How Does a pH Electrode Work?

Understanding how a pH electrode works is essential for accurate pH measurement in various applications. This article reviews what a pH electrode is, how it works, various types of pH electrodes, and their applications.

What is a pH Electrode

A pH electrode is a sensor for measuring the potential of hydrogen ions (H⁺) in a solution. Its measurement indicates the acidity or alkalinity of the solution on a logarithmic scale ranging from 0 to 14. On this scale, lower values (less than 7) signify higher acidity, whereas higher values (over 7) represent alkalinity. At a neutral pH of 7, the concentration of hydrogen ions is equal to that of hydroxide ions (OH⁻), which is the pH of pure water.

pH Electrode Working Principle

The operation of a pH electrode relies on the generation of an electrical potential difference between two electrodes in a solution. Although different sensor manufacturers may develop unique measurement technology, they usually follow a similar process, which the following sections highlight.

pH Electrode Construction an Ion Exchange

Generally, pH electrodes consist of two main components: a sensing electrode and a reference electrode. The sensing electrode typically consists of a thin-walled glass bulb that is sensitive to changes in H⁺ activity. On immersing a pH electrode into a solution, hydrogen ions interact with the glass membrane, thus, generating a potential difference. On the other hand, the reference electrode is there to provide a stable reference potential. It typically contains a silver/silver chloride (Ag/AgCl) electrode in a potassium chloride (KCl) solution. When the reference electrode is in contact with the test solution, it allows ionic contact without affecting the sample pH. The pH and reference electrode are connected to a pH meter which calculates the difference in voltage between the two electrodes. Then, it converts this voltage into a pH reading for display on the meter.

Voltage Production

Due to the ionic interaction at the Sensing electrode glass bulb interface, there is voltage production, which is proportional to the pH of the solution. For example, a higher concentration of hydrogen ions leads to a higher positive voltage, indicating a lower pH (more acidic). Conversely, a lower concentration of hydrogen ions results in a lower positive voltage, which corresponds to a higher pH (more alkaline).

Signal Interpretation from a pH Electrode

One key aspect of pH measurements is in interpreting the hydrogen ion activity from the voltage measurements. At the heart of this is the Nernst equation, which describe the relationship between ion concentration and potential difference as follows:

   

In the equation above, E represents the measured voltage, E₀ is the reference potential, R is the gas constant, T is the temperature in Kelvin, n is the number of electrons transferred, and F is the Faraday constant.

Types of pH Electrode

Different applications require various types of pH electrode designs, highlighting the versatility of this essential tool. The following sections highlight some of the most common designs.

Combination pH Electrodes

Combination electrodes are the most widely used type, integrating both the measuring and reference electrodes into a single unit. This design simplifies the measuring process, making it ideal for routine pH measurements in laboratories and industry.

Single-junction vs. Double-junction Reference Electrodes

• Single-Junction Electrodes: These electrodes have a single reference junction, suitable for general-purpose applications. However, they can be problematic when measuring solutions containing reactive substances like heavy metals or proteins.
• Double-Junction Electrodes: Featuring two reference junctions, these electrodes provide additional protection against contamination of the reference system. As a result, they are ideal for harsher environments, prolonging the lifespan of the electrode.

Micro pH Electrodes

Micro pH electrodes are designed for small sample volumes or confined spaces, such as in microbiology or chromatography applications. These electrodes allow for precise measurements in small vessels, thus, ensuring accurate results without requiring larger volumes of liquid.

Specialized Electrodes

The design of these electrodes allows them to take measurements in specific environments, such as in harsh chemicals, or deep immersion. For example, antimony electrodes are suitable for measuring pH in hydrofluoric acid solutions, and flat surface electrodes are ideal for gel or paste samples. Another common special electrode is the high-temperature pH electrode, which is effective in environments exceeding 100°C. Thus, making them suitable for processes like sterilization and chemical manufacturing.

Solid-State and ISFET Electrodes

Solid-state pH electrodes, such as Ion Selective Field Effect Transistors (ISFET), utilize a semiconductor chip instead of glass. These electrodes are more robust and can provide quick response times, therefore, making them suitable for food samples or harsh environments where traditional glass electrodes may break or fail.

Applications of pH Electrodes

Because pH is an important parameter in various laboratory, biomedical, and industrial processes, its application cuts across almost all industries. The following sections just highlight a few.

Water Treatment

In water treatment facilities, maintaining optimal pH levels is essential for both the quality of drinking water and the effectiveness of wastewater treatment processes. So, pH electrodes are key to monitoring and controlling pH levels to ensure efficient chemical dosing and regulatory compliance.

Food and Beverage Industry

"(《世界人权宣言》) pH levels in food significantly affect their taste, texture, and shelf life. Thus, pH electrodes serve in monitoring acidity during processes like fermentation, pasteurization, and bottling, ensuring product quality and safety.

Chemical Manufacturing

In chemical manufacturing, pH electrodes facilitate real-time monitoring, allowing manufacturers to adjust processes dynamically and deliver products according to pH specifications.

Pharmaceutical Applications

The pharmaceutical industry utilizes pH electrodes, especially for drug formulation and quality control. For example, in labs and production lines, having a pH electrode is necessary to ensure that active pharmaceutical ingredients are within acceptable acidity ranges.

环境监测

Environmental scientists use pH electrodes to study water bodies, assessing the impact of pollution and natural processes on aquatic ecosystems. Monitoring changes in pH levels can indicate shifts in water quality, guiding conservation efforts.

Maintenance of pH Electrodes

To ensure accurate and reliable measurements, regular maintenance of pH electrodes is essential. Key practices include:

• Calibration: Regular calibration with standard buffer solutions (e.g., pH 4, pH 7, and pH 10) is crucial for maintaining accuracy.
• Cleaning: Depending on the measurement solution type, electrodes may need cleaning to remove residues or contaminants that can affect performance.
• Hydration: pH electrodes must be kept moist. If an electrode dries out, it can damage the glass membrane, leading to inaccurate readings.
• Proper Storage: When not in use, pH electrodes should be stored in a suitable solution (usually a storage solution or pH buffer). This will ensure that they remain hydrated and ready for accurate measurements.

Selecting the Right pH Electrode with Alpha

At Alpha, we offer a variety of standard and customizable pH electrode options with features that make our products stand out in the industry. Some of these include:

• Wide pH Range: Measure acidity and alkalinity from 0 to 14, with an accuracy of ±0.02 pH.
• Fast Response Time: Achieve readings in just 10 seconds for timely adjustments in critical processes.
• Minimal Drift: Enjoy a low drift of ±0.05 pH over 24 hours, ensuring consistent reliability.
• Versatile Temperature Range: Operate effectively in temperatures from -5 to 135°C, accommodating diverse environments.
• Durability Options: Select from HF resistant and Tuff-Tip options to withstand harsh conditions.
• Custom Mechanical Assemblies: Tailor your electrode configuration to meet specific requirements.
• Diverse Housing Materials: Choose from various materials such as Ryton®, Epoxy, Kynar®, and others for optimal performance.
• Advanced Reference Systems: Benefit from both single and double junction designs for dependable stability.
• Customizable Electrolytes and Sensor Shapes: Adapt to your application needs with options for different shapes and electrolyte types.
• Analogue and Digital Output: Easily integrate into systems with a range of output options including millivolts, 4 – 20 mA, and RS-485 MODBUS RTU.