Most people have some sense of what acids and bases are. Advertisements are full of references to excess stomach acid and pH-balanced shampoo. We know that drain cleaner (a strong base) and battery acid can cause serious chemical burns. But what exactly do "acidic" and "basic" mean, and how can they be quantified?
An acid is any proton donor, a molecule that releases a proton (H+) in water. A base is a proton acceptor. Since hydroxide ions (OH-) accept H+, many bases are substances that release hydroxide ions—sodium hydroxide (NaOH), for example. A base does not have to be a hydroxide donor, however. Ammonia (NH3) is also a base. It does not release hydroxide ions, but it readily accepts hydrogen ions to become the ammonium ion (NH4+).
Acidity is expressed in terms of pH, a measure derived from the molarity of H+. Molarity is represented by square brackets, so the molarity of H+ is symbolized [H+]. pH is the negative logarithm of hydrogen ion molarity—that is, pH = -log [H+]. In pure water, 1 in 10 million molecules ionizes into hydrogen and hydroxide ions: H2O ^H+ + OH-. Pure water has a neutral pH because it contains equal amounts of H+ and OH-. Since 1 in 10 million molecules ionize, the molarity of H+ and the pH of water are
[H+] = 0.0000001 molar = 10-7 M log [H+] = -7 pH = -log [H+] = 7
The pH scale (fig. 2.12) was invented in 1909 by Danish biochemist and brewer Soren Sorensen to measure the acidity of beer. The scale extends from 0.0 to 14.0. A solution with a pH of 7.0 is neutral; solutions with pH below 7 are acidic; and solutions with pH above 7 are basic (alkaline). The lower the pH value, the more hydrogen ions a solution has and the more acidic it is. Since the pH scale is logarithmic, a change of one whole number on the scale represents a 10-fold change in H+ concentration. In other words, a solution with a pH of 4 is 10 times as acidic as one with a pH of 5 and 100 times as acidic as one with a pH of 6.
Slight disturbances of pH can seriously disrupt physiological functions and alter drug actions (see insight 2.2), so it is important that the body carefully control its pH. Blood, for example, normally has a pH ranging from 7.35 to 7.45. Deviations from this range cause tremors, fainting, paralysis, or even death. Chemical solutions that resist changes in pH are called buffers. Buffers and pH regulation are considered in detail in chapter 24.
Saladin: Anatomy & I 2. The Chemistry of Life I Text I I © The McGraw-Hill
Physiology: The Unity of Companies, 2003 Form and Function, Third Edition
68 Part One Organization of the Body
— 1 M hydrochloric acid
Figure 2.12 The pH scale. The pH is shown within the colored bar. H molarity increases tenfold for every step down the scale.
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