pH measurement is the indication of hydrogen ion activity in liquid solution, implying its acidity or alkalinity. A liquid solution with low pH value is acidic, and high pH value is described as caustic. pH measurement is important in industrial applications like chemical processing, pharmaceutical, manufacturing, food production etc.
The most common method to obtain a pH measurement is by measuring a voltage. To measure the voltage, two points with different electrical potential values are required. Therefore, both the reference electrode and the hydrogen ion selective electrode (ISE) are needed when determining pH.
- The reference electrode is designed to maintain a constant electrical potential that is independent of the sample composition and temperature.
- The hydrogen ion selective electrode (ISE) with glass membrane provides an electrical potential that is dependent upon the activity of hydrogen (H+) ions in the sample solution.
A typical modern pH sensor is a combination electrode, which combines both the glass and reference electrodes into one body. Combination electrode is mainly used in laboratories and industrial applications.
The most common type of reference electrode used is the silver/silver chloride (Ag/AgCl) system. The reference electrode is immersed in reference electrolyte. The reference electrolyte must have good electrical conductivity and be chemically neutral. Potassium chloride (KCl) is one of the most used electrolyte solutions.
In a combination pH electrode, the space surrounding the measurement electrode is filled with reference electrolyte and contains the internal reference system. A reference junction (diaphragm) near the bottom of the electrolyte chamber serves as the pathway between the KCl electrolyte solution and the sample liquid. Since the reference electrolyte is a conductive medium, it completes the electrical circuit to the measurement electrode.
- Liquid electrolyte: Liquid electrolyte can typically be refilled, ideal for electrodes with refillable openings. Liquid electrolyte can easily be drained and replaced if the electrolyte becomes contaminated.
- Gel electrolyte: Gelling agent is added to prevent the electrolyte from leaking into the sample solution via the reference junction during measurement. Gel electrolyte is easier to maintain as there is no loss and does not require to refill.
- Polymer electrolyte: Polymer electrolyte is solid and can directly contact the sample solution during measurement. This is usually used in industrial standard sensors electrodes because it is high resistance to pressure and pressure changes and requires less maintenance.
Due to the nature of construction of glass membrane, pH sensor needs to be handled with care during storage, cleaning and calibration.
- Storage: During storage, it is necessary to prevent the glass membrane from drying out. Always keep the glass membrane hydrated and in storage solution (KCl electrolyte).
*IMPORTANT: Do not store electrode in distilled or deionized water, because this will cause ions to leach out. - A pH sensor that has not been used for a long time needs to be conditioned. Soak in 4.0 pH buffer for one day prior to usage.
- Calibration: pH sensors are electrochemical they will lose accuracy and responsiveness over time. This effect can be corrected with calibration to establish the slope and zero point of the electrode. The frequency of calibration depends on the application, with some applications requiring daily calibration while others may require only weekly or monthly calibration. More frequent calibration is recommended when measuring in heavily contaminated, low-ion, acidic, and high temperature liquid solutions.
- 3-point calibrations are typically performed when the sample pH conditions are not well understood. It is performed in the according order: pH 7.0, pH 4.0 and pH 10.0
- Use fresh, unused buffer solution for calibration. Used buffer solution are contaminated, leading to slow pH electrode response or the inability to calibrate.
- Expired buffer solutions should not be used. Carbon dioxide in the air can change the pH of basic buffer solutions.
- Cleaning: Routine maintenance is recommended to ensure accurate calibration and measurement. Salt crystals around the fill cap or by the protective cap are common. Rinse with distilled or deionized water to remove the salt build up. Never wipe the pH electrode with a cloth or any other type of material. Any static formed will harm the electrode.
- Lifespan: pH sensors are consumables that will degrade over time, especially in demanding environments for industrial applications. Oxidizing agents, reducing agents and temperature effects will affect the lifespan of the sensor. Extended exposure to chemicals can result in the formation and accumulation of deposits, coatings, or fouling on the sensor’s surface.
