[blueplutojp]

 Hello

Please tell me why Hg cathod dissolves. I know Ag anode can dissolve. However, there is no reason why Hg cathod dissolve by means of electrolysis because the electrode is reduced at a cathod. On the other hand the anode metal will be able to be oxidized at anode and dissolved.

[Borek]

Please describe experimental setup you are referring to, as now trying to understand what you are really asking about is a guesswork.

[blueplutojp]

 Hg(cathode)  :  HNO3  N2H4 and UO2(NO3)2 aqueous solution : Pt(anode)

External Voltage  2V.   at room temperature

I tried to reduce U(VI) to U(IV) by means of the electrolysis. But I observed Hg cathode dissolved into acid solution in some experiments. 
 

[Borek]

How concentrated HNO₃? And how do you know cathode dissolved - have it dissappeared?

[blueplutojp]

HNO3: about 4 M or more

I observed the Hg dissolution by the weight of the solution. The acid solution became heavier after the electrolysis.

Thanks

[Borek]

No idea.

Two things that come to mind first - but both dubious - is that mercury is dissolved by nitric acid in the presence on N₂H₄ (assuming N₂H₄ is able to to complex mercury strong enough - but I don't have complexation constants here) or that you did some experimental error (like reversed polarization, or simply error when weighting solution). Have you checked the solution for Hg cation presence?

Why do you use Pt and Hg as electrodes? I mean - mercury is good when reduced metal dissolves in it, or when you need to regenerate clear electrode surface with every drop (like in polarography). Doesn't sound like you need mercury for either reason.

[blueplutojp]

Al cathode is also observed to dissolve in acid solution liberating hydroben at the surface by other investigators according to the literature. So, Hg cathode must be dissolved by the electrolysis. Hydgazine has probably nothing to do with the dissolution.
  Hg cathode is usually employed to reduce uranyl. We had better employ a compartmented cell for the electrolysis because uranous(IV) can be reoxidized at Pt anode.

  I think that the dissolution mechanism of electrode has not been solved yet globally.

[Borek]

I have forwarded your question to a friend of mine (who happens to be an elctrochemistry professor, and a good one ) - his take it that mercury dissolves in nitric acid.

[Alberto_Kravina]

Mercury dissolves in nitric acid.

If there's an excess Hg⁰ the Hg₂²⁺ ion forms.
If there's an excess HNO₃ the Hg²⁺ ion forms.

2 Hg  ? Hg₂²⁺ + 2e^-
HNO₃ + 3e^- + 3 H⁺ ? NO + 2 H₂O

Hg  ? Hg²⁺ + 2e^-
HNO₃ + 3e^- + 3 H⁺ ? NO + 2 H₂O

[blueplutojp]

Mercury
may thermodynamically dissolve in HNO3.

But It is inert in HNO3 with N2H4 because hydrazine scaveges nitrous acid(HNO2) which acts as an autocatalyst in the HNO3 oxidation.

Hg catode must be electrochemically dissolved in HNO3 solution. The mechanism , however is unknown in general.