Adsorption, of iodine by starch

Adsorption, of iodine by starch, I, 256 molecular configuration and, I, 277 by starch, I, 252, 255 Adynerin, I, 148, 171 Aerobacler aerogenes, in fermentation of wood sugars, IV, 184 Aerobacler levanicum, levan formed by, II, 228... 

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 ... 

J. Walker and S. A. Kay showed that the brown product of the action of iodine in aq. or chloroform soln. on magnesia is a case of adsorption analogous to the adsorption of acids by silk, and of iodine by starch. P. Guichard studied the adsorption products with iodine vapour and alumina, silica, magnesia, and beryllia. 

Its value for the description of experimental data is convincingly elucidated in the well-known monograph by McBain [2, p. 5]. This formula was introduced originally by analogy with the distribution of a substance between phases in which it exists in various states of association. However, experimental values very soon showed the impossibility of this explanation it allows neither fractional values of n which vary smoothly with temperature, nor such values as n = 10 for the adsorption of iodine on starch [3]. 

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. 

The precipitated Cul and the adsorption of iodine by the Cul make the end-point difficult to detect. This is mitigated by adding KSCN just before the end-point Pipette 25.0 cm of -0.1 M Cu solution into a conical flask and add 10 cm of 10% KI solution. Titrate the liberated iodine with a standardised 0.05 M sodium thiosulphate solution until the iodine colour fades to a straw colour. Then add 2 cm of freshly prepared starch solution indicator and continue titration until the blue colour just fades. Then add 10 cm of 10% thiocyanate solution when the blue colour becomes more intense. Complete the titration quickly until the blue colour does not reappear within a minutes. The flesh-coloured precipitate Cul remains at the end-point. 

Adsorption of Iodine from Aqueous I2-KI Solutions by Starch of Various Origins69... 

The yields of A-fraction by the above method are substantially higher than the 22-23% previously reported by butyl alcohol fractionation, since the latter method does not give as complete a separation. From the iodine adsorption of raw corn starch and of the purified A-fraction, Bates, French and Rundle have calculated the content of A-fraction in corn starch as 22%. This low value is due to the use of incompletely defatted starch, and its agreement with the yield by butyl alcohol precipitation is purely coincidental. Under preferred methods of testing, exhaustively defatted corn starch adsorbs 5.3% iodine. Dividing this latter value by the 19.0% iodine adsorption for the recrystallized A-fraction, a theoretical content of 28% is calculated for corn starch, in agreement with the yields by Pentasol fractionation. 

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... 

Huebner and Venkataraman69 demonstrated that starch can adsorb iodine from nonaqueous and not necessarily polar solvents, as shown in Table IV. Iodine adsorption by starch in aqueous ethanol increases as the ethanol content increases. Again, the solvent effect depends on the origin of the starch (Table II). The varying effects observed using benzene, ethanol, and chloroform may also be ascribed to the use of starches of different origin... 

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. 

As originally reported, selective precipitation of the A-fraction was accomplished by saturating a starch paste with butyl alcohol, then autoclaving for 2-3 hours at 18-20 lb. pressure. On cooling to room temperature, the A-fraction separated in characteristic crystalline form and could be collected by supercentrifuging. The yield was 22-23% frona either corn or potato starch. The iodine adsorption of the crude precipitated A-fraction was 16.5% for the non-precipitated B-fraction it was 1.5-1.7%. 

When the A-fraction from corn starch is potentiometrically titrated with iodine, its affinity for the latter is sharply reduced in the presence of small amounts of fatty acid. Thus the iodine adsorption of recrystallized A-fraction (originally 18.7%) is reduced to 12.4%, 3.5% and 0% by the addition respectively of 2%, 5% and 10% of palmitic acid. Raw corn starch contains approximately 0.66% of fatty acids, corresponding to approximately 2% on the basis of the linear A-fraction. Thus, a third of the linear component in raw com starch is inactivated. In a sense, Taylor and coworkers were correct in assuming an association between a-amylose and fatty acid, but they erred in presuming the combination to be an ester. 

The effect of temperature on adsorption equilibrium Is worth mentioning. Rise of temperature decreases the extent of adsorption. This is in spite of the fact that surface tension decreases with a rise in temperature. The reason behind this observation is that molecular cohesion, a fundamental cause of adsorption, is disrupted by thermal agitation. For example, when iodine is added to starch solution, a blue colour appears due to formation of starch iodide. However, upon heating, the blue colour disappears due to the failure of iodine to remain adsorbed on the starch particles at higher temperatures. 

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. 

When examining curves of this type, one involuntarily inquires if both phases of the reaction are caused by the same enzyme or if the second phase is not an action of traces of /3-amylase. But if malt extracts are heated to temperatures causing inactivation of the /3-amylase, a variation of time and temperature does not alter the relation between the velocities of the two phases nor the relation between these velocities and the time necessary to change the starch so that it is not colored by iodine. These relations also are not altered if the a-amylase is partly removed from the solutions by adsorption on bentonite, activated carbon, aluminum... 

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