Chemical Processing - July 2008 - (Page 33)

pH sensor forms an ionic bridge completing the circuit (Figure 3). The voltage ratio calculated between the two electrodes provides the pH value. Glass electrode. This also is known as the measuring electrode. A special glass acts as a membrane interacting with process H+ ions and the alkali metal ions in the glass membrane. The interaction creates an electrical potential on the glass surface that changes as the hydronium ion concentration changes in the process fluid. Two kinds of glass membranes are mainly available, ordinary and “B” types. Ordinary glass has cross-sensitivity towards other cations such as sodium ions. In salt solutions the pH may be neutral; however excess sodium ions from NaCl ionization can fool the ordinary glass membrane. The Na+ ions mimic the effects of hydrogen ions. As such, the sensor reads a lower than actual pH. This is common with such membranes in highly caustic fluids and is commonly referred to as sodium ion error. “B” glass membranes, which contain a lithium impurity, are less sensitive to alkali metal ions. “B” glass formulation can minimize sodium ion error. Reference electrode. The pH measurement is a half-cell reaction and requires an electrical circuit to be completed. The electrolyte from the reference cell completes the circuit with an ionic bridge. This electrode provides a reference voltage for comparison with the potential delivered from the glass electrode. The electrolyte is pressurized up to 6 bars. Often it’s in polymerized gel form to prevent rapid depletion. Reference junction. The process fluid and the reference electrolyte interface at the reference junction. The junction permits the electrolyte to create the ionic cloud around the glass electrode to complete the electrical circuit. Several types of junction interfaces are available; the longevity of a sensor depends upon selecting the proper reference junction. This can be as simple as an open hole allowing electrolyte interface with the process, or can use porous materials such as ceramic, polytetrafluoroethylene (PTFE) or wood, which allow the electrolyte to dilute into the process. + + + ++ + + + - Silver/Silver chloride reference Potassium chloride Reference junction Ion cloud - Inernal buffer 7.0 Glass membrane H+ Figure 1. A liquid junction pH sensor includes both measurement and reference elements. then use hot pH 4 buffer. There’re many other ways to clean a pH sensor; cleaning is application specific. It’s also important to note that deionized (DI) or inductive data demineralized (DM) water isn’t necessary to rinse the probe; clean tap water works fine. In addition, don’t store the pH sensor in DI or DM water. Doing so actually will shorten the sensor’s life. When the probe is in DI water, the reference fluid that’s in contact with DI water will equalize its ionic strength. This will inductive transmission of energy deplete the electrolyte — it becomes diluted, resulting in shorter probe life. Reference electrode challenges. When the reference voltage deviates from 0 mV, an error in pH measurement occurs. Depending on the process fluid characteristics, the reference electrolyte can get diluted (contaminated) through the interface with the proSenSor iSSueS The nature and care of the sensor and its need for main- cess. The rate of reference contamination can be fast or slow depending on the permeability of the reference tenance and calibration pose a number of challenges. diaphragm. For example, open aperture reference Measuring glass issues. The measuring glass memjunctions may get diluted comparatively faster — but brane can become dehydrated or can get coated with process material. The glass membrane sensitive to pH also have relatively faster response to the pH changes in the process environment. In extreme cases, where changes requires periodic service in many process the reference diaphragm is a large disk-type surface, conditions. To clean, soak it in pH 4 buffer and dab it with a clean cloth. If it doesn’t respond or calibrate, scraping can expose a new surface. 33 chemicalprocessing.com July 2008 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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