EDTA Titration Curves - Online Tutor, Practice Problems & Exam Prep

EDTA titrations involve the complexation of metal ions, such as barium (Ba²⁺), with EDTA, which has a net charge of -4. The equivalence point occurs when the moles of metal ion equal the moles of EDTA. Calculating concentrations before and after this point requires understanding the conditional formation constant (K_f) and using equilibrium expressions. The concentration of the metal EDTA complex and excess EDTA are crucial for determining free metal ion concentrations, which can be found using the negative log of the concentration.

EDTA Titrations

Now we take a look at EDTA titrations. Now, we've done acid-based titrations in the past. Now we're going to be taking a look at EDTA serving as our titrant. Now, 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 Mach 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 plus 2 here, minus 4 here. The net charge at the end would be minus 2.

We'd write it as BaY2- as our barium EDTA complex. Now, we're going to be dealing with these types of titrations at a specific pH. Because of that, we'll have to determine what our conditional, constant is. Our conditional formation constant is kf. 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. Remember, 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'd 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 the 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.

So 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. So we'd say that the amount of metal ion in moles is greater than the amount of EDTA. 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's 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's, 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 will cancel out and we'd see that the equivalence volume for my ETA would be 100 milliliters.

We would know that before we get to 100 ml's, 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.

EDTA Titrations

We're now starting with our titration. We're slowly adding EDTA titrant to our barium solution. If we take a look, we have the titration of 50 mL of 0.100 molar barium ion. It's buffered to a pH of 9.00 with 80 mL of 0.050 molar EDTA. Now remember, we require 100 mL of EDTA to reach the equivalence point.

We're before the equivalence point at this juncture in our titration curve. Here, because we're before the equivalence point, we need to determine the amount of our free-floating barium ion as we're slowly adding our EDTA titrant. Here, we're going to say that the concentration of our metal ion equals the equivalence volume that we calculated from before minus the volume of EDTA added, divided by the equivalence volume. This represents the fraction of our free-floating metal that remains upon titration. Now, we're going to multiply it by the initial concentration of our metal ion times the volume of our metal ion divided by the volume of the solution.

This ratio here represents our dilution factor. Taking to heart what we've just seen, we're going to say that the concentration of my barium ion at this point equals the equivalence volume, which we said was 100 mL minus the volume of EDTA, which is 80 mL, divided by 100 mL. We're going to multiply by the initial concentration of our metal ion, which is 0.100 molar, times the volume of our metal ion which is 50 mL, divided by the volume of our solution which is 50 mL from my metal ion plus 80 mL from EDTA. When we plug all that in, we're going to get a concentration of 7.69×10-3 molar for our barium ion. Now that we have the concentration of barium ion, we just need to take the negative log to find our final answer.

Here we're going to take the negative log of my barium ion solution and that gives me 2.11 at this point. Realize the more EDTA we add, the higher the negative log of our metal concentration will go. This is the amount that's remaining after we started originally with 0.100 molar of the metal ion. Remember, the EDTA is grasping on to any free-floating barium ions which is why we see a drop in the concentration. We'll continue onward in the next video and see what happens when we're at the equivalence point itself.

What happens to the negative log of the concentration of barium ion?

EDTA Titrations

At the equivalence point, we have equal moles of our metal ion and moles of EDTA. At this point, we'd set up a basic ICE chart setup in order for us to organize our equilibrium expression. Now here, if we take a look we'll have to determine what our concentration for a metal EDTA complex will be. Here would be the initial concentration of my metal ion times the initial volume of the metal divided by the volume of the solution as a whole. If we take a look, barium with EDTA would be BaY_2-.

It would equal the initial concentration of barium ion which is 0.100 times the volume of barium ion divided by the total volume of the solution, which is 150. The concentration of our barium EDTA complex at this point would be 0.033 M. That's the concentration that we would place here within our basic ICE chart. Then we'd say that since there is an initial amount of that, we're going to be decreasing it over time.

Initially, we don't have any concentrations of our metal ion and our EDTA because we've converted it into our complex itself. These will be building up over time, so we have them as plus.

Now remember, this again is our generic equation to show how our metal ion is binding with our EDTA molecule to form our metal EDTA complex. Because we're buffered at an exact pH, we'd have to determine what our conditional formation constant will be which is k'_f. To do that, we multiply the fraction of the basic form of our EDTA times the formation or stability constant here.

If we look that up, we'd see that at pH 9.0, α equals 0.041. Because we're dealing with barium ion, we'd say that the log of K_f for barium ion equals 7.88. And so K_f for barium ion equals 10 to the 7.88 which here equals, when we plug all this into here, 0.041 times 10^7.88 equals 3.11 times 10⁶. So that would be our conditional formation constant. Now we would take that formation constant.

Now, we would take that formation constant and we'd plug it into our equilibrium expression here. So that would equal 0.033 M, which is the concentration we calculated for EDTA complex, minus x divided by x². We have to solve for x here. So you'd multiply both sides by x². So 3.11 times 10⁶ times x² equals 0.033 minus x.

