3.6: Acids and Bases

Whether a liquid is acidic or basic (alkaline) depends on the amount of hydrogen ions in it. Acidity and alkalinity are expressed as pH (pH literally means hydrogen power from the French pouvoir hydrogene). If a liquid has 0.01 gram (g) of hydrogen ions per liter (about a quart), its pH is 2 because the decimal point is 2 places to the left of 1.0 g of hydrogen ions. A liquid with 0.001 g hydrogen ions per liter has a pH of 3 (decimal is 3 places to the left of 1.0 g).* When it has between 0.01 and 0.001 g hydrogen ions per liter, the pH is between 2 and 3.

Water-based liquids have a pH between 0 and 14. Pure water has a pH of 7.0 (0.0000001 g of hydrogen ions per liter) and is neutral in pH. Liquids with a pH less than 7.0 (more hydrogen ions than pure water) are acidic. Liquids with a pH higher than 7.0 (fewer hydrogen ions than pure water) are basic:

Acids release hydrogen ions. An acid’s strength depends on how easily the hydrogen ions are released. Hydrochloric acid (HCl, stomach acid) is strong because it easily separates into hydrogen and chloride ions (Figure 3.1), thus releasing a lot of hydrogen ions. Stomach fluid is about pH 2. Acetic acid, the acid found in vinegar, is weak because it doesn’t easily release its hydrogen ions. Distilled vinegar is about pH 3.

Keep in mind that the difference between pH 2 and pH 3 is 10-fold (0.01 vs. 0.001 g hydrogen ions per liter). Battery acid has a pH near 0, lemon juice between 2 and 3, orange juice about 4, coffee about 5, and milk is slightly acid with a pH between 6 and 7. We’re familiar with an acid taste, as in the tart taste of lemon, or stomach acid backing up into the throat.

Bases remove hydrogen ions. Concentrated lye (NaOH, sodium hydroxide), sometimes used to clear drains, is very basic (about pH 14). Sodium hydroxide easily separates into sodium ions (Na+) and hydroxide ions (OH-) when dissolved in water. The hydroxide ions (OH-) combine with hydrogen ions (H+) to form water (H2O), thereby removing hydrogen ions. Household ammonia is about pH 11, and a solution of baking soda is about pH 9.

Strong alkaline solutions can break down organic material—like the grease in kitchen drains and the hair in shower drains. A cotton rag will “dissolve” in full-strength chlorine laundry bleach (very alkaline). Rub some of this bleach between your fingers, and it feels slippery—it’s breaking down the surface of your skin.

Chemical reactions in the body take place in liquids and are affected by pH. The enzymes that catalyze the body’s chemical reactions generally function within only a narrow range of pH.

The pH of blood and tissues is slightly basic, between 7.35 and 7.45, the range at which their enzymes function optimally. The body has many safeguards to keep the pH within this narrow range. Certain blood proteins, for instance, act as buffers—substances that can take up and release hydrogen ions, making it harder to change the pH. The medical term for an abnormally low pH is acidosis; for an abnormally high pH, alkalosis. A blood pH below 7.0 or above 7.7 is fatal.

^{*In technical terms, pH is the negative log of the hydrogen ion concentration: A hydrogen ion concentration of 0.00000001 g per liter is 10-8 g per liter. The negative log of 10-8 is 8 (pH = 8).}