Can someone help me with this problem? I know the formula is pH=pka + log [A-]/[HA]. Are the pk values the same as pka? What is the acid value and conjugate base value? I'm not sure how to go about this problem. Any insight would be appreciated!

 

the amino acid histidine has 3 hydrogens that could be titrated with pk values of 1.82, 6.0 and 9.17. Describe the pH ranges in which histidine could be used as a buffer and explain why histidine might be a good candidate for buffering the intracellular pH of cells

The symbol pK in this context is the same thing as pK_a. I am not sure what you mean by acid value and conjugate base value. What do you know about buffers?

I guess I am trying to figure out what values I put in to solve for pH. If I put in 6.0 as the pk value, what values do I put in for A- and HA?

Really; what do you know about buffers?

Ok. Buffers maintain the pH by acting like a half-full reservoir. They either donate or accept protons in order to keep the pH in balance. The Henderson-Hasselbach equation is the 1/2 way point for the buffer. I know the pK is supposed to be close to the pH needed to maintain, K being the equilibrium constant. Am I on the right path?

This bit's good

"I know the pK is supposed to be close to the pH needed to maintain"

Ok. Is my answer as simple as stating that histidine is a good buffer for maintaining the intracellular pH of 6.8 because the pk value of 6.0 is close enough to maintain it?

Yep. though that difference, 0.8 pH units, is probably about as big as you can get away with calling a good buffer.

Great! Thanks for guiding me through it!