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An electrochemical cell is shown here.

$$\ce{Ag(s)~|~AgCl(s)~|~KCl(1M)~|~Hg2Cl2(s)~|~Hg(l)~|~Pt}$$

I am used to seeing cell representations with a $||$ for a salt bridge between the two electrode solutions (generally with the anodic solution written first followed by the cathodic solution).

I am seeing the above type of representation for the first time. Can someone tell me what it means? (including which are the anodic and cathodic solutions). Also, how do we write the reactions at anode and cathode for the above cell?

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This is an example of an electrolytic concentration cell.

Both the cathode and the anode have the same electrolyte present and both are in same solution. Thus, there is no need to separate the electrolyte in two different solution and connect by a salt bridge.

The anode and cathode both are metal-sparingly soluble salt-salt anion electrode. The $\ce {KCl}$ solution here is the common electrolyte.

The anode reaction is:$$\ce {Ag(\text{metal})(s) + Cl^-(\text{salt anion})(aq) -> AgCl(\text{sparingly soluble salt})(s) + e^-}$$

The cathode reaction is also similar: $$\ce {Hg2Cl2(s) + 2e^- -> 2\ce{Hg}(l) + 2Cl^-(aq)}$$

Observe that the $\ce {Cl^-} $ is present in both the reactions as a common electrolyte's ion.

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  • $\begingroup$ is the order in which the species are written relevant in such a notation? $\endgroup$
    – harry
    Commented Jun 29, 2020 at 17:53
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    $\begingroup$ Yes, definitely to some extent, to represent which species is getting oxidized to which and which species is getting reduced to what. $\endgroup$
    – Soumik Das
    Commented Jun 30, 2020 at 18:43
  • $\begingroup$ Then how are cell reactions understood from a notation like this couple; $Cl^- | PbCl_2 | Pb $, or for cells like the Weston cell; $Hg | 3CdSO_4, 8H_2O(solid)|$ satd. solution of $CdSO_4|| Hg_2SO_4(s) | Hg$? These seem unrelated to the actual reactions I found on looking them up. $\endgroup$
    – harry
    Commented Jul 1, 2020 at 6:33
  • $\begingroup$ Just go through different types of electrodes used generally. There are many variations of them. For example the cell notation you have written above is an example of Metal-Metal Insoluble salt-Soluble anion type electrode. Just look through what reactions happen in them actually. You'll understand. $\endgroup$
    – Soumik Das
    Commented Jul 2, 2020 at 7:06
  • $\begingroup$ Can we say $E_{cell}=E^°_{cell}$ for this example? $\endgroup$
    – Apurvium
    Commented Oct 1, 2021 at 17:11

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