Arrhenius acid-base theory. An Arrhenius acid is a hydrogen-containing compound that, in water, produces hydrogen ions. An Arrhenius base is a hydroxide-containing compound that, in water, produces hydroxide ions.

Brønsted - Lowry acid - base theory. A Brønsted - Lowry acid is any substance that can donate a proton (H+) to some other substance. A Brønsted - Lowry base is any substance that can accept a proton from some other substance. Proton donation (from an acid) cannot occur unless an acceptor (a base) is present.

Conjugate acids and bases. A conjugate acid - base pair is two species that differ by one proton. The conjugate base of an acid is the species that remains when the acid loses a proton. The conjugate acid of a base is the species formed when the base accepts a proton.

Polyprotic acids. Polyprotic acids are acids that can transfer two or more hydrogen ions during an acid - base reaction.

Strengths of acids and bases. Acids can be classified as strong or weak in terms of the extent to which proton transfer occurs in aqueous solution. A strong acid completely transfers its protons to water. A weak acid transfers only a small percentage of its protons to water.

Salts. Salts are ionic compounds containing a metal or polyatomic ion as the positive ion and a nonmetal or polyatomic ion (except hydroxide ion) as the negative ion. Ionic compounds containing hydroxide ion are bases rather than salts.

Neutralization. Neutralization is the reaction between an acid and a hydroxide base to form a salt and water.

Self-ionization of water. In pure water, a small number of water molecules (1.0 ´ 10-7 mole/L) donate protons to other water molecules to produce small concentrations (1.0 ´ 10-7 mole/L) of hydronium and hydroxide ions.

The pH scale. The pH scale is a scale of small numbers that are used to specify molar hydronium ion concentration in an aqueous solution. Mathematically, the pH is the negative logarithm of the hydronium ion concentration. Solutions with a pH lower than 7.0 are acidic, those with a pH higher than 7.0 are basic, and those with a pH equal to 7.0 are neutral.

Buffer solutions. A buffer solution is a solution that resists pH change when small amounts of acid or base are added to it. The resistance to pH change in most buffers is caused by the presence of a weak acid and a salt of its conjugate base.

Acid - base titrations. Titration is a procedure in which an acid - base neutralization reaction is used in determining an unknown concentration. A measured volume of an acid or a base of known concentration is exactly reacted with a measured volume of a base or an acid of unknown concentration.