A buffer solution is one that resists pH change on the addition of acid or alkali. Such solutions are used in many biochemical experiments where the pH needs to be accurately controlled.
Buffer solution definition:
A buffer solution (more precisely, pH buffer or hydrogen ion buffer) is an aqueous solution consisting of a mixture of a weak acid and its conjugate base, or vice versa. Its pH changes very little when a small amount of strong acid or base is added to it.
From the Henderson-Hasselbalch equation, the pH of a buffer solution depends on two factors: one is the pKa value and the other the ratio of salt to acid.
This ratio is considered to be the same as the amount of salt and acid mixed together over the pH range 4-10, where the concentration of hydrogen and hydroxyl ions is very low and can be ignored. Let us take as an example acetate buffer consisting of a mixture of acetic acid and sodium acetate:
CH3COO–H+ ⇔ CH3.C00– + H+
CH3COO–Na+ → CH3C00- Na+
Since acetic acid is only weakly dissociated, the concentration of acetic acid is almost the same as the amount put in the mixture: likewise, the concentration of acetate ion can be considered to be the same as the concentration of sodium acetate placed in the mixture since the salt is completely dissociated.
Types of Buffer Solutions
There are two types of buffer solutions,
- Solutions of single substances: The solution of the salt of a weak acid and a weak base. Example: ammonium acetate (CH3 COONH4, acts as a buffer.
- Solutions of Mixtures: These are further of two types,
- Acidic buffer: It is the solution of a mixture of a weak acid and a salt of this weak acid with a strong base. Example: CH3COOH+ CH3COONa–
- Basic buffer: It is the solution of a mixture of a weak base and a salt of this weak base with a strong acid. Example: NH4OH + NH4Cl–.
- Buffer action: Buffer action is the mechanism by which added H+ions or OH– ions are almost neutralized; so that pH practically remains constant. Reserved base of buffer neutralizes the added ions while the reserved acid buffer neutralizes the added OH–
- Examples of buffer solutions:
- Phthalic acid + potassium hydrogen phthalate
- Citric acid + sodium citrate.
- Boric acid + borax (sodium tetraborate).
- Carbonic acid (H2CO3) + Sodium hydrogen carbonate (N2HCO1). This system is found in blood and helps in maintaining of the blood close to 7.4 ( value of human blood lies between 7.36 – 7.42; a change in pH by 0.2 units may cause death).
- N2H2PO4 + Na1PO4
- NaH2PO4 + Na2PO4
- Glycine + HCl
- The pH value of gastric juice is maintained between 1.6 and 1.7 due to the buffer system.
Buffer capacity is a measure of the efficiency of a buffer in resisting changes in pH. Conventionally, the buffer capacity is expressed as the amount of strong acid or base, in gram-equivalents, that must be added to 1 litre of the solution to change its pH by one unit.
Buffer solution calculations
Below are basic Buffer solution calculations
What is the pH of a mixture of 5 ml of 0.1 M sodium acetate and 4 ml of 0.1 M acetic acid?
Concentration of CH3COO– = 5/9 x 0.1M
Concentration of CH3COOH = 4/9 x 0.1M
pkd of acetic acid at 25°C = 4.76
Therefore pH 4.76 + log 5/4
pH = 4.76 + (+ 0.097)
pH = 4.86
How is the pH changed by adding 1 ml of 0.1 N HCI to the above mixture?
The addition of HCI provides H+, which combines with the acetate ion to give acetic acid.
This reduces the amount of acetate ion present and increases the quantity of undissociated acetic acid, leading to an alteration in the salt acid ratio and hence to a change in pH.
Concentration of CH3C00– = 5/10 x 0.1 x 1/10 x 0.1 = 0.04M
Concentration of CH3COOH = 4/10 X 0.1 X 1/10 X 0.1 = 0.05M
pKa = 4.76
pH = 4.76+ log 0.04/0.05
pH = 4.76 + (–0.097)
pH = 4.66
The pH of the solution has been reduced from 4.86 to 4.66 a change of only 0.2 of a unit, where if the HCl had been added to distilled water, the pH would be 2. The solution has therefore acted as a buffer by resisting pH change on the addition of acid.
What is Buffer Value?
Buffer solutions vary in the extent to which they resist pH changes, and in order to compare different buffer solutions.
Van Slyke introduced the term buffer value. When acid or alkalis are added to a buffer solution, a titration curve is obtained.
The slope of this curve is given by dB/DPH where dB is the increment of strong acid or base added in g equ/1 and DPH and the change in the pH increment in a positive pH change, Likewise, dB is negative when acid is added causing a negative change in pH.
As can be seen from the titration curve illustrated, the buffer value is a maximum at the pKa and in practice, buffer solutions are used over a pH range of 1 on either side of the pKa value.