What is a pH buffer solution?
The mission of the pH buffer solutions is to maintain the pH of the solution containing them invariable (constant) in the against of dilutions and addition of acids or bases.
These solutions are made up of mixtures of:
-Weak acid + salt of weak acid-strong base: CH3COOH + CH3COONa
-Weak base + salt of a strong acid-weak base: NH4OH + ClNH4
Example 1
Let us see how a buffer acts to maintain the pH of a solution:
Buffer solution CH3COOH/CH3COONa
Acetic acid dissociation
CH3COOH -> CH3COO– + H+
Ka = [CH3COO–]·[H+]/[CH3COOH]
(shifted to the left)
Dissociation of sodium acetate
CH3COONa -> CH3COO– + Na+
(shifted to the left)
the dissociation of the acid can be considered to be strongly shifted to the left as a consequence of the presence of CH3COO– from CH3COONa.
On the other hand, the hydrolysis process of the CH3COO–
CH3COO– + H2O -> CH3COOH + OH–
It is strongly shifted to the left due to the coexistence in the medium CH3COOH
If a strong acid is added, the protons from the dissociation of the acid disappear from the medium because they react with the acetate ions to form poorly dissociated acetic acid, the CH3COO– ions constituting the so-called alkaline reserve of the solution.
CH3COO– + H+ -> CH3COOH
On the other hand, if a strong base is added, the OH– ions disappear and are neutralized by the acetic acid, the CH3COOH molecules constituting the acidic reserve of the solution.
CH3COOH + OH– -> CH3COO– + H2O
Let’s see what is the pH of this buffer solution:
Ka = [CH3COO–]·[H+]/[CH3COOH]
[H+] = Ka·[CH3COOH]/[CH3COO–]
pH = -log Ka –log[CH3COOH]/[ CH3COO–]
= pKa + log([CH3COO–]/[ CH3COOH])
in general
pH = pKto + log([sal]/[ácido])
Example 2
Buffer solution NH4OH/ClNH4
-dissociation of the base:
NH4OH -> NH4+ + OH–
Kb = [NH4+]·[OH–]/[ NH4OH]
(shifted to the left because of the presence of NH4+ from total dissociation of ClNH4
ClNH4 -> NH4+ + Cl–
(shifted to the left)
El proceso de hidrólisis del NH4+
NH4+ + H2O -> NH4OH + H+
It is highly shifted to the left because it coexists in the medium NH4OH
If a strong acid is added, the process takes place:
NH4OH + H+ -> NH4+ + H2O
NH4+ ions constitute the so-called alkaline reserve of the solution.
If a strong base is added:
NH4+ + OH– -> NH4OH
the NH4+ molecules constitute the acid reserve of the solution.
Let’s see what is the pH of this buffer solution:
Kb = [NH4+]·[OH–]/[NH4OH]
[OH–] = Kb·[NH4OH]/[NH4+]
pOH = -log Kb –log[NH4OH]/[NH4+]
= pKb + log([NH4+]/[NH4OH])
14 –pH = pKb + log([NH4+]/[NH4OH])
pH = 14 – pKb + log([NH4OH]/[NH4+])
in general
pH = 14 – pKb + log([base]/[salt])
Video about pH Buffer Solutions
FAQ
What is a buffer solution and what is it used for?
A buffer solution is a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. Buffer solutions serve to help maintain a stable pH value of another solution that is mixed with the buffer solution.
What is buffer solution?
Also known as buffering capacity, this feature helps stabilize the pH when an acidic or alkaline element is added to the soil. Its changes can affect plants, decreasing the fraction of nutrients available to them.
