When a small amount of acid is added to a buffer, the conjugate base part soaks it up. When a small amount of base is added, the weak acid part soaks it up instead. Because of this, the ratio between the two parts shifts only slightly, and the pH barely moves.
Buffering works best when the two components are present in roughly equal amounts. This happens when the solution’s pH is close to the pKa. The pKa is a number that describes how strong the weak acid is.
A buffer eventually fails once one of its two parts runs out. It also fails if too much strong acid or base is added at once. This maximum capacity is called the buffer capacity.
It depends on how much weak acid and conjugate base are present in the solution. Buffers are essential for keeping pH steady in many settings. Biological fluids, like blood, depend on buffers to stay within a safe pH range.
Phosphate buffers are common inside living cells, helping control pH during many biochemical reactions. Laboratory reactions and many industrial processes also rely on buffers to keep conditions stable and predictable.