The influence of thiocyanate ions on the formation of the starch-iodine complex

The redox reaction between thiocyanate and iodine species present in an aqueous solution of KI results in the formation of thiocyanogen, (SCN)(2). This reaction interferes with the formation of inclusion complexes of a potato amylose with iodine species such as I-3(-) and I-5(-). As these ions react with thiocyanate ions (SCN-) the blue colour of the amylose-iodine complex diminishes. The rate of this decrease depends upon the concentration of both the SCN- and the iodine, and appears to follow pseudo-second order kinetics. Using a UV/visible spectrophotometer the reaction of iodine with KSCN can be followed, with or without amylose, and shows a lowering in concentration of iodine species absorbing at 290-300 nm, together with a lowering in absorbance at 630 nm, due to a lesser amount of the amylose-iodine complex. Interference with iodine complex formation was also noted with a potato amylopectin. The technique provides the basis for the study of other reactions of iodine such as that with selenocyanate (SeCN-).