Water

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, <x<x, when the concentration is expressed as 1×10xM1\times10^{-x}\;{\rm M}. In other words, in a substance that has a pH of 5, the concentration of H+ ions is 1×105M1\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, 1×105M1\times10^{-5}\;{\rm M} and 1×109M1\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+:
pH=log[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.

pH Scale

Familiar substances have a range of pH values.