Ionization and pH
The covalent bonds of water molecules sometimes break spontaneously. When this happens, one hydrogen atom nucleus (a proton) dissociates from the rest of the water molecule, leaving behind its electron. Because its positive charge is no longer balanced by an electron, it is a positively charged hydrogen ion, H1. The remaining part of the water molecule now has an extra electron. It is therefore a negatively charged hydroxyl ion, OH-. This process of spontaneous ion formation is called ionization:
H2 OH- + H+
Only very few water molecules are ionized at a single instant in time. Scientists calculate that the fraction of water molecules dissociated (ionized) at any given time in pure water is 0.0000001. This tiny number can be written another way by using exponential notation. This is done by counting the number of places to the right of the decimal point. Because there are seven places, this number is written as 10-7. The minus sign means that the number is less than 1.
To indicate the concentration of H+ ions in a solution, scientists have devised a scale based on the slight degree of spontaneous ionization of water. This scale is called the pH scale. (The letters pH stand for the power of the hydrogen ion [H+]). The pH values of this scale generally range from 0 to 14. The pH of a solution is determined by taking the negative value of the exponent of its hydrogen ion concentration. For example, pure water has a hydrogen ion concentration of 10-7 and therefore a pH of 7. When water ionizes, hydroxyl ions (OH-) are produced in a concentration equal to the concentration of hydrogen ions. In pure water, the concentrations of H+ and OH- ions are equal to 10-7. Because these ions join spontaneously, water, at pH 7, is neutral.
Any substance that dissociates to form H+ ions when it is dissolved in water is called an acid. The more hydrogen ions an acid produces, the stronger an acid it is. Although an acid produces a higher concentration of H1 ions than pure water (0.00001 as opposed to 0.0000001, for example), its pH is lower. Using the above numbers to illustrate: The first number (0.00001 or 1025) represents the hydrogen ion concentration of an acid. The negative value of its exponent results in a pH of 5. The second number (0.0000001 or 10-7) represents the hydrogen ion concentration of water. The negative value of its exponent results in a pH of 7. Each one-unit decrease in pH, however, does not correspond to a onefold increase in acidity. In fact, depending on the acid and how completely it ionizes, a change of one unit may correspond to as much as a tenfold increase in acidity.
Figure 6 shows many common acids and their pH values. The pH of champagne, for example, is about 4. This low pH is due to the dissolved carbonic acid that causes champagne to bubble. Some bodies of water are acidic, such as peat bogs (pH 4 to 5). The hydrochloric acid (HCl) of your stomach ionizes completely to H+ and Cl-. It forms a strong acid with a pH of 2 to 3. Stronger acids are rarely found in living systems.
Figure 6 The pH scale.
Any substance that combines with H+ ions, as OH- ions do, is said to be a base. Any increase in the concentration of a base lowers the H+ ion concentration. Bases therefore have pH values higher than water's neutral value of 7. For example, the environment of your small intestine is kept at a basic pH of between 7.5 and 8.5. Strong bases such as sodium hydroxide (NaOH) have pH values of 12 or more. As with acids, a change of 1 in the pH value of a base may reflect up to a tenfold change in pH, depending on the base.