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

摘要

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 KIO_3 and Na_2S_2O_3. Iodide content generally increased with increase in relative humidity.However, the analytical method was incapble of providing reliable iodide values for complexesof 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 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 (33.1wt% of total iodine, I/I~- = 3.65). The behavior of corn amylose toward iodine was found to be dependent upon its metod of prepration from corn starch. One method of prepration yielded an amylose that, under anhydrous conditions, was very resistance to complexation with iodine, but which was very reactive at 30-100 RH, producing a complex that contiained as much as 18.5 wt% of total iodine (I/~- = 3.49). A different procedure for preparating amylose gave a starch that reacted readily under anhydrous conditins 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 10% RH, with concurrent elimination of cyclodexanol, 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.