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)
In my opinion should be:
1. Centrifugation (separate the nanometric phase)
2. Filtering (filter the micrometer residual)
3. Extraction (very slowly, avoid turbolence in the fluid)
4. Measure with pH-meter
In this case is better for you to measure the zeta potential and the pH of the whole soultion. Then, according with literature, you can do your deductions.
The true question for me should be: what is the pH of colloidal suspension? You can see the colloidal suspension as a single-phase fluid or multi-phase interacting and stable solution.
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