Starch, complexes iodine adsorption

Although the dipolar and resonating nature of the interaction of amylose and iodine is well established, Schlamowitz173 regards the iodine in a starch complex as being in a predominantly non-polar form, and Meyer and Bern-feld174 refute the helix theory and consider that adsorption of iodine occurs on colloidal micelles in amylose solutions. Most of the experimental facts which Meyer presents can, however, be satisfactorily explained on the helical model. 

Iodates, III, 138, 157, 178 Iodic acid, as oxidant, III, 178 Iodine, adsorption by starch, I, 256 colors of products formed from starch by amylases, V, 261 as oxidant, III, 151, 169 solubility of, III, 136 Iodine starch complex, dichroism of flow of, I, 266... 

Investigating the reaction between hydroxylammonium chloride and iodine Weigh out accurately about 0.86 g of purest HO.NH3.CI and make up to 250 cm in a volumetric flask. Pipette 25.0 cm aliquot of the solution into a conical flask and dilute to about 150 cm Warm the solution on a hot plate, fitted with a magnetic stirrer. Add 0.15 g purest MgO (used to avoid acidity of the solution) and add slowly (standardised) 0.05 M iodine solution from a burette keeping the suspension warm. When the colour of iodine fades slowly, add 2 cm of freshly prepared starch solution to the conical flask and continue the titration slowly until the blue colour of the starch/iodine adsorption complex persists for 30 seconds. From the average of two concordant titres, deduce the molar ratio I2 to (HO.NH3) and hence an equation representing the reaction, based on changes in oxidation numbers. 

As with iodine, starch forms an inclusion complex with bromine vapor.205 Depending on the starch variety, different colors are developed by the complex. Maize and wheat produce an ochre color, rice produces a light-buff color, potato and sago develop a pale-yellow color, and cassava forms a cream color.69 Iodine cyanide (and bromine)-amylose complexes are brown-black and dark brown, respectively.206 The adsorption of chlorine and iodine proceeds according to the Freundlich isotherm. A discontinuity on the Freundlich isotherm plot is reported, which possibly results from the swelling of starch granules454... 

The instability of starch sols can be traced primarily to the presence of the linear A-fraction. The adsorptive affinity of this component is manifest in all the reactions of starch pastes and sols. It is responsible for retrogradation, the blue coloration with iodine, and the adsorption of various polar materials. The latter reaction has been utilized to effect a quantitative precipitation of the A-fraction. Thus, when a starch sol is treated with polar organic substances containing a hydrophilic group (such as hydroxyl or carboxyl) attached to a hydrophobic residue, the A-fraction adsorbs this material by polar attraction for the hydrophilic group. The resulting adsorption complex is insoluble by reason of its hydrophobic loading and consequently separates from solution. 

The adsorption of iodine by amylose chains to give a deep blue complex is universally used to qualify starch, and to quantify amylose. For some industrial uses, it is imperative that starch hydrolysates contain no starch, and this can be confirmed by cooling the matrix to 5-10°C, adding 0.02moll iodine drop-wise, and observing whether a blue color appears the detection limit is 50 mg per kg. 

Repeat the above precipitation of lead chromate. No need to use a weighed dried crucible. After filtration and washing the precipitate, place it in a 250 cm beaker, add 1 1 HNO3 to dissolve the chromate followed by 20 cm of 10% KI solution, stir to liberate iodine, add water and titrate with standardised 0.05 M thiosulphate solution until the colour of iodine fades. Then add 2 cm of freshly prepared starch solution until the dark iodine/starch adsorption complex disappears, leaving a green Cr(Ill) solution. Alternatively, filter through a GF filter paper, transfer it quantitatively and carefully, after washing, to a conical flask and continue as above. 

Place each mixture in a shaking thermostat at 25"C (or a large 2-4 dm beaker containing water at room temperature) as quickly as possible after mixing and note the time on a stopwatch when the blue colour of iodine/starch adsorption complex appears. Plot the time against l/[A][B]. 

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