I would try the „Ross-Electrode“ (sold by Thermo). This construction used a redox-system as internal and external reference half cells. Thus, avoiding the leakage of silver- or mercury-ions into the sample solution (problematic in biochemistry). Traditional Ag/AgCl or SCE will show a clogging of the diaphragm since complexed silver- and mercury-ions will precipitate as chlorides within the liquid junction due to the dilution with the ultra-pure water. Make sure that the pH-meter has an input impedance of at least 10 to minus 14 amperes.

Thank you Dr. Cammann. I have not tried Ross-Electrode for pH measurement in high purity water. The main criteria of pH measurement in high purity water whose ionic strength is much low is to provide a control minor leaking path of KCl from the pH electrode to the solution. I have got excellent result using a particular brand of Metrohm electode which is designed for that purpose. But the electrode is expensive one and its life period is one year.

Try a combination of a flow-through cell and the electrode you are now employing.

Thank you Dr. Okawa. Will you please elaborate your answer by giving guidelines what type of flow-through cell to be used and how to combine pH electrode to that flow-through cell. If you quote some reference for this I will be very happy.

For example, see this page: http://www.coleparmer.com/Product/pH_electrode_combination_double_junction_sealed_for_flow_through_cell_05662_48/EW-05662-51

You may design and prepare the similar structure for your electrode.

The important point is that the fresh sample water must be flowing continuously. The flow rate is not important.

If you measure the pH in an open vessel, it can be determined indirectly. In this case, the main ionogenic component in solution is CO2. So enough to measure electrical conductivity and temperature. Hydrogen ion concentration is calculated from the equation system