Acids and Bases: Salts
pH and Tooth Decay
Teeth are made up of a soft material called dentin covered by a hard enamel layer. The outer layer is mostly hydroxyapatite (Ca10(PO4)6(OH)2), a sparingly soluble salt. Tooth decay occurs when the teeth become demineralized (or the hydroxyapatite dissolves) and cavities form.
You were surely told as a child that eating sugar causes cavities. This could be true if sugar causes hydroxyapatite to be more soluble. Does sugar do this?
The answer is yes and the explanation lies in plaque. Plaque is the substance that sticks to your teeth and consists mostly of bacteria. Bacteria live on energy obtained by decomposing sugars. One product of sugar decomposition is lactic acid (CH3CHOHCO2H). The more sugar that enters the mouth, the more the bacteria thrive and the more lactic acid is produced.
The equation for the solubility of hydroxyapatite is shown below. Will the addition of acid cause the equilibrium to shift?
Ca10(PO4)6(OH)2(s) 10 Ca2+(aq) + 6 PO43-(aq) + 2 OH-(aq)
Are any products in the above reaction bases? If so, they will react with the added acid. How will this affect the equilibrium?
The two anions present in hydroxyapatite are bases and will react with any added acid:
PO43-(aq) + H3O+(aq) HPO42-(aq) + H2O(l)
OH-(aq) + H3O+(aq) 2 H2O(l)
This removes them from the equilibrium, causing the equilibrium to shift to the right (Le Châtelier's Principle). Hence, the more sugar one eats, the more lactic acid produced. As more lactic acid is produced, the protective enamel coating becomes more soluble. As the enamel dissolves, tooth decay is very likely.
There are many other insoluble salts that contain bases as anions. The addition of an acid will increase the solubility by reacting with the base.
The following salts are insoluble in water. Click on those whose solubility will be enhanced in an acidic solution.
Cl- is the very weak conjugate base of the strong acid HCl and will not react with acid to a measureable degree. Therefore, adding acid to PbCl2 will not enhance its solubility.
Good! CO32- is a the weak conjugate base of HCO3- and will react with acid, causing CaCO3 to be more soluble in acid than pure water.
Good! SO42- is the weak conjugate base of HSO4-. It will react with added acid, enhancing the solubility of BaSO4.
Are there any others?