[reproman]

By various zwitterionic buffers in varying ratios, one can create a buffer solution to meet their specific needs (optimize maximal buffering capacity...or pKa).  How would one calculate the new pKa value after combining various buffers.  For example, if I combine TES and MOPS at a 2:1 ratio (pKa of each at 7.4 and 7.2 at 25C), what is the final pka of the solution?

[Borek]

pKa is not the property of the solution.

[reproman]

not sure i understand.  one can determine pKa by doing a dose titration.   i was simply wondering if one could calculate this without the dose titation if you knew the pka values of the buffers ahead of time (sigma and other companies list the pKa values for the buffers they sell...so it seems like it's a property).

is it as simple as averaging (1:1 of HEPES:MOPS pKas of 7.2 and 7.4 (given for the compounds my manufacturer).  Would the new pKa be 7.3)

[Borek]

one can determine pKa by doing a dose titration

You determine pKa of what?

Or perhaps better question: what is pKa?

[savy2020]

pKa is the property of a particular substance. Not of a mixture.

Do you mean to calculate pH of the resultant solution?

[reproman]

yes.  if i mix one buffer (TES), which has a pKa listed of 7.4 and another buffer (MOPS), which has pKa of 7.2, and put them into solution, what is the resulting pKa of that buffering solution.  If one would dose titate a TES buffered solution, you would see one curve and you could take the midpoint of the steep slope and it would align to ~7.4...or the pKa value.  The same would hold true for MOPS, which would be ~7.2.  If I mixthe two buffers, you would see the plot moves to a point between the TES-only and MOPS-only buffers.  The resulting pKa of the mixed buffer solution is between that of the single buffer solutions.  This is a way to fine-tune or adjust the pKa or optimal buffering capacity of a buffer solution. I was simply wondering if one could calculate this new pKa value of the mixed-buffer solution, rather than doing the tedious dose-titrations.  Is ist as easy as simply averaging the reported pKa values? 

[Borek]

what is the resulting pKa of that buffering solution

You were told several times and by different people that pKa is not the solution property, yet you are still trying the same approach.

Do you know what pKa really is? And no, it is not a number on the buffer bottle.