Starch Iodine reaction

Starch in helical conformation is indicated by the blue color developed with iodine. In a typical test, a tissue or extract is submerged in a KIS solution. DP heterogeneity is a factor the higher the DP, the higher is the I2 absorption and the more intense is the violet-blue color. The starch-iodine reaction is sensitive enough for starch to be a titrimetric indicator of I2, mindful of the nonstoichiometry and nonspecificity of the reaction. 

To avoid an excess of ozone, the gas is metered by using conventional flow meters. Otherwise, the introduction of ozone is maintained until ozone appears in the exit gas. The presence of ozone is easily recognized by the starch-iodine reaction. The exit gas is passed through an aqueous solution containing 5% of alkaline iodide, 5% of sulfuric acid, and a few drops of a starch solution. Iodine, which is liberated by ozone, forms a dark blue color with starch [7i], Another sign of the end of ozonization is the light-blue tinge of the ozonized solution. 

The discovery of the starch-iodine reaction is ascribed to the French chemists j. j. Colin and H. F. Gaultier de Claubry (1814). The chemist F. Stromeyer from Gottingen studied this reaction independently, writing as follows on January 15th to Professor Gilbert, the editor of the Annalen der Physik ... 

The metachromatic reaction of heparin and heparinoids with dyes is an important key to the riddle of their chemical nature. A similar reaction is the starch-iodine reaction. This is due to the fixation of the iodine atoms in the starch micelle in an orderly array and is a well-known example of a clathrate, in which atoms or molecules are held in channels of larger molecules by strong, non-polar forces— hydrogen bond and London forces. It is the possession of a similar structure but with highly polar groups (NH SO4, O SO3, COO ) in close proximity, which endows the mucopolysaccharides and heparinoids with their highly specific, characteristic properties. 

Preparation of m-nitrophenol from m-nitroaniline 557 A cold mixture of water (450 ml and concentrated sulfuric acid (330 ml) is poured, with stirring, over finely powdered ra-nitro-aniline (210 g), and ice (800 g) is added. When the mixture has become homogeneous, a solution of sodium nitrite (105 g) in water (250 ml) is run in during 8-10 min from a dropping funnel until the starch-iodine reaction remains positive. The temperature is kept between 0° and 5° and the mixture is stirred for a further 5-10 min. Then the m-nitrobenzenediazonium sulfate is allowed to settle, the supernatant liquid is poured off and the solid washed, if necessary, with water by decantation in order to purify the salt filtration is necessary only when gross impurity is present. 

It should be noted that the starch-iodine reaction is more sensitive the lower the temperature. Blue starch-iodine solution loses its color when heated the color reappears on cooling. The sensitivity is decreased by large amounts of alum, magnesium sulfate, alkali sulfate, and the like. Certain organic materials (proteins, resorcinol), when present in large amounts, completely prevent the starch-iodine reaction. Cyanides, which impair the test by formation of cyanogen iodide,... 

A good technical grade of carbon tetrachloride contains not more than the following amounts of impurities 1 ppm acidity as HCl, 1 ppm carbon disulfide if manufactured by carbon disulfide chlorination, 20 ppm bromine, 200 ppm water, and 150 ppm chloroform. The residue should not exceed 10 ppm on total evaporation. The product should give no acid reaction with bromophenol blue, and the starch iodine test should indicate the absence of free chlorine. 

The dependence of reaction rates on pH and on the relative and absolute concentrations of reacting species, coupled with the possibility of autocatalysis and induction periods, has led to the discovery of some spectacular kinetic effects such as H. Landolt s chemical clock (1885) an acidified solution of Na2S03 is reacted with an excess of iodic acid solution in the presence of starch indicator — the induction period before the appearance of the deep-blue starch-iodine colour can be increased systematically from seconds to minutes by appropriate dilution of the solutions before mixing. With an excess of sulfite, free iodine may appear and then disappear as a single pulse due to the following sequence of reactions ... 

Documentation is carried out as soon as the iodine-colored chromatogram zones can be readily recognized. Then the adsorbed iodine can be allowed to evaporate in the fume cupboard or vacuum desiccator, so that the same chromatograms can be subjected to further reactions and separation steps (e. g. SRS techniques, 2-D separations, coupling techniques such as TLC/GC etc.). The chromatogram zones can also be stabilized by spraying with 0.5 to 1 percent starch solution [4, 5] the well-known blue clathrates that are formed (starch-iodine inclusion compounds) remain stable for months. 

