So we already know that some acids are polyprotic acids, and all that means is that they have multiple acidic hydrogens. And we know that each acidic hydrogen is going to have its own pKa value. Also, the titration curves of polyprotic acids have multiple inflection points as well as multiple equivalence points. And so, there's going to be a set of an inflection point and an equivalence point for each acidic hydrogen. So the number of acidic hydrogens determines the number of inflection points and the number of equivalence points. And so if there are 2 acidic hydrogens, there's going to be 2 inflection points and 2 equivalence points. If there are 4 acidic hydrogens, then there's going to be 4 inflection points and 4 equivalence points. And so recall from our previous videos that each inflection point, and also recall each inflection is also known as a midpoint. Each inflection point indicates the pKa of an acidic hydrogen. And also recall from our previous videos the Henderson-Hasselbalch equation, which is going to be really helpful for our titration of the acid, which has a chemical formula of H3PO4. And phosphoric acid is a polyprotic weak acid, so it actually has 3 acidic hydrogens, and those 3 acidic hydrogens are here. And so notice that the first acidic hydrogen has a pKa of 2.2, the second acidic hydrogen has a pKa of 7.2, and the third has a pKa of 12.7. And so over here, what we have is the titration curve of phosphoric acid with a strong base.
On the y-axis, we have the pH of the analyte solution, and on the x-axis, we have the amount of titrant that's being added. And so notice that because phosphoric acid has 3 acidic hydrogens, there are 3 inflection points shown by the dotted black lines, and there are also 3 equivalence points shown by the green-dotted lines. And so, notice that when the inflection point occurs, it indicates the pKa value of an acidic hydrogen. So the 1st acidic hydrogen we said has a pKa value of 2.2. So notice that the inflection point, which occurs right here, corresponds with a pH value of 2.2, and this is where the pKa is. And so when the pH is equal to 2.2, the concentration of conjugate acid equals the concentration of conjugate base.
Here, because we have H3PO4 as our conjugate acid, our H3PO4 conjugate acid is going to equal our conjugate base of H2PO4-. And so when we get to our first equivalence point where we add exactly 1 molar equivalent, notice that this is going to be where we completely neutralize H3PO4 has been eliminated. So H3PO4 is essentially eliminated, or completely neutralized at this equivalence point. And so at the equivalence point, H2PO4- is at its maximum concentration.
Now notice that at the second inflection point, which is shown up here, there is a pH of 7.2, which indicates the pKa value of 7.2. And so when the pH is equal to 7.2, the concentration of conjugate acid is going to equal the concentration of conjugate base. And here, at this point, what we've done is, the conjugate acid is H2PO4-. And so, the concentration of conjugate acid, H2PO4-, is going to equal the concentration of conjugate base, which is HPO42-. And so, that would be at this point here, this midpoint. And so at the second equivalence point over here, what that means is we've added exactly 2 molar equivalents of the titrant to neutralize the conjugate acid. So at this point here, what we've done is we've neutralized H2PO4-. So at this point, H2PO4- is completely gone and HPO42- is at its highest concentration. So then we continue to add titrant, and we get to our 3rd pKa value, our 3rd inflection point.
And so notice that this inflection point occurs at a pH of about 12.7, which indicates that the pKa is 12.7. And so when the pH is equal to 12.7, the concentration of conjugate acid is going to equal the concentration of conjugate base. And so here our conjugate acid is HPO42-. So we can put that in here, HPO42-. And that's going to equal the concentration of conjugate base, which is PO43-. And so, as we continue to add more and more titrant, eventually, we get to our 3rd equivalence point. And our 3rd equivalence point is when we add exactly 3 molar equivalents of titrant. And that means that we've completely eliminated HPO42-, and that PO43- is at its highest concentration of hydronium ions in the solution. In our next video, we'll be able to get a little bit of practice analyzing."