[Jamie]

Hey,

Usually I find the lab work easy enough to work out when I use the Internet and text books for assisstance.  A bunch of questions from the latest one give me headaches though.

Okay, it was an experiment about buffers.  We put sodium acetate and histidine in two seperate experiments, and analyzed how the pH changed as the volume of HCl added increased.  We also had NaOH in the solution with the buffer.

Okay, so the questions I'm not sure about...

How can you tell the region of maximum buffering on a Ph vs. volume graph for a buffer?

How can you work out how many regions of buffering in total there are on the same sort of graph?  Once you do know how many regions (I think it's 4 by the way, but I'm really not sure) how would you know what pH(s) of the histidine to suggest would be the most suitable buffer?   We've been given data on the fact four forms of histidine exist, at four different pH's - 12, 8, 4 and 2.
Also, we need to use this graph to work out exact pKa values for these forms of histidine.

Finally, two equivelants of NaOH were added along with the histidine monohydrochloride?  Anyone have any idea why it'd be neccesiary to do that?

Also, Does anyone know how to solve this question...

The combined concentrations of H2CO3 and HCO3- in blood plasma is approximately 1.25mmol.L-1 (pKa for the H2CO3/HCO3- equilibrium is 6.1).  Calculate the concentration of HCO3- in plasma at 7.4.

I think it might involve the Henderson Hasselbach equation, but I'm not sure how to use it for this.


Thanks in advance for any help

[Arkcon]

There's more than a few questions in your text block, and I'm not ready, right now, to pick them out.  Lets start with the basics --

How can you tell the region of maximum buffering on a Ph vs. volume graph for a buffer?

Ok, what do you expect a buffer graph to look like, vs a non buffer graph.  Nevermind what you did -- you have two beakers, one with water and one with a buffer, you add strong acid (or base) to each, dropwise.  You monitor pH, with a meter or indicator solution.  What do you expect to see, in each case, and how does your experiment resemble these theoretical experiments.