Glossary of Ionic Equilibrium

Amphoteric substance. A molecule which can act both as an acid and as a base.
Autoprotolysis constant. The equilibrium constant for the reaction in which one solvent molecule loses a proton to another, as 2H₂O ⇌ H₃O⁺ + OH^–.
Amphiprotic solvent. A solvent which possesses both acidic and basic properties.
Aprotic solvent. A solvent which is neither appreciably acidic or basic.
Bronsted acid. A substance which furnishes a proton.
Bronsted base. A substance which accepts a proton.
Buffer capacity. A measure of the effectiveness of a buffer in resisting changes in pH; the capacity is greater the concentrations of the conjugate acid-base pair.
Buffer solution. A solution which contains a conjugated acid-base pair. Such a solution resists large changes in pH when H₃O⁺ or OH^– ions are added and when the solution is diluted.
Charge-balance equation. The equation expressing the electroneutrality principle; i.e., the total concentration of positive charge must equal the total concentration of negative charge.
Common-ion effect. The effect produced by an ion, say from a salt, which is the same ion produced by the dissociation of a weak electrolyte. The "common" ion shifts the dissociation equilibrium in accordance with Le Chatelier's principle.
Central metal atom. A cation which accepts electrons from a ligand to form a complex ion.
Conjugate acid-base pair. An acid-base pair which differ only by a proton, as HCl and Cl^–.
Diprotic acid. An acid which furnishes two protons.
Electrolyte. A compound which produces positive and negative ions in solution. Strong electrolytes are completely dissociated, whereas weak electrolytes are only partially dissociated.
Hydrolysis. An acid-base reaction of a cation or anion with water.
Isoelectric point. The pH at which there is an exact balance of positive and negative charge on an amino acid.
Indicator. A visual acid-base indicator is a weak organic acid or base which shows different colors in the molecular and ionic forms.
Ligand. An anion or neutral molecule which forms a complex ion with a cation by donating one or more pairs of electrons.
Nonelectrolyte. A substance which does not dissociate into ions in solution.
pH. The negative logarithm of the hydrogen ion concentration.
pK. The negative logarithm of an equilibrium constant.
Polyprotic acid. An acid which furnishes two or more protons.
Range of an indicator. That portion of the pH scale over which an indicator changes color, roughly the pK of the indicator ± 1 unit.
Salt. The product other than water which is formed when an acid reacts with a base; usually an ion solid.
Simultaneous equilibria. Equilibria established in the same solution in which one molecule or ions is a participant in more than one of the equilibria.
Solubility product constant, K_sp. The constant for the equilibrium established between a slightly soluble salt and its ions in solution.
Stability constant. The equilibrium constant for a reaction in which a complex is formed. Also called a formation constant.