According to the
Bronsted-Lowry Acid-
Base Model, an acid is any substance that contributes
protons (
Hydrogen ions) in
solution. Equally, a
base accepts
protons from
solution.
Polyprotic substances can both contribute and donate
protons and thus can act as
buffers in the maintenance of
pH. Another type of
buffer system relies on the use of both a
weak acid (an acid with a Keq value that isn't ridiculously high) and its
Bronsted Lowry base conjugate (just the acid with one
hydrogen removed and contributed to
solution. If there are more than one
hydrogen ion, then the
substance is called
polyprotic.
More simply,
Arhhenius described an acid as any
substance that releases
hydrogen ions into
solution and a
base as any
substance that releases
hydroxide ions. Since
hydroxide ions couple with
hydrogen ions to form
water, with a neutral
pH, any material that releases them in solution will lower the concentration of
hydrogen ions and raise the
pH.