What Does "pH" Mean?

The term "pH" is used to measure acidity. Strong acids, like hydrochloric acid or sulfuric acid, have very a very low pH while weaker acids, like acetic acid (vinegar), have pH readings that are higher. A "neutral pH" is 7.0, this is close to the pH value of the cytoplasm in living cells.

Higher pH values are "alkaline" rather that acidic. The highest pH value is usually shown as 14 and the lowest pH value is shown as 0.

Acidity is a function of the concentration of hydrogen ions (H⁺), or protons. The strength of alkaline solutions is measured by the concentration of hydroxide ions (OH^-).

There's a reciprocal relationship between the concentrations of these two ions because we're dealing with aqueous solutions (water). Water molecules dissociate into H⁺ and OH^- ions so in pure water there will always be equal concentrations of both ions.

The extent of this dissociation determines the concentration of these ions in pure water. We express the extent of dissociation using a term called the equilibrium constant (K_eq) that is defined as the concentration of the products of a reaction over the concentration of the reactant(s).

For the dissociation of water, the actual equation is ...

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