Titrations
Titration is a volumetric procedure for determining the
concentration of an unknown species by adding a
carefully measured volume of a known species that
reacts with the unknown.
The equivalence point in a titration occurs when the
requirements of the balanced equation for reaction are
exactly satisfied.
An indicator is used to find the end-point pf the titration.
The end-point is the point where the indicator changes
colour. This may not be at the equivalence point
For an acid base titration the equivalence point is the point
where neutralisation occurs.
Indicators must have the following properties :
a) the color change must be sharp. i.e. the indicator must
change colour with the addition of one drop of solution
from the burette.
b) change colour as close to the equivalence point as
possible
c) Give a distinct colour change
Titration Curves
Strong Acid-Strong Base Titrations
The equivalence point of the titration is the point at which exactly enough titrant
has been added to react with all of the substance being titrated with no titrant left
over. In other words, at the equivalence point, the number of moles of titrant added
so far corresponds exactly to the number of moles of substance being titrated
according to the reaction stoichiometry.
Titrations Involving a Weak Acid or Weak Base
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There are three major differences between this curve
(in blue) and the one we saw before (in black):
1. The weak-acid solution has a higher initial pH.
2. The pH rises more rapidly at the start, but less
rapidly near the equivalence point.
3. The pH at the equivalence point does not equal
7.00.
POINT OF EMPHASIS : The equivalence point for a
weak acid-strong base titration has a pH > 7.00.
Weak Base / Strong Acid
1. The weak-base solution has a lower initial pH.
2. The pH drops more rapidly at the start, but less rapidly near
the equivalence point.
3. The pH at the equivalence point does not equal 7.00.
POINT OF EMPHASIS : The equivalence point for a weak basestrong acid titration has a pH < 7.00.
Choosing an indicator
In the strong acid titration, both indicators begin to change colour
at the equivalence point (50 mL of base) so both work equally well.
In the weak acid titration, thymol blue changes colour at the
equivalence point, but methyl red begins to change colour after
only 15mL of base are added, which is far from the equivalence
point, illustrating the importance of choosing an appropriate
indicator.
Primary Standards
A primary standard is a reagent that is extremely pure, stable, has no waters of
hydration, and has a high molecular weight.
Some primary standards for titration of acids:
sodium carbonate: mol wt. = 105.99 g/mol
tris-(hydroxymethyl)aminomethane (TRIS or THAM): mol wt. = 121.14 g/mol
Some primary standards for titration of bases:
potassium hydrogen phthalate (KHP): mol wt. = 204.23 g/mol
potassium hydrogen iodate: mol wt. = 389.92 g/mol
Some primary standards for redox titrations:
potassium dichromate mol wt. = 294.19 g/mol
Secondary Standards
A secondary standard is a standard that is prepared in the laboratory
for a specific analysis. It is usually standardized against a primary
standard
.
Some secondary standards for titration of acids
Sodium hydroxide
Potassium hydroxide
Standardisation Calculations
Sodium carbonate (anhydrous) is used as a primary standard. A
solution of sodium carbonate of concentration 0.100 mol/L is used to
standardise a solution of hydrochloric acid. 25.0 ml of the standard
solution requires 35 ml of the acid for neutralisation. Find the
concentration of the acid.
Na2CO3
+
2HCl

2 NaCl
+
CO2
+
1 mole of Na2CO3 neutralises 2 moles of HCl
No of moles of Na2CO3 in 25 ml = 0.1 x 25 = 2.5 x 10 -3 mol
1000
No of moles HCl in 35 ml = 2 x 2.5 x 10 -3 = 5.0 x 10 -3 mol
Conc of the acid = 5.0 x 10 -3 x 1000 = 0.143 mol/L
35
H2 O