Analysis of the reasons affecting the use of electrodes

2.1 Temperature Factors 2.1.1 Effect of Temperature on Glass Electrodes 1. From the expression of the electromotive force of the original battery, it can be seen that the electrode potential is directly proportional to the solution temperature. Within the temperature range used for electrode calibration, compensation can generally be achieved in the converter feedback circuit through temperature electrodes (pt100 or pt1000). 2. Glass electrodes have a high internal resistance (industrial glass electrodes generally have a resistance of less than 500M Ω), which is not only related to the composition and thickness of the glass film, but also to temperature (exponentially increasing resistance for every 10 ℃ decrease in temperature). 3. High temperature will promote the dissolution of soluble parts in the hydration layer on the surface of sensitive glass film, affecting the electrode potential and leading to electrode aging. The aging cycle depends on the composition and temperature of the medium. In the same medium, assuming an active cycle of 100% at 25 ℃, 20% at 80 ℃, and only 5% at 120 ℃. 2.1.2 The Effect of Temperature on Reference Electrode 1. At high ambient temperatures, KCl crystals often precipitate inside the flow rechargeable reference electrode (filled with saturated KCl solution), causing unstable liquid to ground potential of the reference electrode; At the same time, crystallization may block the ceramic plug at the bottom of the electrode, preventing the electrolyte from seeping into the measurement solution and blocking the electrical path. 2. The calomel electrode is susceptible to temperature changes and should be avoided in media with high temperatures or large temperature fluctuations, while the silver chloride electrode can work at much higher temperatures and has higher stability. 2.2 The effect of micro osmotic pressure on the flow reference electrode: The ceramic plug at the bottom of the reference electrode generates an intermediate impedance in the electrical path. When this impedance is greater than 0.1M Ω, it will cause the reference electrode potential to be unstable or drift. Very polluted media can contaminate the electrode surface and block the ceramic plug. For flow reference electrodes, the formation of electrical channels relies on the micro osmotic pressure of the electrolyte inside the electrode, allowing the electrolyte to penetrate into the measurement solution. When the pressure or concentration of the medium is high, the replenishment channel is not smooth, or there are bubbles present, it may hinder the leakage of the electrolyte, increase the impedance in the middle of the electrical path. If the medium infiltrates back into the electrode, it may contaminate the salt bridge and even cause chemical reactions with the electrolyte or internal electrode (such as AgCl+sulfide Ag2S), leading to electrode poisoning. 2.3 The effect of solution acidity and alkalinity on electrodes Glass electrodes do not have a good linear relationship outside of pH 2-pH 9, and are prone to the formation of a large number of hydrated hydrogen ions H3+O in strongly acidic solutions, resulting in a relative decrease in the number of H+ions reaching the electrode surface and an increase in pH. Na+in a strong alkaline medium also participates in the exchange process between H+in the solution and H+on the electrode hydration layer, resulting in an increase in the glass electrode potential and a lower pH value. In addition, in strongly oxidizing media, the loss of alkaline substances (mainly monovalent cations) in sensitive glass films can damage the hydration layer and cause electrode poisoning. An acid resistant electrode can be selected, and the special process measures used in its manufacturing process (special addition of ion formula) enhance the acid resistance of the glass film. At the same time, the zero potential of the electrode corresponds to pH 0=2, thus correcting the linearity within the acidic range. 2.4 Activity of Sensitive Glass Membrane When the pH value of the solution inside the glass electrode is equal to that of the external solution, the potential difference between the two sides of the glass membrane should be zero. However, in reality, there exists an asymmetric potential Ea, which depends on the composition, thickness, and manufacturing conditions of the glass. After soaking the glass electrode in distilled water or acidic solution (0.1N dilute hydrochloric acid) for 24 hours, a hydration layer will form on the surface of the glass film, greatly reducing Ea. At this time, the electrode is in an active state. Correspondingly, when Ea is large, it is referred to as electrode aging. In order to ensure accurate measurement, glass electrodes should be activated before use and regularly activated during use. Due to the small range of electromotive force E generated by the electrodes (corresponding to 60mV for each pH), in order to ensure measurement accuracy, the internal resistance of the measurement system must be much greater than that of the original battery for the impedance of the 2.5 signal cable to ground. The internal resistance of the glass electrode can reach up to 100M Ω at 20 ℃, and the input impedance of the converter can reach 1012 Ω. The coaxial cable connecting the electrode and the measurement system also has high impedance (greater than 107 Ω). If the cable connection plug is contaminated or flooded, or if the cable is corroded or damaged, causing a decrease in impedance, the signal will be short circuited during transmission, making it impossible to measure correctly. 2.6 Environmental magnetic field interference: Due to the high resistance of glass electrodes, even small electromagnetic induction can cause a voltage drop and attach to E, resulting in measurement errors.

