Though chemists apply the sophisticated structural definitions for acids and bases that we have discussed, there are also more "hands-on" methods for identifying a particular substance (including complex mixtures) as an acid or base. Many of these make use of the pH scale, developed by Danish chemist SØren SØrensen (1868-1939) in 1909.
The term pH stands for "potential of hydrogen," and the pH scale is a means of determining the acidity or alkalinity of a substance. (Though, as noted, the term "alkali" has been replaced by "base," alkalinity is still used as an adjectival term to indicate the degree to which a substance displays the properties of a base.) There are theoretically no limits to the range of the pH scale, but figures for acidity and alkalinity are usually given with numerical values between 0 and 14.
A rating of 0 on the pH scale indicates a substance that is virtually pure acid, while a 14 rating represents a nearly pure base. A rating of 7 indicates a neutral substance. The pH scale is logarithmic, or exponential, meaning that the numbers represent exponents, and thus an increased value of 1 represents not a simple arithmetic addition of 1, but an increase of 1 power. This, however, needs a little further explanation.
The pH scale is actually based on negative logarithms for the values of H 3 O + (the hydronium ion) or H + (protons) in a given substance. The formula is thus pH = −log[H 3 O + ] or −log[H + ], and the presence of hydronium ions or protons is measured according to their concentration of moles per liter of solution.
The pH of a virtually pure acid, such as the sulfuric acid in car batteries, is 0, and this represents 1 mole (mol) of hydronium per liter (l) of solution. Lemon juice has a pH of 2, equal to 10 −2 mol/l. Note that the pH value of 2 translates to an exponent of −2, which, in this case, results in a figure of 0.01 mol/l.
Distilled water, a neutral substance with a pH of 7, has a hydronium equivalent of 10 −7 mol/l. It is interesting to observe that most of the fluids in the human body have pH values in the neutral range blood (venous, 7.35; arterial, 7.45); urine (6.0—note the higher presence of acid); and saliva (6.0 to 7.4).
At the alkaline end of the scale is borax, with a pH of 9, while household ammonia has a pH value of 11, or 10 −11 mol/l. Sodium hydroxide, or lye, an extremely alkaline chemical with a pH of 14, has a value equal to 10 −14 moles of hydronium per liter of solution.
The most precise pH measurements are made with electronic pH meters, which can provide figures accurate to 0.001 pH. However, simpler materials are also used. Best known among these is litmus paper (made from an extract of two lichen species), which turns blue in the presence of bases and red in the presence of acids. The term "litmus test" has become part of everyday language, referring to a make-or-break issue—for example, "views on abortion rights became a litmus test for Supreme Court nominees."
Litmus is just one of many materials used for making pH paper, but in each case, the change of color is the result of the neutralization of the substance on the paper. For instance, paper coated with phenolphthalein changes from colorless to pink in a pH range from 8.2 to 10, so it is useful for testing materials believed to be moderately alkaline. Extracts from various fruits and vegetables, including red cabbages, red onions, and others, are also applied as indicators.
The tables below list a few well-known acids and bases, along with their formulas and a few applications
Of course these represent only a few of the many acids and bases that exist. Selected substances listed above are discussed briefly below.
As its name suggests, citric acid is found in citrus fruits—particularly lemons, limes, and grapefruits. It is also used as a flavoring agent, preservative, and cleaning agent. Produced commercially from the fermentation of sugar by several species of mold, citric acid creates a taste that is both tart and sweet. The tartness, of course, is a function of its acidity, or a manifestation of the fact that it produces hydrogen ions. The sweetness is a more complex biochemical issue relating to the ways that citric acid molecules fit into the tongue's "sweet" receptors.
Citric acid plays a role in one famous stomach remedy, or antacid. This in itself is interesting, since antacids are more generally associated with alkaline substances, used for their ability to neutralize stomach acid. The fizz in Alka-Seltzer, however, comes from the reaction of citric acids (which also provide a more pleasant taste) with sodium bicarbonate or baking soda, a base. This reaction produces carbon dioxide gas. As a preservative, citric acid prevents metal ions from reacting with, and thus hastening the degradation of, fats in foods. It is also used in the production of hair rinses and low-pH shampoos and toothpastes.
The carboxylic acid family of hydrocarbon derivatives includes a wide array of substances—not only citric acids, but amino acids. Amino acids combine to make up proteins, one of the principal components in human muscles, skin, and hair. Carboxylic acids are also applied industrially, particularly in the use of fatty acids for making soaps, detergents, and shampoos.
