Do we find the pka’s for these?

Question

To separate the proteins, you need to prepare 1600 mL of a 182 mM buffer at pH 7.8. You check the shelf and find the following reagents: Develop a detailed protocol for preparing this buffer. Note that there are several correct approaches.

Compound	Molar Mass	pKa
Bicine	163.2	8.35
Cacodylic Acid	138	6.27
PIPES	302.4	6.76
Sodium cacodylate trihydrate	214.03
Monosodium PIPES	324.35
Sodium bicine	185.15

The sodium bicine pka is blank space!!!

Answer 1

To prepare an effective buffer at pH 7.8, you want to use a buffer that has a pKa within + or - 1 pH unit of the desired pH, ie pKa = 6.8 - 8.8. So, looking at the pKa values of the available we see that bicine and perhaps PIPES. We will choose bicine buffer as it is clearly within the +/- 1 pH unit desired.

Now, for the preparation of 1600 ml (1.600 L) of 182 mM (0.182 M) buffer.

1.600 L x 0.182 mol / L = 0.291 mols bicine required.

Using the Henderson Hasselbalch equation, we can calculate the ratio of bicine / sodium bicine

pH = pKa + log [Na-bicine] / [bicine]

7.8 = 8.35 + log [Na-bicine] / [bicine]

log [Na-bicine] / [bicine] = -0.55

[Na-bicine] / [bicine] = 0.282

Now that we know the ratio of Na-bicine / bicine, we can calculate moles of each.

Let x = moles Na-bicine

0.282 = x / 0.291 - x

x = 0.0640 mols of Na-bicine

0.291-x = 0.227 mols bicine

Finally, we can now calculate the mass of each (bicine and Na-bicine) needed to make this buffer.

For Na-bicine: 0.0640 mols x 185.15 g / mol = 11.8 g Na-bicine

For bicine: 0.227 mols x 163.2 g / mol = 37.0 g of bicine

Procedure for preparing the buffer:

Weigh out 11.8 g sodium bicine. Weigh out 37.0 g bicine. Dissolve in water and bring the final volume to 1.600 liters.