[gambit]

Out of curiosity, how could you tell from a titration curve, if an the acid had more than one ionizable hydrogen.

[Arkcon]

Well, what do you see if it has just one?  And can you take  a guess what happens when there are two ionizable hydrogens?  Some unlucky bastard had to figure this out for themselves a hundred or so years ago, so whe could all have the answer now.

[gambit]

Well, when there is one ionizable hydrogen atom then there is one dramatic change in pH. If there are two hydrogen's then would be two dramatic changes.

[Borek]

There can be two visible changes, there can be one, depending on the dissociation constants for both protons. So while in some cases this approach will work, in others it will not.

Think about neutralisation stoichiometry and mass of equivalent, as compared to molar mass.

[cliverlong]

Well, when there is one ionizable hydrogen atom then there is one dramatic change in pH. If there are two hydrogen's then would be two dramatic changes.

Dramatic change IF at least one of the acid/alkali is strong. See Borek's middle diagram for titrating a weak acid against a strong alkali

Look at section
Titration curves for weak acid v weak base
in
http://www.chemguide.co.uk/physical/acidbaseeqia/phcurves.html
for example of a pH curve without a dramatic change


Hello Borek,

Sorry I don't (fully)  understand why sulphuric acid has one equivalence point - even though it is a diprotic acid. Is it because when sulphuric acid ionizes in water, both protons pass into solution and are available to combine with OH^- ?


Thanks

Clive

[Borek]

Dramatic change IF at least one of the acid/alkali is strong.

More like "strong enough". Maleic acid has pKa1=1.8, so it it is not very strong.

I am not aware of a simple rule that will help you say whether the acid will have two distinct inflection points or not. Such a rule will have a form "first pKa at least x and difference between pKas at least y", you may try to find it out by yourself using BATE (no need to purchase the program, free trial will suffice).

Sorry I don't (fully)  understand why sulphuric acid has one equivalence point - even though it is a diprotic acid. Is it because when sulphuric acid ionizes in water, both protons pass into solution and are available to combine with OH^- ?

pKa2=2.0, so it is still relatively strong acid. At pH 2.0 it is already 50% neutralised.