Reference Electrodes - Online Tutor, Practice Problems & Exam Prep

The silver-silver chloride electrode (SSCE) and calomel reference electrode (SCE) are crucial in electrochemistry. The SSCE operates on the redox couple of silver chloride and silver ion, with a cell potential influenced by chloride ion activity. The Nernst equation describes this relationship: $E^{°}=E^{°}-\frac{0.05916}{n}log\left(a\right)$. The calomel electrode, based on mercury and mercury(I) chloride, similarly relies on chloride ion concentration, affecting its voltage and stability under varying conditions.

Silver-Silver Chloride Reference Electrode

So here we have our silver silver chloride reference electrode, which is one of the most versatile and most widely used reference electrodes in the field. Now we're going to say it is typically constructed as a thin tube that is subsequently dipped into solution. So we saw previously this half-cell reaction that was closed off by this dotted box, this represents our SSCE or silver silver chloride electrode reference electrode. We're going to say here that when it comes to the components of it, we'd say here that this is our wire lead, and basically here this is where we have our just our silver wire. We have our solution of potassium chloride and silver chloride here.

This is just our silver chloride paste and then we have some KCl solid here as well as some silver chloride solid. Here, this is part of our salt bridge and this is our porous opening. If the liquid gets too high, it basically comes out here. Remember we said that the silver silver chloride electrode is our reference electrode so its concentration would not change. Here we'd have our indicator electrode.

Now when we say that the chloride concentration approaches unity, so that means it comes close to 1, we're gonna have a cell potential equal to this. That's because when we take into account the reference electrode, which in this case is the silver silver chloride reference electrode, we're gonna say that our nonstandard potential is equal to our standard cell potential minus 0.05916nlog[Cl^-] Here, if our chloride ions approach unity, that means that this is gonna become 1. Log of 1 is equal to 0, and so all of this drops out, and therefore my cell potential under non-standard conditions equals my cell potential under standard conditions. Now here we're going to see the reference electrode is based on the redox couple, between silver chloride and silver ion.

So here we have the reaction, and what we're going to say here what happens is an electron is absorbed by the solid, chlorine is the one to accept it and becomes chloride ion. Now here the activity of the chloride ion determines the potential of the electrode. So remember your activity, which is a, takes into account the activity coefficient, as well as the concentration of the ion in order to figure out the activity. And again, remember as it approaches 1 or unity, we're going to have sub-potential under non-standard conditions equals sub-potential under standard conditions. We're going to say here, when dealing with our saturated chloride solution, we have to keep in mind how our concentrations can affect many things for our solution.

So we're going to say here when it's saturated we get a voltage of 0.197 volts, and when it's dipped into a concentration that's greater than that, it increases to 0.205 volts. So like other reference electrodes, concentrations can be manipulated, in order to increase or decrease our overall voltage. Here we'd say that this reference electrode, so this would be the potential of our reference electrode, and then over here we deal with the potential of our indicator electrode. So here would be silver solid, here we have a phase boundary, and then we have silver chloride solid. Potassium chloride aqueous, and then we have the activity of chlorine, of chloride ion depending on what it is.

So remember there are different types of reference electrodes. This just happens to be one of them. Here it may not be the most widely used, but it is one of the best versions to use. The hardest of the reference electrodes we'll talk about which deals with SHE, the standard hydrogen electrode. That one is much more difficult to use because of its implementation, setup, and cost in running it.

Saturated Calomel Reference Electrode

So here we're dealing with a calomel reference electrode. We're going to say the calomel reference electrode is based on the following redox couple reaction between mercury(I) chloride and mercury itself. So here we have mercury(I) chloride solid. It absorbs 2 moles of electrons to give us 2 moles of mercury liquid plus 2 moles of chloride ion. Here we have our typical reference calomel reference electrode.

It's usually abbreviated as SCE. Now, in terms of the components, we can say here that it starts off with a wire lead, and it leads down to this portion which is our platinum wire. This is just our inert electrode. We're going to say here we have this is just a hole to allow drainage, and in this compartment here's where we actually have the mercury liquid, and if it gets too high, the mercury liquid will drain outside of this hole. In this compartment here, we have our reaction going on.

We have our mercury, we have mercury(I) chloride, and we have potassium chloride also involved. The potassium chloride can help release some chloride ions here, and on the bottom here, we just have some, basically some glass wool and an opening. In here we have our saturated solution of KCl. Here we have this is our KCl solid and this is just our porous plug, which forms part of the salt bridge.

Now we're going to say that this is the most common type of reference electrode that we can use. Basically, it's just a slurry of the mercury and mercury(I) chloride with the saturated KCl in there, and we're going to say here that there are a few things about this particular reference electrode we can mention. So, when we're talking about its Nernst equation, we're going to say that its non-standard cell potential equals the standard cell potential of its reduction reaction minus 0.05916 volts divided by the number of electrons transferred. Here we have the log of the concentration of chloride ions. Here we just put A for activity.

Remember your activity can equal your activity coefficient times the concentration of the particular ion, if your activity coefficient reaches unity, then you can just substitute in chloride ion here. Okay. So when your activity reaches 1, you can say that represents the concentration of chloride ion. Here it's squared because in the original equation up above, we have a 2 here. Mercury is not included and mercury(I) chloride is not included because they're liquids and solids respectively.

We ignore liquids and solids within our equilibrium expressions. Here this is our standard cell potential for that half-reaction. Now, we're going to say from the equation it is determined that the potential of the electrode is based on the activity of the chloride ion because this is really what's changing and that can affect my overall cell potential. We're going to say the concentration of chloride ions is determined by the solubility of KCl. So the more soluble KCl is, the more chloride ions you can release, which will have an impact here on this.

