Iodine-starch complexes formation

Figure 16-6 (a) Schematic structure of the starch-iodine complex. The amylose chain forms a helix around l6 units. [Adapted from A T. Calabrese and A. Khan, "Amylose-lodine Complex Formation with Kl Evidence for Absence of Iodide Ions Within the Complex." J. Polymer Sci. 1999, A37,2711.] (fc>) View down the starch helix. Showing iodine inside the helix.8 [Figure kindly provided by R. D. Hancock, [rower Engineering, Sett Lake City.]... 

Both the amylose and amylopectin components of starch form complexes with iodine, but early studies showed that there is no connection between the iodine reaction and the reducibility of starch fractions.63 The complex of amylose is pure blue, whereas the complex of amylopectin is blue-violet.5864 Thus, the varying amylose-to-amylopectin ratio can be one of the factors responsible for the various shades of blue color exhibited by various varieties of starch. Amylopectin takes up less iodine than does amylose. Also the course of complex formation uptake is different, as is evident65 from Fig. 2. 

Probably the best known polysaccharide complex is the blue, starch-iodine complex. This reaction between iodine and the amylose component of starch involves the polymerization of iodine atoms inside the linear cavity of helically coiled, amylose macromolecules. The kinetics of complex-formation for many starches follow the same rate-equation as that for pure amylose, and, thus, the presence of amylopectin does not significantly affect the reaction. The differences between the... 

Iodine-metal alginate compound... Sodium alginate reacts with iodine and under acid gives a blue stained adduct like a starch-iodine inclusion complex. In this paper results for film form metal alginate are presented. The effect of pH on the formation of a blue stained adduct is shown in Figure 4A. At about pH 1 the formation reaches the maximum. The amount of the adduct shows an increase with iodine concentration and... 

The starch-iodine(iodide) complex has been known for centuries. The presence of iodide, iodine and a sufficient amount of water [58] is necessary for the formation of the deep blue complex. Bundle [59] studied its structure by X-ray diffraction, and his results suggest a sixfold symmetrical helical conformation. Starch forms helical complexes not only with triiodide but also with many organics such as butanol or fatty acids, and this property can be used to separate amylose, which forms the helical complex, from other polycarbohydrates (amilopectins) which do not. Without complexing agents the helical conformation of amylose, called amylose-V, is stable only in the crystalline state. The structural parameters of the amylose-iodine(triiodide) complex were determined by Saenger etal. [60,61] in experiments on several model compounds. They found that six monomer units form a turn of the... 

The resulting iodine (or triiodide if we account for complex formation in the presence of excess I ) can then be titrated with sodium thiosulfate using starch as the endpoint indicator... 

The great merit of starch is that it is inexpensive. It possesses the following disadvantages (1) insolubility in cold water (2) instability of suspensions in water (3) it gives a water-insoluble complex with iodine, the formation of which precludes the addition of the indicator early in the titration (for this reason, in titrations of iodine, the starch solution should not be added until just prior to the end point when the colour begins to fade) and (4) there is sometimes a drift end point, which is marked when the solutions are dilute. 

The yield is higher in basic solution in the presence of an oxidizing agent. Iodine forms a blue complex with P-amylose in starch. A hnear array of I5 species consisting of I2 -1 - I2 units bound to the amylose hehx causes blue color formation. 

Color Plate 12 Formation of Diffusion Layer During Electrolysis (Box 17-3) (a) Cu electrode (flat plate, left) and Pt electrode (mesh basket, right) immersed in solution containing KI and starch, with no electric current, (b) Starch-iodine complex forms at surface of Pt anode when current flows. 

I hen iodine is mixed with starch, I the amylose stTands coil around the iodine molecules, forming a starch-iodine complex that has a characteristic dark blue color. The more starch present in a solution, the deeper the blue. The formation of this color is used to identify the presence of starch. In this activity, you will use an iodine solution to test for the presence of amylose and the amylose-digesting action of your saliva. 

The compression of starch in the presence of some mineral salts (CoCl2 and FeCl3) and of elemental iodine leads to the formation of starch-additive inclusion complexes. Among many salts tested, some form only physical mixtures, as proved by thermal analysis.63,64 (See Appendix for additional text.)... 

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