Bring everything over to the left side, so add plus x, subtract 0.033 from both sides. So we'd have 3.11 times 10⁶ x² plus x minus 0.033. You would set up your basic quadratic formula in order to solve for x. And when you set up the quadratic formula, we find that x equals 1.03 times 10^-4 for the concentration. That x there would give us the concentration of the metal ion itself.

We've determined the concentration of barium ion as being this value here. Because we know that, we can just take the negative log of it to find our final answer. As we predicted, we should see an increase in the negative log of the concentration of our metal. Here it's 3.99 that we get. Remember, it's imperative that you can calculate the equivalence volume of our titrant EDTA to determine where exactly within the titration our calculations will take us.

At the equivalence point, we have a lot more work to do, not just simply plugging into one formula. We have to set up our concentration of our EDTA complex. From there, set up our equilibrium expression. You can set up a basic generic ICE chart in order to help organize the layout for your equilibrium expression here. Remember, because we're dealing at a buffered pH, we'd have to figure out what our conditional formation constant, which is k'_f.

Utilizing the expression, the concentration calculated, and our equilibrium expression helps us to determine what x is. With x, you know what the concentration of your metal ion will be. From there, we can take the negative log. Finally, we'll go on to our last video in terms of our titration to see what happens after the equivalence point and see what steps need to be taken to determine the concentration of the metal at that point in our titration curve.

EDTA Titrations

We have finally reached the point where we have to consider calculations after the equivalence point in our EDTA titration curve. At this point, we utilize our equilibrium expression where our conditional formation constant kf' equals our metal EDTA complex divided by the concentration of the metal times the concentration of EDTA. At this point, we have enough information. We've already calculated our conditional formation constant from a previous part in our titration curve. We'll be able to calculate our concentration of metal EDTA complex, and we'll be able to determine what our EDTA excess will be in terms of its concentration.

With these three pieces of information, we can finally determine what the remaining concentration of our metal ion will be after the equivalence point. Here we have the titration of 50 ml of 0.100 molar barium ion still buffered to are 9 for pH with a 112 ml of 0.050 molar EDTA. What we need to do is determine what the concentration of my EDTA is and the concentration of my metal say here that the concentration of EDTA equals the initial concentration of EDTA which is 0.0 50 molar times the excess volume of EDTA. We only needed 100 ml's to get to the equivalence point. We are 12 ml's beyond that point.

So, 12 ml's is our excess volume of EDTA. Then we're going to divide it by the total volume. That's 50 ml's plus 112 ml's. When we do that, that gives us a concentration of 0.00370 molar. Next, we need to determine what the concentration of our metal EDTA complex will be, which in this case is Ba²⁻.

So we have the initial concentration of our metal, which is 0.100 molar times the initial volume of the metal ion, so 50 ml's. Again, divided by the total volume, which is 50 ml's plus 112 ml's. So that gives me 0.0309 molar. We're going to use the conditional formation constant we found previously. Remember, we said that kf'=3.116, so the concentration of my metal EDTA complex which is 0.0309 molar divided by the concentration of the metal ion, which we're looking for, which is barium ion, times the concentration of EDTA at this point.

So there we go. And all we have to do is solve for that missing variable there. So when we rearrange this expression, we'd see that the metal ion concentration, which is barium, equals the concentration of my barium EDTA complex divided by my conditional formation constant times the concentration of my excess EDTA. So plug in the values that we found for each one and we'd see that we get a concentration for barium of 2.69×10−6. When we take the negative log of that, we'll have our answer.

So that gives us 5.57 at this point. Again, it's all based on first determining what our equivalence volume for EDTA titrant will be. Once we do that, we'll know where exactly in our titration curve our calculations will take us. Are we before the equivalence point, at the equivalence point, or after the equivalence point? Just remember, before the equivalence point we'll have an excess of barium ion.

At the equivalence point, all of it has been reacted with our EDTA where they're equal moles of both. We just have to determine what our remaining barium ion will be. That's free-floating. Then finally, after the equivalence point, we have an excess of EDTA, but we still have some free-floating metal ions. In that case, we'd have to determine what that final amount would be.

Just remember, where in the equivalence, well in the titration curve our calculations take us, and then what formulas are necessary to determine our concentration of our metal ion.

EDTA Titration Calculations

Here we need to calculate the negative log of Manganese 3 ion for the titration of 30 ml of 0.0100 molar EDTA with 50 ml of 0.0200 molar of Manganese 3 Phosphate when the pH is 10.00. What we need to do first is to determine if this titration is occurring before, at, or after the equivalence point. We're going to need to determine what our equivalence volume for our EDTA titrant will be. We're going to say here that the molarity of our metal, which is manganese 3 in this case, times the volume of manganese 3 equals the molarity of EDTA times the equivalence volume of EDTA. Plug in what we know.

The molarity of manganese 3 ion is 0.0200 molar. The volume is 50 ml equals we're going to have 0.0100 molar. And we don't know what the equivalence volume is yet. Divide both sides here by 0.0100 molar. So the molarities cancel out and we'll see that our equivalence volume equals 100 ml of EDTA.

