The problem:
Measuring the pH value in seawater with glass electrodes requires a special sensor design to avoid errors due to the high ionic strength of the
seawater. When using conventional reference electrodes with ceramic diaphragms, problems can also occur due to the high and variable junction potentials that arise under pressure. For this reason,
pH electrodes have previously only been offered for depths of up to 1,500 m. In addition, it was sometimes a bit cumbersome to use two channels of the probe system for pH measurement – one for the glass electrode and one for the reference electrode.

The solution: A new combination electrode for submersible probe systems

The pH combination sensor consists of a reference electrode and a pH-sensitive glass electrode in one housing to save a free channel of the probe system. In order to enable accurate measurements even in the deep sea, a double membrane was used for the reference electrode to avoid problems with signal stability under pressure changes.

The interface reference electrode/sample is realized in this case by means of a hole membrane.
Inside the reference electrode is a KCl-containing gel
with a special built-in second membrane containing the Ag/AgCl reference system
in a potassium chloride solution. The pH combination sensor for
In-situ determination of pH is available as a deep-sea version (up to 600
bar) without an integrated temperature sensor.