Lecture 1.105- Calomel Electrode

Now there is a very common tradition with science teaching, you told the drawback of something only when, we have some improvement to tell you and since, we discussed a lot of issues we are having with the SHE(again, just a coincidence), we will go for some improvement, now there are several kind of secondary reference electrode, but we will start with Calomel Electrode, most commonly used.

Let’s see some analogies, just like we took a certain potential for a SHE, at certain conditions we have something we similar processing with the calomel electrode as well, But since we have to rectify the problems we were having,

We will find the cause and try to remove it.

Dependence on Pressure – Now that problem is due to the electrode being a gas, so we will make it liquid and we minimize the pressure dependence, Now we have a idea, Engineering is half done, Next we have a theory for it, what will look in reality.

If we look at the diagram we see that the electrode is a closed system, and since no reaction is going on inside it producing or consuming any gas, pressure is constant.calomel-electrdoe

Can’t work with certain some – Now this problem is already solved in the above change, If I isolate it from reacting from other chemicals, I don’t care what are those unwanted reactions because they aren’t happening anymore.

Concentration and Temperature dependence, it not an issue to worry actually, because we can fix it very easily, Room temperature is same for the reactions.

Now we can built a better Reference electrode, The construction of the Calomel Electrode is as shown in picture.

Reactions involved are,  Hg2Cl2 (s) < ——- > Hg2+2 + 2Cl–                                                                                                                          Hg2+2  (s) + 2e < —— > 2Hg

Net reaction –                  Hg2Cl2 (s) + 2e < ——- > 2Hg + 2Cl

From the above reactions, and the Nerst Equation we have,

E electrode = E0Hg/Hg2Cl2 – K * log [Cl]2

, here K is a temperature dependent constant we can get easily from Nerst equation.

So, we can conclude that the electrode potential of calomel electrode depend on conc. of KCl. Where K = 2.303 RT/nF from the nerst equation.

With Saturated KCl solution  0.02412V and with 1M KCl solution we have .28V, the numbers can be justified with more Cl we have lower Electrode potential. The following are some of application and advantages of Calomel electrode-

  • It is easy to construct and transport comparatively
  • Since it don’t change its potential considerably with varying temperature, it find its place at top reference electrode in chemistry lab.
  • It is used in corrosion studies(later in corrosion)calomel-elecrrode-app
  • To find the unknown H+ ion concentration in the given solution using the setup shown by side, function of which is just like the working of electrochemical cell but just this time the calomel is fixed and we have changing Hydrogen ion concentration which varies the Electrode potential of the cell.


Abhishek kumar jha

(Chemistry at Utkarshini)





downlordable lecture notes-1-105-electrochemistry-calomel-electrode


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