2. Aum Sri Sai Ram Department of Chemistry ELECTROCHEMICAL MONITORING OF PHOTOLYTIC INTERMEDIATES Hari Kumar Y
3. Introduction: Photochemical reactions often involves species that are electrochemically interesting, for example excited states , radicals , and metals with light influenced oxidation states are very common intermediates or products. “So we can study photochemical processes with electrochemical tools”
4. Experimental techniques are divided into different types based on the way in which the solution containing compounds to be studied is irradiated. Different approaches are: 1.Irradiating the solution continuously by an intense source. 2.Irradiating the solution with a short burst of light of high intensity.
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6. This is done so that the overall photochemical reaction hapens at steady state approximately.
7. Any electrometric method can be used to monitor the intermediates or products. Some techniques used are: (1) Potentiometric measurements (2) Photopotential studies + Faradic pproaches (thesehave more sensitivity and selectivity) These Faradic approaches include polarography and amperometry at the DME.
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9. Absorption by the photo active solute creates a high initial concentration of the primary intermediate, its decay with time often leads to the rise and fall of other transient species that appear later in the reaction scheme.
10. Because these time dependencies tell much about the photolysis mechanism flash methods are immensely valuable to photochemistry and have become very common.
11. The intermediates are monitored by various methods . (1) Spectroscopic methods like UV or visible absorption spectroscopy. (2) Electrochemical monitoring (like observing the current transient arising from the electrolysis of flash generated species at an eletrode held at constant potential ) (3) Cascaded approaches like using both spectroscopic approaches + electrochemical approaches simultaneously
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13. The current decay is not linked simply to a time dependent bulk concentration of the electroactive substance. Superimposed on that time function is the normal cottrell decay from the diffusion process.
14. These difficulties are eliminated through a technique called time delayed potentiostatic analysis.COMPLICATIONS INVOLVED IN MONITORING:
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16. At time τ the switch closes, electrolysis begins and for a period thereafter one see a current decay that is governed by a corttell relation unperturbed by reactant disappeareance caused by decay.
17. An observation proportional to the bulk concentration of the transient species can be obtained by sampling the current at a fixed interval following τ.
18. Repeating the experiment with different potentials or different τ values enables one to derive current – potential curves for a sequence of delay times, hence we can obtain both qualitative and quantitative information about the transients.
19. SimuLtaneous electrochemical and optical monitoring of flash transients provides more information TIME DELAYED POTENTIOSTATIC ANALYSIS
22. The modifications are: (1)RRDE design in which discs are replaced by a quartz window through which the light was directed downward (2)RRDE design featuring a transparent thin film platinum disc through which the photolytic flux was directed. Advantages of these approaches are: a. Measurements are simpler than flash methods b. For strongly absorbing solutions they feature a localized photolysis in a region near the electrode. c. They also provide methods to study photolytic processes in substances that are produced electro- chemically. REVIVED CONTINUOUS IRRADIATION APPROACHES
