Pulp fibers get caught in the area around the glass, clogging the electrode. Once clogged with pulp fibers the reference and glass to no longer have contact with the process, so pH measurements wander. There are two different electrode modifications to help alleviate the clogging problem. One is a flat pH glass tip (IC Controls sensor option – 6). Benefits of Flat Glass: – reduces crevices to clog, – minimizes direct impact on glass. Disadvantages of Flat Glass; surface is parallel to flow so does not contact liquid sample as well, – glass has 90º angle which can break do to thermal stress. Another proven pH sensor design incorporates a domed pH glass in a pulp modified tip (IC Controls sensor option – 73), to ensure better contact with the solution while protecting the tip from the shearing action of the stock and clogging. Benefits of Pulp Modified: – Domed pH Glass stronger in thermal stress applications, – Tip Protected from shearing of dense stock, – De watering on the electrode edge for better pH response, – Flow over surface on domed glass provides more accurate pH results.
To combat reference contamination, double junction and/or plasticized references are used to slow down the migration of the sulfide or cyanide to the reference wire. The second junction (IC Controls sensor option – 7) will to increase the path length of the contaminants to the reference wire. The longer the migration path, the longer it will take to reach the reference wire, extending the life of the reference. The second junction also forms a chamber which will slow the migration ions, again extending reference life. A second contamination deterrent is a plasticized reference solution (IC Controls sensor option – 8). The plasticized reference impedes ion migrates as a solid as opposed to a liquid. The more solid the reference solution, the longer contaminant migration will take thus protecting the silver/silver chloride wire.
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.
If the pH response becomes sluggish and the slope of the probe starts to get too low, IC Controls Electrode Renew Solution can be used (P/N A1100092). The electrode renew solution will skim off a micro-layer of the coated glass, thereby exposing a new glass surface which will exhibit better pH response.