3. Common troubleshooting methods

3.1 Aging phenomenon of glass electrodes: response lag; Decreased sensitivity; Zero drift. Processing: 1. Regularly perform electrode calibration. 2. Electrode activation: Aging electrodes can be soaked in a mixed solution of 1M acetic acid and 1M potassium chloride (1:1), activated for 10 minutes, and then removed and cleaned. Electrodes with mild aging can be soaked in distilled water or 0.1N dilute hydrochloric acid solution for 24 hours to activate. 3.2 Glass electrode contamination phenomenon: decreased sensitivity; Measurement deviation. Treatment: 1. The alkaline precipitate on the electrode can be washed away in dilute hydrochloric acid solution, and then the electrode can be cleaned with distilled water. 2. The grease attached to the electrode can be cleaned with hot water and household detergent, or wiped with gauze. 3. Colloidal dirt can be cleaned with strong hydrochloric acid solution, and after washing, pay attention to cleaning the electrode with distilled water. After cleaning with acid, alkaline solvents or organic solvents, the electrode should be soaked in distilled water for a period of time to restore the damaged hydration layer. 3.3 Reference electrode contamination (ceramic plug blockage) phenomenon: measurement value is too high; The indication is unstable. Treatment: Can be cleaned with hot water and household detergent; Oil or alkaline pollutants can be cleaned with organic solvents (such as alcohol) or dilute hydrochloric acid solution; If the pollution is severe, it can be washed with a soft bristled brush or wiped with a filter paper strip. For non flow electrodes, the electrode can be placed in an electrolyte at 80 ℃ until the electrolyte cools down. 3.4 Reference electrode poisoning phenomenon: incorrect indication; Unstable indication; Unable to calibrate. Solution: If only the electrolyte inside the reference electrode is contaminated, replace the electrolyte; If the non flow electrode or internal electrode is poisoned, the electrode needs to be replaced. 3.5 Signal cable to ground impedance reduction (＜ 107 Ω) phenomenon: indicates instability and jumping; Display out of range, unable to measure. Solution: Replace the cable and dry the cable plug or junction box with a hair dryer.

4.1 Quickly determine if the electrode is in good condition. 1. Glass electrode: Connect it to a known good reference electrode and converter, test with two standard buffer solutions, read mV and pH respectively, and verify if it is 60mV/pH. 2. Reference electrode: Connect it to another known good reference electrode and the converter. Connect the test electrode to the working electron (note that both electrodes must be of the same reference system). Immerse both electrodes in a buffer solution at the same time, and through the "zero adjustment" operation, the converter should be able to read a stable 0mV (pH7). 4.2 Electrode Protection 1. Regular Calibration: The electrode should be calibrated regularly during use to ensure the accuracy of measurement values, and faults can be detected in a timely manner through status checks. The calibration cycle depends on the application situation. The glass electrode does not have a Nernst response, and the standard solution pHS used for calibration should be as close as possible to the measuring medium pHX. 2. Regular activation: After using the electrode for a period of time, it should be actively removed for activation and replaced with another set of electrodes for use. This rotation can extend the service life, and the replacement cycle should be 1-2 months lower than the aging cycle. 3. Long term soaking can lead to a decrease in the hydrogen function of the glass film due to the dissolution of soluble parts in the glass film. Therefore, if the glass electrode is not used for a long time, it is better to clean it and store it dry. 4. For electrodes that have been activated after aging or poisoning, they should not be used in process conditions with poor medium environment and high detection requirements. Instead, they can be used for pH detection in aqueous media, which can fully utilize the electrodes and extend their service life. When the electrode sensitivity is below 25mV/pH, it is not suitable to continue using.