Solving for x, we find x = 1022 = [I2]. That is, free iodine of this enormous concentration would be required to prevent oxidation of hydroiodic acid in molar solution by the oxygen of the air at room temperatures. It is obvious that hydroiodic acid must be extremely sensitive to oxidation by exposure to air.
One might estimate, in a similar way, the extent to which hydroiodic acid, of a given concentration, would be oxidized by air before equilibrium would be reached. The process would involve simultaneous changes in three factors—iodide-ion is destroyed, iodine is formed and hydroxide-ion increases, as the result of the neutralization of hydrogen-ion by the hydroxide-ion formed in the action (see above). The solution of the equilibrium equation is too involved for the elementary purposes of this discussion: it leads to the same qualitative conclusion as was just reached.
Oxidation of Potassium Iodide by Air.
| [I−]4 × [O2] | = | 1 × (1/5) × (1 / 23.9) | = 4E9. |
| [I2]2 × [HO−]4 | y2 × (2 y)4 |
Solving for y, we find y = 0.007. That is, in molar solution, about 1.4% of the iodide[608] would be oxidized (carbonic acid and other acids being excluded); in 5 c.c. (see Lab. Manual, p. 73) 9 milligrams of iodine[609] would be liberated to reach a condition of equilibrium.[610]
It is thus clear that the conditions for equilibrium between a solution of an iodide and air would be satisfied, in the case of an alkali iodide, by the liberation of a mere trace of iodine, whereas, as was previously shown, in the case of hydrogen iodide, a very large proportion of iodine must be liberated before equilibrium could obtain. A careful comparison of the two developments shows that the difference in result[610] is plainly due to the higher oxidizing power, the higher potential of oxygen (p. [280]), in acid solutions, containing only a minute concentration of hydroxide-ion, as compared with its efficiency in neutral or slightly alkaline solution.
Chapter XVI Footnotes
[589] In the few cases when there is interference, it is provided against.
[590] Cf. Fresenius, Qualitative Analysis, p. 520.