1. Experiments in Analytical Chemistry
-Spectroelectrochemistry
-Observation of spectral change of o-tolidine

2. Spectroelectrochemistry
Combination of optical and electrochemical techniques
Two quite different techniques are combined
Both spectroscopic and electrochemical information can be acquired at the same time
Provides very reliable information concerning the system under investigation
Frequently used for studying the redox chemistry of inorganic, organic, and biological molecules

3. Various methods Transmission Reflectance (specular, internal) Raman
Fluorescence

4. Representative applications of spectroelectrochemistry

5. Optically transparent electrode
OTEs enable light to be passed directly through the electrode and adjacent solution.
Electrode transparency is necessary for spectroelectrochemistry.
First type: Thin film of conductive materials such as Pt, Au, C, etc on a transparent substrate such as glass or plastic, or quartz.
Second type: OTE with small holes in the electrode.
Minigrid electrodes
Porous reticulated vitreous carbon
Minigrid electrode
Reticulated vitreous carbon

6. Optical absorption spectroelectrochemistry
The most frequently used optical technique in spectroelectrochemistry
Thin-layer spectroelectrochemistry
Solution thickness < 1 mm
Optical beam is passed directly through the transparent electrode and the solution.
Oxidation states of species is precisely controlled.
Applied potential controls the ratio of O and R.
Formal potential and number of electrons, n, can be determined.

7. Optical absorption spectroelectrochemistry
-example
Spectra recorded during thin-layer spectropotentiostatic experiment on 1.04 mM [Tc(III)(dmpe)2Br2]+, 0.5 M TEAP in DMF.
Nernst plot at 499 nm

8. Optical absorption spectroelectrochemistry
Semi-infinite diffusion spectroelectrochemistry
The electrode is in contact with an electrolyte solution that is much thicker than the diffusion layer.
Used primarily to measure fast homogeneous chemical reactions of electrogenerated species.
In above situation, absorbance change depends on t and k.

9. Other spectroelectrochemical cells
In situ IR cell
In situ Raman cell
In situ X-ray cell in transmission mode

10. In our experiments Species: o-tolidine
Working electrode: Pt gauze electrode
Reference electrode: Ag/AgCl
Counter electrode: Pt wire
Electrolyte: pH 7.0 buffer
Tolidine(red) ↔ Tolidine(ox) + ne- E0’
(no color) (colored)
E = E0’ log{[Tolidine(ox)]/[Tolidine(red)]}

11. Instruments: UV-vis spectrophotometer, potentiostat
log{[O]/[R]} = log{(A2-A1)/(A3-A2)}
A1: absorbance of reduced tolidine
A2: measured absorbance
A3: absorbance of oxidized tolidine

12. In your report,
Drive the equation, [O]/[R] = (A2-A1)/(A3-A2)
Obtain molar absorptivity of oxidized o-tolidine. Path length will be given.
Plot E vs. [O]/[R] and calculate number of electrons in a tolidine redox reaction.