So was reading a magazine for competitive exams and this question is bugging me a lot.

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How is pka values of a base associated with the pH of the solution. The solution for it is that the Nitrogens would remain this way and only the H from OH would be dissociated. That's obvious because the medium is basic and the acidic H would be dissociated, but what about the nitrogen atoms? How is the pka of a base represented? Is that the equilibrium constant for it's conjugate base?


The pKa's quoted for the amine groups are for the protonated forms of the amines. The protonated forms would add a +1 charge to the molecule in solution. Since amines are fundamentally basic, the protonated amine is a conjugate acid.

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  • $\begingroup$ But we use pka for bases too right? And how does it explain which amine will be protonated and which would not? $\endgroup$ – jatin Mar 20 '16 at 21:24
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    $\begingroup$ No the pKb value is for a base to accept a proton. For aqueous solutions: $$\mathrm{pKa + pKb = 14}$$ $\endgroup$ – MaxW Mar 20 '16 at 21:35
  • $\begingroup$ Ok I get that but how would you explain which amine will protonated and which not at a particular pH? $\endgroup$ – jatin Mar 20 '16 at 22:00
  • $\begingroup$ That is a different question which I side stepped on purpose. The short answer is that I would look at the information provided in the question (image). // The longer and more complicated answer is that histidine has a side chain imidazole group. The imidazole group is a planar aromatic heterocycle which is amphoteric. see the wikipeda article en.wikipedia.org/wiki/Imidazole $\endgroup$ – MaxW Mar 20 '16 at 22:16
  • $\begingroup$ Hey in that article its written pka for imidazole and pka for its conjugate acid given separately. But you told that pka is only for acids and conjugate acids?? $\endgroup$ – jatin Mar 20 '16 at 22:26

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