Acids Bases and pH

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 sodium hydroxide Oven cleaner, lye

- Household ammonia

- Milk of magnesia

Household bleach

-Bile

Blood Pure water

- Cow's milk, saliva Urine

- Black coffee

- Tomatoes

- Soft drinks, citrus juices, vinegar

Gastric juice

— 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.

_Think About It_

A pH of 7.20 is slightly alkaline, yet a blood pH of 7.20 is called acidosis. Why do you think it is called this?

stomach, for example, it is uncharged and passes easily through the stomach lining into the bloodstream. Here it encounters a basic pH, whereupon it ionizes. In this state, it is unable to pass back through the membrane, so it accumulates in the blood. This effect, called ion trapping or pH partitioning, can be controlled to help clear poisons from the body. The pH of the urine, for example, can be manipulated so that poisons become trapped there and thus rapidly excreted from the body.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

6. What is the difference between a mixture and a compound?

7. What are hydrophilic and hydrophobic substances? Give an example of each.

8. Why would the cohesion and thermal stability of water be less if water did not have polar covalent bonds?

9. How do solutions, colloids, and suspensions differ from each other? Give an example of each in the human body.

10. What is one advantage of percentage over molarity as a measure of solute concentration? What is one advantage of molarity over percentage?

11. If solution A had a H+ concentration of 10—8 M, what would be its pH? If solution B had 1,000 times this H+ concentration, what would be its pH? Would solution A be acidic or basic? What about solution B?

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Responses

  • sara
    Did sorenson derived pH scale in 1909?
    8 years ago
  • gabriella
    WHAT IS THE pH OF 0.0000001MOLAR NaOH SOLUTION?
    3 years ago

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