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> pH measurement faces acid test
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 solutions1. 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. The most alkaline solution has an H+ ion concentration of 10-14 mole/L, hence a pH of 14. The scale runs from 0 to 14. At pH 14, the sensor is measuring H+ ions beyond the PPQ level.
How do pH sensors work?
To answer this question, it’s important to understand and recognize the functions of the pH sensor and its parts. Figure 1
The glass membrane responds to changing hydronium 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. Therefore, 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 combination electrodes, with the reference electrode and measuring electrode within a single glass body. This pH electrode is immersed in the process liquid and electrolyte flows out from the reference junction and forms an ionic bridge completing the circuit (Figure 3). The voltage ratio calculated between the two electrodes provides the pH value.