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There are several definitions for base:
There are several other (very technical) definitions, but those are beyond the scope of what one usually sees on a Safety Data Sheet.
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.
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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:
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.
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. The pH of a substance (if applicable/known) will be found in Section 9 (physical and chemical properties) of the Safety Data Sheet and warnings about incompatibility will found in Section 10 (stability and reactivity).
pH is also important to know in case you spill the material on your skin or eyes. Whenever a substance enters the eyes, flush them with water for 15 minutes, preferably with an ANSI-approved eyewash unit. Most bases have a very strong affinity for eye tissue and may not be fully flushed from the eye even with such first aid treatment, so always consult a medical professional after such exposure. Take the SDS with you to the emergency room, if possible, to minimize any treatment delays.
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See also: acid, amine, pH.
Additional definitions from Google and OneLook.
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