Redox Titration, Lecture notes of Analytical Chemistry

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Redox Titration

Redox Titration

• Redox titration is based on the redox reaction (oxidation-reduction) between analyte and titrant.

Ce4+^ + Fe2+^ → Ce3+^ + Fe3+

Determine the end point

• Indicator electrode

• Redox indicators – the indicator has

different color at reduction and oxidation

state.

• Non redox indicator – change color when

excess amount of titrant exists, e.g.

Starch changes to deep blue color when

excess amount I 2 remains

Redox indicator

Adjustment of oxidation state

• Sometimes the oxidation states of

analytes need to be adjusted before

titration – oxidants need to be removed.

• Pre-oxidation:
  • S 2 O 8 2-^ (peroxydisulfate, persulfate): S 2 O 8 2-^ + 2e  2SO 4 2- S 2 O 8 2-^ +2H 2 O → 4SO 4 2-^ +O 2 + 4H+^ (boiling) sometime, Ag+^ is needed as catalyst S 2 O 8 2-^ + Ag+^ → SO 4 2-^ + SO 4 -^ + 4Ag2+
  • H 2 O (^2)

Adjustment of oxidation state

• Pre-reduction
  • SnCl 2 , etc
  • Reductor: Jones reductor: Zn-Zn amalgam Walden reductor: Ag/AgCl

Methods involving Iodine

• Iodine is hardly soluble in water, but very soluble in Iodide solution I 2 + I -^ ⇄ I 3 -^ (triiodide)
• Iodimetry: I 3 -^ as titrant Iodometry: I 3 -^ is produced by adding oxidating analytes into excess amount of Iodide (I - )
• Indicator starch, added near the end point (Iodometry), but at the beginning for Iodimetry.
• Standardization (pure enough for primary standard, but evaporates during weighting) - Weight and dissolve in I - - Use Arsenious Oxide (As 4 O 6 ) or Soduim Thiosulfate (Na 2 S 2 O 4 ) for standardization
• Storage, no light, no oxygen

Can be used to analyze oxidants

and reductants

Thermodynamic and Kinetic

• If there is no net current passing through the

electrochemical cell, the system is at

thermodynamically equilibrium state. The

potential can be calculated by Nernst equation.

• If there is net current passing through, the

system is away from the thermodynamic

equilibrium state, thus the potential can not be

calculated by Nernst equation

Kinetics of electrochemical system

• When Current passing through the system, the potential will move away from that of equilibrium state for three reasons - Overpotential - IR drop - Concentration polarization

IR drop

• Inevitably, the electrochemical system

will have ohimic resistance, when the

current runs through the system, the

voltage will drop due the ohmic

resistance:

∆V= IR

Concentration Overpotential

• Due to the concentration gradient from

the surface of the electrode to the bulk of

the solution.