Question

3-16. Tris base was added to water to the final pH 10.0. Calculate the concentration of Tris in the solution (use Table 3-4).

3-17. Calculate the amount of NaOH that should be added to 1 L of 0.1 M solution of HEPES in order to shift the pH from 7.4 to 7.5.

Common name	Full compound name	pKa at 25 oC	Buffer range
TAPS	3-{[tris(hydroxymethyl)methyl]amino}propanesulfonic acid	8.43	7.7-9.1
Bicine	N,N-bis(2-hydroxyethyl)glycine	8.35	7.6-9.0
Tris	tris(hydroxymethyl)methylamine	8.1	7.5-9.0
Tricine	N-tris(hydroxymethyl)methylglycine	8.05	7.4-8.8
HEPES	4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid	7.55	6.8-8.2
TES	2-{[tris(hydroxymethyl)methyl]amino}ethanesulfonic acid	7.4	6.8-8.2
MOPS	3-(N-morpholino)propanesulfonic acid	7.2	6.5-7.9
PIPES	piperazine-N,N'-bis(2-ethanesulfonic acid)	6.76	6.1-7.5
Cacodylate	dimethylarsinic acid	6.27	5.0-7.4
SSC	saline sodium citrate	7.0	6.5-7.5
MES	2-(N-morpholino)ethanesulfonic acid	6.15	5.5-6.7

Answer 1

3-16. The acid-base equilibrium of TRIS is:

With a Ka value of:

The equilibrium constant is given by:

Where, due to the stoichiometry of the reaction and the pH = 10:

So, the concentration of Tris is:

So, the total added concentration of Tris was:

3-17. We can use the Henderson-Hasselbach equation:

We can re-arrange the equation to get:

We also know that:

Combining these equations:

So, we have that:

This is the concentration of conjugate base we need in our buffer system, The way to achieve it is adding NaOH to deprotonate the original HEPES. Since we have 1 L and NaOH weights 40 g/mol: