Take Some Basic Steps with pH Measurements

By Bhupen R. Patel, and Fred Kohlmann, Endress+Hauser, Inc.

ChemicalProcessing.com

A number of factors contribute to achieving high performance of pH loops

Learn more about pH measurement by visiting these articles, books and whitepapers > pH measurement faces acid test > Whitepaper: Theory and practice of pH measurement > Cotton, A. F. and G. Wilkinson, “Advanced inorganic chemistry, a comprehensive text,” 3rd ed., p. 320, Wiley Eastern, Mumbai, India (1972). > “Handbook of chemistry and physics,” 66th ed., p. F-253, CRC Press, Boca Raton, Fla. (1985-86). Kohlmann, F., “Understanding pH in practical terms,” internal technical document, Endress+Hauser, Greenwood, Ind. (2007). [Available upon request.] > Skoog, D. A., “Principles of instrumental analysis,” 3rd ed., p. 574, Thomson Learning, Stamford, Conn. (1985). > Slowinski, E., and W.L. Masterton, “Chemical principles,” 2nd ed., p. 428, W.B. Saunders, Philadelphia (1969). > Tanis J.N., “ Procedures of industrial water treatment,” Ltan Inc., Ridgefield, Conn. (1987).

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.

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.