Analysis of common problems encountered in the use of industrial pH meter electrodes
Effect of Temperature on Glass Electrode
1. It can be seen from the expression of the electromotive force of the primary battery that the electrode potential is proportional to the solution temperature. In the temperature range of the electrode calibration, it can generally be compensated in the converter feedback circuit through the temperature electrode (pt100 or pt1000).
2. The glass electrode has a high internal resistance (industrial glass electrode resistance is generally less than 500MΩ), its size is not only related to the composition and thickness of the glass film, but also related to the temperature (exponential relationship, every time the temperature drops by 10 ° C, the resistance value approximately doubled).
3. High temperature will promote the dissolution of the soluble part in the hydration layer on the surface of the sensitive glass membrane, which will affect the electrode potential and cause the electrode to age. Its aging cycle depends on the medium composition and temperature. In the same medium, it is assumed that the activity cycle is 100% at 25°C, 20% at 80°C, and only 5% at 120°C.
Effect of Temperature on Reference Electrode
1. In the case of high ambient temperature, KCl crystallization will often occur inside the flow-type rechargeable reference electrode (filled with saturated KCl solution), resulting in unstable liquid junction potential of the reference electrode; at the same time, the crystallization may block the ceramic at the bottom of the electrode The plug prevents the electrolyte from leaking into the measurement solution and blocking the electrical pathway.
2. Calomel electrodes are susceptible to temperature changes and should be avoided in media with high temperature or large temperature fluctuations, while silver-silver chloride electrodes can work at much higher temperatures and have higher stability.
Influence of micro-osmotic pressure on flow reference electrode
The ceramic plug at the bottom of the reference electrode produces an intermediate impedance on the electrical path. When this impedance is greater than 0.1MΩ, it will cause the reference electrode potential to be unstable or drift. Very dirty media contaminate the electrode surface and block the ceramic plug. the
For flow reference electrodes, the formation of electrical channels depends on the micro-osmotic pressure of the electrolyte in the electrode, allowing the electrolyte to penetrate into the measurement solution. When the pressure or concentration of the medium is high, the rehydration channel is not smooth, or there are air bubbles, etc., it may hinder the extravasation of the electrolyte and increase the intermediate impedance of the electrical path. If the medium reverses into the electrode, it will pollute the salt bridge and may even Chemical reaction with electrolyte or internal electrode (for example: AgCl + sulfide → Ag2S) to poison the electrode.
Effect of pH of solution on electrodes
The glass electrode does not have a good linear relationship outside pH2~pH9, and it is easy to form a large amount of hydronium ions H3+O in a strong acid solution, so that the number of H+ reaching the electrode surface is relatively reduced, and the pH value increases. Na+ in the strong alkaline medium will also participate in the exchange process of H+ in the solution and the H+ on the electrode hydration layer, resulting in an increase in the potential of the glass electrode and a low pH value.
In addition, in a strong oxidizing medium, the loss of alkaline substances (mainly monovalent cations) in the sensitive glass membrane will damage the hydration layer and cause electrode poisoning. An anti-acid electrode can be selected, and the special technological measures (specially added ion formula) adopted in the manufacturing process have enhanced the anti-acidosis ability of the glass membrane, and at the same time, the zero potential of the electrode corresponds to pH0=2, so that the linearity in the acid range can be obtained. Correction.
Activity of sensitive glass membranes
When the pH value of the solution in the glass electrode is equal to the pH value of the outer solution, the potential difference between the two sides of the glass membrane should be zero, but in fact there is an asymmetric potential Ea, and its size is related to the composition, thickness and production conditions of the glass. After immersing 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 membrane, thereby greatly reducing Ea, and the electrode is in an active state at this time. Correspondingly, when Ea is large, it is called electrode aging. In order to make the measurement accurate, the glass electrode should be activated before use, and must be activated regularly during use.
