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| What an encyclopedia says about pH-value, acids and bases, how to measure them, air pollution and acid rain: |
What is the PH-value?
A term indicating the hydrogen ion concentration of a solution, a measure
of the solution's acidity. The term (from Fr. pouvoir hydroge ne, "hydrogen
power") is defined as the negative logarithm of the concentration of H+
ions (protons):
pH = -log 10 [H+], where [H+] is the concentration of H+ ions in moles
per liter (see Mole). Because H+ ions associate with water molecules to
form hydronium (H3O+) ions (see Acids and Bases), pH also is often expressed
in terms of the concentration of hydronium ions. In pure water at 220
C (720 F), H3O+ and hydroxyl (OH-) ions exist in equal quantities; the
concentration of each is 10-7 moles/liter. Consequently, the pH of pure
water is -log (10-7), which equals log 107, or 7. If an acid is added
to water, however, an excess of H3O+ ions is formed; their concentration
can range between 10-6 and 0.10 moles/liter, depending on the strength
and amount of the acid. Therefore, acid solutions have a pH ranging from
6 (for a weak acid) to 1 (for a strong acid). Inversely, a basic solution
has a low concentration of H3O+ ions and an excess of OH- ions, and the
pH ranges from 8 (for a weak base) to 14 (for a strong base). The pH of
a solution can be measured by titration, which consists of the neutralization
of the acid (or base) by a measured quantity of base (or acid) of known
concentration, in the presence of an indicator (a compound the color of
which depends on the pH). The pH of a solution can also be determined
directly by measuring the electric potential arising at special electrodes.
What are Acids and Bases?
two classes of chemical compounds that display generally opposite characteristics.
Acids taste sour, turn litmus (a pink dye derived from lichens) red, and
often react with some metals to produce hydrogen gas. Bases taste bitter,
turn litmus blue, and feel slippery. When aqueous (water) solutions of
an acid and a base are combined, a neutralization reaction occurs. This
reaction is characteristically very rapid and generally produces water
and a salt. For example, sulfuric acid and sodium hydroxide, NaOH, yield
water and sodium sulfate.
Early Theories:
Modern understanding of acids and bases began with the discovery in 1834
by the English physicist Michael Faraday that acids, bases, and salts
are electrolytes. That is, when they are dissolved in water, they produce
a solution that contains charged particles, or ions, and can conduct an
electric current (see Ionization). In 1884 the Swedish chemist Svante
Arrhenius (and later Wilhelm Ostwald, a German chemist) proposed that
an acid be defined as a hydrogen-containing compound that, when dissolved
in water, produces a concentration of hydrogen ions, or protons, greater
than that of pure water. Similarly, Arrhenius proposed that a base be
defined as a substance that, when dissolved in water, produces an excess
of hydroxyl ions, OH-. A number of criticisms of the Arrhenius-Ostwald
theory have been made. First, acids are restricted to hydrogen-containing
species and bases to hydroxyl-containing species. Second, the theory applies
to aqueous solutions exclusively, where as many acid-base reactions are
known to take place in the absence of water.
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