Acids and Bases

Some molecules form ions when they are dissolved in water. For example, the HCl molecule comes apart (it dissociates) and produces H+ and Cl-. The electron that was normally with the H remains with the Cl. The H now has a positive charge because it no longer has an electron. Similarly, the Cl has a negative charge because it has the electron from the H atom.

HCl -----> H+ + Cl-

Acids are substances that dissociate to produce hydrogen ions and a negative ion (anion). HCl is therefore an acid.

Bases are substances that combine with hydrogen ions, thus lowering the concentration of hydrogen ions. Substances that produce hydroxide ions ( OH-) are bases because hydroxide ions are capable of combining with hydrogen ions to form water, thus lowering the concentration of hydrogen ions. Bases are therefore proton acceptors.

OH- + H+ -----> H2O

When NaOH is dissolved in water, an electron from the sodium atom remains with the OH. This produces a sodium ion (Na+) and a hydroxide ion (OH-). NaOH is therefore a base.

NaOH -----> Na+ + OH-

Most bases produce hydroxide ions and a cation when dissolved in water.

Water molecules have a slight tendency to dissociate, forming both H+ and OH- as shown below. Water is neutral when it ionizes because the number of H+ equals the number of OH-.

H2O «----- H+ + OH-

pH
The measure of the strength of an acid or base is called the pH. It is a measure of the concentration of hydrogen ions (H+). It is the negative log (base 10) of the hydrogen ion concentration.

pH = -log10[H+]

The brackets above are used to indicate concentration. It is measured in moles per liter.

Water has a hydrogen ion concentration of 0.0000001 X 10-7 moles per liter. The pH of pure water is therefore 7.

The product of hydrogen ion concentration and hydroxide ion concentration is always 1 X 10-14, so if the number of one kind of ion is known, the other can be calculated. For example, a solution with a pH of 4 (or 1 X 10-4 hydrogen ions) will have 1 X 10-10 hydroxide ions because 10-4 X 10-10 = 10-14. The table below shows the concentration of hydrogen and hydroxide ions at different pH levels.

pH..[H+]........[OH-].....[H+] X [OH-]
0- 1 X 10-0 1 X 10-14 1 X 10-14
1- 1 X 10-1 1 X 10-13 1 X 10-14
2- 1 X 10-2 1 X 10-12 1 X 10-14
3- 1 X 10-3 1 X 10-11 1 X 10-14
4- 1 X 10-4 1 X 10-10 1 X 10-14
5- 1 X 10-5 1 X 10-9 1 X 10-14
6- 1 X 10-6 1 X 10-8 1 X 10-14
7- 1 X 10-7 1 X 10-7 1 X 10-14 neutral
8- 1 X 10-8 1 X 10-6 1 X 10-14
9- 1 X 10-9 1 X 10-5 1 X 10-14
10- 1 X 10-10 1 X 10-4 1 X 10-14
11- 1 X 10-11 1 X 10-3 1 X 10-14
12- 1 X 10-12 1 X 10-2 1 X 10-14
13- 1 X 10-13 1 X 10-1 1 X 10-14
14- 1 X 10-14 1 X 10-0 1 X 10-14

The pH scale ranges from 0 to 14. An acid has a pH less than 7. A base has a pH greater than 7. A pH of 7 is neutral. A substance that has the same number of hydrogen ions and hydroxide ions is neutral. The equation below shows that when water ionizes, it produces one hydrogen ion and one hydroxide ion. Water is therefore neutral. The table above shows that it has the same concentration of hydroxide ions as hydrogen ions.

H2O «---- H+ + OH-

The pH scale is a logarithmic scale; each decrease of 1 pH unit corresponds to a 10-fold increase in the concentration in hydrogen ions.

Buffers
Buffers are substances that prevent the pH of a solution from changing. They do this by either accepting hydrogen ions if the solution becomes acidic or by releasing hydrogen ions if the solution becomes basic.

Carbonic acid is an important buffer in biological systems. The equation below shows that carbon dioxide dissolved in water produces carbonic acid and carbonic acid further dissociates to produce hydrogen ions and bicarbonate ions.

CO2 + H2O -----> H2CO3 -----> H+ + HCO3-

Carbonic acid is a weak acid; at any point in time there are some molecules that are not dissociated and others have dissociated to form hydrogen ions and bicarbonate ions. If the solution becomes acidic, the reaction shown above will move toward the left, that is, hydrogen ions combine with bicarbonate ions to form carbonic acid. The concentration of hydrogen ions therefore does not change as much as it would change without the presence of bicarbonate ions.

H2CO3 -----> H+ + HCO3-

If the solution becomes more basic, the equation moves toward the right. Carbonic acid dissociates to release hydrogen ions and bicarbonate ions.

H2CO3 -----> H+ + HCO3-