The Starch-Iodine Complex

Starch-guest molecule compounds in inclusion complexes are usually nonstoichiometric species. On the other hand, in the case of amylose such compounds are stoichiometric, but their composition is not repeatable. Starch complexes may consist of partly physical mixtures, adsorbates, and true inclusion complexes formed by direct involvement of dipolar interactions, host-guest hydrogen bonds, and/or clathration-like interactions within the starch matrix. 

Inclusion complexes of amylose are rather well defined, and a consistent theory of such complexes is available that explains amylose complexes with iodine, fatty acids, alcohols, and other guest molecules.4,5 This subject is surveyed in this article because of the growing interest and importance of such complexes in pharmacology and in the food industry. It is probable that starch in its biological sources (tubers, granules) exists in the form of native complexes with proteins, lipids, mineral salts, and water. 

This topic has already been reviewed in several sources,6-15 most recently in 1984. 

Adsorption of iodine by starch was quantitatively analyzed by Harrison23 as well as by Angulescu and Mirescu.33 Both studies entailed the use of I2 in solutions of potassium iodide. Adsorption took place according to the Freundlich isotherm  

Bear46 as well as Katzbeck and Kerr47 established a solution to this problem. The authors observed that it is the so-called V-type (pregelatinized) starch that adsorbs iodine, whereas starch with the A- and B-crystallographic patterns is incapable of adsorbing iodine. The experiments of Rundle et al.48,49 appeared to be crucial in this respect and have shown that starch pretreated with 1-butanol adsorbs iodine vapor rapidly, even if the starch is thoroughly dried. In addition, if there is some inclusion of 1- 

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Suppose 25.00 mL of an aqueous solution of iodine was titrated with 0.0250 M Na2S20,(aq), with starch as the indicator. The blue color of the starch-iodine complex disappeared when 27.65 mL of the thiosulfate solution had... 

Schnepfe [83] has described yet another procedure for the determination of iodate and total iodine in seawater. To determine total iodine 1 ml of 1% aqueous sulfamic acid is added to 10 ml seawater which, if necessary, is filtered and then adjusted to a pH of less than 2.0. After 15 min, 1 ml sodium hydroxide (0.1 M) and 0.5 ml potassium permanganate (0.1M) are added and the mixture heated on a steam bath for one hour. The cooled solution is filtered and the residue washed. The filtrate and washings are diluted to 16 ml and 1ml of a phosphate solution (0.25 M) added (containing 0.3 xg iodine as iodate per ml) at 0 °C. Then 0.7 ml ferrous chloride (0.1 M) in 0.2% v/v sulfuric acid, 5 ml aqueous sulfuric acid (10%) - phosphoric acid (1 1) are added at 0 °C followed by 2 ml starch-cadmium iodide reagent. The solution is diluted to 25 ml and after 10-15 min the extinction of the starch-iodine complex is measured in a -5 cm cell. To determine iodate the same procedure is followed as is described previously except that the oxidation stage with sodium hydroxide - potassium permanganate is omitted and only 0.2 ml ferrous chloride solution is added. A potassium iodate standard was used in both methods. 

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.]... 

Lime gives a vdute turbidity on addition of saturated ammonium oxalate solution, and sulphates with barium chloride acidified with hydrochloric acid. A useful reagent for nitrites is metaphenylene diamine, 5 grams of which are dissolved in water, acidified with dilute sulphuric acid, and made up to one litre. It may be necessary to previously decolorise the solution with charcoal. If nitrites are present in the water to be tested, on addition of the diamine, a yellow colour is produced, either immediately or upon standing. Starch-iodide solution acidified with dilute sulphuric acid may also be used, the characteristic blue colour of the starch-iodine complex indicating nitrites, but this test is not altogether satisfactory. 

The starch-iodine complex of amylose. The amylose helix forms a blue charge-transfer complex with molecular iodine. 

The formation of the starch-iodine complex is affected by alcohols. This effect is interpreted as the result of the isolation of polyiodide chains inside the amylose helices as molecules of alcohol also occupy their interior.119198... 

Mokhnach and Rusakova11 have shown that amylose-iodine and also starch-iodine complexes absorb at 226, 288-290, 344-360, and 585-620 nm. The first band appears only when iodine is added to the solution simultaneously with KI, and it is absent when only iodine is added. The longest-wavelength absorption may be indicative of the molecular size of the carbohydrate portion of the complex, as demonstrated106 in Table VIII. Spectra of iodine-amylopectin complexes were measured by Archibald et al.146 The UV absorption spectrum of the starch-iodine complex is shown... 

Fig. 16.—Absorption of the starch-iodine complex at various concentrations of iodine. The concentration of starch is 0.046 g in every case and the concentration of the solution of 0.01 N I in 100 mL KI varies from 1, 2, 4, 4, and 16 to 32 mL in the case of curves A through F, respectively. (From Lampitt el al.2m)...

Fig. 20.—The iodine-129 Mossbauer spectrum of the starch-iodine complex. [Reprinted with permission from Teitelbaum et a/.128 Copyright (1980) American Chemical Society.]...

