When there's a weak base substance (hydrophobic powder), and it needs to be completely protonated by adding an acid (eg: HCl), how does one know when it's completely protonated? Since the powdered base substance isn't soluble until protonated, doesn't that mean that it won't be reflected when measuring the pH (via the commonly used pH test strips)?

I'm under the impression that when the base is suspended in water, as HCl is added and given time to react, the pH would float round 7 or 8. Then when it's fully protonated, the pH will start to dip below 7, which would mean there's now an excess of HCl since there's nothing for it to bind with, which would be too much.

I don't exactly have a specific substance base substance in mind. This is more a general question about acid-base reactions involving a solid base.

Thank you


1 Answer 1


Yes, that is correct. As you add HCl, the pH will remain more or less constant at around 5, since in solution there is a salt of a strong acid and a weak base. Then when all the base has been protonated, the pH will fall rapidly.

If you need to know how much insoluble base you have, it must be done through back titration. You add an excess of HCl to the sample and then titrate the remaining free HCl.

  • $\begingroup$ Ah!.. "back titration", exactly what i was looking for! Thank you very much. $\endgroup$
    – Justin
    Commented Oct 29, 2018 at 14:01

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