Question

In the case of phosphate buffer, what are the correlation between buffer ph, pKa of the weak acid, and the maximum amount of buffer capacity?

What are we expect to see in the "buffer capacity vs initial ph" plot?

Does change the concentration of the buffers affect buffer capacity at a given Ph? why?

Answer 1

Solution:

The pH of the phosphate buffer is calculated by Hendersen-Hasselbalch equation,

pH = pka + log [salt] /[acid]

Maximum buffer capacity (βmax) is obtained when pH = pka,

Thus,

βmax = 0.576 x C

Where, C = concentration

>>Buffer capacity can be defined as quantity of strong acid or base that must be added to change the pH of one liter of solution by one pH unit.

Thus, increase or decrease in buffer capacity will increase or decrease the pH to one unit.

Thus, the plot between initial pH and buffer capacity (β) gives straight line with respect to pka of acid.

>> Since buffer capacity (βmax) is related with concentration as,

βmax = 0.576 x C

Thus, β max increases with increasing concentration C.