Page 8 - Pharmaceutical analytical chemistry |

A-1. Acids and bases are defined according to different
theories, as follows:

1) Arrhenius theory
An acid is a substance that produces H+ when it dissolves in H2O

while a base is a substance that produces hydroxide ions, OH-, eg. HCl
and NaOH, respectively.

2) Bronsted-Lowry theory

They consider acid-base reactions as proton-transfer reactions. An

acid is the species (molecule or ion) that donates a H+ to another species

in a proton-transfer reaction. A base is the species (molecule or ion) that

accepts a proton in a proton-transfer reaction. The dissolution of nitric

acid in water is an example

HNO3 (aq) + H2O (l) ⎯→ NO3- (aq) + H3O+ (aq)

acid base hydronium ion

Proton donor Proton acceptor

Bear in mind that the H+ (aq) (hydrogen ion) and the H3O+ (aq)

(hydronium ion) represent precisely the same physical ion.

(3) Lewis theory
Lewis proposed a new concept of acids and bases based on the electronic
theory of valence. He defined a base as any substance that has a pair of
electrons that can donate to establish a coordinate bond and an acid as
any substance that may accept an electron pair to form such a bond.

2. Acids and bases strength

Acids and bases are classified as strong or weak, depending on
whether they are strong or weak electrolytes.

7