Ammonia Milk of magnesia

Intestinal fluid

Blood Water


Vinegar Stomach acid figure 2-14

Some of your body fluids are acidic, and others are alkaline. A solution with a pH above 7 is alkaline, and a solution with a pH below 7 is acidic. Each unit on the pH scale reflects a 10-fold change in acidity or alkalinity.

This solution then contains more hydroxide ions than hydronium ions and is therefore defined as a base. The adjective alkaline refers to bases. Bases have a bitter taste; however, never taste a substance to test for alkalinity. They tend to feel slippery because the OH~ ions react with the oil on our skin to form a soap. In fact, commercial soap is the product of a reaction between a base and a fat.

Scientists have developed a scale for comparing the relative concentrations of hydronium ions and hydroxide ions in a solution. This scale is called the pH scale, and it ranges from 0 to 14, as shown in Figure 2-14. A solution with a pH of 0 is very acidic, a solution with a pH of 7 is neutral, and a solution with a pH of 14 is very basic. A solution's pH is measured on a logarithmic scale. That is, the change of one pH unit reflects a 10-fold change in the acidity or alkalinity. For example, urine has 10 times the H3O+ ions at a pH of 6 than water does at a pH of 7. Vinegar, has 1,000 times more H3O+ ions at a pH of 3 than urine at a pH of 6, and 10,000 times more H3O+ ions than water at a pH of 7. The pH of a solution can be measured with litmus paper or with some other chemical indicator that changes color at various pH levels.


The control of pH is important for living systems. Enzymes can function only within a very narrow pH range. The control of pH in organisms is often accomplished with buffers. Buffers are chemical substances that neutralize small amounts of either an acid or a base added to a solution. As Figure 2-14 shows, the composition of your internal environment—in terms of acidity and alkalinity— varies greatly. Some of your body fluids, such as stomach acid and urine, are acidic. Others, such as intestinal fluid and blood, are basic or alkaline. Complex buffering systems maintain the pH values in a normal healthy body.

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