Organic and inorganic acids are categorized by whether or not their conjugate bases are organic or inorganic compounds. Organic compounds are those that include both carbon and hydrogen atoms.
Common examples of organic acids include acetic acid, lactic acid and citric acid. Common inorganic acids include hydrochloric acid, nitric acid and boric acid. Although this is not always the case, inorganic acids are usually stronger than organic acids.
It is possible to determine whether an acid is organic or inorganic by looking at the conjugate base. The conjugate base is formed when the acid is dissolved in water. Acids in water will dissociate, which means that the hydrogen atom separates from the rest of the acid. These dissociated hydrogen ions are what causes the pH of the solution to decrease and are what is measured when the pH of a solution is measured. The remaining molecule is called the conjugate base.
For example, when HCl dissolves in water, the H+ ion dissociates, leaving a Cl- ion. Since the Cl- does not have both carbon and hydrogen in it, this is an inorganic acid. When acetic acid (CH3COOH) dissociates in water, the remaining conjugate base is CH3COO-. Since this base contains both carbon and hydrogen, acetic acid is an organic acid.