Problems that should be paid attention to in the measurement of ph meter
Before use, first check that the electrodes are in good condition. Most of the supporting electrodes used on existing acidity meters (PH meters) are composite electrodes, and the old generation of acidity meters made of calomel electrodes are still in use. In view of the wide application of composite electrodes, the composite electrodes are mainly introduced below.
The composite electrodes used in the laboratory are mainly two types: fully enclosed and non-enclosed, and the fully enclosed type is relatively small, mainly produced by foreign companies. Check whether the glass bulb is broken before use. If there is no break, use pH buffer solution for two-point calibration. The positioning and slope buttons can be adjusted to the corresponding pH value, which is generally considered to be usable. Otherwise, perform electrode activation according to the instructions. The activation method is to immerse in 4% hydrogen fluoride solution for 3~5 s, take it out and rinse it with distilled water, then soak it in 0.1 mol/L hydrochloric acid solution for several hours, rinse it with distilled water, and then calibrate it, that is, use the pH value 6.86 (25°C) buffer positioning, after adjustment, choose another pH buffer to adjust the slope, if it cannot be adjusted, replace the electrode. An external reference solution, namely 3 mol/L potassium chloride solution, must be added to the unclosed composite electrode, so it is necessary to check whether the potassium chloride solution in the electrode is more than 1/3, otherwise it is necessary to add a 3 mol/L potassium chloride solution. If the potassium chloride solution is outside the small hole, remove the excess potassium chloride solution and place it under the small hole, and check whether there are air bubbles in the solution. If there is any, flick the electrode to completely remove the air bubbles .
During application, the rubber skin on the top of the electrode should be peeled off to expose the small hole. Otherwise, negative pressure will be generated during analysis, which will cause the potassium chloride solution to fail to pass through the glass ball and perform ion exchange with the measured solution, resulting in inaccurate measurement data. precise. After the measurement is taken, the rubber should recover and seal the small hole. Before cleaning the electrode with distilled water, it should be immersed in 3 mol/L potassium chloride solution to keep the electrode bulb moist; if it is found that the protective solution has been lost before use, it should be immersed in 3 mol/L potassium chloride solution In the solution for several hours to allow the electrode to reach the best measurement state. During use, it is found that some analysts use the composite electrode as a glass electrode and soak it in distilled water for a long time. This practice is incorrect. It will greatly reduce the concentration of potassium chloride solution in the composite electrode, resulting in poor electrode reaction during measurement. Sensitivity will eventually lead to inaccurate measurement results, therefore, the composite electrode should not be soaked in distilled water for a long time.
Caution when using electrodes
The glass electrode socket should be kept dry and clean, and should not be in contact with acid mist, salt mist and other harmful gases, and should not be stained with aqueous solution to ensure high input impedance of the instrument.
When not measuring, the input signal should be short-circuited to avoid damage to the instrument.
New or unused electrodes must be soaked in distilled water for several hours before use. Make the asymmetric potential energy of the electrode drop to a stable level and reduce the internal resistance.
When measuring, the electrode bulb must be fully immersed in the solution to be measured.
When in use, the internal reference electrode should be immersed in the reference solution, and the internal reference solution must not be poured into one end of the electrode cover, causing the internal reference to be suspended.
When in use, the rubber plug of the electrolyte filling port of the reference electrode should be pulled out, so that the reference electrolyte (salt bridge) can maintain a certain flow rate of seepage by means of gravity, and communicate with the solution to be tested. Otherwise, reading drift will occur.
