how do i create a buffer system with ph=6.4?

Question

Creating a buffer system: 

 

Desired pH = 6.4

What is the best acid to use?

What is the pKa of the acid?

Ratio Ha/A- 

Ratio A-/HA 

I was thinking to use Si(OH)4 because it has a pH of 6.4 but I'm not sure that's right.

edit: Ok, I changed my mind to H2CO3 since its pKa is 6.37? still confused on ratios.

Answer 1

Good decision. That's the idea. Find a weak acid with a pKa close to the desired pH. I didn't look up the pKa for carbonic acid, but I'll trust you that it is 6.37. Now, how to make the buffer.

First, decide on a concentration and a volume. The concentration is somewhat arbitrary but may relate to cost, solubility, or effect on the biological system being studied. Assuming none of these are factors in your decision, we can arbitrarily chose 0.1 M. This usually refers to the combined concentration of the weak acid and the conjugate base. We will also choose a volume of 1 liter. So, to find the ratio of HCO3- / H2CO3, we proceed as follows:

pH = pKa + log [HCO3-]/[H2CO3] ... Henderson Hasselbalch equation

6.4 = 6.37 + log x where x = ratio [HCO3-]/[H2CO3

log x = -0.03

x = 1.0

So, you want to use the same moles of HCO3- and H2CO3 and in the total moles should add to 0.1 if we make 1 L. Thus you would use 0.05 moles H2CO3 and 0.05 moles NaHCO3 in a final volume of 1 L.