pH electrodes are pretty much just "wet cell" batteries, usually made of a thin film of a proprietary chemical deposited on glass or ceramic material. They output a small voltage that varies with the activity of a specific ion (H+ in the case of water). A very high-impedance amplifier would be able to convert the output of an electrode to an impedance and level compatible with a common ADC. Electrodes are made for immersion and also to be inserted into a pipeline, for example to monitor wastewater discharge leaving a plant. They usually have a BNC or similar two-conductor connector.
