There are several definitions for base: (1) The Arrhenius definition is a substance that ionizes in water to produce OH- (hydroxide) ions. (2) The Brnsted definition is a substance that is a proton (H+) acceptor. This does not require the substances to be in aqueous (water) solution. (3) The Lewis definition is a substance that can donate a pair of electrons. This does not require a proton or aqueous solution. There are several other (very technical) definitions, but those are beyond the scope of what one usually sees on an MSDS. A basic solution is defined as one that has a pH greater than 7.0. Basic materials or solutions are sometimes called caustic or alkaline. Additional Info: The following are examples of strong bases, meaning that they completely dissociate into ions and form OH- in aqueous (water) solution. For example NaOH Na++ HO-. All of these will cause severe burns upon skin contact: Formula: NaOH, Name: Sodium hydroxide; Formula: KOH, Name: Potassium hydroxide; Formula: Ba(OH)2, Name: Barium hydroxide; Formula: Ca(OH)2, Name: Calcium hydroxide. Weak bases?do not dissociate completely to form hydroxide (OH-) ions in water. Examples of these include ammonia (NH3), amines, fluoride ion (F-), and acetate ion (CH3COO-). Water can also act as a weak acid or base. Here is a representative example of a chemical reaction of a weak base in water. The two arrows below are together called an equilibrium arrow, which means that the reaction takes place in both directions simultaneously and at some point the concentration of each component will a steady value. NH3 + H2O NH4++ OH-. Just because an base is weak does not mean that it can't harm you. For example, ammonia can cause severe burning of the lungs and death if enough is inhaled and is extremely irritating to the eyes. MSDS Relevance: It is important to know the pH of substances because they may be corrosive or react with incompatible materials. For example, acids and bases should not be stored or used near each other as their accidental combination could generate a huge amount of heat and energy, possibly resulting in an explosion. pH is also important to know in case you spill the material on your skin or eyes. Whenever a substance enters the eye, flush with water for 15 minutes and get prompt medical attention. See also: Acid, Amine, pH