Junction Potential

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 bridges purpose is to help to create a counterbalance to the electrons that travel from the an ode to the cathode by releasing an ions that flow from the cath outside to the an outside 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 gonna 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 off of two things, It's based on the concentration of the solutions and differences in mobility of the ions. Now here we're gonna 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 semi permeable 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 one molar On the right side, we have a concentration of .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. So what's gonna happen is our H plus ion and RVR minus ions will traverse the semi permeable membrane and go from the area of high concentration to the area of low concentration. Notice the difference in length of the arrow, That's because these two ions are not the same size, so they're gonna both travel at different rates towards the right side. Because hydrogen is smaller, it'll move faster. What what does this cause in terms of my semi permeable membrane, we'll realize here that we have positive ions that are crossing over much more quickly than negative ions. So there's gonna be a build up of positive ions here and that's because hydrogen is crossed over faster, so more of them going across there, which helps to build up the amount of H plus on that side. So it'll be positively charged in terms of the right side of my semi permeable membrane. At the same time my bromide ions are bigger and slower, so they're gonna be left behind. There's gonna be a slow build up of bromide ions because again the H plus ions are leaving quickly enough and the br minuses are not leaving quickly enough that there's gonna be a build up of negative ions on the left side of my semi permeable membrane. This here is an example of a building up of potential in terms of the semi permeable 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 sl equals cathode minus an out or in this case indicator cell versus the reference electrode. So the indicator electrode versus the reference electrode, we also have to take into account the junction potential that can 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 built up with 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 the mobility of 36.30 times 10 to the negative eight. And we have bromide ions Which have only 8.13 times 10 to the -8 H plus is smaller than B r minus. That's white 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 gonna 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 K C L. Because the two ions have similar mobility values. So there's gonna be a small built up of potential, so so small that it's negligible and the sub potential overall is just again catholic minus a node or in this case the indicator electrode minus the reference electrode here. You want to make sure you choose ions that are similar in size so that you have a potential that is as small as possible. From the options given here, we can see that the smallest difference in potential is given in this option here and again. When it comes to the right types of ions to use for my salt bridge, potassium chloride is the best type of ionic compound to use because K positive and cl minus are so close in size. Now that we've talked about this attempt to do the example problem left on the bottom of the page based on the junction provided, determine which side will become more negative

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 semi permeable 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 bromide ion. So that means it's not gonna play a factor because both sides have bromide ions and both have the same concentration of bromide ions. So really the difference is that looking is looking at the positive ions, they're different ions so they're 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 potassium ion with the mobility of this and we have a sodium ion with the mobility of this, we can see that the potassium ion is actually moving faster here now because it moves faster, it's gonna be able to cross the semi permeable membrane at a faster rate and sodium is moving much slower. So it's not gonna be able to cross over to the other side this side here of the semi permeable membrane as quickly. So what's gonna 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 gonna have a build up of positive ions on this side. So overall the left side would have a higher concentration of positive ions. So that side will 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 gonna have ions leaving that side to go to the other side at a faster rate. But if the concentrations are tied we then look at the number of ions, the type of ions, the higher the mobility of that ion, the faster can cross over the semi permeable membrane and create a charge imbalance on the other side of the semi permeable membrane or junction here, we can see that potassium and sodium definitely move at different rates because potassium can move faster. It's gonna cross over to the left side and 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