Iodine adsorption potassium iodide

Liquid-phase adsorption methods are widely used for quaUty control and specification purposes. The adsorption of iodine from potassium iodide solution is the standard ASTM method D1510-83 (2). The surface area is expressed as the iodine number whose units are milligrams of iodine adsorbed per gram of carbon. It is quite fortuitous that the values of iodine numbers turn out to be about the same as the values for surface areas in square meters per gram by nitrogen adsorption for nonporous carbon blacks. 

If freshly precipitated, magnesium hydroxide is tinted deep brown-red by a solution of iodine in potassium iodide. The color fades on digestion with potassium iodide, alcohol, potassium hydroxide or other solvents for iodine. It is also discharged by treatment with sulfite or thiosulfate. Likewise, when magnesium hydroxide is formed in the presence of free iodine, the precipitate is brown-red. It is obviously an adsorption compound of magnesium hydroxide and iodine. The most suitable conditions for the formation of this colored product (even in dilute solution) are attained when an iodine solution is decolorized with sodium or potassium hydroxide immediately before the addition of the magnesium salt. In the hypoiodite solution, the following equilibrium prevails ... 

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. 

Adsorption of iodine by starch was quantitatively analyzed by Harrison23 as well as by Angulescu and Mirescu.33 Both studies entailed the use of I2 in solutions of potassium iodide. Adsorption took place according to the Freundlich isotherm ... 

So far we have considered the adsorption to be influenced solely by attractive forces to the carbon surface, but often of equal or even greater significance are the forces that solutes can exert on solution status of one another. Potassium iodide increases the solubility of iodine in water and thereby reduces the adsorption of iodine. Conversely, the adsorption of fatty acids is increased when the solubility is reduced by the addition of sodium chloride. The effect on fatty acids is in sharp contrast to that observed with mercuric chloride. The adsorption of mercuric chloride may be diminished 60% when sodium chloride is present this action is ascribed to the formation of a weakly adsorbable complex, Na2HgCI4.28... 

Chaney13 observed a correlation between the retentivity of a carbon and the adsorptive power for iodine, which he termed the activity. The activity is obtained by determining the amount of iodine which 1 gram of dried 200-mesh carbon will adsorb from SO milliliters of 0.2 N iodine-potassium iodide solution. The mixture is agitated for 3 minutes, filtered, and the first portions of the filtrate are discarded. The iodine concentration of an aliquot portion of the filtrate is determined by titration. Highly active carbons for gas-adsorption will remove approximately 90 % of the iodine. 

The need to know the chemical nature of the adsorptive is well illustrated by two simple studies, that of iodine (I2) from an aqueous solution in potassium iodide (KI) and of acetic acid (CH3COOH) from an aqueous solution. The Kl acts as a reservoir for the iodine by forming KI3 (K and IJ). It is necessary to do this because of the limited solubility of iodine, as such, in water. What is needed to be known, for the construction of appropriate isotherms is the chemical nature of the adsorptive, be it (I2) or (If) for the iodine system, or CH3COOH or CH3C00H(H20) for the acetic acid system. Further, there is the need to elucidate if (a) the iodine is adsorbed via pore filling or via capillary condensation mechanisms and (b) to demonstrate uncertainties which exist when adsorbing species can be associated with water molecules in aqueous solution, as for adsorption of acetic acid (Hill and Marsh, 1968). 

A range of iodine concentrations was used with controlled potassium iodide concentrations and equilibrated over known weights of carbon. To know extents of adsorption of iodine by the carbon, the latter was centrifuged from the solution and the equilibrated iodine desorbed from the carbon by shaking the carbon with benzene and titrating the benzene with sodium thiosulfate solution. Total iodine concentrations ([I2] + [I ]) were measured by direct titration, with sodium thiosulfate, of the solution as separated from the carbon. Concentrations of I2 were calculated from the equilibrium constant for the reaction [I2 + 1 I ] taken as 870 L mol at 298 K. 

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