pH probes, also called pH sensors or pH electrodes, are devices used to measure the liquid pH levels, i.e., the acidity or alkalinity of a solution. pH refers to a measure of the hydrogen ions (H+) concentration in a solution, expressed from 0 to 14 on a scale, where 7 is considered neutral. A pH of less than 7 shows acidity, and greater than 7 indicates alkalinity. A pH probe is of great importance as it determines the solution’s chemical conditions. These sensors come in different shapes and are used in conjunction with pH meters for precise pH readability. Additionally, pH probes come in various types, with some models integrating special functionality, including measuring temperatures and auto-calibration features.
How Do pH Probes Work?
pH probes work based on the potentiometry principle, where the potential difference between the sensing element and a reference electrode is measured and used to determine the solution’s pH. They consist of a glass membrane–sensitive to changes in hydrogen ion concentration. This glass membrane creates an electric potential varying with the solution pH. When the probe is immersed in a solution, the hydrogen ion concentration difference between the internal filling and the tested solution causes an electrical potential difference across the glass membrane. A pH meter gives output in values, indicating the alkalinity or acidity of a solution.
Popular Types of pH Probes
There are various types of pH probes based on their electrodes and materials. Some of the popular types include:
- Combination pH Probes – These are the most popular pH probes used, also known as Glass pH Probes. These probes comprise both the glass electrode for pH measurement and a reference electrode. The sensitive glass membrane at the probe tip is selectively permeable to hydrogen ions (H+). When the probe is immersed in a solution, H+ ions interact with the glass membrane, generating a potential difference.
- Sealed Gel-Filled pH Probes – These pH probes are filled with a gel electrolyte that moistens the internal reference electrode and prevents the reference solution from leaking.
- Half-Cell pH Probes – These probes integrate glass electrodes only and do not have an internal reference electrode. Instead, they rely on a separate reference electrode immersed in a reference solution.
- PEI (Polyetherimide) pH Probes – PEI (Polyetherimide) pH probes, also known as solid-state pH probes or ISFET pH probes, are the most advanced pH sensing technology. These probes use a solid-state ion-sensitive membrane made of Polyetherimide or similar materials.
- PVDF (Polyvinylidene fluoride) pH Probes – These are sensors used for measuring the pH level of a solution. PVDF is a type of fluoropolymer with unique properties, making it suitable for various applications, including pH sensing. These probes are widely used in scientific research, industrial processes, environmental monitoring, and other fields where pH measurement is paramount.
pH Probes and pH Meters: The Key Difference
In most cases, these two devices are often used interchangeably; however, it is not the case. A pH probe is an integral part of a pH meter that does not function without a pH meter. A pH probe measures the solution’s pH value, recording the hydrogen ion voltage and converting it into a readable pH value for the pH meter.
Application Versatility of pH Probes
pH probes measure acid and alkaline levels in solutions and their electrical potential, offering a wide range of use in applications, such as:
- Water Quality Monitoring
- Pharmaceutical Industry
- Food and Beverage Industry
- Research Laboratories
- Education Institutions
- Quality Control
- Aquaculture
- Swimming Pools
- Biomedical and Clinical Applications
- Agriculture Industry
- Environmental Monitoring.
