Electrochemical Behavior of Nitrate and Nitrite Ions in the Molten NaOH ‐  KOH  (49 m/o) Eutectic Mixture at 250°C

Abstract

The electrochemical reduction of nitrate ions in molten NaOH-KOH at 250~ is strongly dependent on the acid-base character of the melt and on the nature of the working electrode. On silver electrodes, two well-defined waves are observed; they are attributed to the following reactions NOs-+ 2e-~c~ NO~-+ O ~-NOs-+ e-~ NO + O sOn platinum electrodes the second step appears to be influenced by an electrode surface phenomenon. In acidic solutions, the nitrate reduction occurs through a water diffusion limited process according to following equationNOs-+ H~O + 2e-~ NO2-+ 2 OHIn acidic melts, the nitrate wave is then either split into two partial waves or completely shifted toward more anodic potentials depending on the water concentration of the melt. The nitrite ion reduction is shown to proceed as a quasireversible one-electron process in a basic melt NOs-+ e-~ NO + 0 3.In acidic solutions, this reaction occurs at a potential close to the water reduction and mixed waves are observed.The electrochemistry of water has been investigated in molten chlorides (1-4), carbonates (5), acetates (6, 7), hydroxides, and nitrates (11)(12)(13)(14). In the latter media, water yields a reduction wave at a potential 0.4V less cathodic than the nitrate reduction peak (18). Jordan (16, 17) attributed the so-called "water wave" to the diffusion controlled reaction NO~-+ H20 + 2e-~ NO2--I-2 OH-[1]Zambonin and co-workers (12) describe the reaction as H20 + e-~=~ 89 Hs + OH-[2]Lovering and co-workers (19) investigated a "nitrite free" nitrate melt and concluded that the presence of some nitrite ions in the melt (generated, for instance, by an initial cathodic scan) is necessary to observe the water reduction wave. They suggest the following mechanism H~O + NOs-+ 2e-:~-2 OH-+ NO-NO-+ NOs-~=~ 2NOs-[4] Considering these reactions, it seemed interesting to investigate the electrochemical behavior of nitrate ions in dry and acidic hydroxide melts. The advantage of such a study is to allow independent variations of the concentrations of both solutes: water and nitrate ions. ExperimentalA three-el ectrode el ectrochemi cal cell was used. The indicator electrode was a partially immersed I mm platinum or silver wire. The reference electrode was a platinum quasi-reference electrode or, as previously used, a Teflon reference electrode (15). The counterelectrode was a graphite rod. ~Present address: Diamond Shamrock Electrosearch S.A., CH-1227 Carouge, switzerland. ] 50 I i +1.0 +0.5 A. I D0 100 pA~ 8. ' ' o' .o +'t.O +0.5 EN) ' ' d0 + 1.0 + 0.5 Fig. i. Cyclic voltammetric waves obtained for the reduction of nitrate ions (XKNo3 --1.15-10 -8) in an NaOH-KOH (49 m/o) melt at 250~ (Scan rate = 120 mV/sec; electrode area --0.07 cm2.) A, Basic melt, XNa2o = 2.0 9 10-4; B, acidic melt, XH20 --" 2.4 9 10-4; C, acidic melt, XH2O --3.6 9 10 -3. 123 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.216.129.208 Downloaded on 2015-03-22 to IP