Iodine standard electrode potential

Iodine has the lowest standard electrode potential of any of the common halogens (E = +0.54 V) and is consequently the least powerful oxidising agent. Indeed, the iodide ion can be oxidised to iodine by many reagents including air which will oxidise an acidified solution of iodide ions. However, iodine will oxidise arsenate(lll) to arsenate(V) in alkaline solution (the presence of sodium carbonate makes the solution sufficiently alkaline) but the reaction is reversible, for example by removal of iodine. 

Which atom or ion in this list will reduce iodine to iodide ions Explain youranswer. 6 The list below gives the standard electrode potentials for five half-reactions. 

As an illustration of the use of electrode potentials, consider the classical method of analysis of copper in brass, which involves dissolving the weighed sample in nitric acid to obtain Cu2+(aq), adjusting the pH to a weakly acidic level, allowing the Cu2+ to react completely with excess potassium iodide to form iodine and the poorly soluble Cul, and then titrating the iodine with sodium thiosulfate solution that has been standardized against pure copper by the same procedure ... 

In this equation sJ> j2, j- represents the standard oxidation potential of the iodine electrode (sJ.tija j- — — 0.535 V) and aj- the activity of iodide ions in the solution. The negative value of this potential means that in combination with a hydrogen electrode a spontaneous reduction process will take place at the iodine electrode during which iodine will pass into the solution in ionic form. 

It should be remembered that the standard potential refers to the condition in which all the substances in the cell are in their standard states of unit activity. Gases such as hydrogen, oxygen and chlorine are thus at 1 atm. pressure. With bromine and iodine, however, the standard states are chosen as the pure liquid and solid, respectively the solutions are therefore saturated with these elements in the standard electrodes. For all ions the standard state of unit activity is taken as the hypothetical ideal solution of unit molality or, in other words, a solution for which the product my is unity, where m is the molality of the ion and y its activity coefficient. 

Moreover, the potential shift toward negative values corresponding to the onset potential of massive copper deposition due to the presence of iodine on the surface leads us to conclude that there is a surface formation of the Cul species, which becomes the new active electrode. If we consider that the standard potential of the Cu/Cu2+ electrode is 0.342 V vs. NHE, and the Cu2+ concentration is 0.01 M, the reversible potential for copper deposition will be 0.282 V. In the presence of iodide, a new electrode is formed, which is a Cu/Cul/I interface. Considering its standard potential, —0.185 V vs. NHE in the 3 x 10 4 M iodide concentrations, the reversible potential of the Cu/Cul/I electrode will be —0.032 V. The difference between the reversible potential of Cu/Cul/I and the reversible potential of Cu/Cu2+ is 0.250 V, that is, the potential shift found experimentally. Therefore, the presence of iodide in solution, after adsorption, produces a new interface with completely distinct electrochemical properties. 

The 21 formed in the second reaction is determined either by visual chemical titration with a reagent such as sodium thiosulfate in the presence of a suitable endpoint indicator or by amperometric, coulometric, or photometric titration methods. The most sensitive KF methods for the measurement of iodine are coulometric. For both the volumetric-amperometric and coulometric methods the endpoint is detected by a pair of platinum electrodes called the indicator electrodes. An electrical potential (100-400 mV) is applied across the electrodes to balance the circuit and the endpoint is reached when the concentration of I2 ( 50pmoll ) depolarizes the cathode deflecting a galvanometer. The volumetric method measures the amount of standardized reagent necessary to depolarize the platinum electrodes. The coulometric method utilizes, in addition to the indicator electrodes, a second pair of platinum electrodes (generator electrodes) that electrolytically convert the 1 to I2. The current consumed in this process is used to calculate the amount of water using the equation that describes Faraday s laws of electrolysis. 

The standard iodometric method for peroxide value requires a sample of 5 g for peroxide values below 10, and about 1 g for higher peroxide values. The sensitivity of this method is about 0.5 meq/kg, and can be improved by determining the iodine starch-end point colorimetrically, or the liberated iodine electrometrically by reduction at a platinum electrode maintained at constant potential. 

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