A buffer is a compound or a mixture of compounds such that when dissolved in a solution it becomes resistant to change in pH. A buffer can be composed from a single compound such as hydrochloric acid (HCl) or it can be a mixture of compounds such as acetic acid - sodium acetate (acetate buffer). A buffer solution has a pH that is resistant to change upon addition of small amounts of an acid or a base. It has the ability to tie or neutralize any amounts of hydrogen ions or hydroxyl ions introduced into the solution. For example, if a small amount of HCl is added to an acetate buffer solution then acetate ions will tie the introduced hydrogen ions to produce acetic acid. On the other hand, if sodium hydroxide is added some acetic acid will neutralize the hydroxyl ions to produce acetate ions.
Illustration:
If 1 ml 0.1 M NaOH is added to 100 ml pure water it causes the pH to change from 7 to 11 (4 units change) as described below:
Final volume of solution is 100+1 = 101 ml
Number of NaOH moles = 1 ml × (1 liter/ 1000 ml) × 0.1 (moles/ liter) = 0.0001 moles
Final concentration = (0.0001 moles/ 101 ml) × (1000 ml/ 1 liter) = 0.001 M
pH = 14+ log (0.001) =11
On the other hand, if 1 ml 0.1 M NaOH is added to 100 ml buffer solution containing 0.005 M acetic acid (Ka = 1.75 × 10-5) and 0.005 M sodium acetate then the pH changes as the following:
Starting pH = 4.76 (calculated from equations of single conjugate acid-base pair).
After addition the concentration of acetic acid and acetate ions change and the pH can be calculated as the following:
Number of moles of acetic acid in solution (before addition) = 100 ml × (1 liter/ 1000 ml) × 0.005 (moles/ liter) = 0.0005 moles
Final concentration for acetic acid = (0.0004 moles/ 101 ml) × (1000 ml/ 1 liter) =0.004 M
Final concentration for sodium acetate = (0.0006 moles/ 101 ml) × (1000 ml/ 1 liter) =0.006 M
Accordingly, pH = 4.93 (calculated from equations of single conjugate acid-base pair).
Illustration:
If 1 ml 0.1 M NaOH is added to 100 ml pure water it causes the pH to change from 7 to 11 (4 units change) as described below:
Final volume of solution is 100+1 = 101 ml
Number of NaOH moles = 1 ml × (1 liter/ 1000 ml) × 0.1 (moles/ liter) = 0.0001 moles
Final concentration = (0.0001 moles/ 101 ml) × (1000 ml/ 1 liter) = 0.001 M
pH = 14
On the other hand, if 1 ml 0.1 M NaOH is added to 100 ml buffer solution containing 0.005 M acetic acid (Ka = 1.75 × 10-5) and 0.005 M sodium acetate then the pH changes as the following:
Starting pH = 4.76 (calculated from equations of single conjugate acid-base pair).
After addition the concentration of acetic acid and acetate ions change and the pH can be calculated as the following:
Number of moles of acetic acid in solution (before addition) = 100 ml × (1 liter/ 1000 ml) × 0.005 (moles/ liter) = 0.0005 moles
Number of moles of sodium acetate in solution (before addition) = 100 ml × (1 liter/ 1000 ml) × 0.005 (moles/ liter) = 0.0005 moles
Number of moles of hydroxyl ions introduced to solution = 1 ml × (1 liter/ 1000 ml) × 0.1 (moles/ liter) = 0.0001 moles
Number of moles of acetic acid in solution after addition = 0.0005 - 0.0001 = 0.0004 moles
Number of moles of sodium acetate in solution after addition = 0.0005 + 0.0001 = 0.0006 moles
Final concentration for acetic acid = (0.0004 moles/ 101 ml) × (1000 ml/ 1 liter) =0.004 M
Final concentration for sodium acetate = (0.0006 moles/ 101 ml) × (1000 ml/ 1 liter) =0.006 M
Accordingly, pH = 4.93 (calculated from equations of single conjugate acid-base pair).
So the pH changed from 4.76 to 4.93 ( 0.17 units change) after addition.
In conclusion the presence of the buffer in solution made it resistant to pH change (0.17 units change compared with 4 units change).
In conclusion the presence of the buffer in solution made it resistant to pH change (0.17 units change compared with 4 units change).
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