However, the starch solution should not be omitted completely since the color difference between the chromatogram zones, in which the iodine is reduced to colorless iodide according to the iodine azide reaction mentioned above, and the background colored brown by unreacted iodine is considerably less than the difference in color between the deep blue background provided by the starch-iodine clathrate complex and the pale chromatogram zones. 

Rendleman, J. A. (2003). The reaction of starch with iodine vapor. Determination of iodide-ion content of starch-iodine complexes. Carbohydr. Polym. 51,191-202. 

To determine the amylose content of starch, the iodine reaction has been most commonly used because amylose and amylopectin have different abilities to bind iodine. The methods such as blue value (absorbance at 680 nm for starch-iodine complex using amylose and amylopectin standards), and potentiometric and amperometric titration have been used for more than 50 years. These procedures are based on the capacity of amylose to form helical inclusion complexes with iodine, which display a blue color characterized by a maximum absorption wavelength (kmax) above 620 nm. During the titration of starch with iodine solution, the amount (mg) of iodine bound to 100 mg of starch is determined. The value is defined as iodine-binding capacity or iodine affinity (lA). The amylose content is based on the iodine affinity of starch vs. purified linear fraction from the standard 100 mg pure linear amylose fraction has an iodine affinity of 19.5-21.0mg depending on amylose source. Amylopectin binds 0-1.2mg iodine per 100mg (Banks and Greenwood, 1975). The amylose content determined by potentiometric titration is considered an absolute amylose content if the sample is defatted before analysis. 

Dextrin is a hydrolysis product intermediate betw starches and sugars, such as dextrose (see next item). It does not give the blue iodine reaction of starch and is not fermentable, but is changed to maltose by the action of enzymes (diastase) and to dextrose by the action of acids... 

Discard the filtrate and transfer the ppt quantitatively to an iodine reaction flask by successive washings with 3NHC1 and hot w. Cool, add iO mi of 10% KI solo and titrate with N/10 Na SjOj soln until near discoloration. Add 5 ml of 0.5% starch soln and continue titration to greenish coloration Following reactions take place ... 

In direct opposition to this view may be placed the facts already stated of the combined effect of heat aQd solvents. Thus, if a little potato starch be heated on a metal plate to 3130° or 390°, and then, examined after treatment with a little water, the several layers will be found swollen, and by adding an aqueous solution of iodine they will bo made to appear with considerable distinctness. The question, however, is not yet thoroughly answered, though the mass of evidence is in favor of the view that an outar cuticle surrounds the inclosed matter, from which, as well by the iodine reaction as by its less solubility, such coating differs in many of its properties. 

Starch is used as an indicator in the reaction. The end point is marked by the appearance of a deep blue starch-iodine complex when the first fraction of a drop of unreacted If remains in the solution. 

Here is a coulometric procedure for analysis of total sulfite in white wine. Total sulfite means all species in Reaction (A) and the adduct in Reaction (B). We use white wine so that we can see the color of a starch-iodine end point. 

The sutjhydryl group is determined by reaction with iodine, which is produced in the vessel from potassium iodide, added in excess to the solution, and potassium iodate, added from a buret until the completion of the reaction is shown by the permanent appearance of ihe blue color of starch-iodine. 

Ealy, "The Starch-Iodine Clock Reaction," Chemical Demonstrations, A Sourcebook for Teachers, Vol. 1 (American Chemical Society, Washington, DC, 1988), pp. 107-108. 

An accurately weighted amount of primary standard is dissolved in water containing an excess of potassium iodide. Upon acidification, stoichiometric amounts of iodine are liberated instantly, which are titrated with thiosulfate titrant of unknown strength, decolorizing the blue starch-iodine complex at the end point. With potassium iodate, the ionic reaction is as follows ... 

The reaction then takes place and its progress is checked until the absence of nitrous ions. After 40 minutes, the presence or absence of nitrous ions is checked at regular intervals in the reaction medium. Starch-iodine paper, for example, is used, checking every 5 minutes. After about 60 minutes of reaction, the nitrous acid had been entirely consumed and no more N02" ions remained in the reaction medium. The pH was then adjusted to 7 with pure caustic soda, and the products of the reaction were recovered by the addition of 31 liters of pure ethanol (2 volumes). The precipitate formed was collected by centrifugation, washed with ethanol and dried at 60°C under vacuum. 

---