Electrochemistry pH Measurement of Pure Water: Techniques and Solutions for Reliable Results

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Mettler-Toledo GmbH

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Water is the most commonly used solvent in the laboratory. It is available in several grades of purity. An important parameter here is the pH value.

Water is extremely important in many industries and segments, including Pharma, Food and Beverage, Chemical, Biotechnology, and Electronics. In each of these industries, the measurement of pH is one of the most important criteria in water analysis. At different workplaces within these industries, the pH of water implies varied parameters as in estimation of soluble trace minerals, ability to corrode metals, suitability for aquatic life, quality of potable drinking water and so on.

For industries like packaged bottled water, beverages and snack food manufacturers, water analysis is required to evaluate its purity and efficiency for various process and production activities. Water is used as a solvent in many of these industries. In pharma industries, water is used as an excipient or during synthesis, reconstitution, or as a cleaning agent. The quality of water required for a specific application is as per international standards or Pharmacopeia guidelines.

Water used in laboratories are graded as types of water, based on technical specifications, viz Type 1, Type 2 and Type 3. Type 1 has least electrical conductivity (0.056 μS/cm at 298 K) termed as ultrapure water and has the highest purity.

This article explores the challenges associated with pH measurement in pure water, its key applications, and provides guidance on how to select the right instruments to achieve accurate measurements.

Why is the Accuracy of pH Measurement in Pure Water Important

The accuracy of pH levels in pure water is crucial for many reasons.

Baseline for Calibration: Pure water is often used as a reference point for calibrating pH meters and sensors. The accuracy of its pH level helps ensure the instruments’ reliable readings in other solutions.

Scientific Research: In scientific experiments, many chemical reactions and biological processes depend on specific pH levels. Precise pH measurements are important for reproducibility and consistency.

Water Quality Assessment: pH influences the solubility of minerals and the behavior of contaminants in the water, which can impact environmental health and safety.

Regulatory Compliance: Many industries are subject to regulations that require monitoring of water pH. Measurement accuracy ensures compliance with environmental standards and safety guidelines.

Industrial Processes: In industries, such as pharmaceuticals, food and beverage, and chemical manufacturing, the pH of water used in processes can affect product quality and consistency.

Measurement Challenges

Pure water has a pH value of 7.00 at 25 °C. However, pH measurement of pure water samples requires special attention due to its low ionic nature (conductivity of ≤100 μS/cm). These samples lack both hydrogen ions (H⁺) and other conducting ions. Pure water samples are prone to CO₂ absorption from the atmosphere. This leads to carbonic acid formation and makes the pH value drift to 5.5 pH. ASTM D1293 describes the pH measurement of water samples, and ASTM D5464 is specific for measuring the pH of low-conductivity water.

Measuring pH accurately in pure water is challenging, especially when using a conventional pH electrode. Such electrodes normally cause fluctuations during pH measurement due to the high liquid junction potential and lack of sensitivity of the glass membrane. As a result, users may experience increased response times, leading to inaccurate and non-reproducible results.

The membrane type and shape of the pH sensor can affect the sample interaction. Fluctuations and erroneous results lead to high standard deviations during pH measurement. The outflow of the reference electrolyte depends on the type of junction. For pH determination of low ionic samples in pure water, the outflow should be ideal enough to avoid an unstable junction potential.

Some main challenges when using a conventional sensor include:

• Low-ionic sample nature

• Reading fluctuations

• Presence of atmospheric carbon dioxide

Selecting the Right Sensor

Measuring pH in pure water requires a careful approach, and choosing the right sensor is key to getting accurate results. The Inlab Pure Pro-ISM is specifically designed for this purpose, making it an ideal choice for high-purity water applications. This sensor stands out because it features a built-in temperature probe and Intelligent Sensor Management (ISM) technology, which help capture all important measurement details easily.

What makes this sensor particularly effective is its LoT glass membrane, which has a low resistance of less than 50 MΩ at 25 °C. This low resistance enhances sensitivity, even in low ionic environments. Additionally, the sensor's immovable glass sleeve ensures a consistent flow of electrolyte, which is crucial for measuring pH in water with very few ions. The spherical design of the sensor increases its interaction with the sample, resulting in even better sensitivity.

Another advantage is the use of a bridge electrolyte (1 mol/L KCl), which helps minimize junction potential and speeds up response time in low-conductivity water samples.

For comprehensive guidance on measuring pH in low-conductivity and ultrapure water, including sample preparation and electrode calibration, check out the full video on Youtube. To access the application note or white paper regarding pH measurement in pure water, go to the Mettler Toledo website and look for “pH of Pure Water: Specialized Sensors Increase Measurement Performance.”

If you have questions for the author, feel free to email pHmatters@mt.com.

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