Acids and bases in
The pH drop that occurs during tooth decay
is confined to the mouth; it does not affect the
body. Unlike the mouth, the body maintains a
tight pH range; if it didn’t, enzymes that catalyze biochemical reactions in the body would
not work well. Throughout the body, blood
is always slightly alkaline, with a pH of 7. 4, a
situation that is maintained by the lungs, kidneys, and several other organs that help keep
the blood’s pH constant.
The lungs and the kidneys, for instance,
work together to help maintain a pH of 7. 4 by
keeping the following chemical reactions in
the blood in a state of equilibrium:
CO2 + H2O ⇄ H2CO3
H2CO3 ⇄ HCO3– + H+
In the first chemical equilibrium, carbon
dioxide (CO2) reacts with water (H2O) to produce carbonic acid (H2CO3); and in the second
chemical equilibrium, carbonic acid is ionized
in the blood in the form of a bicarbonate ion
(HCO3–) and a hydrogen ion (H+).
Whether either reaction moves to the right
or to the left depends on the relative abundance of carbon dioxide, water, and carbonic
acid in the first chemical equilibrium and
carbonic acid, bicarbonate ions, and hydrogen
ions in the second equilibrium.
The two chemical equilibria above can be
combined as follows:
CO2 + H2O ⇄ H2CO3 ⇄ HCO3– + H+
So, how can these equilibria shift? For
example, when you exercise heavily, your
muscles produce lactic acid (CH3
CHOHCOOH), so hydrogen ions are added to the
blood. When this happens, the concentration
of hydrogen ions increases, which disturbs
the two chemical equilibria above. To reestablish them, the concentration of hydrogen ions
needs to decrease.
In general, a chemical equilibrium is rees-
tablished according to a principle called Le
Châtelier’s Principle, which states that a sys-
tem that is shifted away from equilibrium acts
to restore equilibrium by reacting in opposi-
(shown as A ⇄ B in Fig. 3), this means that if
the concentration of either reactants or prod-
ucts is changed, the additional reactants or
products will cause the equilibrium to shift left
or right until a new equilibrium is established
(but for which the concentrations of the reac-
tants and products will be different than the
In the case of the two equilibria described
before, some of the excess hydrogen ions will
react with bicarbonate ions to form carbonic
acid, and the additional carbonic acid will be
converted into carbon dioxide and water. So,
the equilibria shift to the left and, in time, new
equilibria are formed.
If not for this shift in equilibria, the pH
would drop dramatically, which could seriously damage the lungs, heart, and other
organs. Instead, the pH drops slightly, generally to 7.36.
2. More blue circles are added.
The system is now out of equilibrium.
3. To reestablish equilibrium, some of the
blue circles and red stars undergo chemical
reactions and end up on the other side
(as blue stars and red circles, respectively).
4. & 5. These chemical reactions continue
to occur until a new equilibiurm is
established, with a different number of
circles and stars than in the original
equilibrium but with the same ratio of
circles vs. stars (2-to-1), as in 1.
1. System at equilibrium
The number of blue circles to red stars are
in a ratio of 2-to-1 (six blue circles and
three red stars).
Figure 3. Schematic representation of Le Châtelier’s Principle in the case of a chemical equilibrium
between a reactant (A) and a product (B). When more reactant A is added to the solution, it disturbs the
equilibrium. To restore the equilibrium, the additional reactant causes the forward reaction to occur,
resulting in more product, and some of the additional product to decompose into the reactant. Ultimately,
a new chemical equilibrium is established. S H E L L E
Figure 2. Our
gain and lose
called mineralization and demineralization,
respectively, which occur at equal rates in our
mouths and result in no net loss of these ions from
our teeth. But if acidic food leftovers bind to our
teeth and are not cleared, they increase the rate of
demineralization, which can lead to tooth decay.