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

recalibratedBUT• 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 TitrationsHarris 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 indicatorAs iodine is generated it is used up by the reaction with arsenicOnce 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 polymerof 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