Buffers

If you need a solution to maintain its pH, you could prepare a buffer solution.  

For example, you now know that standing water becomes more acidic because it absorbs carbon dioxide (CO₂). The use of a buffer in solution could prevent that acidification.

Basic impurities in glass could leach into your solution if it was stored in a glass bottle, thereby increasing the pH. The use of a buffer in solution could prevent the solution from becoming more basic.

Buffered solutions are capable of maintaining a fairly constant pH, even when small amounts of strong acid or base are added to the solution.