So, the larger that is, the larger this overall value will be, which will subtract from this and bring down your overall cell potential and make you less spontaneous. So the potential of 0.268 is when the activity of chloride ion approaches unity, so reaches 1, because when we have log of 1 for this that equals 0, so all of this would drop out. Now when the concentration of KCl is varied, that can have a direct impact on my voltage. When it's 0.100 molar, the voltage would be 0.336 volts, but if I increase it, then my voltage decreases, because remember we said that the larger this value is, the bigger this overall will be, and the more it'll subtract from this standard cell potential here. Now we're going to say here that the potential can also be affected by temperature.

So we're going to say at 25 degrees Celsius it is approximately 0.2444 volts, and at 35 degrees Celsius it's 0.2376 volts. Okay. So, notice here that we have basically dropping of our voltage because here temperature is affecting the basic concentration, which will affect the overall cell potential. And when it comes to the cell notation for this half-reaction or this reference electrode, we're going to say here it's equal to the mercury as a liquid, and then here we have our phase boundary, and here we have mercury(I) chloride solid times KCl saturated, and then we have our physical boundary. So here, this would be the reference electrode compartment for my cell notation.

Remember here we'd have the rest we could talk about our indicator electrode over here, whatever we're comparing it to. So remember the SCE electrode is basically the best out of all of them in terms of use and preparation and cost. But we have to keep in mind that things such as temperature as well as the concentration of our potassium chloride do have a big impact on the overall cell potential.

Here’s what students ask on this topic:

What is a silver-silver chloride reference electrode and how does it work?

A silver-silver chloride reference electrode (SSCE) is a widely used reference electrode in electrochemistry. It operates based on the redox couple between silver chloride (AgCl) and silver ion (Ag⁺). The electrode typically consists of a silver wire coated with AgCl, immersed in a solution of potassium chloride (KCl) and AgCl. The cell potential of the SSCE is influenced by the activity of chloride ions (Cl^-). The Nernst equation describes this relationship:

$E_{\mathrm{cell}}=E_0-\frac{0.05916}{n}log\left(a\right)$

where $E_{\mathrm{cell}}$ is the cell potential, $E_0$ is the standard cell potential, $n$ is the number of electrons transferred, and $a$ is the activity of chloride ions. When the activity of Cl^- approaches unity, the cell potential under non-standard conditions equals the standard cell potential.

What is a calomel reference electrode and how does it function?

A calomel reference electrode (SCE) is based on the redox couple between mercury (Hg) and mercury(I) chloride (Hg₂Cl₂). It consists of a platinum wire, mercury, Hg₂Cl₂, and a saturated KCl solution. The redox reaction is:

${\mathrm{Hg}}_2{\mathrm{Cl}}_2\left(s\right)+2e-=2\mathrm{Hg}\left(l\right)+2\mathrm{Cl}-$

The potential of the SCE is influenced by the activity of Cl^- ions, described by the Nernst equation:

$E_{\mathrm{cell}}=E_0-\frac{0.05916}{n}log\left(a\right)$

where $E_{\mathrm{cell}}$ is the cell potential, $E_0$ is the standard cell potential, $n$ is the number of electrons transferred, and $a$ is the activity of chloride ions. The potential can vary with temperature and KCl concentration.

How does the concentration of chloride ions affect the potential of a silver-silver chloride reference electrode?

The potential of a silver-silver chloride reference electrode (SSCE) is influenced by the activity of chloride ions (Cl^-). According to the Nernst equation:

$E_{\mathrm{cell}}=E_0-\frac{0.05916}{n}log\left(a\right)$

where $E_{\mathrm{cell}}$ is the cell potential, $E_0$ is the standard cell potential, $n$ is the number of electrons transferred, and $a$ is the activity of chloride ions. When the activity of Cl^- approaches unity (1), the log term becomes zero, and the cell potential under non-standard conditions equals the standard cell potential. Variations in Cl^- concentration can thus alter the electrode potential, with higher concentrations generally increasing the potential.

What are the advantages and disadvantages of using a calomel reference electrode?

The calomel reference electrode (SCE) has several advantages and disadvantages. Advantages include its stability, ease of preparation, and cost-effectiveness. It is widely used due to its reliable and reproducible potential. The SCE is also less sensitive to changes in temperature compared to other reference electrodes.

However, there are disadvantages. The use of mercury poses environmental and health hazards, requiring careful handling and disposal. Additionally, the potential of the SCE can be affected by the concentration of potassium chloride (KCl) and temperature variations. For instance, at 25°C, the potential is approximately 0.2444 V, but it decreases to 0.2376 V at 35°C. These factors must be controlled to ensure accurate measurements.

How does temperature affect the potential of a calomel reference electrode?

The potential of a calomel reference electrode (SCE) is influenced by temperature. As temperature increases, the solubility of potassium chloride (KCl) changes, affecting the concentration of chloride ions (Cl^-) in the solution. This, in turn, impacts the electrode potential. For example, at 25°C, the potential is approximately 0.2444 V, while at 35°C, it decreases to 0.2376 V. The Nernst equation describes this relationship:

$E_{\mathrm{cell}}=E_0-\frac{0.05916}{n}log\left(a\right)$

where $E_{\mathrm{cell}}$ is the cell potential, $E_0$ is the standard cell potential, $n$ is the number of electrons transferred, and $a$ is the activity of chloride ions. Temperature changes can thus affect the electrode's potential, requiring careful control during measurements.