Influence of the medium ( strongly acid or basic ) over the electrochemical properties of the nitrate selective electrode

A nitrate selective electrode was prepared for use in an aggresive medium (high acidic or basic concentration). It is demonstrated that the depending E graph with respect to pNO3 has not a Nernstian response in concentration acidic range upper 0.1 mol/L H2SO4. The observed behaviour is supposed to be due to the formation of a dimeric anion HN2O6 -.


Introduction
Nowadays the determination of nitrate concentrations in different objects 1,5,6,7 has an extraordinary practical and theoretical importance for Analytical Chemistry. It is important to be able to relate the nocive effect of the nitrate in the human being base on the nitrate interaction of the environmentalhuman organism. For ionometry it requires to know the effects of the matrix above the electrochemical behaviour of the electrode, 2,4,8 specially in the highly aggresive medium, this aspect has not been profundly studied yet.
In our work we intend to study the effect of the aggresiveness of the medium in the electrodic function of the nitrate selective electrode.

Materials and methods
For the study of the behaviour of the electrode an plastificated ion selective electrode EM-NO 3 --01 was used. An Ag/AgCl electrode was used as reference electrode; both joint to universal ionemeter EV-74 type with exactitude of ± 1 mV and temperature of 273 ± 1 K. The reagents used in experiments (KHSO 4 , KNO 3 , NaOH, H 2 SO 4 ), were analytical grade. A 12 mol/L H 2 SO 4 solution was prepared by employing sulphuric acid of the density 1.84 g.cm -3 , later for successive dilution were prepared until 1 mol/L in H 2 SO 4 . Solutions of NaOH were prepared by successive dilution from another 12 mol/L solution previously weighed until exactitude of 0.1 g. In this medium the response and behaviour of the electrode were investigated, just as restoring of the electrode response with respect to nitrate anion.

Results and discussion
In Figure 1 is observed the dependence of the nitrate selective electrode potential with respect to H 2 SO 4 concentration, in the range given from 1 -10 mol/L , to fixed nitrate concentration equal to It's known the HNO 3 presents the following equilibrium: as the H 2 SO 4 increases; the association of H + with NO 3 is favoured, increasing the HNO 3 concentration in the medium to impose conditions for the reaction between nitric acid and the nitrate ion in solution, as expected that the equilibrium: gives place to dimeric anion HN 2 O 6 highly hydrophobic. This compound would interfere to the characteristics of the nitrate electrode. Already a similar behaviour was observed in the work. 3 In Figure 2   An evidence about formation of HN 2 O 6 anion is observed in Table I, where it can be seen the dependence of nitrate selective electrode potential in presence of fixed nitrate concentration equal 0.1 mol/L with respect to KHSO 4 and H 2 SO 4 concentrations, if the dependence of the potential would be due to the presence of HSO 4 in both cases the same phenomen would been possible, wich isn't like that.
When investigating the electrode response in an alkaline medium (up to 10 mol/L in NaOH) the same characteristics are not observed like that in H 2 SO 4 due presumably to the used plastificant -dibutilphtalate. The plastificant in presence of high alkali concentration (NaOH) saponificates originating a new component which presents electrodic activity with respect to Na + cation. In this moment a new function begins to respond to Na + cation that is occasion so that the variation in the electrodic potential has been observed: In this form the component II in the plastificated electrode matrix acts like an active substance sensitive to changes in the activities of Na + .
In the restablish of practical response time, it was observed that after working in H 2 SO 4 medium one hour was required to take to the adecuate potential, this is due to the adsorption in the phase membrane the ion HN 2 O 6 -.
However the restablishment in the alkaline medium was fast (1-5 min) in successive washes.

Conclusions
In this work it was demonstrated the abnormal behaviour of the electrode response of EM-NO 3 --01