[601] The exact structure of the complex acid is not known.
[602] In the Laboratory Manual a second, similar method is also given.
[603] It is considered that water has a constant concentration in a dilute solution and that for its active components [H+] × [HO−] is a constant (p. [176]).
[604] In the calculation which follows, which is meant merely for a rough survey, no account is taken of the formation of complex ions I3−, or of the tendency of hydroiodic acid to decompose spontaneously into iodine and hydrogen: 2 H+ + 2 I− ⇄ H2 + I2, a reaction which could also be studied profitably with the aid of the equilibrium constants for I2 ⇄ 2 I− and for H2 ⇄ 2 H+. The value of the iodide constant is also uncertain (see p. [273]).
[605] 4 K+ + 4 I− + O2 + 2 HOH ⇄ 2 I2 + 4 K+ + 4 HO−.
[606] The formation of complex ions I3− and other secondary reactions (formation of hypoiodite, iodate, etc.) are ignored.
[607] y has so small a value that we may consider [I−] practically unchanged.
[608] 0.007 I2 = 0.014 I−.
[609] A mole of I2 = 2 × 127 = 254 grams; 0.007 × 254 × 5 / 1000 = 0.009 gram.
[610] The tendency of iodine to form hypoiodous acid, iodates, etc., is not taken into consideration here and involves another relation.