Acids, Bases, and pH
For thousands of years people have known that vinegar, lemon juice and many other foods taste sour. However, it was not until a few hundred years ago that it was discovered why these things taste sour - because they are all acids. The term acid, in fact, comes from the Latin term acere, which means "sour".
In the seventeenth century, the Irish writer and amateur chemist Robert Boyle first labeled substances as either acids or bases (he called bases alkalies) according to the following characteristics:
While the bonds in water molecules are strong, at any given time a tiny fraction of those bonds might break, forming a positively charged hydrogen ion, H+, and a negatively charged hydroxide ion, OH-. For the chemical processes of life to work correctly, the right balance of H+ ions and OH- ions is critical.
H2O ---> H+ + OH-
As a result, pure water always has a low concentration of hydrogen ions and hydroxide ions, which are present in equal numbers.
Some chemical compounds form additional H+ ions when added to water, while others reduce the number of H+ ions.
A compound that forms H+ ions in a solution is called an acid. When an acid is added to water, the concentration of hydrogen ions in the solution is increased above that of pure water. An example is hydrochloric acid (HCl), the acid in your stomach. In an aqueous solution, hydrochloric acid breaks apart completely into H+ and Cl- ions.
A compound that reduces the concentration of H+ ions in an aqueous solution is called a base. Many bases form hydroxide ions, OH-, when dissolved in water. Such bases lower the concentration of hydrogen ions because hydroxide ions react with hydrogen ions to form water molecules. In essence, a base is the opposite of an acid.
The pH Scale
As you have learned, both acids and bases are related to the concentration of hydrogen ions present. Acids increase the concentration of hydrogen ions, while bases decrease the concentration of hydrogen ions. The pH scale measures the concentration of hydrogen ions, H+, in solutions.
All solutions have a pH value between 0 and 14. Pure water has equal amounts of H+ and OH- ions and is said to be neutral at a pH value of 7. Acidic solutions have pH values below 7, and basic solutions have pH values above 7.
Each whole number on the pH scale represents a factor of 10 on the scale. For example, lemon juice at pH 2 has 10 times more H+ ions than an equal amount of grapefruit juice at pH 3. The pH of the solution inside most living cells is close to 7.
|A solution having a pH of 7 is neutral.
Many fruits have pH values less than 7, making them acidic.
Many household cleaners have pH values greater than 7, making them basic.
Health Perspective: pH and your blood
It is very important for your blood pH to stay within the normal range. At higher or lower pH values, your body does not function properly. Fortunately, you can regulate the pH of your blood simply by breathing!
Blood is a watery solution that contains many solutes including the dissolved gases carbon dioxide and oxygen. Carbon dioxide appears in your blood because it is produced by respiration. Recall that respiration is the combustion of sugar by your body. You breathe in oxygen to get this process going. The end products of this reaction are energy, water, and carbon dioxide.
C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
The rate at which you breathe controls the concentration of carbon dioxide in your blood. For example, if you hold your breath, more carbon dioxide enters your blood. If you hyperventilate, you blow off carbon dioxide, so that significantly less is in your blood. These two processes influence blood pH. The equation below illustrates how carbon dioxide dissolves in an aqueous (watery) solution like blood:
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-
carbon dioxide + water ↔ carbonic acid ↔ hydrogen ion + bicarbonate ion
When CO2 dissolves, H+ ions are produced in solution. Therefore, the more CO2 in your blood, the more acidic your blood will become. If you breathe slowly, the added CO2 makes your blood more acidic. However, if you breathe too often and too quickly (hyperventilating), the loss of CO2 makes your blood more basic. You can offset this effect by breathing into a paper bag. This forces you to re-breathe carbon dioxide. When you breathe normally, your blood pH ranges between 7.35 and 7.45.