Question

Over what pH range(s) could valine be used as an effective buffer? Explain your answer, supporting it with what we have discussed about buffers and amino acids

Answer 1

Buffer solution is used to preserve the pH of a medium.

Buffer solutions are of two types. A buffer solution is made of either (a) a weak acid and its salt with a strong base

or (b) a weak base and its salt with a strong acid.

The pH of a buffer solution is governed by the Henderson-Hasselbach equation:

for the buffer of kind (a)

for the buffer of kind (b)

where, pKa = - log(Ka) , Ka = acid dissociation constant

  pKb = - log(Kb) , Kb = base dissociation constant

[salt] = concentration of salt

[acid] = concentration of acid

Now, for a buffer, the buffer capacity () is defined as the amount (number of moles) of an acid or a base which when added to one litre of the guven buffer solution changes its pH by unity.

Buffer capacity,   [positive sign as pH will increase on addition of Base]

or   [negative sign as pH will decrease on addition of Acid]

We will always try to prepare a buffer solution with the maximum buffer capacity possible so that it can be most resistive towards its pH change [well its only job is to preserve the pH!].

Now, for maximum buffer capacity, we will have to take the 1st derivative of   with respect to the added acid or base concentration and equate this to 0 (zero) [remember maxima-minima chapter from mathematics?].

Let, a and b are the concentrations of weak acid and strong base respectively which were mixed to make a buffer solution, where a > b.

Therefore, the concentration of salt(produced form neutralisation of weak acid with strong base) = b

(Since all of the strong base will abstract proton from the acid and form salt)

the concentration of remaining acid = (a - b) (since acid concentration is more than base concentration.)

Now, from Henderson-Hasselbach equation:

Differentiating with respect to b, we get

Therefore,

For , equate

or,

or,

or,

Therefore, for getting maximum amount of buffer capacity we have to take equal amounts of weak acid or base and its salt.

Again from Henderson-Hasselbach equation:

Therefore, for making a buffer solution of a particualr pH (with maximum buffer capacity) we have to take a acid or base with pKa around that pH range.

Similar calculation for buffer solution of kind (b) would yield  

Therefore,

We know, the pKa for alpha-carboxylic group of valine = 2.32

   & the pKa for alpha-amine group of valine = 9.62

Therefore, the pH ranges over which valine can be used as buffer are - (a) 1.32 to 3.32   

(b) 8.62 to 10.62   

The range comes from plugging in the ratio and seperately to get

and respectively in the Henderson-Hasselbach equation.