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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?

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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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