Laboratory pH meter to measure the pH of a solution? How to calibrate the pH meter?
1. Working principle Leici PHS-3E pH meter is composed of main box, keyboard, large-screen LCD liquid crystal display, multi-functional electrode holder, E-301 pH compound electrode and power switch. It measures pH using the potentiometric method. The potentiometric method uses the selective response of the glass electrode to the change of the concentration of hydrogen ions in the aqueous solution. It is combined with the reference electrode in the solution to form a "chemical battery", and the pH value of the solution is determined by measuring the change of its electromotive force.
2. Working environment
The ambient temperature required for the normal use and storage of the pH meter is 5~40°C, and the relative humidity does not exceed 85%. It needs a stable power supply, no other magnetic field interference except the earth's magnetic field, and no vibration and corrosive gases around.
The operating environment of the instrument should be kept clean, and the experimental instruments should be placed neatly. Laboratory countertops need to be dry and hygienic to prevent water samples from being artificially polluted. Smoking is strictly prohibited in the laboratory, food or other irrelevant items cannot be brought into the laboratory, and fire safety should be paid attention to.
Acid rain observers must have a serious work attitude, a meticulous work style, and neat and clean experimental habits. Observers wear special work clothes when entering the laboratory, and wash hands in time before and after work. Pay attention to your own safety during operation, and do not splash the chemical reagent solution on the body, equipment and work surface. Before use, make sure that the instrument is installed correctly, the grounding is good, the connection between the power supply and the electrode must be firm and reliable, and be careful not to be polluted by corrosive gases. After the operation is over, it is necessary to disconnect the power supply in time and insert the short-circuit plug into the socket to prevent dust and water vapor from entering; when the instrument is not in use for a long time, it should be covered with a clean rag or a special cloth to prevent dust from entering the instrument.
3. Precautions during measurement Before starting the equipment, check whether the power supply is firmly connected, and pay attention to check whether there is enough protective liquid inside the composite electrode and the electrode protection cap. When measuring, it must be carried out in accordance with the regulations and the measurement steps stipulated by the station, and the measurement action should be light and steady. The leading wire of the electrode should be kept still, otherwise it will cause unstable measurement. After preheating, the electrode needs to be calibrated before each use of the instrument. The Leici PHS-3E pH meter has the ability to automatically identify standard buffer solutions, and can quickly and automatically identify 4.00pH, 6.86pH, and 9.18pH standard solutions. ” key, you can directly press the “OK” key to complete the calibration without adjusting the data.
There are two methods of calibration: one-point calibration and two-point calibration. The article uses the two-point calibration method. When calibrating, the order of "positioning first, then slope" must be followed. Generally, the neutral standard liquid is used for positioning, but the slope of the standard liquid should be selected according to the range of the precipitation pH value of each station over the years, and the standard liquid close to this range should be selected for slope calibration. Sometimes the pH reading is unstable, in addition to the instrument itself, it may be caused by the wrong calibration method. At this time, the method of two-point calibration can be adopted repeatedly to make the instrument reach the best state.
The measurement accuracy of the pH meter will drift with time and the number of times of use. The station should regularly go to the department with relevant verification qualifications for verification every year to ensure the normal operation of the instrument.
4. The use and maintenance of composite electrode The composite electrode is an important component of this model of pH meter. Whether it is normal or not is directly related to the accuracy of the measurement data. During daily operation and maintenance, you should pay attention to the following.
First of all, the composite electrode should be discarded in time when it leaves the factory for 2a or reaches 1a after use. Secondly, before using the composite electrode, the sensitive glass bulb of the electrode should be checked. Under normal circumstances, the electrode should be transparent without cracks; the sphere should be filled with solution without air bubbles, otherwise it will affect the measurement accuracy, if there are air bubbles, the internal air bubbles should be removed like a thermometer. Since the glass bulb is so thin, any breakage will render the electrode useless. When it is confirmed that everything is normal, remove the electrode protective cover, and avoid the sensitive electrode glass bulb from contacting with hard objects during operation, and avoid long-term immersion in distilled water, protein solution and acid fluoride.
When in use, the rubber stopper on the top of the composite electrode should be removed; the height of the filling liquid inside the electrode is generally at 2/3, if the filling liquid is too little, an external reference supplementary solution of saturated potassium chloride should be added. After cleaning the electrode, do not wipe the glass membrane with filter paper, and use filter paper to absorb moisture to prevent cross-contamination and damage to the glass membrane. In addition, the leading end of the composite electrode must be kept clean and dry to prevent short circuit at both ends of the output, otherwise it will cause measurement inaccuracy or failure.
At the end of the measurement, the electrode protection should be installed in time, and a small amount of external reference replenishment liquid should be placed in the electrode sleeve to keep the electrode bubble moist. If the composite electrode is not used for a long time (more than two weeks), before using it again, the measuring end of the electrode should be immersed in 3mol/L potassium chloride solution for more than 2 hours before use, otherwise it will cause potential drift.
