Starch-iodine blue complex

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. 

Substances containing active chlorine or bromine oxidize iodide ions — if necessary under the influence of UV light - to iodine, which reacts with starch to yield the well-known intense blue starch-iodine inclusion complex. 

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... 

X-ray fiber diffraction data of film form metal alginate showed broad peaks corresponding to around 7.2A, 8.7A. The characteristic fiber peak observed in the blue stained film prepared at pH 1 corresponds to 3.13A. This value is similar to 3.06A for I-I distance of iodine packed in helical cylinder of starch-iodine inclusion complex. 

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... 

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. 

The disappearance of iodine at the end point is detected by the addition of fresh starch solution which gives a blue complex as long as iodine is present. 

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... 

Peroxides Potassium iodide + starch Peroxides release free iodine which forms a blue complex with the starch. [17, 33]... 

Peroxides release free iodine which forms a blue complex with the starch. 

It should be noted that molecular complexes of the cyclodextrins may be isolated as crystalline solids for example, a crystalline complex is obtained with iodine (which resembles the well known blue complex between iodine and starch) as well as with a large number of other inorganic and organic guests. 

SAQ 8.9 Iodide reacts with thiosulphate to form elemental iodine. If the reaction solution contains a tiny amount of starch solution, then this I2 is seen by eye as a blue complex. The data below were obtained at 298 K. Determine the order of reaction, and hence its rate constant k. 

Often, to make the endpoint of an iodine titration more obvious, an indicator solution that contains starch is added to the solution being titrated. Starch forms a deep blue complex with triiodide. Is", but it is colourless with l . As long as there is unreacted vitamin C in solution, no triiodide ions will be present in solution. Therefore, the blue colour will appear only at the endpoint. 

APHA Method 4500-CL02-B, iodometric titration analysis, measures the concentration of chlorine dioxide in water by titration with iodide, which is reduced to form iodine. Iodine is then measured colorimetrically when a blue color forms from the production of a starch-iodine complex. The detection limit for this method is 20 pg/L (APHA 1998). 

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. 

The reactions of potassium iodide in aqueous solutions are those of iodide ion, r. In iodometric titration I combines with iodine to form triiodide ion, I3. The latter adds to (i-amylose fraction of the starch to form a blue complex. 

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. 

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. 

Numerous analytical procedures are based on redox titrations involving iodine. Starch8 is the indicator of choice for these procedures because it forms an intense blue complex with iodine. Starch is not a redox indicator it responds specifically to the presence of I2, not to a... 

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. 

In both starch and glycogen the glucose emits of the main chains are linked with a-1,4 linkages. An extended conformation is not possible and the chains tend to undergo helical coiling. One of the first helical structures of a biopolymer to be discovered (in 1943)76 77 was the left-handed helix of amylose wound around molecules of pentaiodide (I5 ) in the well-known blue starch-iodine complex78 (Fig. 4-8). Tire helix contains six residues per turn, with a pitch of 0.8 nm and a diameter of nearly 14 nm. Amylose forms complexes of similar structure with many other small molecules.79... 

Ozone is an extremely powerful oxidizing agent. In fact, of the common oxidizing agents, only F2 is more potent. A standard method for detecting ozone in polluted air is to pass the air through a basic solution of potassium iodide that contains a starch indicator. The ozone oxidizes iodide ion to iodine, I2, which combines with the starch to give the deep blue starch-iodine complex ... 

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 ... 

Starch is composed of macromolecular components, a-amylose and (i-aim -lose. The former reacts irreversibly with iodine to form a red adduct. (i-Aim losc. on the other hand, reacts with iodine forming a deep blue complex. Because this reaction is reversible, [3-amyl0sc is an excellent choice for the indicator. The undesired alpha fraction should be removed from the starch. The soluble starch that is commercially available, principally consists of (3-amylose. (3-Amylose is a polymer of thousands of glucose molecules. It has a helical structure into which iodine is incorporated as I5. ... 

When thiosulfate or PAO titrant is slowly added to the deep blue solution of starch-iodine complex, the reducing agent takes away iodine from the helix. At the end point, when all the iodine is lost, the solution becomes colorless. 

When all sulfite in the sample is consumed, the excess iodine reacts with starch to form blue coloration. Thus, at the end point of the titration, when no sulfite is left, iodine produced in situ from the addition of a single drop of standard iodide-iodate titrant into the acid solution forms the blue complex with starch previously added to the sample. 

We can detect the appearance of iodine with the aid of starch indicator this reagent forms a blue complex with iodine. The time it takes for the blue color to suddenly appear indicates when all the bisulfite was used up in the first reaction. That s why the name iodine clock. Thus you should measure the time (with a stopwatch, if available) elapsed between mixing the two solutions and the appearance of the blue color. Place the reactants in two separate 150-mL beakers according to the outline in Table 20.1. 

As vitamin C is oxidized by iodine, I2 becomes reduced to I . When the end point is reached (no vitamin C is left), the excess of I2 will react with a starch indicator to form a starch-iodine complex which is blackish-blue in color. 

I2 + starch — iodine — starch complex (blackish-blue)... 

Complex Negative Ions, (a) To 1 cc. 1N CuS04 add 10 cc. water and 2 cc. of 1A KI. Note that the solution turns brown and that a precipitate is formed. Let the precipitate settle, pour off the brown solution and note that a drop of it will turn some starch paste blue, showing the presence of iodine. Wash the precipitate by decantation and note that it is white after the brown solution is removed. To the precipitate suspended in about 2 cc. of water add a small crystal of potassium iodide. This dissolves quickly, giving a fairly concentrated solution. Note that the precipitate dissolves in the KI solution. 

Starch exists in two major forms, amylose and amylopecttn. Although amylose is a linear polymer like cellulose, the a stereochemistry enables the chains to twist and to assume a helical shape. Small molecules such as l2 fit inside the helix and form a complex. This is the basis for the deep blue color in the starch-iodine test. This non-straight or bent conformation is sug-... 

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 helical structure of amylose also serves as the basis for an interesting and useful reaction. The inside of the helix is just the right size and polarity to accept an iodine (I2) molecule. When iodine is lodged within this helix, a deep blue starch-iodine complex results (Figure 23-19). This is the basis of the starch-iodide test for oxidizers. The material to be tested is added to an aqueous solution of amylose and potassium iodide. If the material is an oxidizer, some of the iodide (I-) is oxidized to iodine (I2), which forms the blue complex with amylose. 

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

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