In colloid solutions your instrument may not measure what pH is- a negative log of concentration (activity) of hydrogen ions. Perhaps you could say more about your slurry.
m777182
I assume that you are using a glass electrode as a pH sensing element. In a colloidal suspension (or solution) containing cationic macromolecules the surface of the glass electrode will be covered partly by H ions and partly by cations and colloids with their particular ionic athmosphere. So it is hard to say what is the contribution of H ions to the potential that the electrode measures.You can easily check it by adding some flocculant: same solution but different pH. Perhaps it would be fair to mention that surfactants could poison your glass electrode for a long time, they adsorb tightly.
The concept of pH was applied to water solutions without solids, colloids or surfactants at the begining where it still works fine and was later extended to practically everything (like meat, cheese, emulsions and even non water based solutions), where it is not allways clear what are you measuring actually: your instrument will allways give some potential, even with good repeatability and stability of the signal. Now you have to decide if you call it pH or something else.
m777182
