Abstract: An ion sensor probe for constant monitoring of the pH level of a solution in a scrubbing system. The probe includes a tubular housing having a plug member inserted at one end thereof. The plug member supports an ion sensor cell and a reference cell in a side-by-side relationship. Each of the cells has one end exposed for contacting the solution. The plug member encloses the one end of the ion sensor cell to form a chamber which communicates with a bore in the plug member. A cleaning fluid of water and an acidic solution can be independently admitted into the chamber to cleanse the one end of the ion sensor cell. The probe is inserted into a well member shaped to receive the probe and having one end configured so as to permit only a portion of the solution to contact the ion sensor cell.