Fig. 25.—The shift of the maximum of the specific rotation of the starch-iodine complex as the function of the concentration (c) of iodine (in I2-C6H10O5, mol/mol). (Reprinted with permission from R. C. Schulz, R. Wolf, and H. Mayerhoefer, Kolloid-Z., Z. Polym., 227 (1968) 65-72.)...

Boric acid is known to complex starch (see Section IV), and the same complexation takes place with the starch-iodine complexes. Figure 29 diagrams the assumed position of attached boric acid.159 As with starch, starch blue is capable of sorbing some dyes (Acridine Orange, Methylene Blue, and 1,9-dimethyl-methylene blue).225 This phenomenon is discussed in more detail in the accompanying article (p. 345). 

A method of determining airborne iodine has also been reported.241 Here, iodine is absorbed into 5% aqueous KI and spectrophotometrically determined at 590 nm in the form of its complex with starch. This method is selective with respect to bromine and chlorine, and the sensitivity of this method is 0.25 mg of I2 per m3 of air. The concentration of the, 31I isotope in water can be determined by a method involving isotope exchange in the starch-iodine complex.242 Flow-injection determination of ascorbic acid (0.1-40 mg/mL) has been proposed.243 Iodine is generated in the flow system as I3- ions, which are in turn exposed to starch to produce a steady signal at 350 and 580 nm. Ascorbic acid provides inversed maxima which are measured. This method is recommended for analysis of ascorbic acid in fruit juice, jam, and vitamin-C preparations. Use of the blue complex has also been reported for determination of sodium dichloro-isocyanurate in air.244 Obviously the blue reaction is applicable in the determination of amylose, amylopectin, and starch,245-252 as well as modified starches.245,253-255... 

Use of the starch-iodine complex has been reported for the disinfection of wounds and for the artificial insemination of animals.310 Food containers may also be disinfected with this complex.311 Uses of the starch-iodine complex in the iodization of food312,313 and ruminant fodder314 have also been described. This complex has been found effective in the storage preservation of cabbage and carrots,315 potatoes,316 and apples 317 The addition of 0.0005 to 0.005% of the complex to bee honey also improves its properties it was reported that the application of a 1% aqueous solution of the complex for spraying bee houses increased the yield of honey.318... 

According to Hollo et al.,709 the low stability of starch-alcohol complexes (lower than that of the starch-iodine complex) is caused by the relatively small amount of space inside the amylose helix that is available for the hydrophobic moiety of the alcohol. The data in Table XXX appear to confirm this assumption and indicate iodine sorption amounts by starch complexes with subsequent members of the homologous series of alcohols. With the exception of ferf-butanol, the iodine uptake decreases as the alcohol inside of the helix becomes more bulky. 

Solid-state cellulose can also be noncrystalline, sometimes called amorphous. Intermediate situations are also likely to be important but not well characterized. One example, nematic ordered cellulose has been described [230]. In most treatments that produce amorphous cellulose, the whole fiber is severely degraded. For example, decrystallization can be effected by ball milling, which leaves the cellulose as a fine dust. In this case, some crystalline structure can be recreated by placing the sample in a humid environment. Another approach uses phosphoric acid, which can dissolve the cellulose. Precipitation by dilution with water results in a material with very little crystallinity. There is some chance that the chain may adopt a different shape (a collapsed, sixfold helix) after phosphoric acid treatment. This was concluded because the cellulose stains blue with iodine (see Figure 5.12), similar to the sixfold amylose helix in the starch-iodine complex. 

Because the characteristic blue color of the starch-iodine complex is not formed at high concentrations of hydrochloric acid, starch is not a suitable indicator. Andrews used a few milliliters of an extraction solvent such as carbon tetrachloride or chloroform to obtain a highly sensitive end point. The main disadvantage is the inconvenience of shaking the reagent with the extraction solvent after each addition near the end point. As internal indicators, amaranth, brilliant ponceau, and / -ethoxychrysoidin have given the best results. 

Tlie (deep blue-)black solution in the first beaker is due to the starch-iodine complex formed in the reaction the red color formed in the second beaker is caused by the indicator phenolphthalein and the p-nitrophcnol component in an alkaline medium, while elemental sulfur and A.S2S3 are mainly responsible for the golden yellow color generated in the third beaker. 

The oscillation process, the theoretical principles of which are described in Experiment 96, leads to the periodic formation of free iodine, which is characterised by the blue color of the starch-iodine complex. ... 

Resonance Raman spectra (a) and M6l5bauer spectra (b) of the starch-iodine complex (A) and the model substance (trimesic acid H20)re Hl (B)... 

The combination of the blue color of the starch-iodine complex and excess iodine gives a dark brown solution. 

The molar absorptivity of the starch-iodine complex (after iodate amplification) is... 

In a hydrochloric acid medium (optimum concentration -0.5 M HCl), thallium(I) is oxidized with bromine to thallium(III). Thallium(III) oxidizes iodide to iodine, which gives a blue complex with starch. The molar absorptivity of the starch-iodine complex is 3.9-10 (a = 0.19) at 590 nm. 

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