# pH and the pH Scale The pH scale measures the hydrogen ion concentration in a solution, and identifies a solution as acidic, neutral, or basic.

As pure water dissociates (breaks up into H+ ions and OH ions), the relative concentrations of hydrogen ion (H+) and hydroxide ion (OH) remain equal. However, if additional H+ ions or OH ions are added to the solution, it can cause an imbalance in the relative concentrations of the two ions. The measure of the concentration of H+ (or H3O+) ions in solution is called the pH. The pOH of a substance is a measure of the concentration of OH ions in solution.

The pH scale ranges from zero to 14, with values under 7 considered acidic and values over 7 considered basic. Acidic solutions have a higher concentration of H+ (or H3O+) ions, relative to the concentration of OH ions. An acid is any substance that lowers pH by increasing the levels of hydrogen ions. Basic solutions have a lower concentration of H+ (or H3O+) ions, relative to the concentration of OH ions. A base is any substance that increases pH by removing hydrogen ions from a solution. The relationship between H+ ion concentration and pH is logarithmic (base 10). The pH value itself is the value of the exponent, $, when the concentration is expressed as $1\times10^{-x}\;{\rm M}$. In other words, in a substance that has a pH of 5, the concentration of H+ ions is $1\times10^{-5}\;{\rm M}$. The total of the exponents must equal 14, so in a substance with a pH of 5, the pOH must be 9. When the two values are compared, $1\times10^{-5}\;{\rm M}$ and $1\times10^{-9}\;{\rm M}$, the concentration of H+ is greater than that of OH by four orders of magnitude.

The following equation relates pH to the concentration of H+:
$\rm{pH}=-\log\lbrack{\rm H}^+\rbrack$
The order of magnitude of pH determines the acidity or alkalinity of a solution, which can also determine its impact on living cells. Pure water, for example, has a pH of 7. Milk has a pH of about 6.6. While the difference in pH is only 0.4 because the pH scale is logarithmic, milk is more acidic than pure water. Milk is considered a weak acid. Solutions that are important to life, such as rainwater, seawater, blood, and tree sap all tend to stay near the neutral range with a few exceptions, such as the acidic human stomach. Drifting too far in pH in either direction can affect protein function and quickly lead to trouble for an organism.