So here we state that a junction potential is created at the interface between two ionic solutions. Now, realize from our previous discussions on electrochemical cells that we have the salt bridge, and the salt bridge's purpose is to help create a counterbalance to the electrons that travel from the anode to the cathode by releasing anions that flow from the cathode side to the anode side. This opposite movement of negatively charged ions helps to complete the circuit for our given electrochemical cell in terms of a galvanic cell or voltaic cell. Now, we're going to say at both ends of the salt bridge, we have the building up of these ions. And depending on how quickly these ions pass as they move from the salt bridge to the solution, there is a certain amount of potential that builds up, and this is when we talk about the junction potential. Now, we're going to say here that this junction potential is based on two things. Based on the concentration of the solutions and differences in mobility of the ions. Here we're going to say this creates a negligible amount of voltage at the end of the salt bridge connecting the two half reactions. So, let's take a look at our image on the left. In this image, we have two solutions that are separated from one another by a semipermeable membrane, which means we have the movement of ions from one side to the other side. On the left side, we have a concentration of 1 molar. On the right side, we have a concentration of 0.1 molar. Now what's going to happen is our ions will travel to the side that is less concentrated. So, this is a form of dispersion where we go from high concentration to low concentration. What's going to happen is our H+ ion and our Br- ions will traverse the semipermeable membrane and go from the area of high concentration to the area of low concentration. Notice the difference in lengths of the arrow. That's because these two ions are not the same size. So they can both travel at different rates towards the right side. Because hydrogen is smaller, it'll move faster. What does this cause in terms of my semipermeable membrane? Well, realize here that we have positive ions that are crossing over much more quickly than negative ions. There is going to be a build-up of positive ions here because hydrogen has crossed over faster. More of them are crossing there, which helps to build up the amount of H+ on that side. It will be positively charged on the right side of my semipermeable membrane. At the same time, my bromide ions are bigger and slower, so they are going to be left behind. There's going to be a slow buildup of bromide ions because the H+ ions are leaving quickly enough, and the Br- ions are not leaving quickly enough; there's going to be a buildup of negative ions on the left side of my semipermeable membrane. This here is an example of a buildup of potential in terms of the semipermeable membrane. This is what we expect when we're dealing with our salt bridge and the possibility of a junction potential being built up. Now here we're accustomed to seeing that the cell potential, which is e cell equals cathode minus anode or, in this case, indicator cell versus the reference electrode. We also have to take into account the junction potential that cannot get result as my ions move from one side to the other side. Normally, we don't include this junction potential here because we use the right types of ions within my salt bridge to minimize the degree of this up of potential. Now, when we're talking about mobility of ions, we're talking about that based on the size of the ions themselves. The bigger the ion is, the slower it will move. So, as we can see here, we have the mobility of different ions. In our example, we have the hydrogen ion which has mobility of 3.63×10-8, and we have bromide ions, which have only 1.13×10-8. H+ is smaller than Br- that's why it moves faster. We're also going to see that we have junction potentials that can result based on the concentrations as well as the identity of the ions used. Now we're going to say here we can see that there's a building up of potentials as we compare different ions to one another and different concentrations to one another. What this is showing me is that when it comes to creating the perfect junction potential, it's best to use KCl because the two ions have similar mobility values. There's going to be a small buildup of potential, so small that it's negligible and the sub potential overall is just again cathode minus anode or, in this case, th
Junction Potential - Video Tutorials & Practice Problems
Understanding Junction Potentials
Junction Potential
Video transcript
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Mobility of ions is based on their size and the greater the difference in sizes between the two ions the greater the potential.
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Junction Potential
Video transcript
So here it asks which side of the 0.5 molar sodium bromide or 0.5 molar potassium bromide junction will be more negative. Alright. So we have our junction here or semipermeable membrane here. We have 0.5 molar sodium bromide on this side, and we have 0.5 molar potassium bromide on this side. Realize here that both of these contain the same negative ion, the bromide ion. So that means it's not going to play a factor because both sides have bromide ions and both have the same concentration of bromide ions. So, really the difference is looking at the positive ions. They are different ions so they are going to move at different rates and that's the key to determining which side will be more negative. If we take a look here, we have a potassium ion with a mobility of this, and we have a sodium ion with a mobility of this. We can see that the potassium ion is actually moving faster here. Now because it moves faster, it's going to be able to cross the semipermeable membrane at a faster rate, and sodium is moving much slower. So, it's not going to be able to cross over to the other side of the semipermeable membrane as quickly. So what's going to happen is we have some sodium ions that are left back here because they can't move fast enough, and then we also have potassium ions crossing over much more quickly. So we're going to have a buildup of positive ions on this side. So overall, the left side would have a higher concentration of positive ions. So that side would become more positive, and if the left side is more positive that would mean that it's this side here, the side with the potassium bromide, that will be more negative. So when it comes to questions like this, we have to look at two things. We look at concentration and we look at the ions themselves. The one with the greater concentration is going to have ions leaving that side to go to the other side at a faster rate. But if the concentrations are equal, we then look at the number of ions, the type of ions. The higher the mobility of that ion, the faster it can cross over the semipermeable membrane and create a charge imbalance on the other side of the semipermeable membrane or junction. Here we can see that potassium and sodium definitely move at different rates. Because potassium can move faster, it's going to cross over to the left side and make that side more positively charged. Here we're looking for which side is more negative. So if the side with the sodium ion is more positive, by default, the side with the potassium would have to be more negative. More ions are leaving that side at a quicker rate leaving it depleted of positive ions and therefore more negative.