Cadmium , redox with complexes

Cathodic electrodeposition of microcrystalline cadmium-zinc selenide (Cdi i Zn i Se CZS) films has been reported from selenite and selenosulfate baths [125, 126]. When applied for CZS, the typical electrocrystallization process from acidic solutions involves the underpotential reduction of at least one of the metal ion species (the less noble zinc). However, the direct formation of the alloy in this manner is problematic, basically due to a large difference between the redox potentials of and Cd " couples [127]. In solutions containing both zinc and cadmium ions, Cd will deposit preferentially because of its more positive potential, thus leading to free CdSe phase. This is true even if the cations are complexed since the stability constants of cadmium and zinc with various complexants are similar. Notwithstanding, films electrodeposited from typical solutions have been used to study the molar fraction dependence of the CZS band gap energy in the light of photoelectrochemical measurements, along with considerations within the virtual crystal approximation [128]. 

In the most sensitive eiectroanaiytical methods, exclusively treated within this context, the analyte ion is electrodeposited on an electrode from an electrically conducting sample solution. Current and potential of subsequent redissolution are due to the concentration and the kind of ion to be determined. For thallium, the reversible redox couple TI /TI° at about -0.5 V versus saturated calomel electrode is used (Bellavance and Miller, 1975). Infinite tolerance towards alkali, alkaline earths and halogenides are great merits for the analysis of biological materials. Because of the preconcentration step included, thallium determination is more sensitive than atomic spectrometric methods. For thallium, the multielement capabilities of the method can hardly be used, because lead and frequently cadmium have to be masked with excess of complexants, leaving just Tl in the potential... 

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