There are plenty of acids found in the human body, including hydrochloric acid or stomach acid—which, in large quantities, causes indigestion, and the need for neutralization with a base. Nature also produces acids that are toxic to humans, such as sulfuric acid.
Though direct exposure to sulfuric acid is extremely dangerous, the substance has numerous applications. Not only is it used in car batteries, but sulfuric acid is also a significant component in the production of fertilizers. On the other hand, sulfuric acid is damaging to the environment when it appears in the form of acid rain. Among the impurities in coal is sulfur, and this results in the production of sulfur dioxide and sulfur trioxide when the coal is burned. Sulfur trioxide reacts with water in the air, creating sulfuric acid and thus acid rain, which can endanger plant and animal life, as well as corrode metals and building materials.
The alkali metal and alkaline earth metal families of elements are, as their name suggests, bases. A number of substances created by the reaction of these metals with nonmetallic elements are taken internally for the purpose of settling gastric trouble or clearing intestinal blockage. For instance, there is magnesium sulfate, better known as Epsom salts, which provide a powerful laxative also used for ridding the body of poisons.
Aluminum hydroxide is an interesting base, because it has a wide number of applications, including its use in antacids. As such, it reacts with and neutralizes stomach acid, and for that reason is found in commercial antacids such as Di-Gel™, Gelusil™, and Maalox™. Aluminum hydroxide is also used in water purification, in dyeing garments, and in the production of certain kinds of glass. A close relative, aluminum hydroxychloride or Al 2 (OH) 5 Cl, appears in many commercial antiperspirants, and helps to close pores, thus stopping the flow of perspiration.
Baking soda, known by chemists both as sodium bicarbonate and sodium hydrogen carbonate, is another example of a base with multiple purposes. As noted earlier, it is used in Alka-Seltzer™, with the addition of citric acid to improve the flavor; in fact, baking soda alone can perform the function of an antacid, but the taste is rather unpleasant.
Baking soda is also used in fighting fires, because at high temperatures it turns into carbon dioxide, which smothers flames by obstructing the flow of oxygen to the fire. Of course, baking soda is also used in baking, when it is combined with a weak acid to make baking powder. The reaction of the acid and the baking soda produces carbon dioxide, which causes dough and batters to rise. In a refrigerator or cabinet, baking soda can absorb unpleasant odors, and additionally, it can be applied as a cleaning product.
Another base used for cleaning is sodium hydroxide, known commonly as lye or caustic soda. Unlike baking soda, however, it is not to be taken internally, because it is highly damaging to human tissue—particularly the eyes. Lye appears in drain cleaners, such as Drano™, and oven cleaners, such as Easy-Off™, which make use of its ability to convert fats to water-soluble soap.
In the process of doing so, however, relatively large amounts of lye may generate enough heat to boil the water in a drain, causing the water to shoot upward. For this reason, it is not advisable to stand near a drain being treated with lye. In a closed oven, this is not a danger, of course; and after the cleaning process is complete, the converted fats (now in the form of soap) can be dissolved and wiped off with a sponge.
"Acids and Bases Frequently Asked Questions." General Chemistry Online (Web site). <http://antoine.fsu.umd.edu/chem/senese/101/acidbase/faq.shtml> (June 7, 2001).
"Acids, Bases, and Salts." Chemistry Coach (Web site). <http://www.chemistrycoach.com/acids.htm> (June7, 2001).
"Acids, Bases, and Salts." University of Akron, Department of Chemistry (Web site). <http://ull.chemistry.uakron.edu/genobc/Chapter_09/title.html> (June 7, 2001).
ChemLab. Danbury, CT: Grolier Educational, 1998.
Ebbing, Darrell D.; R. A. D. Wentworth; and James P. Birk. Introductory Chemistry. Boston: Houghton Mifflin, 1995.
Haines, Gail Kay. What Makes a Lemon Sour? Illustratedby Janet McCaffery. New York: Morrow, 1977.
Oxlade, Chris. Acids and Bases. Chicago: Heinemann Library, 2001.
Patten, J.M. Acids and Bases. Vero Beach, FL: Rourke Book Company, 1995.
Walters, Derek. Chemistry. Illustrated by Denis Bishopand Jim Robins. New York: F. Watts, 1982.
Zumdahl, Steven S. Introductory Chemistry A Foundation, 4th ed. Boston: Houghton Mifflin, 2000.