Consider a cell with both nickel electrodes and a solution of nickel nitrate ($\ce{Ni(NO3)2}$). Current is passed through the solution. The $ E° $ values predict that nitrate in the solution should get reduced to nitrogen oxide. But a question regarding this has the answer given that the concentration does not change. Why is the concentration of nickel nitrate constant?

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    $\begingroup$ Are both electrodes made up of nickel and immersed in the same solution of nickel nitrate? What type of cell is it? If you can elaborate on the question, then it would be easier to answer. $\endgroup$ – Shoubhik R Maiti Dec 31 '18 at 13:43
  • $\begingroup$ Yes both are immersed in the same solution of nickel nitrate. $\endgroup$ – Harsh Dec 31 '18 at 15:32
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    $\begingroup$ Negatively-charged nitrate ions travelling to the also negatively-charged cathode doesn’t seem to be efficient. $\endgroup$ – Faded Giant Dec 31 '18 at 15:50
  • $\begingroup$ Also, the reduction of nitrate requires acid: $$\ce{NO3- + 2H+ + e- -> NO2 + H2O}$$ $\endgroup$ – Ben Norris Dec 31 '18 at 16:03
  • $\begingroup$ Okay then the next possibility is reduction of $ H+ $ . Are there any other reactions taking place? $\endgroup$ – Harsh Dec 31 '18 at 16:09

To plate nickel out of a 1 molar Ni(NO3)2 solution should require 0.25 volts (plus a little more). But the fact that a nickel anode in H2O could generate 0.25 volts suggests that a small current could carry nickel from one electrode to the other at a voltage significantly lower than 0.25 volts. The total effect is a mechanical (physical) shift of the metal, not a change in the chemistry.

The potential for reduction of NO3- is high in 1 molar H+, but this concentration will not be nearly approached in the nickel nitrate solution (pH will probably be about 4-5). In the Nernst equation, a factor of 10000 in the hydrogen concentration and a factor of 4 in the exponent predicts that the reduction of NO3- ion will not take place at voltages which would be enough to move nickel from one electrode to the other.

Now if the current is increased by raising the voltage very high, I think that not only would the nickel be transferred faster, but some nitrate could be reduced to NO.

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