Well, the volume in the question itself is only 30 ml, so this is dealing with calculations before the equivalence point. Since we're dealing with calculations before the equivalence point, that means we're going to have an excess of our manganese 3 ion. What we have to do here is calculate what that excess amount of manganese 3 ion will be. We're going to say the concentration of manganese 3 ion equals your equivalence volume minus the volume of EDTA divided by the equivalence volume. This will give us the amount of the manganese 3 ion remaining, the fraction remaining times the initial concentration of manganese 3 ion times its dilution factor.

That would be the volume of manganese 3 ion divided by volume of the solution. Plug in what we know. Our equivalence volume that we calculated for EDTA is 100 ml's minus the 30 ml's that we're actually using divided by 100 ml's. Next, multiply by the concentration of manganese 3 ion times the volume of manganese 3 ion divided by 80 ml's, which is the volume of our solution, which is 30 ml's plus 50 ml's. When we work all that out, we get a concentration for manganese 3 ion as 8.75×10-3 M.

Now that we have that concentration, we can take the negative log of it to find our final solution. Negative log of 8.75×10-3 gives me 2.06 as my final answer. Remember, for questions like this, we need to determine where exactly within our titration curve our calculations will take us. Here we're dealing with calculations before the equivalence point. We don't have to worry about things such as the conditional formation constant, the use of equilibrium expressions.

All that stuff occurs at and after the equivalence point. Here, since we have an excess of our metal ion, just determine what that concentration is. Take the negative log and you'll have your final answer. Now that you've seen this example, attempt to do the practice question that's left here. Here it's a bit trickier because if we do need to calculate our conditional formation constant, we're not dealing with a whole integer for our pH.

That means that you'll have to utilize some of the formulas we've seen previously to determine what our alpha of our basic form of EDTA will be. Once you do that, then you'll be able to find your formation constant or your conditional formation constant. Good luck and attempt to do this one. Once you do, come back and see how your answer matches up with mine.

Calculate the pNi²⁺ for the titration of 50.0 mL of 0.120 M EDTA with 15.0 mL of 0.100 M NiCl­₂ at pH = 8.22. 

• A. 1.90x10^-17
• B. 17.7
• C. 16.7
• D. 17.4

Here’s what students ask on this topic:

What is the role of EDTA in titrations?

EDTA (ethylenediaminetetraacetic acid) acts as a chelating agent in titrations, binding to metal ions to form stable complexes. This process is used to determine the concentration of metal ions in a solution. EDTA has a net charge of -4 and can form a 1:1 complex with metal ions, such as Ba²⁺. The equivalence point in an EDTA titration is reached when the moles of EDTA added equal the moles of metal ions present in the solution.

How do you calculate the equivalence volume in an EDTA titration?

To calculate the equivalence volume in an EDTA titration, use the formula:

$V=\frac{nMV}{}$_mnE

where $V$ is the equivalence volume, $n$ is the number of moles, $M$ is the molarity of the metal ion, $V$_m is the volume of the metal ion solution, and $E$ is the molarity of EDTA. For example, if you have 50 mL of 0.100 M Ba²⁺ and 0.050 M EDTA, the equivalence volume is 100 mL.

What is the conditional formation constant (K_f') in EDTA titrations?

The conditional formation constant (K_f') in EDTA titrations is an equilibrium constant that accounts for the fraction of EDTA in its basic form at a given pH. It is calculated by multiplying the formation constant (K_f) by the fraction of EDTA in its basic form (α). The formula is:

$K_f\text{'}=\alpha \times K_f$

For example, if α = 0.041 at pH 9.0 and K_f for Ba²⁺ is 10^7.88, then K_f' = 0.041 × 10^7.88 = 3.11 × 10⁶.

How do you determine the concentration of metal ions before the equivalence point in an EDTA titration?

To determine the concentration of metal ions before the equivalence point in an EDTA titration, use the formula:

$C=\frac{V}{}$_eq-V_EDTAV_eq×C_m×V_mV_m+V_EDTA

where C is the concentration of the metal ion, V_eq is the equivalence volume, V_EDTA is the volume of EDTA added, C_m is the initial concentration of the metal ion, and V_m is the volume of the metal ion solution. For example, if V_eq = 100 mL, V_EDTA = 80 mL, C_m = 0.100 M, and V_m = 50 mL, the concentration of the metal ion is 7.69 × 10^-3 M.

What happens at the equivalence point in an EDTA titration?

At the equivalence point in an EDTA titration, the moles of EDTA added equal the moles of metal ions in the solution. This means that all the metal ions have reacted with EDTA to form a metal-EDTA complex. To determine the concentration of the metal-EDTA complex, use the formula:

$C$_complex=C_m×V_mV_m+V_EDTA

where C_complex is the concentration of the metal-EDTA complex, C_m is the initial concentration of the metal ion, V_m is the volume of the metal ion solution, and V_EDTA is the volume of EDTA added. For example, if C_m = 0.100 M, V_m = 50 mL, and V_EDTA = 100 mL, the concentration of the metal-EDTA complex is 0.033 M.