Chemical Processing - July 2008 - (Page 34)

5 50 55 60 Reference electrolyte contamination. e eluting of the reference electrolyte out of the reference diaphragm into the process to form an ionic cloud can lead to dissimilar ions in the reference electrolyte. When the junction potential is changing due to reference contamination, it will appear as a drift in pH. e solution to this ranges from using a diﬀerent type of junction material, to multiple junction reference assemblies or a pressurized free-ﬂowing reference/junction. Pseudo halide ions situation. When reduced sulfur or cyanide is present, with porous or open aperture junctions these pseudo halide ions migrate into the reference electrode, contaminate the reference electrolyte and combine with the elemental silver wire to form silver sulﬁde. Excessive sulﬁde ions may form a highly water insoluble silver sulﬁde coating on the wire. Using double junction reference electrodes (Figure 4) with KNO3 in the ﬁrst reference chamber will help overcome the sulﬁde and cyanide ion problems. A potassium nitrate solution prevents the sulﬁde or cyanide ions from getting into KCl reference chamber. Solid state sensor. It’s possible to avoid glass issues by opting for a sensor based on the ion selective ﬁeld eﬀect transistor (ISFET), which was developed in the early 1970s. Such sensors particularly suit food and pharmaceutical processes. However, ISFET technology poses its own set of challenges. e sensor’s measuring surface is reduced to approximately a 2-mm-dia. circle. Processes with high suspended solids can create problems with this limited measuring surface area. Installing the sensor at a 45° angle to the process ﬂow can Completing the circuit mitigate this situation. Signal transmission. Most sensors provide analog mV signals; such signals are susceptible to galvanic interference, which we’ll Glass electrode discuss shortly. It’s possible to avoid such Ag/AgCI problems by choosing KCI electrolyte a sensor that digitizes the signal before sending it to the analyzer. Figure 2. Most pH probes combine the two electrodes within a single glass body. Figure 5 shows such a sensor that uses our Memosens technology. ese sensors also feature an inductive coupling of the pH sensor and cable, allowing the data and power transmission to travel bidirectionally. THE PROCESS ENVIRONMENT Proper sensor selection is the key to long probe life and good performance. Combination pH probes are expected to respond very quickly, almost 90% of a pH step change in 10 seconds. is requires a constant free-ﬂowing and pure reference electrolyte and a hydrated measuring glass. In choosing a sensor, you must consider process dynamics, conditions and surroundings. Factors to watch out for include: • coating of the glass membrane; • liquid junction plugging; • ﬂuid velocity; • temperature and pressure of the measured ﬂuid; • abrasion, drying and aging of the glass membrane; • galvanic isolation; • calibration and storage; and • hypoionic processes (low ionic strength). Let’s look at several of these. Coating. As the sensor remains in contact with process ﬂuid, that ﬂuid starts depositing onto the measuring glass surface. Periodic cleaning of the glass membrane with soap and water will remove such deposits. Automated cleaning and calibration systems are available and may be useful in explosion-proof locations or ones posing health hazards. Transmitter Reference electrode Ag/AgCI KCI electrolyte t t pH glass membrane diaphragm JULY 2008 CHEMICALPROCESSING.COM 34 http://chemicalprocessing.com

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Chemical Processing - July 2008 Contents From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor Who’s a Big Hit? Succeed with Condensate Control Take Some Basic Steps with pH Measurements Steam Projects Provide Fast Payback Process Puzzler Plant InSites Equipment & Services Product Spotlight/Classifieds/Ad-Lits Ad Index End Point

Chemical Processing - July 2008

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