Triiodide ion

The best known polyhalide is the triiodide ion, Ij", found when iodine dissolves in the aqueous solution of the iodide of a large unipositive cation (usually K ) ... 

The hberated iodine, as the complex triiodide ion, may be titrated with standard thiosulfate solution. A general iodometric assay method for organic peroxides has been pubUshed (253). Some peroxyesters may be determined by ferric ion-catalyzed iodometric analysis or by cupric ion catalysis. The latter has become an ASTM Standard procedure (254). Other reducing agents are ferrous, titanous, chromous, staimous, and arsenite ions triphenylphosphine diphenyl sulfide and triphenjiarsine (255,256). 

Figure 4 Free energy surfaces of triiodide ion in various solutions. Contours correspond to isoenergy lines of IkgT, Ik T, and 3kgT, respectively.

Computed free energy surfaces of the triiodide ion in its ground state in acetonitrile, methanol, and aqueous solution are presented in Figure 4, in which the two I—I bond... 

The observed rate coefficient will be less than the true rate coefficient since some iodine is converted to triiodide ion through the equilibrium for which AT[IJ ] = DUDn, ie- 2 = D/Pzl which reduced to kota = k2Kf(K+[l ])... 

V02+) by chromium(VI) (HCrO ), when carried out in the presence of iodide ions, results in formation of triiodide ions, I3,4 This reaction occurs rather rapidly, whereas both HCrO and vanadium(V) (VOJ, a product) oxidize I- so slowly that these reactions can be ignored. The net reactions with and without I" are... 

L.19 Iodine is a common oxidizing agent, often used as the triiodide ion, I3. Suppose that in the presence of HCI(aq),... 

Sei f-Test 12.2B When iodide ions react with iodate ions in basic aqueous solution, triiodide ions, I,, are formed. Write the net ionic equation for the reaction. (Note that the same product is obtained in each half-reaction.)... 

Self-Test 13.4A The reaction S2082 (aq) + 31 (aq) — 2 S042 (aq) + I3 (aq) is commonly used to produce triiodide ion in a clock reaction," in which a sudden color change signifies that a reagent has been used up. Write the rate law for the consumption of persulfate ions and determine the value of k, given the following data ... 

Although a catalyst does not appear in the balanced equation for a reaction, the concentration of a homogeneous catalyst does appear in the rate law. For example, the reaction between the triiodide ion and the azide ion is very slow unless a catalyst such as carbon disulfide is present ... 

Step 2 The complex then reacts rapidly with triiodide ion in a series of fast... 

Notice that the rate law is first order in the catalyst, carbon disulfide, but zero order in triiodide ion, which appears only in the fast step following the slow step. 

When iodine dissolves in organic solvents, it produces solutions having a variety of colors. These colors arise from the different interactions between the I2 molecules and the solvent (Fig. 15.21). The element is only slightly soluble in water, unless I ions are present, in which case the soluble, brown triiodide ion, I,, is formed. Iodine itself has few direct uses but dissolved in alcohol, it is familiar as a mild oxidizing antiseptic. Because it is an essential trace element for living systems but scarce in inland areas, iodides are added to table salt (sold as iodized salt ) in order to prevent an iodine deficiency. 

C09-0023. The fourth molecular shape arising from a steric number of 5 is represented by the triiodide anion I3. Determine the molecular geometry and draw a three-dimensional picture of the triiodide ion. 

The ion reductions of iodine, triiodide ion and bromine are all simple second-order , with no acidity dependence. The rate and activation data can be summarised ( = 1.0 M) as... 

Inasmuch as the soil used was slightly alkaline (pH 7.70) it was believed that some of the triiodide produced in the soil distillate by Reaction 1 would be lost. The addition of a small quantity of acetic acid to this distillate considerably increased the percentage of ethylene dibromide recovered in the analysis (Table I). However, the addition of acid aids the oxidation of iodide ion to triiodide ion by oxygen of the air according to Equation 3. 

By adjusting the acid concentration it was possible to minimize both Reaction 3 and the loss of triiodide ion and thus modify the procedure of Brenner and Poland (1) sufficiently to obtain a 59.6% recovery when 0.125 mg. of ethylene dibromide had been added to 70 grams of air-dried soil. Larger quantities could be recovered more completely. 

At the end of this period the solution was removed from the condenser while still hot and titrated immediately with 0.002500 N sodium thiosulfate before any appreciable oxygen could be absorbed and oxidize iodide ion to triiodide ion. The disappearance of the yellow color of triiodide ion against a white background was used for the end point. These solutions usually had a slight brown tint at the end point, which was assumed to be organic matter distilled over from the soil. Accordingly, the blank was usually titrated first and its final color was used as a standard end point color for the other three solutions run with it. 

