The plasmachemical properties of an electric discharge in humid air are considered by examining the oxidation of Fe(II) to Fe(III) aquo-ions in a solution exposed to the discharge. The reaction product is characterised and quantified by means of the thiocyanate complex FeSCN2+. The reaction mechanism shows two distinct steps: a rapid step directly depending on the treatment time and a slow post-discharge step, which are both ascribed to species formed in the discharge (i.e., respectively to OH radical and to hydrogen peroxide or nitrate ions). These concerned species are able to react either at the liquid surface (OH) or to diffuse into the solution and react as solutes. Experimental arguments in favour of zero and first order reaction steps are given. A particular reactor design involving a multibladed electrode is tested at the juncture and its use considered for further industrial applications.