Why is it difficult to read the pH of pure water?

A. It can be very difficult to get reproducible pH reading in samples of water that have a low ionic strength, such as distilled or deionised (DI) water. It is common to attain different pH values even with new probes that calibrate perfectly. This is because water has a low conductivity so is of low ionic strength. Water is a bad conductor which means electrostatic interference could potentially occur.

So how do you measure water that is of low ionic strength?

1. Use pH probes that are refillable and are shielded from electrostatic interference. These can be expensive and require daily maintenance but they are specifically designed for the measurement of pure water.
2. Take a small sample of the water to be measured rather than measuring direct into large volumes to avoid any likelihood of electrostatic interference.
3. Minimise the exposure of the sample to air as CO₂ absorption can decrease pH.
4. Add a very small amount of high purity KCl to the sample which will improve response time because the ionic strength will be greater.
5. Keep the glassware of the probe clean. Calibrate daily using high quality calibration solutions and calibrate to three points. Rinse the probe in DI water between uses and during calibration.
6. Use meters that have temperature compensation.