Triiodide-starch complex

In this step the reddish brown color of the triiodide begins to fade to yellow and finally to clear, indicating only iodide ions present. However, this is not the best procedure for determining when all of the I3 has disappeared since it is not a sensitive reaction and the change from pale yellow to colorless is not distinct. A better procedure is to add a soluble starch solution shortly prior to reaching the end point, since if it is added to soon, too much iodine or triiodide ion may be present forming a complex that may not be reversible in the titration. The amount of thiosulfate is proportional to the amount of hypochlorite ion present. 

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. 

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. 

Add a drop of the reaction mixture to a piece of starch-iodide paper. Any unreacted hypochlorite will cause the appearance of the blue starch-triiodide complex. Add 1-mL portions of saturated sodium bisulfite solution to the reaction mixture until the starch-iodide test is negative. 

Specific indicators owe their behavior to a reaction with one of the participants in the titration. The best-known specific indicator is starch, which forms a dark blue complex with triiodide ion. Also, potassium thiocyanate is used as a specific indicator, for example, in titration of iron(III) with solutions of titanium(III) sulfate. 

It has been known for almost 200 years that starch gives a deep blue color when a solution of potassium iodide and iodine is added [47]. More than a century later it was suggested that the complex consisted of a helical polysaccharide, with triiodide in the center of the helix [48]. Using flow dichroism, it was demonstrated that the triiodide was stacked in a linear structure, as required for the helical model [49]. Another study of the optical properties of crystals of the amylose-triiodide complex showed it to be consistent with a helical structure [50] and X-ray diffraction showed the triiodide complex gave the dimensions of a unit-cell of a helix with six glucose residues per turn [51]. This confirmed a helical structure for the amyiose complex with triiodide that predated the helical models proposed by Pauling for polypeptides [52] and the double helical model for DNA by Watson and Crick [53] by 10 years. 

A small amount of starch solution is added as an indicator because it forms a deep-blue complex with the triiodide solution. Disappearance of the blue color thus signals the completion of the titration. Suppose 53.2 L of a gas mixture at a temperature of 18°C and a total pressure of 0.993 atm is passed through a solution of potassium iodide until the ozone in the mixture has reacted completely. The solution requires 26.2 mL of a 0.1359-M solution of thiosulfate ion to titrate to the endpoint. Calculate the mole fraction of ozone in the original gas sample. 

Staich, which forms a blue complex with triiodide ion, is a widely used specific indicator in oxidation/reduction reactions involving iodine as an oxidant or iodide ion as a reductant. A starch solution containing a little triiodide or iodide ion can... 

Figure 3. Turing patterns in the chlorite-iodide-malonic acid reaction. Dark areas show high concentrations of starch-triiodide complex. Each frame is approximately 1,3 mm square. Images courtesy of Patrick De Kepper,...

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). 

Figure 6.14 Patterns observed in the chlorite-iodide-malonic acid reaction in a Couette reactor. The CSTR composition, flow rate, and rotation rate are held fixed, except for chlorite composition in one CSTR, whieh serves as the bifurcation parameter. In each frame, the abscissa represents the position along the reactor and the ordinate represents time. The dark color results from the presence of the starch- triiodide complex. (Adapted from Ouyang et al., 1991.)...

Figure 14.5 Schematic depiction of the formation of the starch triiodide complex.

The intensely blue color is due to a charge-transfer complex formed by the triiodide ion and starch. It results from the inclusion of tri-iodide ions into the straight-chain fractions of a-amylose of starch, which form a helix. The formed clathrate exhibits a narrow band of charge transfer near 620 nm (Fig. 18.2). 

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 resulting iodine (or triiodide if we account for complex formation in the presence of excess I ) can then be titrated with sodium thiosulfate using starch as the endpoint indicator... 

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