Starch-iodide-iodine complex

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. 

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. 

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). 

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.]... 

Iodine makes blue colored complexes with many substances such as starch [1, 2] nylon-6 [3], poly(vinyl pyrrolidone) [4], poly(vinyl alcohol) PVA [5, 6], From the application point of view, the blue PVA-Iodine complex is the most important among them, for it is widely used for film polarizers [7,8]. The polarizers are prepared by soaking PVA films in a solution of iodine and potassium iodide (KI) with boric acid, and subsequent drawing to cause the high... 

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 ... 

Potassium iodide and starch Hydrogen Peroxide, H202 If sample is previously acidified by dilute sulfuric acid, a deep blue coloration occurs attributed to the production of iodine complexation with starch... 

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. 

Fig. 2.—Absorption Spectra of Polysaccharide-Iodine Complexes. (I) Mytiltis edulis glycogen, (II) rabbit liver glycogen, (III) waxy-maize starch (amylopectin). [Solutions contained 0.01% of polysaccharide and 0.02% of iodine in 0.2% of potassium iodide, and were read against an iodine—potassium iodide reference solution.]...

Free iodine reacts with starch solution to give the blue-black amylose-iodine complex, which contains iodine in the form of an Ij" chain. The aiiiylopecliii also present in starch reacts with iodine to give a brown violet color. In concen Hated aqueous alkaline iodide solutions and in alcoholic solution iodine has an intense brown color at high dilutions this color changes to yellow because of... 

A dithizone method, a very sensitive thio-Michler s ketone method, and a less sensitive iodide method are discussed below. A simple modification of the iodide method gives a very sensitive method involving the formation of the blue starch-iodine complex. 

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. 

The parallel between the cyloamyloses and V-amyloses helped to explain the well-known blue colour of starch-iodine complexes a series of intensely coloured complexes of cyclohexaamylose, iodine and metal iodides had chains of iodine atoms in the central cavity of stacks of cyclohexaamyloses. The blue colour was thought to arise from charge transfer interactions amongst the I2 molecules and the linear and ions trapped in the central cavity. 

Chlorine (Cl"). A dark blue color of starch-potassium iodide paper, due to the formation of a starch-iodine complex, indicates the presence of CI2. Be careful reddish-brown NO2 gas and Br2 gas may cause a faint test. 

The starch and iodine form a blue complex. The solution becomes colorless when all the iodine has been reduced to iodide by the thiosulfate. The iodine formed in reaction [IV] will range from a strong yellow color (indicating a high level of oxygen) to a dark brown precipitate from reaction [II] to no iodine (colorless solution), corresponding to zero oxygen. 

Turing strucmres have been widely studied in CIMA reaction [67] and its derivatives. When the CIMA reaction is performed in gel media by using starch as an indicator, some striped and hexagonal (spotted) structures [68] are observed. These strucmres are shown in Fig. 1.7. First, a starch-iodide complex is formed. The activator (iodine species) and the starch-triiodide complex generate Turing strucmres which diffuse much more slowly in the gel medium than inhibitor species (chlorite or chlorine dioxide). 

OmpF was successfully reconstituted into Upid vesicles (186,218), which could be demonstrated using an encapsulated enzyme, -lactamase, which is able to hydrolyze the antibiotic ampicillin. The hydrolysis product, ampicillinoic acid, can reduce iodine to iodide, which can be monitored by iodometry, ie via the decol-orization of a starch/iodine complex in the exterior solution. The enzyme and the membrane channel have been shown to preserve their activity in the presence of hydrophobic methacrylate monomers before and after their cross-linking polymerization. Polymerization of hydrophobic iV-butylmethacrylate and ethylene glycol dimethacrylate (186) lead to partial expulsion of some reconstituted channels during the cross-linking reaction, thereby decreasing the enzymatic activity. 

Because the concentration of sodium hypochlorite in bleach depends on the age of the solution, it is important to monitor the oxidation of 1 with starch/iodide test paper to ensure that excess oxidant is present. The test paper should turn blue-black owing to the formation of the complex of starch and iodine that is produced upon oxidation of iodide ion by the hypochlorite ion. On the other hand, commercially available calcium hypochlorite is quite stable and maybe stored at room temperature without decomposition. Therefore, the amoimt of oxidant used is easy to determine, but it is still good practice to ensure that an excess of oxidant is present by applying the starch/iodide test, especially if the reaction mixture is heated. Excess oxidant may be reduced at the end of the experiment using sodium bisulfite, NaHSOj. 

