14. Electrodes and Potentiometry
Junction Potential
Video duration:
5mSkip to main content
So here we state that a junction potential is created at the interface between 2 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 to 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 gonna say at both ends of the salt bridge, we have the building off 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 gonna say here that this junction potential is based off of 2 things. It's based on the concentration of the solutions and differences in mobility of the ions. Now, here we're going to say this creates a negligible amount of voltage at the end 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 2 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 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. So 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're going to 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? Well, realize here that we have positive ions that are crossing over much more quickly than negative ions. So there's going to be a build up of positive ions here and that's because hydrogen is crossed over faster. So more of them are going to cross there which helps to build up the amount of h plus on that side. So it will be positively charged in terms of 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 again the H plus ions are leaving quickly enough and the are not leaving quickly enough that there's going to be a buildup 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 are 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. 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 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 a mobility of 36.30 times 10 to the negative 8, and we have bromide ions, which have only 8.13 times 10 to the negative 8. H plus is smaller than b r minus that is 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 2 ions have similar mobility values. So there's going to be a small build up of potential. So so small that it's negligible and the sub potential overall is just again cathode minus anode or in this case the indicator electrode verse minus the reference electrode. Here, you wanna 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 used for my salt bridge, potassium chloride is the best type of ionic compound to use because k positive and c l 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.
05:58
