There has long been a debate among my colleagues about measuring the pH of aqueous colloids. As I see there is no general truth in the matter, so I ask this question below.
What is the best and most reliable method for measuring the pH of
- aqueous surfactants,
- aqueous polyelectrolyte and
- aqueous surfactant/polyelectrolyte systems?
You can use a regular glass electrode pH meter.
However, if you are at high ionic strength (>10mM) you need to account for hydrogen ion activity. This is true for any pH measurement - not just colloids - but no-one these days seems to know that.
Refer to: Sørensen,S. P. L.; Linderstrøm-Lang, K.; Lund,E. The Influence of Salts upon the Ionisation of Egg Albumin. J. Gen.Physiol. 1927,8(6), 543−599.
For modern application and importance, see: https://dx.doi.org/10.1021/acs.langmuir.0c01694
(Determination of Protein Charge in Aqueous Solution Using Electrophoretic Light Scattering: A Critical Investigation of the Theoretical Fundamentals and Experimental Methodologies)
Hello, first use paper indicator for preliminary detection of pH, for prevention any damage of electrode. If your pH from 2 to 12 range you can measure by pH meter with glass electrode as mentioned above. Good luck!
Dear John Francis Miller
I support your answer, but want to comment further your " if you are at high ionic strength (>10mM) you need to account for hydrogen ion activity." In pH measurements you don't need to make any corrections for ionic strength. By definition pH is -log10(activity of H+), but not -log10[H+]. pH meter measures the-log10(activity of H+). The concept of pH is much more complex "but no-one these days seems to know that." By convention, pH-meters are calibrated by the standard solutions with the ionic strength 0.1M. This is beyond of the title question and could be discussed in another thread. You can open it by asking a question "what is a definition of pH and how to measure it."
My answer to the original question. Yes, you can measure the pH of aqueous colloids by a pH meter, but you have to understand what a pH meter measures.
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