Sensor for measuring a PH value

A sensor for measuring a pH value of a measuring liquid, the senor comprising:

• a sensor element, which includes a first surface adapted to contact the measuring liquid;
• at least one radiation source configured to emit electromagnetic transmission radiation incident on the sensor element, wherein at least a portion of the transmission radiation is converted into measurement radiation in a region of the first surface by reflection and/or scattering;
• at least one radiation receiver configured to receive the measurement radiation and convert the measurement radiation into electrical signals;
• and
• a measuring circuit connected to the radiation receiver, the measuring circuit configured to determine a measured value representing the pH value of the measuring liquid based on the electrical signals of the radiation receiver,
• wherein the sensor element is configured such that a wavelength of at least a portion of the transmission radiation generates charge carriers in at least one surface region or a near-surface region of the first surface adapted to contact the measuring liquid to effect a photoelectrochemical reaction with formation of hydrogen in the measuring liquid adjacent the first surface adapted to contact the measuring liquid.