The error introduced by Reaction 3 was further diminished by flushing out much of the oxygen from the refluxing system by boiling the contents before adding the potassium iodide. Crystals between 2 and 5 mm. were used because smaller ones had a tendency to stick to the walls of the wet condenser. Early in this work it was noticed that crystals of potassium iodide which were not washed down were often partly oxidized to iodine, which worked down into the flask and caused high results. Thus it was important to wash all the potassium iodide down the condenser with water. Crystals of potassium iodide were used rather than a solution because this avoided the possibility of such a solution being oxidized to contain triiodide ion. 

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. 

Apart from recapture of the injected electrons by the oxidized dye, there are additional loss channels in dye-sensitized solar cells, which involve reduction of triiodide ions in the electrolyte, resulting in dark currents. The Ti02 layer is an interconnected network of nanoparticles with a porous structure. The functionalized dyes penetrate through the porous network and adsorb over Ti02 the surface. However, if the pore size is too small for the dye to penetrate, that part of the surface may still be exposed to the redox mediator whose size is smaller than the dye. Under these circumstances, the redox mediator can collect the injected electron from the Ti02 conduction band, resulting in a dark current (Equation (6)), which can be measured from intensity-modulated experiments and the dark current of the photovoltaic cell. Such dark currents reduce the maximum cell voltage obtainable, and thereby the total efficiency. 

The presence of the peak at 362 nm could witness, also in this case, the formation of the triiodide ion. 

Linear triatomic anions The pioneering crystallographic studies of Odd Hassel140 on trihalides and related donor-acceptor species led to a far-reaching analysis of such Hassel compounds by Henry Bent.141 The triiodide ion (I3-, stable in aqueous solution) and other known linear trihalide XYZ- species also served as the prototype for Pimentel s incisive three-center MO analysis of hypervalency. We shall therefore begin with NBO/NRT investigation of a series of hypervalent and non-hypervalent triatomic anions in order to make contact with these classic studies. While ab initio studies add many quantitative details to the understanding of these species, the basic picture sketched by Bent and Pimentel is found to be essentially preserved. 

Returning to the extraction of iodine from an aqueous solution of iodine and sodium chloride, the effect of adding iodide to the system is to involve the iodine in formation of the triiodide ion... 

The third class of redox species are couples located near the conduction band of WSe2- The only outer-sphere example found, which is suitable for use in aqueous electrolytes, is Ru(NH3)e3+. Its reduction is characterized by an immediate onset upon accumulation in the semiconductor and a tafel slope of 130 mV/decade. The reduction mechanism appears to be direct reduction of the Ru(NH3)e3+ by electrons from the accumulation layer. The only member of the forth class of redox species is triiodide ion. It is characterized by adsorption onto the semiconductor surface as was demonstrated by the first application of chronocoulometry to a semiconductor electrode (another demonstration of the reproducibility and low background currents on... 

However, iodine is not very soluble in water. Therefore, an aqueous solution of iodide ion is added to the iodine to form the complex ion, triiodide ion, I3 (aq). In dilute concentrations, the triiodide ion is yellow and in concentrated solutions it is a dark reddish-brown. 

Finally, the triiodide ion is titrated with a known solution of thiosulfate ions, which forms iodide ions ... 

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. 

Explanation Iodine is sparingly soluble in water but undergoes rapid dissolution in the presence of potassium iodide due to the formation of the corresponding triiodide ion ... 

A TLC method for determination of quaternary ammonium antiseptics was proposed, using silanized silica plates in combination with triiodide ions and UVV densitometry at 400 nm. The method was applied to cetylpyridinium chloride, cetrimide (197c) and the isomers of benzalkonium chloride (197b)420. 

Structural analyses of the triiodide ion, in crystals of this ion with various counterions, show that the I3 unit is always linear, or nearly so, and that there is considerable variety in the I I distances, which range from 2.67 A (covalent bond length) to 4.30 A (nonbonded contact distance). Further, there is strong correlation between the two I I distances in the I I I ion. Very similar correlations were obtained for the S-S S grouping in the thia-thiophthenes 37, for the O-H O groupings in a number of hydrogen-bonded... 

Oxidative microcoulometry has become a widely accepted technique for the determination of low concentrations of sulfur in petroleum and petroleum products (ASTM D3120). The method involves combustion of the sample in an oxygen-rich atmosphere followed by microcoulometric generation of a triiodide ion to consume the resulting sulfur dioxide. It is intended to distinguish the technique from reductive microcoulometry, which converts sulfur in the sample to hydrogen sulflde that is titrated with coulometrically generated silver ion. 

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. 

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