Starch triiodide color

The disappearance of the faint yellow color of triiodide ion was used to determine the end point of the titration with sodium thiosulfate. This gave better results than were obtained when the solution was diluted with sufficient water to use the starch triiodide end point in the presence of ethyl alcohol. 

The method the iodine reagent to obtain the blue color of starch contains 0.2% (w/v) iodine in a 2.0% (w/v) potassium iodide solution [172]. Usually a solution with 10-times this concentration is prepared and then diluted 1 10 immediately before use. The analysis is carried out by the addition of 25 pL of the diluted iodine/iodide reagent to 50 pL of each sample in a microtiter plate well and the absorbance is measured at 600-620 nm, using a microtiter plate colorimeter. The starch triiodide blue color is most stable under acidic conditions (pH 1-2) and will not form under alkaline conditions (pH 8 and higher). 

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.)...

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. 

Bassam Z. Shakhashiri, "Color Variations of the Landolt Reaction," Chemical Demonstrations, A Handbook for Teachers of Chemistry, Vol. 4 (The University of Wisconsin Press, Madison, 1992), pp. 26-28. Three pairs of colorless solutions are mixed. After 10 seconds the three mixtures turn red, yellow, and blue, respectively. Each clock reaction is the reduction of iodate by bisulfite. The observed colors are related to the amount of triiodide produced, which depends on the relative amounts of iodate and bisulfite reacted, and whether or not starch is present. 

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. 

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). 

To enhance the visibility of the oscillations and spatial structures in the CIMA reaction, starch is used as a color indicator of triiodide in most experiments. In the open spatial gel reactors developed in Bordeaux and Texas polyacrylamide gel (PAA) was used as the reaction medium to prevent convective motion. The starch is introduced into the acrylamide monomer solution before poly-... 

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