pH meters measure the acidity or alkalinity of a solution by detecting the concentration of hydrogen ions (H+) in the solution. The pH scale ranges from 0 to 14, where a pH of 7 is considered neutral, lower values indicate acidity, and higher values indicate alkalinity. pH meters work by measuring the potential difference (voltage) between a reference electrode and a glass electrode immersed in the solution.

The electrical potential difference across the glass electrode is influenced by the activity of hydrogen ions in the solution, which is related to the pH of the solution. However, in waters with varying conductivities, there can be some challenges that affect the performance and accuracy of pH measurements. Here's why:

To address these challenges, it's important to properly calibrate and maintain pH meters for different types of water samples. Calibration involves adjusting the meter's readings using standard buffer solutions with known pH values. Additionally, using electrodes specifically designed for high ionic strength solutions or employing correction factors for conductivity can help improve accuracy.

In summary, the conductivity of a solution can impact the behavior of a pH meter due to the interactions between ions and the electrode surface. When working with waters of varying conductivities, careful calibration, electrode selection, and consideration of the specific water chemistry are essential for obtaining accurate pH measurements.

Measuring pH in pure water may seem simple in theory. For example, pure water should be neutral-pH 7.0 and there should be no interferences. Measuring the pH of pure water is challenging because the pH electrode response tends to drift and may be slow, non-reproducible and inaccurate. Measurements in these samples are more difficult because of the low conductivity of the sample, differences between the low ionic strength solutions and normal ionic strength buffers, changes in the liquid junction potential and the absorption of carbon dioxide into the sample. Since pure water solutions have a low conductivity, the solution will tend to act as an “antenna” and the electrode response can be noisy.

The three most common challenges when testing pH in pure water and other low conductivity waters are:

1. Electrode Drift

Low conductivity water is a high resistance sample. This may lead to noisy readings and signal drift.

2. Contamination of Sample

Due to the low concentration of ions in these waters, they are also poorly buffered and so are subject to contamination (e.g. from CO2 or ammonia absorption, and/or cross contamination from other sources, during sampling, transport, storage, handling, and testing).

3. Inaccurate Measurement

The large difference in ionic strength between the electrode filling solution inside and the sample outside the electrode may lead to significant junction potentials. This can affect accuracy, cause long stabilization times, and lead to poor precision.