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So when we perform electrolysis of say an aqueous solution of silver nitrate with silver electrodes, silver ions having a greater reduction potential than water, accepts electrons and deposits at the cathode.At the anode however, silver just leaves the electrode and enters into its solution. The anode must just be a metal rod whose sea of electrons is much poorer than what it must be.Why does the silver get oxidized and leaves the rod as an ion? Water could get oxidized instead and that appears to be a more logical expectation.

Secondly why does this not happen if we do the same reaction using platinum electrodes? What is preventing the metal from leaving the anode as an ion,moving to the cathode causing gradual thinning of one electrode and thickening of another?

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The reason why silver dissolves (or water is oxidized) is well known. The reason why platinum does not dissolve is more elusive. Thanks to Maurice's comment for pointing this out.

Let's assume your solution is 1M in AgNO3 and 1M in acid ($H^+$). The potential required to oxidize silver is 0.80 V; the voltage required to split water (oxidize it) is 1.23 V, but there are complications of overvoltage, mostly with hydrogen, but also with oxygen. To sum up: it is easier to grab an electron from silver than to pump another hydrogen ion into solution. AHA! How about reducing the acid concentration, or not adding any at all? (Of course, the reason for adding the acid was to make the silver dissolve easier.) If the pH is increased so that hydrogen ion concentration is reduced to give a lower oxidation potential than silver, then water will be oxidized, hydrogen ion will be produced, and eventually, silver will become more easily oxidized. This electrolysis wil go thru a phase where hydrogen and silver are codeposited, probably making a porous, weak deposit of silver.

For platinum to yield 2 electrons requires 1.188 V. Although this is less than required for oxidizing water, overvoltage effects due to oxygen and hydrogen are apparently important. Although platinum can be electroplated with a platinum anode, apparently the process fails to work well; older methods of platinum plating were kept refreshed with additions of platinum salts rather than relying on dissolution of the anode (Ref 1). More modern methods use a pulsed electropotential, but their success seems to be from a solving difficulties at the cathode rather than getting the anode to dissolve (Ref 2). They deposit a monolayer of platinum attached to a monolayer of hydrogen, then oxidize off the hydrogen and repeat. The electrode potentials strongly suggest that increasing the cell potential, if you had a platinum anode, would result in dissolving platinum metal, but the catalytic reactivity of platinum with respect to H2O2 suggests that maybe the oxidation reaction occurs in the water layer, and the platinum acts merely as an electronic conductor.

Ref 1. https://www.technology.matthey.com/article/25/1/32-41/

Ref 2. https://www.chemistryworld.com/news/platinum-plating-at-the-flick-of-a-switch-/5720.article

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  • $\begingroup$ hmmm... I didn't realise that silver oxidation was the thermodynamically favoured option. I was asking the question expecting that there would be a particular effect that occurred only when the same metal was used in the electrolyte as well as the electrode.If I were to choose a different solution, say, Copper Sulphate, would we still expect a silver electrode to get oxidised instead of water? $\endgroup$ – Lucifer Aug 17 at 12:22
  • $\begingroup$ @Lucifer: Two silver electrodes in a copper sulfate solution (without applied potential) should be stable, because silver is less active than copper. When you apply a potential, copper is the only cation available, and so plates out at the cathode. The anode has the same situation as previously, except sulfate ion carries the current, not nitrate (which is a complication I did not discuss in the original answer). Under very acidic conditions, silver will dissolve; without acid, silver will dissolve - then go to the cathode and plate out preferentially. $\endgroup$ – James Gaidis Aug 17 at 15:58
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When current flows in an electrolytic cell, the bulk solution has to remain electrically neutral. If one positive "ion" is deposited at the cathode as a neutral atom, another positive ion has to be generated. What is the source of the positive ion? The metallic anode.

Coming to your other question: What if the anode is a non-metal instead of any metal (an extreme case)? Say, it is made of graphite. In that case, water begins to oxidize, and a hydrogen ion is generated.

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  • $\begingroup$ even in the silver nitrate case, water could get oxidized to maintain the balance. In the case of graphite, i understand that water has to get oxidized, but my question is concerning why the metal ion leaves here instead of that. and what is stopping such behavior on say a platinum electrode. $\endgroup$ – Lucifer Jul 14 at 8:11
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    $\begingroup$ If I understand your question, you are saying that if we have Pt anode and a silver cathode. The electrolyte is silver nitrate. Why does Pt not dissolve? $\endgroup$ – M. Farooq Jul 14 at 14:48
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    $\begingroup$ yeah.why does Pt not dissolve while silver does $\endgroup$ – Lucifer Jul 15 at 12:23
  • $\begingroup$ Probably nobody knows why Platinum does not dissolve at the anode. $\endgroup$ – Maurice Jul 15 at 12:41

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