Good
Titrations:
Volumetric Analysis-1
Ch6-2 CHEM 3369
1. Volumetric Analysis
A titration is a procedure in which
increments of the known reagent solution –
the titrant – are added to analyte until the
reaction is complete.
Titrant is usually delivered from a buret.
Each increment of titrant should be
completely and quickly consumed by
reaction with analyte until the analyte is
used up.
Common titrations are based on:
• acid-base reactions
• oxidation-reduction reactions
• precipitation reactions
• complex formation reactions
Ch6-3 CHEM 3369
How to Determine the Reaction Has
Gone to Completion
(1) detecting a sudden change in voltage or current
between a pair of electrodes (Chapters 10 & 14)
Methods of determining when analyte has been consumed include:
(2) monitoring the absorbance of light by species in the
reaction (Section 19-3)
(3) observing an indicator color change (Sections
6-6, 9-6, 10-4, 13-3, and 16-2)
Ch6-4 CHEM 3369
When the Titration Reaction Stops
Equivalence point: the point at which stoichiometric amounts of
reactants have been added (theoretical):
moles of analyte
originally in flask
moles of titrant
added from buret
=
End point: what we actually measure – marked by a sudden
physical or chemical change in solution.
?end point - ?equivalence point = ?titration error
® want to make titration error as small as possible
® choose appropriate end point detection
® do a blank titration
If one mole of titrant reacts with one mole of analyte
Ch6-5 CHEM 3369
Example 1: An Acid-Base Titration
After an initial buret reading,
base (OH– ions) is slowly added
to the acid (H+ ions).
H+(aq) + Cl–(aq) + Na+(aq) + OH–(aq) ® H2O(l) + Cl–(aq) + Na+(aq)
Near the end of the titration, the
indicator momentarily changes
to its base color but reverts to its
acid color with swirling.
When the end point is reached, a
tiny excess of OH– is present,
shown by the permanent change
of color of the indicator.
Base
Acid plus
phenolphthalein
(colorless
in acid
& pink
in base)
Ch6-6 CHEM 3369
change occurs
over ~2 pH units
An acid/base indicator is a weak organic
acid or base, whose undissociated form
differs in color from its conjugate base or its
conjugate acid form
low
basic
blu
mixture of yellow and
yel
e
lueColor change of the indicator bromothymol blue
acidic

Ch6-7 CHEM 3369
Na+(aq) + Cl–(aq) + Ag+(aq) + NO3–(aq) ® AgCl(s) + NO3–(aq) + Na+(aq)
AgNO3(aq)
NaCl(aq)
Prior to the
titration,
the solution
is yellow,
due to the
dichlorofluorescein
indicator.
Milky-white
AgCl
suspension
appears
during the
titration.
The pink
suspension
appears at
the end
point, when
the anionic
indicator
becomes
adsorbed to
the cationic
particles of
precipitate.
Example 2: A Precipitation Titration
COLOR PLATE 1 The Fajans titration of Cl– with Ag+.
Ch6-8 CHEM 3369
A crystal growing in the presence of
excess lattice anions (anions that belong
in the crystal) has a negative charge
because…
See Figure 6-6 Ions from solution are adsorbed on the surface
of a growing crystallite.
… anions are
predominantly
adsorbed.
A crystal growing in the
presence of excess lattice
cations has a positive
charge and…
…can
therefore
adsorb
a negative
indicator ion.
Ch6-9 CHEM 3369
Example 3: A Redox Titration
5C
2O42– + 2MnO4– + 16H+ ® 10CO2 + 2Mn2+ + 8H2O
Up to the
equivalence point,
all the added
permanganate is
consumed by
oxalate, and the
titration solution
remains colorless.
After the
equivalence
point, unreacted
permanganate
builds up until
there is enough
to see.
Titrant is also
an indicator
Ch6-10 CHEM 3369
Two Types of Volumetric Titration
In a direct titration, titrant is added until the end point is observed.
analyte
Unknown
titrant
Known
+ ® product
Titrant (All colorless)
Permanganate
(purple)
Analyte
Oxalic acid
(colorless)
5HO–C–C–OH + 2MnO4– + 6H+ ® 10CO2 + 2Mn2+ + 8H2O
O
||
O
||
Example: addition of permanganate titrant to oxalic acid analyte