The End Point

So we've talked about the endpoint in previous chapters. When we discuss asset and baste it rations. Remember with an acid based filtration. The endpoint represented the range in which our indicator would change colors. This was used as an estimate to help us determine what the equivalence point would be for tight rations. Remember in the equivalence point we'd have equal moles of our acid and base at that point. Now with endpoints dealing with redox tight rations, we're gonna say within a redox tight rations, we can utilize indicators and electrodes to determine the endpoint. In this case the end point, we're not really using it as an indicator to determine what the equivalence point is. Rather, we're using it to help us determine what our potential would be our greatest jump in potential in terms of a redox titrate. Now, when it comes to redox indicators, we're gonna say when we add a redox indicator to the analytic or what's called the tight trend, the indicator will change colors based on the solutions potential. Now, the way it works is we're gonna have our tight Trent that we're adding this tight trend here will basically affect my solutions potential. Oftentimes causing an increase in the potential of the solution. What this is going to do is the tight trend will be interacting with the indicator. In this case, the indicator can either be reduced or oxidized by the tight trend that we're using. If we're using an oxidizing um re agent, then that oxidizing reagent would oxidize my indicator, removing an electron from it. If we're dealing with a reducing agent, then that reducing agent would add an electron and therefore reduce my indicator. So we're gonna say here the indicator will have a change in the oxidation state. Okay so let me just write out the word change change and oxidation state by having a change in its oxidation state. This will cause a color change within the indicator. And that color change signifies that we are at the end point in terms of my redox titrate. Now here we're gonna say the reduction half reaction for a redox indicator can be seen as we have the oxidized form of the indicator here we can add some number of electrons and could represent one to any other larger whole number which are added to my oxidized form of my indicator as a result of this my indicator is now in its reduced form here we can use this half reduction reaction to create our nurse equation. In this form we'd say that the nerds equation is the self potential under nonstandard conditions equals the cell potential Under standard conditions minus 0.5916 volts divided by the number of electrons transferred here. We're gonna have times log of the indicator in its reduced form, divided by the indicator. In its oxidized form here we've seen this type of ratios before when we've dealt with the Henderson Hasselbach equation like the Henderson Hasselbach equation. This ratio also has a range, it's best if they don't differ from one another by a magnitude of 10. So if we had the indicator itself That's in its reduced form was one. And let's say are oxidized format, most can be 10 When we do the log of that, that gives me -1. Or we could have a blog of 10/1 And that would give me one. This range of negative 1 to positive one is what we deal with when we're discussing are indicators and how it's being affected by the titrate added. Because of this minus one plus one range. We can recreate our nerds equation in this form in which we have plus or minus 10.5916 volts divided by N. And again it says by assuming that the indicators color change from the oxidized state to the reduced state. When the ratio changes from .1.1, meaning when it's 1/10 2, 10 words 10/1. That's how we're able to come up with this new range in which our indicator can operate. Now we're gonna say the indicator transition range should overlap the portion of the curve that has the sharpest increase in potential. Just like an acid based filtration. We were able to determine what the ph was at the equivalence point by looking at the highest increase in my ph Now we're looking at the highest increase in my potential here. We can also use a grand plot to help us identify the um end point in which we use the maximum value of the first derivative, which is a change in your charge, your potential divided by your change and volume. Here we can take a look at a basic filtration curve in which we're dealing with 50 mls of 500.100 Moeller mv two plus. So this would be my an elite or tight trend. And here we're adding to it Siri um four plus, Syrian four plus is an example of an oxidizing agent. It represents my titrate here. We're using two different indicators would indicate that we're using is diphenyl amine symphonic acid. Here in this color region we see that this is where the color change of my indicator takes place. We also have here our second indicator here. It's color change happens within this range here and it coincides with a large increase in my potential. So that's when we've reached basically um are equivalent volume, which causes a sharp increase in the potential for my reaction here. Out of the two indicators would say that the second one would be better in terms of determining the correct endpoint because it lies within the region where we have the greatest increase in our potential. Realize here that endpoints whether they're dealing with acid base indicators or dealing with redox tight rations. They help us to determine what the endpoint of my titillation reaction will be for acid base indicators. We can correlate this with our equivalence point where the mold of acid equal moles of base and for redox reactions, we can core this with the endpoint, which represents the state in which my indicator goes from its oxidized form to its reduced form in this case. So keep that in mind. When we're looking at any type of filtration curve in which we have to spot the endpoint, look for the sharp increase in ph or potential, and then look to see where the color change happens for the indicator as an estimate of your endpoint.