Abstract: To prevent the formation of encrustations on a pH sensor in a combustion exhaust or flue gas washing apparatus, alkaline additives are added to the wash water which, upon contacting SO.sub.2 in the gases, form sulfite particles. The sulfite particles are prevented from depositing at the bottom of a collection vessel (11) in form of a mud, and, in accordance with the invention, rather are continuously stirred and recirculated in a closed recirculation loop to a spray device (25). The particles, upon being recirculated, will grow to form a granulate which, eventually, will deposit, the granulate, however, being constituted by granules substantially in excess of those which form a mud; the larger granules also prevent deposition of encrustations along the walls, and, for example, a pH sensor (47).