Pure or distilled water has relatively no buffering capacity. The buffering capacity of water can vary somewhat if the water comes from an impure source such as a river or pond.
Buffering capacity refers to how well a solution can maintain a narrow pH range when an acidic or alkaline solution is added. Some buffering capacity test kits measure KH, also known as carbonate hardness. A higher reading from the test kit, indicates a high KH and a high buffering capacity, meaning the solution provides better resistance to pH changes. A result under 4.5 dH indicates a lower KH and a limited buffering capacity.