Investigations are carried out by adding solutions of thiosulfate ions of different concentrations to reaction mixtures containing peroxodisulfate and iodide ions. Some starch solution is also added. The time from mixing to the appearance of the blue-black starch-iodine complex is again measured and 1/t is used as a measure of the initial reaction rate. 

The levels of amylase in normal urine and sera have been measured using Phadebas Starch as a substrate. The sera of patients undergoing haemodialysis or treatment with heparin appear to show a rise in the levels of amylase when the amylase is measured by a starch-iodine procedure. A corresponding increase was not observed in in vitro experiments, and it was concluded that interactions between lipoproteins and starch in vivo prevent the starch-iodide complexes from forming, thereby giving a false measurement. 

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... 

The starch-iodine complex does not have a well-defined stoichiometry. The ratio [I ]/([l2]+[IJ]) must be greater than zero in aqueous solution. The actual stoichiometry depends on the polymer chain length, the iodide and salt... 

The Turing mechanism requires that the diffusion coefficients of the activator and inlribitor be sufficiently different but the diffusion coefficients of small molecules in solution differ very little. The chemical Turing patterns seen in the CIMA reaction used starch as an indicator for iodine. The starch indicator complexes with iodide which is the activator species in the reaction. As a result, the complexing reaction with the immobilized starch molecules must be accounted for in the mechanism and leads to the possibility of Turing pattern fonnation even if the diffusion coefficients of the activator and inlribitor species are the same 62. 

PuUy hydroly2ed poly(vinyl alcohol) and iodine form a complex that exhibits a characteristic blue color similar to that formed by iodine and starch (171—173). The color of the complex can be enhanced by the addition of boric acid to the solution consisting of iodine and potassium iodide. This affords a good calorimetric method for the deterrnination of poly(vinyl alcohol). Color intensity of the complex is effected by molecular weight, degree of... 

Wet-Chemical Determinations. Both water-soluble and prepared insoluble samples must be treated to ensure that all the chromium is present as Cr(VI). For water-soluble Cr(III) compounds, the oxidation is easily accompHshed using dilute sodium hydroxide, dilute hydrogen peroxide, and heat. Any excess peroxide can be destroyed by adding a catalyst and boiling the alkaline solution for a short time (101). Appropriate ahquot portions of the samples are acidified and chromium is found by titration either using a standard ferrous solution or a standard thiosulfate solution after addition of potassium iodide to generate an iodine equivalent. The ferrous endpoint is found either potentiometricaHy or by visual indicators, such as ferroin, a complex of iron(II) and o-phenanthroline, and the thiosulfate endpoint is ascertained using starch as an indicator. 

Molecular Interactions. Various polysaccharides readily associate with other substances, including bile acids and cholesterol, proteins, small organic molecules, inorganic salts, and ions. Anionic polysaccharides form salts and chelate complexes with cations some neutral polysaccharides form complexes with inorganic salts and some interactions are stmcture specific. Starch amylose and the linear branches of amylopectin form inclusion complexes with several classes of polar molecules, including fatty acids, glycerides, alcohols, esters, ketones, and iodine/iodide. The absorbed molecule occupies the cavity of the amylose helix, which has the capacity to expand somewhat to accommodate larger molecules. The starch—Hpid complex is important in food systems. Whether similar inclusion complexes can form with any of the dietary fiber components is not known. 

A solution of iodine in aqueous iodide has an intense yellow to brown colour. One drop of 0.05M iodine solution imparts a perceptible pale yellow colour to 100 mL of water, so that in otherwise colourless solutions iodine can serve as its own indicator. The test is made much more sensitive by the use of a solution of starch as indicator. Starch reacts with iodine in the presence of iodide to form an intensely blue-coloured complex, which is visible at very low concentrations of iodine. The sensitivity of the colour reaction is such that a blue colour is visible when the iodine concentration is 2 x 10 " 5 M and the iodide concentration is greater than 4x 10 4M at 20 °C. The colour sensitivity decreases with increasing temperature of the solution thus at 50 °C it is about ten times less sensitive than at 25 °C. The sensitivity decreases upon the addition of solvents, such as ethanol no colour is obtained in solutions containing 50 per cent ethanol or more. It cannot be used in a strongly acid medium because hydrolysis of the starch occurs. 

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