or colloidal solutions. If there are adherents on the surface of the calomel electrode ceramic sand core, you can use emery paper or add water to the oil stone to gently remove it.

⑺ Regularly check the stability of the calomel electrode, and measure the potential of the tested calomel electrode and another intact calomel electrode with the same internal fluid in anhydrous or in the same water sample. The potential difference should be less than 2mV, otherwise a new calomel electrode needs to be replaced.

45. What are the precautions for temperature measurement?

At present, the national sewage discharge standards do not have specific regulations on water temperature, but water temperature is of great significance to conventional biological treatment systems and must be paid great attention to. Both aerobic and anaerobic treatment are required to be carried out within a certain temperature range. Once this range is exceeded, the temperature is too high or too low, which will reduce the treatment efficiency and even cause the failure of the entire system. Particular attention should be paid to the temperature monitoring of the inlet water of the treatment system. Once the inlet water temperature changes are found, we should pay close attention to the changes in water temperature in the subsequent treatment devices. If they are within the tolerable range, they can be ignored. Otherwise, the inlet water temperature should be adjusted.

GB 13195--91 specifies specific methods for measuring water temperature using surface thermometers, deep thermometers or inversion thermometers. Under normal circumstances, when temporarily measuring the water temperature in each process structure of the wastewater treatment plant on site, a qualified mercury-filled glass thermometer can generally be used to measure it. If the thermometer needs to be taken out of the water for reading, the time from when the thermometer leaves the water to when the reading is completed should not exceed 20 seconds. The thermometer must have an accurate scale of at least 0.1oC, and the heat capacity should be as small as possible to make it easy to reach equilibrium. It also needs to be calibrated regularly by the metrology and verification department using a precision thermometer.

When temporarily measuring water temperature, the probe of a glass thermometer or other temperature measurement equipment should be immersed in the water to be measured for a certain period of time (usually more than 5 minutes), and then read the data after reaching equilibrium. The temperature value is generally accurate to 0.1oC. Wastewater treatment plants generally install an online temperature measuring instrument at the water inlet end of the aeration tank, and the thermometer usually uses a thermistor to measure the water temperature.