Careful measurement of the pH of pure water samples is critical due to their low conductivity, which can lead to fluctuations in the liquid junction potential of the pH electrode system. This can ultimately cause drift in response time and inaccurate, irreproducible results. Successful measurements depend on the choice of sensor. The type of glass membrane, the shape of the glass tip, and the type of junction are all factors that play an important role in the suitability of a sensor to the application.
pH measurements with a typical sensor membrane, which has low sensitivity to hydrogen ions, result in compromised accuracy and variable readings. It requires specialized sensors, such as InLab Pure Pro-ISM or InLab Pure, are required to address the technical aspects of pH measurement of pure water with respect to accuracy, precision and response time.
In order to support pH analysis of pure water samples, METTER TOLEDO offers an Application Note and a White Paper. With recommendations for the best-suited sensors and tips and tricks on best measurement practice, as well as correct electrode maintenance, these documents will help you to obtain reliable results and prolong sensor life.
Below, we explore the challenges that may result when measuring the pH of pure water samples, and provide information on why the pH of pure water should be measured and guidelines for a correct sensor selection for every application.
What challenges do you face when measuring the pH of pure water?
Watch this video to learn how using the right sensor can simplify measurements while ensuring accurate and reliable results.
Why are pH measurements of pure water useful?
The pH of water serves as a proxy for its purity, revealing the presence of soluble trace minerals and indicating the likelihood that the water will cause corrosion, can sustain aquatic life or is safe to drink. Hence, to address this concern, the American Society for Testing and Materials (ASTM) describes the pH measurement of water and low-ion water samples in the D1293 and D5464 standards.
Where is the pH of pure water measured?
Water purity is an important parameter in a number of industries and sectors, including pharma, food and beverage, life science, power generation and many others – which all require pure water for day-to-day laboratory testing and activities.
The measurement of pH in pure water samples requires special attention due to their low ionic nature (conductivity of 100 μS/cm). Pure water has a pH value of 7.00 at 25 ºC; however, samples are especially prone to CO2 absorption from the atmosphere, which alters the pH from the expected value of 7.00 toward 5.20 due to carbonic acid formation. To minimize this effect, METTLER TOLEDO recommends pH readings in a flow-through cell.
Finding the right sensor
One way to mitigate the challenges associated with measurement of pure water is to use a specialty electrode. METTLER TOLEDO offers two electrodes ideally suited to samples with low ionic concentration. With the InLab Pure Pro-ISM and InLab Pure sensors, the measurement of purified water samples is simplified and the results are more precise compared to measurements with standard pH electrodes.
To learn more about pH measurement of pure water samples, download the following documents: