Preparation of di oxygen (O2) from water using nitrate (NO3-) ion and partially inert iron (Fe) electrodes
Preparation of H2 is possible by electrolysis of neutral aqueous solutions of every salt of the alkali metals as well as alkaline earth metals. Any conductor (both inert and active) can serve as the electrodes; e.g. when common salt solution (NaCl) is electrolyzed using Cu wires (acting as the electrodes) and a battery, H2 is invariably liberated at cathode. This is evident from the fact that as soon as the current is switched ON, bubbles start forming at cathode. But O2 is never obtained. Every time the metallic anode ionizes to give the corresponding hydroxides which are generally insoluble. In the above example Cu anode ionizes to Cu2+ and combines with OH- to give insoluble Cu(OH)2. This occurs for any salt (chlorides, nitrates, nitrites, sulphates e.t.c) of alkali or alkaline earth metals. Concentration of the solutions can make no difference. H2 is liberated at cathode and metal hydroxide is obtained at anode. So the anode needs to be replaced. This happens because metals like Cu, Ni, Zn all have a greater tendency to lose electrons than OH-. The only way to obtain O2 is to use an inert anode like graphite, platinum (Pt). But Ca(NO3)2.4H2O and rusted Fe can alter the picture. When a highly concentrated solution of calcium nitrate (made saturated by heating) is electrolyzed using old Fe (may be nails) having rust on their surface, O2 is obtained at anode. Fe becomes partly inert as will be discussed in the subsequent headings. Very pure Fe(OH)3 is also obtained as a by product which can be easily reduced to Fe.
Parantap Nandi. Preparation of di oxygen (O2) from water using nitrate (NO3-) ion and partially inert iron (Fe) electrodes. International Journal of Chemical Science, Volume 1, Issue 2, 2017, Pages 45-47