The kinetics of oxidation
of nitrite to nitrate by chlorine in aqueous acid solution (pH 0-1) have been
studied using a spectrophotometric stopped flow technique. The rate law is
���������������� -d[Cl2]a/dt =([Cl2][NO2-]/Ka[Cl-]2)(f+g[HNO2])
where [Cl2]a
= [Cl2]+[Cl3-] and where Ka is the
ionization constant for nitrous acid. At 298.2 K and ionic strength 2.75M, f =
60.8�0.5 mol2 l-2 s-1 and g = (2.35�0.05)x105
mol l-1 s-1: the associated activation energies are 68�3
and 44�2 kJ mol-1 respectively. A mechanism is proposed involving
the reversible initial step:
�������������������������� NO2-+Cl2
↔ NO2Cl+Cl-
with the NO2Cl
undergoing two parallel subsequent reactions, one a unimolecular decomposition
and the other an attack by HNO2 upon NO2Cl. ��� Oxidation of nitrite by the three halogens,
Cl2, Br2, I2, is discussed.