Triiodide/iodide reference electrodes

The triiodide/iodide electrode (Pt/lJ/I ) has been recommended as reference electrode in MeCN and propylene carbonate [223]. It was found to have a higher exchange current... 

On the basis of extensive screening of hundreds of ruthenium complexes, it was discovered that the sensitizer excited state oxidation potential should be at least - 0.9 V versus Saturated calomel reference electrode (SCE), to inject electrons efficiently onto Ti02 conduction band. The ground state oxidation potential should be about 0.5 V versus SCE, to be regenerated rapidly via electron donation from the electrolyte (iodide/triiodide redox system) or a hole conductor. A significant decrease in electron injection efficiencies will occur if the excited and ground state redox potentials are lower than these values [32]. 

Because of possible interference of the iodide or triiodide ions with sample solutions a KCl bridge electrolyte is introduced for practical applications. In this case a very small and constant liquid-junction potential is obtained at the inner junction of triiodide-iodide solution and the KCl solution and at the outer bridge diaphragm the same junction potentials with KCl solution are obtained as with other reference electrodes too. 

Figure 11. Principle of operation of the dye-sensitized nanocrystalUne solar cell. Photoexcitation of the sensitizer (S) is followed hy electron injection into the conduction band of an oxide semiconductor film. The dye molecule is regenerated by the redox system, which itself is regenerated at the counter-electrode by electrons passed through the load. Potentials are referred to the normal hydrogen electrode (NHE). The energy levels drawn match the redox potentials of the standard N3 sensitizer ground state and the iodide/triiodide couple. (Redrawn from Gratzel [187] with permission from publisher, Elsevier. License Number 2627070632803).

Equation (5.5.5) describes the relationship between the electrode potential and the concentrations of dissolved triiodide and iodide ions at a given temperature. The manufacturing of this redox electrode is easy and gives reproducible reference potentials if suitable housing materials like glass, platinum, and ceramic are used. 

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