Coulometric Titration
Answers for prelab and write up
Titration
• Use titrant of known concentration
and identity. Should be stable.
• Titrate into known volume of
analyte and determine the volume of titrant
required to reach an end point. (Close to
equivalence point)
• The reaction between titrant and analyte
must be fast, quantitative and known.
What if Iodine is the desired titrant?
• Standard solution must be constantly
recalibrated
BUT
• Can generate it in situ electrochemically.
• Can generate it as it reacts
• Stop generating when endpoint is reached
• Measure the moles of electrons that were
required to generate it
• Therefore know the moles of titrant
required
Iodimetric titrations p351 on
• Iodimetry = titration with Iodine
• As3+ titrated with I3• An iodine solution made up in KI will be
unstable because a KI solution is unstable
– iodine is formed, especially in the light.
• So solutions would have to be
standardized before use – and not even
just once when first made up
Iodometric titrations
• Titration of iodine (I3-) produced by analyte
• We will put bleach with iodide to form I3• Then add excess thiosulfate and titrate
with iodine – back titration
• Two reasons for the back titration here –
allows us to do a coulometric titration
• Allows us to see a colourless to blue color
change for starch (better than watching
blue fade)
Coulometric Titrations
Harris p 369,370
Titrant is generated electrochemically
by constant current
Coulometric titrations are similar to
volumetric titrations:
• The concentration of the titrant is
equivalent to the generating current
•The volume of the titrant is equivalent
to the generating time.
Coulometric titration
• Analyte should react with 100% current
efficiency
• Use a constant current
• Measure current rather than volume
• Current is coulombs/time
• Faraday = coulombs per mole = 96,485
coul/mol
• Moles electrons = current x time / F
• Iodide is oxidized to Iodine
• Then – iodine reacts with arsenic
• KI is present – supplies the iodide for
producing iodine
• Also keeps the iodine in solution by
forming I3-
• Iodine is generated at one electrode
(oxidation)
• Must be a reduction at the other electrode
• Water is reduced to hydrogen gas
• Can see bubbles forming on the electrode
• Electrodes have large surface area so that
the current density is low.
Endpoint - use starch indicator
As iodine is generated it is used up by the
reaction with arsenic
Once all the arsenic has reacted there
will be excess iodine present which will
interact with the starch to give a blue
colour.
The iodine slides
into the starch helix.
Active fraction is
Amylose- a polymer
of α-D-glucose
Endpoint
• How well we determine the endpoint will
determine the precision and the LOD.
• Try and keep the same blue endpoint
colour each time
• Not too dark or cannot see the exact
shade
• To be more precise – use a potentiometric
endpoint. This could help lower the LOD.
• Could then also use a lower current.
We titrate endpoint to endpoint!
• Rather than starting with a new setup for
each titration, once one titration is
complete we add in more analyte and
repeat the titration.
• The overall volume is not important – we
are titrating moles with moles.
• So we start at a blue endpoint and titrate
till the next blue endpoint is reached.
Determining LOD
• We can titrate our blank and see how long
it takes us to see the blue colour.
A buffer is prepared for each
titration vessel
• The optimal pH range for performing the
reaction is 7 - 9. Bicarbonate is used to
restrict the pH to this range
• Complete oxidation of As(III) requires a pH
above 7 (Le Chatelier principle)
• AsO33- + I3- + H2O ↔ AsO43- + 3I- + 2H+
• To avoid disproportionation of iodine the
pH must be smaller than 9.
• 3I2 + 3 H2O ↔ IO33- +5I- + 6H+
Back Titrations
• Harris page 23
• Want to titrate excess thiosulfate with
iodine
• So there is no unreacted iodine present
until endpoint –then we get blue colour
• Endpoint would be difficult to see going
blue to colourless