buffer solution

A buffer solution can maintains a narrow range of pH even when strong acid are added. In our experiment, we can see water is not a buffer, water can undergo very big changes when small amounts of strong acid or strong bases are added. When the strong acid, 100µl HCl was added into the dd water, the pH value changed from 7.38 to 5.83 which mean the dd water has turn into acid. The same phenomenon occurs while strong base, 100µl NaOH was added into the dd water. The pH value changed from 7.07 to 9.48. The result can be obviously seen, as the dd water was turned into acid and base as the strong acid and strong base was added respectively. This mean water has no buffer capacity, its pH changes sharply when even small quantities of acids or bases are dissolved in it.
The pH value of PBS solution didn’t undergo a lot of changes even when we added HCl and NaOH. When we added each 100µl HCl in PBS solution, the pH change was almost 0.06. When we added 100 to 900µl NaCl to PBS solution, the pH value had a small change (range of 0.06-0.19). However, when we added 1000µl NaCl to PBS solution, the pH value had a large increase, because the buffer capacity lost when the limit exceed. Buffer solution work by applying Le Chatelier’s Principle to weal acid equilibrium.
H2PO4- + H2O↔H3O+ + HPO42-
When we add HCl to PBS solution, by Le Chatelier’s Principle, the reaction will shift to the left to minimize the disturbance.
H2PO4- + OH- ↔HPO42- + H2O
When we add NaCl to PBS solution, by Le Chatelier’s Principle, the reaction will shift to the right to minimize the disturbance.
Therefore, the pH value of PBS solution will not have great change.

The buffer limits can be met by:

Buffer capacity will be lost when the limit exceed. Reference
Petrucci, R.H. (2007). General chemistry principles and modern applications. Pearson. Page 750-759