An introduction to Buffer Solutions

Buffer solutions are solutions in which the pH value is fairly constant. They can resist the change in pH with the addition of small amounts of acidic or alkaline components. For this property, buffer solutions are in several chemical, biological, and industrial processes. These chemical reagents particularly help in fermentation processes, electroplating, and the manufacture of cosmetic products. Hence, we use these solutions wherever there is a need to maintain definite pH values.

Among the natural buffer solutions, the most relevant example would be human blood. Blood always maintains a pH value of 7.4 irrespective of what food we intake. For this, blood as buffer solutions uses the action of carbonic acid (H2CO3), bicarbonate ion (HCO3-), and carbon dioxide.  Seawater is another natural buffer solution, which maintains a pH value of around 8.2. The complex buffer action of numerous acidic and basic salts present in it makes it a buffer solution.

Buffer solutions are primarily of two types:

   ACIDIC BUFFERS: The solution of a weak acid and its salt containing a strong base is defined as an acidic buffer. And, the salt in acidic buffer solutions is usually sodium salt. The pH value of these solutions is less than 7. An example of acidic buffer solutions is a solution of equimolar quantities of acetic acid (CH3COOH) and sodium acetate (CH3COONa), which is a salt of acetic acid with a strong base NaOH. This solution has a PH value of 4.74.

   BASIC BUFFERS: The solution of a weak base and its salt containing a strong acid is called a basic buffer. The pH value of these solutions is more than 7. An example of basic buffer solutions is a mixture containing equimolar proportions of Ammonium hydroxide (NH₄OH) and Ammonium chloride (NH4Cl), which is a salt of ammonium hydroxide with strong acid HCl. This solution maintains a pH value of 9.25.

Process of Working of Buffer Solutions:

The main characteristic of buffer solutions is that they can resist the change in pH level on the addition of small portions of acid or base. This ability of a buffer is termed as Buffer action. The working of a buffer solution will help us understand the buffer action in different types of solution

1.           BUFFER ACTION OF ACIDIC BUFFER: We’ll consider the buffer solution of acetic acid and its salt, sodium acetate. Now, acetic acid is a weak acid and hence does not ionise much in the solution. But, sodium acetate, on the other hand, ionises almost completely in the solution.

CH3OOH (aq) + H2O CH3COO- (aq) + H3O+(aq)

CH3COONa(aq) CH3COO-(aq) + Na+(aq)

Due to the common ion effect, the presence of sodium acetate suppresses the dissociation of acetic acid. Here, the acetate ions present in the large excess in the buffer solution immediately combine with the H3O+ions to form weakly ionised acetic acid molecules.

CH3COO-(aq) + H3O+ (aq) CH3COOH(aq) +H2O(l)

Thus practically, H3O+ions given by the acid are used by CH3COO-ions in the form of acetic acid molecules. So, the pH value of the solution does not change. 

On the other hand, if a small amount of base, for example, NaOH is added to it, the OH-ions of NaOH combines with the H3O+ions present in the buffer and form water molecules.

Therefore, we can conclude that an acidic buffer is capable of limiting the change of pH value on the addition of a small quantity of acid or base.

1. BUFFER ACTION OF A BASIC BUFFER: Let us consider the buffer solution of ammonium hydroxide and ammonium chloride. Here, ammonium hydroxide is a weak base, and so does not ionise much in the solution. But, ammonium chloride dissociates completely and so the buffer solution has an excess of NH4+ ions. 

NH4OH(aq)NH 4+(aq) + OH-

NH4Cl(aq)   NH4+ (aq) + Cl-(aq) 

On adding a small amount of NaOH to this solution, the OH-ions liberated by NaOH combined with NH4+ions and form a weakly ionised NH4OH. Thus the pH value of this solution remains unchanged. 

Also, if we add a small amount of HCl, the H3O+ions of HCl combine with OH-ions already present in the buffer solution to form water molecules. 

CAPACITY OF BUFFER SOLUTIONS

The buffer capacity is defined as the capacity of a buffer solution to resist a change in its pH. We can measure the buffer capacity of buffer solutions using the equation:

Buffer Capacity =  Number of moles of acid or base added to 1 litre of solutionChanges in pH

Or,                           = dBd(pH) 

Here,  denotes the capacity of a solution

dB = number of moles of acid or base in one litre of buffer solution

and, d(pH) = Resulting change in pH

So, if the change in the value of pH = 1 then we have from the equation,

Buffer capacity = number of moles of acid or base added to 1-litre buffer solution.

NOTE: It has been found that the buffer solutions having the ratio SaltAcidor SaltBase are chemically most effective and do not change pH by adding a small quantity of acid or base.

Conclusion:

Usually, a solution is unable to maintain the value of its pH for a long time or on the addition of acid or base. Buffer solutions, on the contrary, can keep pH values constant. In the above text, we saw how buffers of different types work differently and learnt to calculate the buffer capacity.