Problems and precautions encountered when using PH electrodes

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1 Immersion of the pH glass electrode: The pH glass electrode must be immersed in a solution before use, for the following reasons. a. The sensitive membrane of a PH glass electrode can respond properly to hydrogen ions only when a very thin layer of hydrated gel is formed on its surface, so that the relationship between potential and PH value can follow the Nernst equation. b. After being immersed in water, the asymmetric potential of a PH glass electrode decreases and stabilizes, while its internal resistance drops. c. For composite electrodes, immersion in solution helps keep the liquid junction wet and unobstructed, thereby maintaining a stable liquid junction potential. The immersion methods for different electrodes vary. a. For non-composite PH glass electrodes, they can generally be soaked in distilled water (or deionized water), a buffer with a PH value of 4.00, or a 0.01 mol/L hydrochloric acid solution. The soaking time depends on the thickness of the sensitive film, its shape, and the degree of electrode aging. With a thick sensitive membrane, the electrodes can be used for a longer period of time, and their immersion time is also longer, typically ranging from 8 to 24 hours. Due to their structural characteristics, conical electrodes require a longer immersion time. Generally, a higher solution temperature yields better soaking results than a lower one, but it is essential to avoid soaking in alkaline solutions. b. For composite PH electrodes, the immersion solution is different from the one mentioned above; they are usually immersed in the same solution as the external reference solution, and must not be immersed in distilled water or deionized water. If the soaking method is incorrect, the performance of good electrodes will deteriorate. When the composite PH electrode is immersed in distilled water or deionized water, the external reference solution leaks into the distilled water or deionized water through the liquid junction. At this point, the silver complex ions dissolved in the potassium chloride solution re-form silver chloride when the external chloride concentration drops suddenly to near zero, precipitating at the liquid junction and blocking it. This prevents the external reference solution from leaking properly, resulting in unstable liquid junction potentials, increased internal resistance of the electrode, and degraded performance of the electrode; in severe cases, it can even render the electrode unusable. 2 Inspection of PH electrodes 2.1 Visual inspection a. Check whether there are scratches or cracks on the sensitive membrane glass. b. Is the internal reference solution of the electrode cloudy or moldy (with flocculent substances)? c. Whether the liquid junction at the external reference electrode of the composite electrode is blocked can generally be determined by observing color changes. d. Check whether the lead wires and plugs of the electrodes are in good condition; in particular, the plugs of the electrodes should be dry and clean. If the above phenomena occur, it will affect the performance of the electrode. 2.2 Performance testing The main technical specifications of PH electrodes include: the PH value at zero potential, internal electrode resistance, alkali error, response time, and percent theoretical slope (PTS). When the percent theoretical slope of the electrode is below 90%, it is recommended to replace the electrode. In conventional analog circuit pH meters, when adjusting the \"slope calibration\" potentiometer and the displayed value does not reach the standard value required for the calibration buffer solution, it is necessary to replace the electrode. In pH meters equipped with microprocessors, the percent theoretical slope is automatically calculated by the computer. When the percent theoretical slope of the electrode is below 90%, it is recommended to replace the electrode to ensure measurement accuracy. 3 Measurement of pH value at low temperatures
a. At low temperatures, the internal resistance of pH glass electrodes increases dramatically (by several orders of magnitude). This mismatch between the instrument’s input impedance and the electrode’s resistance leads to measurement errors. Additionally, the increased internal resistance often gives rise to interference signals. b. At low temperatures, the response of pH glass electrodes slows down, and the time required to reach equilibrium increases. C. At low temperatures below zero degrees, a PH electrode with low internal resistance must be used. 4 Measurement of the pH value of pure water: Due to the low ionic strength of pure water, the electrode responds slowly and it is difficult to reach equilibrium. Moreover, pure water has a low electrical conductivity and high internal resistance; it exhibits insulating properties and is prone to interference from external electromagnetic fields, which affects the stability and accuracy of measurements. When measuring the pH value of pure water, the effect of the junction potential becomes significant. The composition of the electrode calibration solution differs greatly from that of the solution being measured (pure water), and the junction potential of the reference electrode varies considerably between these two solutions, leading to large measurement errors. Therefore, when measuring the pH value of pure water, electrodes with low internal resistance are used to improve response speed and reduce electromagnetic interference. Either seal the measuring cell or employ a flow measurement method to prevent contamination of the pure water and stabilize the liquid junction potential. 5 Measurement of pH value in municipal wastewater: Municipal wastewater mainly falls into two categories: industrial wastewater and domestic wastewater. Urban wastewater contains a large amount of particulate and lumpy suspended solids. These suspended solids are distributed quite uniformly, resulting in high turbidity. Wastewater discharged by industries often contains colorants, high turbidity, oils, oxidizers, reducers, or strong acids and bases, all of which can interfere with measurements. The glass electrode method is commonly used to determine the pH value of such wastewater; this method can eliminate the interference caused by colorants, turbidity, oils, oxidizers, and reducers. However, for wastewater containing strong alkalis—especially when the pH value exceeds 10—a “sodium error” occurs, leading to a lower-than-actual reading. In such cases, a specially designed glass electrode with low sodium error should be employed. The pH value of urban wastewater varies widely and is greatly affected by temperature; therefore, instruments with automatic temperature compensation can be used to reduce errors caused by temperature changes. At the same time, using a glass electrode that is compatible with the pH meter or has the same brand and model can also reduce measurement errors. Before measuring urban wastewater, it is necessary to calibrate the instrument using a standard buffer solution with a pH value similar to that of the water sample, in order to minimize measurement errors. This approach can also reduce the \"sodium error\" caused by strong bases, thereby improving measurement accuracy. When measuring the pH value of urban wastewater, glass electrodes are prone to contamination by organic substances, especially oils. In such cases, they should first be cleaned with dilute hydrochloric acid, followed by rinsing with pure water after removing the inorganic salts; however, anhydrous alcohol should not be used as it will degrade the electrode’s performance. After cleaning, the electrode should be soaked in water for a day and night to allow a hydration layer to form on its surface before use. 6 Lifespan of pH electrodes The lifespan of pH electrodes depends on the operating conditions and whether they are used correctly. a. Measuring medium. Including cleaning solutions, viscous liquids, suspensions, strong acids, strong bases, and non-aqueous solutions. Some of them are highly corrosive to the sensitive membrane of PH electrodes. Such as strong acid and strong base solutions, especially strong base solutions; some of them will adhere to the electrodes, such as viscous liquids. The service life of the electrode in this medium is shorter than that in a clean solution. b. Operating temperature. The operating temperature of the electrode should be selected within the normal operating range; if it is used frequently at the upper limit of this range, the electrode’s lifespan will be shortened and it will suffer severe damage. The higher the temperature, the greater the degree of damage. C. Cleaning and storage. After using the electrode, care should be taken to clean the sensitive membrane; for composite electrodes, extra attention must be paid to cleaning the liquid junction area. Once cleaning is complete, the electrode membrane area should be stored in the specified electrode protection solution. When using PH electrodes, analysts should regularly summarize experiences and lessons, follow standard operating procedures, and maintain the instruments properly in order to provide accurate and reliable analysis results in a timely manner.