The reaction of starch with iodine vapor. Determination of iodide-ion content of starch–iodine complexes

The molecular-iodine and iodide-ion contents of starch–iodine complexes, prepared by subjecting corn amylose (average degree of polymerization, dp 1050), low-molecular-weight amylose (dp 61–69), amylose–cyclohexanol complexes, and native corn starches to iodine vapor for 30 days under different conditions of relative humidity (RH), were determined by a differential method requiring titrations with both KIO3 and Na2S2O3. Iodide content generally increased with increase in relative humidity. However, the analytical method was incapable of providing reliable iodide values for complexes of very low total iodine content (<5 wt%), such as those of ordinary corn starch, waxy maize starch, and low-dp amylose. A granular hybrid corn starch of 64% amylose content yielded, at 100% RH, a complex containing 10.0 wt% of total iodine (I/I−=3.7). Low-dp amylose exhibited low reactivity toward iodine vapor, even at 100% RH; however, a cyclohexanol complex of low-dp amylose reacted readily at 100% RH, with concurrent elimination of cyclohexanol, to produce a starch–iodine complex of unusually high iodine content (33.1 wt% of total iodine; I/I−=3.65). The behavior of corn amylose toward iodine was found to be dependent upon its method of preparation from corn starch. One method of preparation yielded an amylose that, under anhydrous conditions, was very resistant to complexation with iodine, but which was very reactive at 30–100% RH, producing a complex that contained as much as 18.5 wt% of total iodine (I/I−=3.49). A different procedure for preparing amylose gave a starch that reacted readily under anhydrous conditions to produce a complex containing 21.8 wt% of molecular iodine and no detectable amount of iodide ion. A cyclohexanol complex of corn amylose reacted with iodine vapor at 100% RH, with concurrent elimination of cyclohexanol, to form a starch–iodine complex having a total iodine content of 31.6 wt% (I/I−=3.91). For most of the iodine complexes, there was a close similarity between I− content and H+ content, suggesting that the source of iodide ion was hydrolysis of molecular iodine by water of hydration in the starch.