pH sensors with porous reference junctions allow KCl to migrate through the junction into the process to complete the electrochemical connection. Since the junction is porous, some aggressive processes can also migrate ions into the reference, causing contamination and premature failure. As the process migrates into the reference it may attack the silver/silver chloride wire, changing its chemistry which results in the electrode drifting. This is seen as an offset away from zero mV during calibration. There are a number of different constituents that can contaminate the reference electrode. Two major contaminates in the industrial market are sulfide (S2-) and cyanide (CN-). Sulfide is commonly found in kraft pulp and paper mills, mines and oil refineries. All of the different liquors in the kraft process have a high sulfide content and therefore, the pulp stock will have a significant amount of sulfide as well. Cyanide, is typically found in mines, metal refineries, and electroplating. The cyanide is for metal separation in flotation cells. Thus, contamination of the reference is an issue for both pH and ORP sensors which are used in these processes.