Now we take a look at EDTA titrations. We've done acid-based titrations in the past. Now, we're going to be taking a look at EDTA serving as our titrant. We're going to say that the general formula is MN+ where M represents some metal ion with some charge reacting with EDTA. Remember, EDTA for the most part, we're dealing with its basic form which has a negative four charge. When they combine, they form our complex here, MYN4. In our mock titration, we're dealing with barium ions. If we were to write this out, we'd have barium, Ba2+ reacting with EDTA. The net charge at the end, we'd have +2 here, -4 here, so the net charge at the end would be -2. We'd write it as bay2-2 as our barium EDTA complex.

Now, we're going to be dealing with these types of K'f. Remember, that is equal to the fraction of EDTA in the basic form times our formation or stability constant here. Remember, to figure out our fraction of basic form of EDTA, we'd utilize our chart that we've seen on previous pages. If we're dealing with whole numbers, we can just simply look at that chart from 0 to 14 and see what our alpha value would be. As the pH increases, the alpha value will increase as well because the higher the pH becomes, the greater proportion of my total solution will be in the basic form. If we get a pH that is not a whole number, then we'd have to utilize the formula that we've used in the past to calculate what our alpha would be for the basic form of EDTA. When we take a look at our formation or stability constant here, we'd also utilize a chart that gives us the log of Kf. We do the inverse log to find our Kf there. We say here because our conditional formation constant which is this is an equilibrium constant. It also equals products over reactants. It would equal our metal EDTA complex divided by the concentration of my metal times the concentration of EDTA. Realize that in this process of titrating our metal ion, we're adding EDTA as our titrant. This is going to cause the pM of my metal to gradually increase over time. Remember, p here stands for negative log. This side here represents the negative log of the metal concentration.

We're going to say we can reach an equivalence point because we're dealing with a titration. At the equivalence point, we'd say that our metal ion concentration equals the concentration of EDTA. And really, it's the moles that are equal, not necessarily the concentrations. We say the moles of our metal ion equals the moles of our EDTA at the equivalence point. Before the equivalence point is reached, we'd have excess metal ion. After the equivalence point is reached, we'd have an excess of EDTA. EDTA would be greater than the concentration of my metal ion beyond the equivalence point. Now with any types of titrations, it's always important to determine what our equivalence volume for the titrant will be. This allows us to determine are we before the equivalence point, at the equivalence point, or after the equivalence point.

Here we have the titration of 50 mL of 0.100 molar barium ion. It's buffered to a pH of 9.0 and we're going to say with 0.050 molar EDTA. Here, we'd say that the molarity of my metal times the volume of my metal equals the molarity of EDTA times its equivalence volume. Remember, EDTA is serving as a titrant, so we're looking for the equivalence volume of that titrant. We plug in what we have, so the molarity of my barium ion metal is 0.100 molar times its volume, which is 50 mL equals the molarity of my EDTA times the equivalence volume. Divide both sides here by 0.050 molar. So this will cancel with this. The molarities would cancel out and we'd see that the equivalence volume for my EDTA would be 100 milliliters. We would know that before we get to 100 mLs, we'd be dealing with calculations before the equivalence point. Take a look at the next video and see what are the calculations involved for determining the concentration of my metal ion before the equivalence point.