Facilitation of quantitative estimation of the composition of starch‐iodine blue by specific precipitation

Abstract The fact that potassium fluoride specifically and quantitatively precipitates the blue starch‐iodine complex from an excess of starch was used for the direct estimation of the composition of starch‐iodine blue for the first time. The result shows starch‐iodine blue to be extremely stable in nature and with a definite stoichiometry regarding starch, iodine and iodide, which is in good agreement with the result computed from X‐ray data obtained with cyclodextrin in place of starch. The mole ratio of starch monomeric units/iodine atoms was found to be 2,90 and the iodine/iodide ratio 4,28. The spectra of precipitated starch‐iodine blue dissolved in water are identical with those of the original starch‐iodine blue in the UV and visible regions. 1 M KF also quantitatively precipitates the adduct of starch‐iodine blue with the dye 1,9‐dimethyl‐methylene blue (DMMB), thus facilitating the estimation of starch‐iodine and the dye adduct.