Monitoring device for a biogas plant

Monitoring means (24) for a biogas plant (10), comprising: - a removal device (19) which it is able to extract a sample of biogas in a biogas reactor (12) of the biogas plant (10) biogas generated; - to retain a water retention means (20) which is adapted in the biogas sample water contained; - a sensor device (22) which is adapted to determine an acid content in the retained water.