The guidelines below should be applied in the order given to predict acid-base properties.
Note that the first two guidelines can be applied without reference to pKa data.
Anions containing no hydrogen are always the base in an acid-base conjugate pair.
These may be either
basic (if their conjugate acid is weak - p
Ka > 2) or
pH neutral (if their conjugate acid is strong - p
Ka negative).
It is helpful when deciding whether an acid is weak or strong to have learned the common strong acids (HCl, HBr, HNO3, H2SO4, (HSO4– moderate)).
For
anions related by loss of a proton (such as H
2PO
4–, HPO
42– and PO
43–).
The anion that has the
most protons is the
strongest acid.
The anion that has the
fewest protons is the
strongest base.
This analysis can be done simply on the basis of formula - no numbers required.
Hydrogen-containing anions such as H
2PO
4–, HPO
42– may act as either an an
acid or a
base. An aqueous solution containing a potassium or sodium salt of a hydrogen-containing anion is
acidic (pH < 7) if the anion has significant acid strength (p
Ka < 7.5)
basic (pH > 7) if the anion is a very weak acid (p
Ka > 7.5).
Application of this guideline does require pKa data.