Chemistry Reference
In-Depth Information
The range of hydrogen ion concentrations encountered in chemistry is
very large, so it is convenient to adopt the pH notation first developed by
another Scandinavian chemist (Danish this time), Søren Peder Lauritz
Sørensen. He defined pH as 'the negative logarithm (to the base 10) of the
hydrogen ion concentration', or
1
pH
log
10
[H
]
log
10
—--
[H
]
Use of the pH notation allows all degrees of acidity and alkalinity normally
encountered in chemistry to be expressed on a scale from 0 to 14,
corresponding to the concentrations of H
ions contained in the solution.
Solutions with a pH
7 are considered acidic, solutions with a pH
7
are alkaline, while a solution with a pH
7 is neutral.
It should be noted that a sample of water will often give a pH reading
of less than 7, particularly if the sample has been left in an open beaker.
This is due to carbon dioxide present in the atmosphere dissolving in the
water to give carbonic acid (H
2
CO
3
), which dissociates to release H
ions.
Dissociation of weak acids and bases
Acids are compounds that ionise to release hydrogen ions, or protons, to
their surroundings. Bases are compounds that can accept hydrogen ions.
This is called the Brønsted-Lowry definition of acids and bases (named
after yet another Scandinavian chemist, Johannes Nicolaus Brønsted, and
Thomas Martin Lowry, who was British). There are other ways of
explaining acidity and basicity, but the Brønsted-Lowry theory works most
of the time, and will be used throughout this topic.
The dissociation of a weak acid is usually represented as follows:
HA
3
H
A
However, this suggests that protons exist free in solution like little tennis
balls bouncing around chemical reactions. The reality is that protons are
solvated in solution, that is they go around attached to a solvent molecule.
Since the most common solvent in pharmaceutical and biological systems is
water, the ionisation of a weak acid is better represented as
HA
H
2
O
3
H
3
O
A
where H
3
O
is a
hydroxonium
ion, and the ionisation of a base can be
represented as
B
H
2
O
3
BH
OH
