Measuring pH of low ionic strength solutions

pH meters use the amount of dissolved salts in a solution to make measurement. With plenty of salts, measurements can be made using an electrode and meter quickly and precisely. Samples without those salts, with low ionic strength and conductivity such as pure water, can make measurement difficult. 
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​Care needs to be taken in order to measure the pH of low ionic strength/conductivity solutions. The difference between the concentrations of the electrode’s fill solution and of the sample’s can lead to significant junction potentials, which can lead to poor precision, drifting results and slow stabilisation. Selecting the correct pH probe for the job and following a few steps can significantly reduced error in measurement. 

• The first step to minimise error is to select the correct electrode for the job. The TRUEscience low ionic strength electrode, part number 1222328, is specially designed with low resistance glass for low ionic strength samples. 
• Use low ionic strength buffers for calibration. Poorly buffered samples can be easily contaminated and the pH readings will be altered due to cross contamination and CO2 absorption etc.
• Add ionic strength adjustors to the sample, or 0.30 mL saturated (3-4 molar) KCl solution to 100 mL of sample;
• Use temperature correction during measurements
• Calibration in buffers and sample testing should be done at the same temperature

​For more advice on choosing the correct method for measuring low ionic strength/low conductivity samples please email us at and we can give you advice on your individual application.