Nace CP 1, Руководство по эксплуатации

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Basic Chemistry and Basic Corrosion Theory 2:3
CP 1 – Cathodic Protection Tester Course Manual
©
NACE International, 2000
02/01/05
Oxygen
Atom
Hydrogen Atoms
Figure 2.2 Water Molecule
Acidity and Alkalinity (pH)
When discussing an aqueous medium (including soil), the question often
arises as to how acid or alkaline the solution is. This refers to whether there
is an excess of hydrogen (H
+
) or hydroxyl (OH
–
) ions present.
When acids dissociate, the cation produced is the hydrogen ion, H
+
. A
medium is said to be
acidic
when there is an excess of H
+
ions. The strength
of an acid is a measure of the hydrogen ion concentration in an aqueous
solution and is classified according to the pH scale. The definition of pH is
the negative logarithm to the base 10 of the hydrogen ion concentration, or:
pH = –log [H
+
]
When an alkali dissociates, the anion produced is a hydroxyl ion, OH
–
. A
medium is said to be
alkaline
when an excess OH
–
(hydroxyl) ions are
present.
This concept is better understood if we look at pure water, H
2
O. Pure water
will ionize into equal parts of hydrogen ions (H
+
) and hydroxyl ions (OH
–
).
The ionization constant for water is the square root of 10
–14
. That equals 10
–
7
. This is the definition of a negative logarithm. Since water ionizes into
equal parts of hydrogen and hydroxyl ions, 7 represents a neutral solution.
The pH scale is illustrated in Figure 2.3. The neutral point is 7. Acid
solutions have a pH below 7 and alkaline, or basic, solutions have a pH