Chemical Processing - July 2008 - (Page 32)

MANY PROCESSING operations depend upon pH measurement. Yet, too often plants ﬁnd it challenging to get accurate readings. So, we’ll discuss the variety of factors — ranging from the nature of the pH sensor to process conditions — that come into play. First, though, let’s start with some history. In 1909, Søren Peter Lauritz Sørensen proposed a method to describe the acidity or basicity of solutions [1]. It’s based on the interaction of two water molecules to form hydronium and hydroxide ions: H2O + H2O → H3O+ + OH(1) A solution is considered neutral when it contains 10-7 moles/L hydronium (protons or H+ ions); he termed this pH 7. e most alkaline solution has an H+ ion concentration of 10-14 mole/L, hence a pH of 14. e scale runs from 0 to 14. At pH 14, the sensor is measuring H+ ions beyond the PPQ level. dronium ion concentrations to produce an electrical potential proportional to the concentration — this is continuously compared with an internal pH 7 buﬀer. At pH 7, the electrical potential is the same on the outside and the inside of the glass membrane, yielding a zero potential diﬀerence. erefore, we are measuring zero potential at pH 7. (Broken pH sensors also can give zero electrical potential — so, check for ﬂuctuations in pH to indicate the sensor is actually responding.) In alkaline and acidic solutions, the potential reﬂects changes in the concentration of hydrogen ions at the outer surface of the glass measuring electrode. As the diﬀerence in hydronium ion concentration builds up junction potential, the alkali metal in the measuring glass transfers the charge onto a silver wire. (Glass, which sometimes is called an amorphous solid or pseudo solid, possesses the properties of an electrolyte.) Figure 2 depicts the electron ﬂow of a pH measurement circuit loop. In actuality, most pH probes today use combiHOW DO PH SENSORS WORK? nation electrodes, with the reference electrode and To answer this question, it’s important to understand and recognize the functions of the pH sensor and its measuring electrode within a single glass body. is pH electrode is immersed in the process liquid and parts (Figure 1). electrolyte ﬂows out from the reference junction and e glass membrane responds to changing hyJULY 2008 CHEMICALPROCESSING.COM 32

Table of Contents Feed for the Digital Edition of Chemical Processing - July 2008

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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