Acid-Base Titration
and pH
Aqueous Solutions and
the Concept of pH
• In the self-ionization of water, two
water molecules produce a hydronium
ion and a hydroxide ion by the
transfer of a proton
• At 25 degrees C, the
concentration of [OH] and [H3O]
equals 1.0 x 10-7
• The ionization constant for water
is written as Kw and is equal to
[H3O]x[OH]
• Kw = 1.0 x 10-14
• Since the concentrations of
hydroxide and hydronium are equal
in water, water is neutral
– If the hydronium concentration is
greater, it is acidic, if the hydroxide
concentration is greater it is basic
• Since the ionization constant is
known, the hydronium or hydroxide
concentration can be calculated if
the other is known
• The pH of a solution is defined as the
negative of the common logarithm of
the hydronium ion concentration
– pH – French for pouvoir hydrogene
“hydrogen power”
– pH = -log [H3O]
– pOH is the negative log of the
hydroxide ion concentration
• To calculate the pH simply take the
–log of the concentration
– If only the OH concentration is known,
divide this from 1.0 x 10-14 and take the
–log of this number
• To calculate the hydronium ion
concentration from pH, take the
antilog of the pH
– (2nd/log/(-)/pH/enter)
• The molarity and ion concentration
are only equal in strong acids and
bases (weak ones must be
calculated)
Determining pH and
Titrations
• Acid base indicators are compounds
whose colors are sensitive to pH
• They are weak acids or bases
• A pH meter
determines the
pH of a
solutions by
measuring the
voltage
between the
two electrodes
that are placed
in the solution
• A titration is the controlled addition and
measurement of the amount of a solution of
known concentration required to react
completely with a measured amount of a
solution of unknown concentration
• The point at which the two solutions used in a
titration are present in chemically equivalent
amounts is the equivalence point
• The point in a titration at which an indicator
changes color is called the end point of the
indicator
• In titrations of strong acids and strong
bases, the equivalence point will be 7
• In titrations of strong acids and weak
bases, the equivalence point will be less
than 7, since the salt formed is a weak
acid
• In titrations of strong bases and weak
acids, the equivalence point will be
greater than 7, since the salt formed is
a weak base