Using weak base for strong acid titration is completely off. This way you can determine pKb of the base, but only if you will add 50% excess of titrant.Thanks Borek. What does the above mean? Do you mean determining pKa of strong acid is impossible?
Possible, but not by titration. Strong acids have pKa well below 3 (limit I have signalled earlier).
Even after adding 50% more titrant: weak base?
You are so confused I have no idea what to start with...
pH = -log([H+
Strong acid - full dissociation, so if you have solution of strong monoprotic acid concentration of H+
is identical with acid concentration, and pH = -log(acid concentration).
During titration you are neutralizing H+
and what you are left with is a mixture of salt and fully dissociated strong acid. pH = -log(concentration of strong acid left). No place (or need) for pKa. After all acid has been titrated, pH depends only on the added excess of base.
Weak acid - partial dissociation, described by the acid dissociation constant Ka. pKa = -log(Ka). When you add strong (fully dissociated) base you neutralize acid - and you are left with salt and not yet neutralized acid in the solution. It happens that if the acid is weak enough (pKa > 3) pH can be calculated using so called Henderson-Hessalbalch equation
pH = pKa + log(concentration of neutralized acid/concentration of not neutralized acid)
When exactly half of the acid is neutralized pH = pKa. It works reasonably correctly only for acids with pKa within the 3-11 range.
After full neutralization, pH of the solution once again depends only on the concentration of excess base.
This picture is slightly simplified, but it shpould be enough to understand what's going on.
For bases replace Ka with Kb and pH with pOH (pOH+pH=14), otherwise they behave identical.
Can I dilute the acid in question, say ten thousand fold to get a a lower pH before titration?
If you dilute acid pH will go up, not down.
BTW: I was wrong, it should be 100% excess of titrant, not 50%. My mistake. But it still doesn't apply to your problem. You can't easily determine pKa of strong acid with titration, no matter if you use strong or weak base.