Types of Electrodes

Transcript

0:00Introduction

Hello everyone! Let's start with a small puzzle. For silver ions turning into silver, the standard electrode potential is zero point eight zero volt. Now take the same silver wire, coat it with silver chloride, and dip it in a chloride solution. Its standard potential drops to about zero point two two volt. Same metal, same silver! So where did more than half a volt go? Strange, right? The answer lies in the type of electrode. By the end of this lesson, you will solve this puzzle yourself. Electrode questions come up often in JEE, so let's begin.

0:40Metal, gas and redox electrodes

So, type one: the metal, metal ion electrode. Just a metal rod dipped in a solution of its own ions. Zinc, single bar, zinc two plus. Z n is zinc, the two plus up top is the charge, and the single bar is the boundary between the solid rod and the solution. Simple, right? Now think. Hydrogen is a gas. You cannot make a rod out of a gas! So how do we build a hydrogen electrode? Platinum, yes! That is type two: the gas electrode. We dip a platinum plate, P t, into the solution and bubble the gas over it. So the hydrogen electrode is written P t, H two, H plus. The small two below the H is a subscript: two atoms in the molecule. And chlorine works the same way: P t, C l two, C l minus, the chloride ion. Okay? Look at the figure: the standard hydrogen electrode, the SHE, from an earlier lesson. Hydrogen gas at one bar comes in through the tube, over platinised platinum, in H plus solution. The a q in brackets means aqueous, dissolved in water, and small a equals one means unit activity, one molar. The E with the small circle is E naught, the standard potential. For the SHE it is zero by definition. At the bottom, two H plus ions plus two electrons give hydrogen gas, and the double arrow means it goes both ways. Clear? Type three: the redox electrode. Here both forms are ions, iron three plus and iron two plus, in the same solution. The comma between them means the same phase. And again, a platinum wire dips into it. Got it? So, in the gas and the redox electrodes, the platinum itself does not react. It is inert. It only carries the electrons in and out. Right? One more detail. Platinised platinum is coated with platinum black. Platinum black adsorbs hydrogen, holding the gas on its surface, and that speeds up the electrode reaction. With me?

2:52Metal–insoluble salt–anion electrodes

Now type four: the metal, insoluble salt, anion electrode. Coat a silver wire with silver chloride, a salt that hardly dissolves, and dip it in chloride solution. We write silver, single bar, silver chloride solid, single bar, chloride ion: the silver, silver chloride electrode. Here, A g is silver, and the s in brackets means solid. Okay? The second one is the calomel electrode. Mercury, H g, single bar, H g two C l two solid, single bar, chloride ion. H g two C l two is calomel, an insoluble salt of mercury. Clear? Now the key point. These electrodes respond to the anion concentration, here chloride. The square brackets mean concentration in moles per litre. We will see the direction in example one. Right? And their E naught follows from K s p, the solubility product of the salt. E naught for X minus, M X, M equals E naught for M plus slash M, plus zero point zero five nine times log K s p. M is the metal, X minus the anion, and the log is to base ten. This simple form is for a one is to one salt M X with a one electron change, like silver chloride. This holds at two hundred and ninety eight kelvin, for standard reduction potentials. Okay? For silver chloride, K s p is a tiny number, much less than one. So, is log K s p positive or negative? Negative, yes, exactly! The double less than sign means much less than, so log K s p is less than zero: negative. So this electrode has a lower E naught than the plain M plus slash M electrode. That is the first clue to our puzzle. Got it? The SHE is the reference, with E naught zero. So why not use it every time? Remember that figure: hydrogen gas at one bar, over platinised platinum, in one molar acid. Not easy to handle! So calomel and silver, silver chloride are used in place of the SHE, as secondary reference electrodes, because they are easier to handle. With me?

5:08Example 1: calomel electrode reaction and chloride effect

Alright, example one. Write the electrode reaction of the calomel electrode, and state how its potential changes with the chloride concentration. Pause the video, and try it. Done? Okay, let's check. Electrode potentials are reduction potentials, so we write a reduction. Calomel takes two electrons, e minus, and the arrow says it gives two mercury atoms and two chloride ions. The l in brackets means liquid. Charge check: minus two on the left, from the electrons, and minus two on the right, from the chloride ions. Perfect! Now the second part. We add more chloride ions. More ions, so a higher potential, right? No, no, careful! Look at the equation for this electrode. E equals E naught, minus zero point zero five nine times log of the chloride concentration, at two hundred and ninety eight kelvin. Plain E, without the circle, is the potential at any concentration. And see the minus sign in front of the log. Okay? So, as chloride rises, its log rises, and because of that minus sign, E falls. The double arrows mean which gives. Exactly the opposite of the guess! Clear?

6:27Example 2: E naught of Ag/AgCl electrode from Ksp of AgCl

Now example two, and this one solves our puzzle. E naught for silver ion slash silver is zero point eight zero volt, and K s p of silver chloride is one point eight times ten to the power minus ten. Find E naught for silver chloride solid, plus an electron, giving silver solid and a chloride ion. Pause the video, and try it. Done? Okay, let's check. Standard means chloride at one molar; capital M is molar. K s p is silver ion concentration times chloride concentration. So with chloride at one, the double arrow tells us silver ion concentration equals K s p itself. Right? So, use our formula. E naught equals zero point eight zero, plus zero point zero five nine times log of one point eight times ten to the power minus ten. Okay? Now the log. Ten to the power minus ten gives minus ten. So the log is just minus ten, right? Oh no, careful! Don't forget the one point eight. Log of one point eight is about zero point two six. So the log is zero point two six minus ten, which is minus nine point seven four. Got it? Now multiply: zero point zero five nine times minus nine point seven four is about minus zero point five seven five. Clear? So, E naught is about zero point eight zero minus zero point five seven five, which is about zero point two two volt. That's it! And our puzzle is solved! In the silver chloride electrode, the silver wire sees only a tiny silver ion concentration, just K s p. So E naught falls from zero point eight zero volt to about zero point two two volt. See how it all connects?

8:21Quick revision: electrode types, Ksp formula, references

So, quick revision. Metal ion, gas and redox electrodes, where platinum is inert and only carries electrons. Insoluble salt electrodes: E naught equals E naught for M plus slash M, plus zero point zero five nine times log K s p, at two hundred and ninety eight kelvin. And calomel and silver, silver chloride are easier references than the SHE, okay? Now go and try the two practice problems on this page. You can do them. All the best!