Iodide, iodine Concentration

In view of the chromophoric character of the elemental iodine itself, many colorimetric methods have been proposed for the deterrnination of inorganic iodine (88—92). These methods use the visible portion of the spectmm in reading iodine concentrations. In the visible range the extinction coefficient for iodine is not high enough to be used for minute quantities of iodine in water and other solvents (93). Higher sensitivities have been reported for elemental iodine in potassium iodide solutions in the ultraviolet (93,94). 

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. 

Discussion. In addition to a small solubility (0.335 g of iodine dissolves in 1 L of water at 25 °C), aqueous solutions of iodine have an appreciable vapour pressure of iodine, and therefore decrease slightly in concentration on account of volatilisation when handled. Both difficulties are overcome by dissolving the iodine in an aqueous solution of potassium iodide. Iodine dissolves readily in aqueous potassium iodide the more concentrated the solution, the greater is the solubility of the iodine. The increased solubility is due to the formation of a tri-iodide ion ... 

The interpretation of the above data on iodination has been questioned by Buss and Taylor217, and by Grovenstein et a/.218,219. The former workers studied the iodination of 2,4-dichlorophenol at about 25 °C using a stirred flow reactor, the advantages of which are that once a steady state has been reached there is no change in the concentration of the reactive species in the reactor with time and the rate of reaction is simply a product of extent of reaction multiplied by the reciprocal ol the contact time hence it is possible to use unbuffered solutions and low iodide ion concentrations. They found general catalysis by the base component of added phosphate buffers and the observed rate coefficients varied with [H+ ] according to... 

The values of kH/kD for the uncatalysed and catalysed reactions were 4.36 and 4.47 respectively, yet the isotope effect is not necessarily diminished on reducing the concentration of iodide ion to zero and by the arguments elaborated above (p. 95) this implies that molecular iodine is not the iodinating species and that this species is formed in some pre-equilibrium, the function of the base being to form the species and not to remove the proton. This argument assumes, as does the previous discussion of the effect of iodide ion concentration on isotope effects, that a minute concentration of I- is insufficient to compete effectively with the reaction involving proton loss. 

Additional kinetic evidence supporting molecular iodine as an iodinating species is sparse. Li325 found that the iodination of tyrosine in acetate buffers at 25 °C showed the mixed inverse dependence on iodide ion concentration noted above, so that part of the reaction appeared to involve the molecular species. Subsequently, Doak and Corwin326 found that the kinetics of the iodination of (N-Me)-4-carboethoxy-2,5-dimethyl- and (N-Me)-5-carboethoxy-2,4-dimethyl-pyrroles in phosphate buffers in aqueous dioxane at 26.5 °C obeyed equation (162), viz. 

Phenyl iodide chemisorbs dissociatively at a Cu(110) surface at 295 K with structural information being obtained from STM and chemical information from XPS.28 At low exposures (6 L), the surface concentrations of carbon and iodine species, calculated from the intensities of the C(ls) and I(3d) spectra, were in the expected 6 1 ratio and the iodine concentration 5.1 x 1014cm 2. With further exposure, the iodine concentration increased and reached a maximum value of 5.5 x 1014cm-2 after an exposure of 1200L. This was... 

The amounts oi adsorption of the polymer on latex and silica particles were measured as follows. Three milliliters of the polymer solution containing a known concentration was introduced into an adsorption tube(lO ml volume) which contained 2 ml of latex (C = l+.O wt %) and silica(C = 2.0 wt %) suspensions. After being rotated(l0 rpm) end-over-end for 1 hr in a water bath at a constant temperature, the colloid particles were separated from the solution by centrifugation(25000 G, 30 min.) under a controlled temperature. The polymer concentration that remained in the supernatant was measured colorimetrically, using sulfuric acid and phenol for the cellulose derivatives(12), and potassium iodide, iodine and boric acid for PVA(13). From these measurements, the number of milligrams of adsorbed polymer per square meter of the adsorbent surface was calculated using a calibration curve. 

The decrease in amount of bound iodine with increase in iodide-ion concentration is reflected in a decrease in the wave length of maximum absorption of the complex.166 On the basis of Kuhn s theory for polyenes,171 this has been related to the breaking of resonating chains of iodine molecules.172... 

