A buffer solution is water mixed with a chemical to give it special properties in regards to pH (acidity). The chemical, known as a buffer agent, resists pH changes when exposed to acids and bases when properly mixed in a solution. This property makes a buffer solution extremely useful in protecting sensitive equipment, dealing with chemical accidents, and even in balancing the internal processes of living things.

A buffer solution is made by mixing a buffer agent’s acid form (HA) with its conjugate base (A^-) in water. Once a balance exists between HA and A^-, any other acids or bases added to the buffer solution will be neutralized when they turn a portion of the buffer agent into either the HA or A^- form. The change in the buffer agent initially causes little change in the buffer solution’s pH. As the proportion of a buffer agent’s HA and A^- changes due to the addition of other acids and bases, the buffering capacity of the solution decreases. Eventually, a buffer agent can be used up to the point where it can no longer significantly resist pH changes, meaning it is no longer useful as a buffer solution.

Buffer solutions and buffer agents are extremely common. The detergent borax, for instance, can be used to make a simple buffer solution. Many traditional antacid tablets are, in fact, buffer agents that work within the human body. Buffered aspirin is aspirin that has been treated with a buffering agent to help reduce pH changes to the aspirin when it is exposed to stomach acid. pH probes are protected from damage by being stored in buffer solutions.

In the human body, buffer agents play a vital role in both respiration and maintaining the body’s pH level. One of the most important buffer agents in humans is bicarbonate. When carbon dioxide (CO₂) mixes with water (H₂O), carbonic acid is formed (H₂CO₃). Bicarbonate (HCO₃^-) is a dissociation of carbonic acid. Carbonic acid and bicarbonate form an equilibrium in the blood that helps the body adjust to pH changes.

Just like a buffer solution, the bicarbonate/carbonic acid equilibrium will no longer resist pH changes if their proportions are significantly altered. Excesses of carbonic acid are adjusted by forming carbon dioxide through the removal of water from carbonic acid; the carbon dioxide is then exhaled. Excessive amounts of bicarbonate can be reduced by inhaling more carbon dioxide, which is then changed into carbonic acid.