The interaction between starch and I₂ in aqueous medium in the absence of added I^_ has been investigated using Cyclic Voltammetry (CV) and visible absorption spectra. The formation of the familiar blue starch-I₂ complex reduces the peak currents, indicating the complex to be electrochemically inactive. The complex can be treated with an adsorption process that shows an exponential growth, which is qualitatively analogous to a Type-III isotherm. The blue complex shows a broad absorption band with maximum at ~600 nm, which is slightly blue-shifted at higher I₂ concentration. Apart from the blue complex, the evidence of the formation of another complex between uncharged I₂ and starch has been obtained from a shoulder at ~460-470 nm in the absorption spectrum that matches the absorption maximum of I₂ in aqueous medium. In the latter complex, I₂ retains its electrochemical activity and has a slightly less positive redox potential than that of I₂/I^_ couple.

The observation that iodine forms a characteristic blue complex with starch was reported almost two centuries ago (Colin and Claubry, 1814; and Bard and Faulkner, 1980). The starch-iodine complex is still widely used as an indicator in iodometric method of estimation. Extensive studies have been carried out over the years to understand the nature of this complex, which have been reviewed from time-to-time (Isenberg, 1967; French, 1984; and Moulik and Gupta, 1986). The factors affecting its formation as well as its physicochemical properties have also been investigated. The blue complex has been shown to form by absorption of iodine from both solution, mainly aqueous, and vapor phase (Moulik and Gupta, 1986; and Murdoch, 1992).

Starch is a biopolymer of a-D glucose. It consists of two main components called amylose and amylopectin. The former is a linear chain compound, while the latter is a branched chain one. The amylose portion forms a blue complex with iodine and the resulting product was proposed to possess a helical structure (Murphy et al., 1975). Later from spectral and X-ray diffraction studies, it was established that this is indeed an inclusion complex where iodine occupies the annular cavities of more or less regular helical amylose chains. The structurally important helices are composed of six glucose residues per turn and have been found to have a pitch of 0.791-0.817 nm (Rundle and French, 1943; and Rappenecker and Zugenmaier, 1981) and an outer diameter of 1.3 nm. The iodine components appear to be linearly arranged in the 0.5 nm wide inner cavity of the helices with an I-I distance of approximately 0.31 nm (Saenger, 1984).

Starch-iodine complex, Cyclic voltammetry measurements, CV, Absorption spectra, Absorption isotherm, Physicochemical properties, Amylose, Amylopectin, X-ray diffraction studies, Spectrophotometric evidences, Stereochemical Approach, Electrochemical Methods.