Worked Example 3.6. An old medicine bottle is discovered that once contained tincture of iodine. The bottle still contains a trace of solid iodine. Some of the solid is dissolved in an aqueous solution of potassium iodide of concentration 0.1 mol dm . The electrode potential of the I2,1 couple was determined at a platinum electrode immersed in the solution. If = —0.54 V and i,i for the resultant solution is —0.60 V, what is the concentration of the iodine Ignore the presence of the brown 13 ion. 

FIGURE 10. Reaction between polymer-bounded phosphines and methyl iodide relative extinction of 1123cm-1 phosphonium bond in comparison with the iodine concentration in copolymers (styrene-divinylbenzene)104... 

THE EFFECT OF IODIDE ION CONCENTRATION ON THE OBSERVED SECOND-ORDER RATE COEFFICIENT, k%", FOR IODINOLYSIS OF TETRAALLYLTIN BY IODINE IN SOLVENT ACETONE AT 20 °C AND IONIC STRENGTH 0.5 M16... 

The kinetics of iodination of indole and some of its derivatives in aqueous ethanol have been studied over a wide range of iodide ion concentration (4 x 10-4 to 10-1 M) and at different pH values (6.50-7.86).107-109 The observed second-order rate constants are inversely... 

Szllard-Chalmers. In 1954 It was reported by Szilard and Chalmers that radioactive iodine produced by thermal neutron Irradiation of ethyl iodide could be extracted into an aqueous phase. Then by being reduced to iodide and precipitated as silver iodide, the concentrated. radioactive iodide could be obtained (255) ... 

Concentrated sulphuric acid With a solid iodide, iodine is liberated on warming, violet vapours are evolved, which turn starch paper blue. Some hydrogen iodide is formed - this can be seen by blowing across the mouth of the... 

The vapor pressures of iodides of polonium were measured in a closed system. Metalhc polonium was heated in iodine vapor and the pressures were measured. At iodine concentrations below 80 atomic percent and with increasing temperatures, polonium tetraiodide is formed in the condensed phase. The tetraiodide dissociates into P0I2 beginning at a temperature of 473 °K. The evaporation enthalpy of P0I2 is 94.4kj mole. At 80 atomic percent the condensed phase exhibits the presence of Pole. 

Deuterium iodide has been thermally decomposed in the temperature range 660-719 "K by Blagg and Murphy In their experiments the extent of reaction and the rate of the thermal decomposition were determined by measurement of the iodine concentration. At the end of an experiment the products were dissolved in potassium iodide solution and the Ij was titrated with thiosulfate. Table 3 lists the measured rates for the reaction... 

Abel and Stadler re-investigated the reaction under two other conditions of approximately constant iodine concentration (solutions saturated with iodine and also solutions where the iodine concentration was kept low by continuous extraction into benzene). They reported that the process under such conditions was clearly fifth-order, but with one unit of order contributed (in parallel paths) by either iodide or tri-iodide, viz. 

Equation (19-1) describes the behavior of a solution saturated with solid iodine. This half-reaction occurs, for example, toward the end of a titration of iodide with an oxidant such as permanganate, when the iodide ion concentration becomes relatively low. Near the beginning, and in most indirect determinations, an excess of iodide is present, and the half-reaction is more accurately written... 

Soluble starch, available from chemical supply houses, is readily dispersed in water. The iodine-starch complex has limited water solubility, and it is therefore important not to add the starch indicator until near the end point when the iodine concentration is low. Because starch is subject to attack by microorganisms, the solution usually is prepared as needed. Among the products of hydrolysis is dextrose, which can cause large errors because of its reducing action. Various substances have been recommended as preservatives, including mercury(II) iodide and thymol. With formamide a clear solution containing 5% starch is obtained that is stable indefinitely. 

The relatively large quantity of methane formed in the gas phase photolysis of methyl iodide is hard to reconcile with step (3) for which the activation energy would be about 9 kcal.mole . Schultz and Taylor showed that methane formation is independent of temperature over the range 40-100 °C and is also reasonably independent of iodine concentration. They therefore proposed that methane formation proceeds via a hot radical mechanism as follows... 

---