Question

Titration of Cysteine:

The Pka values for Cysteine are 1.8, 8.3, and 10.8. Consider the titration curve for Cysteine shown below. At which point of the curve will the structure be below the >99% structure of Cys in the solution?

Answer 1

In a titration I understand that when 1 mole of base has been added for each mole of acid, the equivalence point is reached, where pH = pKa.

I also know that the percentage of acid + the percentage of base adds up to 100%. I also understand that as the acid is progressively converted to its conjugate base as the titration process proceeds.

I am thinking at point could be found by taking: aka of Cys = 8.3

pH = pKa + log (base/acid)
pH = 8.30 + log (99%/1%)
pH = 10.30

So I am thinking the answer is E or D?

Answer 2

or would it be B because:

Titration fraction tells us how far we have proceeded with the titration. At the equivalence point we have added stoichiometric amount of titrant and titration fraction equals 1 (or 100%, when it is expressed as titration percentage).

It may seem that more convenient will be to use titrant volume to measure titration progres. That's not a case. For example it is much easier to say "pH at 50% titration equals pKa of monoprotic weak acid" then to calculate each time volume of titrant added to get to the same point.

Note that in the case of polyprotic acids 100% titration may either mean that only first proton was fully neutralised, or that acid was completely neutralised. Thus depending on the definition used fully titrated phosphoric acid is titrated either 100% or 300%. We will use the latter number throughout the site.

Answer 3

D is where almost all of the second proton (at the thiol group) would be titrated.