Question

How do you calculate the pH of a buffer solution?

Answer 1

In order to calculate the pH of a buffer solution, we must first have an idea about what a pH is. pH is a convenient way to measure the acidity and basicity of the solution. pH is a negative logarithm scale.

Complete step by step solution:
Let us first understand what a pH is. pH is a convenient way to measure the acidity and basicity of the solution. pH is a negative logarithm scale.
- Logarithm means for every one-unit change in the pH scale, the concentration of ${H^ + }$changes by 10 factors.
- For a temperature at $25^\circ C$, the pH is below 7 for acidic solution, pH is above 7 for basic solution and pH is equal to 7 for neutral solution.
Now let us understand what a buffer solution is. Buffer solution is a solution which consists of a mixture that contains weak acid and its conjugate base or weak base and its conjugate acid. Buffer will help to maintain the pH of a solution. Buffer solution is usually classified into two types. They are:

Acid buffer
Alkaline buffer
Acid buffer is a solution which is used to maintain the acidic environment. This solution will be having an acidic pH. Acid buffer is prepared by mixing weak acid (HA) with its conjugate base (KA) along with the strong base (KOH).

When the weak acid (HA) gets ionized, then the equilibrium reaction can be written as:
$HA + {H_2}O \rightleftharpoons {H^ + } + {A^ - }$
We can write the acid dissociation constant as
${K_a} = \dfrac{{[{H^ + }][{A^ - }]}}{{[HA]}}$
Taking log on both sides we get
$- \log {K_a} = - \log [{H^ + }] - log\left( {\dfrac{{[{A^ - }]}}{{[HA]}}} \right)$
$p{K_a} = pH - \log \left( {\dfrac{{[salt]}}{{[acid]}}} \right)$
pH of acid buffer = $p{K_a} + \left( {\dfrac{{[salt]}}{{[acid]}}} \right)$

Basic buffer is a solution which is used to maintain the basic environment. This solution will be having a basic pH. Basic buffer is prepared by mixing weak base (B) with its conjugate acid (BA) along with the strong acid.

So, pH of basic buffer = $p{K_a} - \left( {\dfrac{{[salt]}}{{[acid]}}} \right)$

Note:
- We have to remember that pH and pOH are different from one another.
- The pH and pOH values are not the same for acidic solutions. When pH is below 7 for acidic solution, then pOH will be below 7 for acidic solution.
- The pH is above 7 for basic solution while pOH is below 7 for basic solution.