A solution that is neutral has a pH of seven, meaning it is neither acidic nor basic. A solution's pH is a measure of the relative proportion of H+ (acid) and OH- (basic) ions in the solution; represented as a number generally ranging from zero (acidic) to 14 (basic), where seven (neutral) represents equal quantities of H+ and OH- ions. Most living things require an internal pH at or near neutral in order to survive. This is because many cellular processes can be disrupted if the pH balance varies by even a little bit. Systems within an organism help maintain the optimal pH for the organism.A solution's pH is a measurement of how acidic or basic a solution is. It can be thought of as parts of hydrogen. Specifically, pH refers to the proportion of H+ (or hydronium ions, H3O+) to OH- (hydroxide ions) in the solution. The pH scale is a base 10 scale. This means that each value is 10 times greater or less than the one next to it. The value is obtained using the equation:
A solution's pH can be measured easily using litmus paper, which is paper coated in special substances that change color depending on the solution's pH. This test is easily carried out by most people and can be done quickly. For more precise measurements, meters that use probes can measure the pH to several decimal places and can monitor changes in real time.
For example, human blood has a pH of about 7.4, which is maintained through the use of carbonic acid. The amount of carbonic acid in blood is controlled by the lungs, which expel carbon dioxide during respiration. Additionally, the kidneys help regulate the pH of blood by selectively excreting acid or base, but much more slowly than the lungs. Other living things have similar mechanisms to maintain a pH at or near neutral. There are a few exceptions to the rule. The pH of stomach acid, which aids in digestion, is very low: between 1.5 and 3.5.In order to maintain a certain internal pH range, organisms use buffers, which resist changes in pH. Every buffer is an aqueous solution (solution containing water) containing a weak acid and its conjugate weak base, or a weak base and its conjugate weak acid. A conjugate acid is the acid formed by the donation of a H+ from a base. Conversely, a conjugate base is the base formed by accepting the proton from an acid. In a diagram, the weak acid is shown on the left side of a reversible equation with the H+ attached, and the weak base is shown alone. The weak acid transfers this H+ to the base. This forms a conjugate base (which donated the H+) and a conjugate acid (which received the H+). This is known as the Brønsted–Lowry theory.