Principal Species - Online Tutor, Practice Problems & Exam Prep

The acid dissociation constant (Ka) indicates acid strength; a Ka > 1 signifies a strong acid, while Ka < 1 indicates a weak acid. The relationship between pH and pKa helps determine the predominant species in a solution. For a monoprotic acid, if pH < pKa, the acidic form predominates; if pH > pKa, the basic form is favored. When pH equals pKa, both forms are present in equal amounts. This concept extends to diprotic and polyprotic acids, enhancing understanding of acid-base reactions and equilibria.

Principal Species

We've learned in the past that our acid dissociation constant, otherwise known as Ka, is a way of gauging the strength of an acid. We know that if our Ka value is greater than 1, then we're dealing with a strong acid. And if our Ka is less than 1, then we're dealing with a weak acid. Now, in addition to this, we can say that the acid constant Ka tells us the numerical value that an acidic hydrogen can be removed. Basically, if we can convert our Ka to pKa, we can compare it to the pH of our solution to determine which one is the principal species within our solution. Is it the acidic form or the basic form that predominates?

Now here, if we're looking at a monoprotic acid, if we have the pH being less than our pKa, then we're gonna say that the acid form is greater than the basic form. And then if we look at it in the other way, if our pH is greater than our pKa, then our acid form is less than our basic form. The way you can think of it is we have these two forms, and A minus. And in between them is this line here. At this line, we can say that this is our pKa. If your pH happens to be less than this pKa, then we'll exist in our acidic form here. But if all of a sudden, we crank up the pH where it surpasses pKa, then we'll be on this side of the fence, so exist in our basic form.

What would your, what will be the principal species if your pH equaled your pKa? Well, if your pH equaled your pKa, you'd be exactly on this dividing line here. On this dividing line, both forms exist together. If your pH equaled your pKa, that would mean that you have 50% of this acid form and 50% of the basic form. That is true when pH equals pKa. This relationship of pH and pKa can also be expanded outwards to diprotic as well as polyprotic acids. We'll take a look at those situations as we investigate each of the example problems left below. Click on the next video and see how we approach example 1.

Principal Species

In this question here, it says, what is the predominant form of the diprotic acid, methionine at a pH equal to 4.18? All right. So we're dealing with the diprotic acid. Remember, when it comes to diprotic species, there are 3 major forms. We have our fully protonated form. We can say that is H₂A. So you could think of this as the acidic form. Then we have our intermediate form, H₂A^-. And finally, we have our basic form after all the acidic hydrogens have been removed. That is A^2-. And remember, when we're talking about borders and things, so in between our acidic form and the intermediate form, exists one border here. We're talking about removing the 1st acidic hydrogen from H₂A to create H₂A^-. Since we're talking about the 1st acidic hydrogen, that means we're dealing with K_A1. Here, we're going to take the negative log of that to find pK_A1. pK_A1 equals the negative log of K_A1. Taking the negative log of that gives me 2.18. Here on this border, pK_A1 equals 2.18. Remember, exactly on that border, we'd have 50% of the acid form and 50% of the intermediate form in this case. The next border that separates my intermediate form from the basic form would be here. In that case, we'd be dealing with pK_A2 because we're removing the second acidic hydrogen to create our basic form. pK_A2 equals negative log of K_A2. That's negative log of 8.3 times 10 to the negative 10. So that gives me 9.08. Here we'd have 50% of the intermediate form and 50 percent of the basic form. Now here we're dealing with the pH equal to 4.18, which means that we've passed beyond this border here and we're not quite at this border here. We're somewhere in the middle. And what form exists predominantly in that space? The intermediate form does. Here, the intermediate form will be the predominant form because it is past the first border. It's blown past the acidic form, but it hasn't quite reached the second border. It's not having any equal amount with the basic form. In this case, our intermediate form of methionine would be the predominant species. Now that you've seen this example, see if you can do example 2. Once you do, come back and see how I approach that same question.

Principal Species

Although we didn't cover one dealing with a triprotic acid, we can apply the same methods that we used in the previous example. Here it says, what is the predominant form of histidine at a pH equal to 8? Here we know it's a triprotic acid because it has 3 pKa values.

Principal Species

If we take a look at this example, we said, "What is the second most predominant form in the previous question?" We know that the predominant form is the intermediate 2 form. The pH of 8, which is here, is within this region. Next to that region is the first intermediate form and our basic form. We're trying to determine which one of these two would be the next highest concentration at a pH of 8.00. Now, I gave the hint of looking up the isoelectric point. Here, if we calculate the isoelectric pH, that'll help us navigate which one will be the second most dominant form. Here, pH equals half pKa1 plus pKa2. Now, which pKas are we dealing with? 8.00 is in between pKa2 and pKa3. Those will be the pKa's that we're using. That equals half of 5.97 + 9.28. We're gonna figure out the pH here. When we do, we get 7.625 as my pH, my isoelectric isoelectric pH. Now, what do we say in terms of this value? Well, we're gonna say here, if we are above a pH of 7.625, that means we are more basic. Right? Because the higher the pH gets, the more basic your solution becomes. And the more basic your solution becomes, the easier it is to deprotonate or take off an H+. At a pH above 7.625, it becomes too basic. So the 3- basic form would be the second most predominant. And then, if we said below a pH of 7.625, that would mean that our pH is lower so it's more acidic. Remember, in a more acidic environment, what do we do? We add an H+ to something. We protonate it. In that form, H₂A^- which is the intermediate one would be second most predominant. This whole isoelectric pH formula is just to help us establish a baseline. All we do now is compare this new baseline to the current pH environment. The current pH environment is 8.00, like we said in example 2. So, the current pH value is higher than our baseline of 7.625. Therefore, it's the basic form that would be the second most predominant form within this solution. That's how we are able to determine who's the second most popular or dominant form within a solution. Refer to the borders, examples for examples 1 and 2 to find the most dominant, and then refer to an isoelectric pH to determine which one is second. Once you do that, you'll be able to get your answers.

Here’s what students ask on this topic:

What is the relationship between pH and pKa in determining the principal species in a solution?

The relationship between pH and pKa is crucial in determining the principal species in a solution. If the pH is less than the pKa, the acidic form (HA) predominates. Conversely, if the pH is greater than the pKa, the basic form (A^-) predominates. When the pH equals the pKa, both the acidic and basic forms are present in equal amounts, meaning the solution contains 50% of each form. This relationship helps in understanding the behavior of acids and bases in different pH environments and is applicable to monoprotic, diprotic, and polyprotic acids.

How does the acid dissociation constant (Ka) indicate the strength of an acid?

The acid dissociation constant (Ka) is a measure of the strength of an acid in solution. A Ka value greater than 1 indicates a strong acid, which dissociates completely in water. Conversely, a Ka value less than 1 indicates a weak acid, which only partially dissociates. The Ka value provides insight into the extent of dissociation and the ability of the acid to donate protons (H⁺). This information is essential for predicting the behavior of acids in various chemical reactions and environments.

What happens to the principal species when pH equals pKa?

When the pH of a solution equals the pKa of the acid, the principal species are present in equal amounts. This means that 50% of the species is in the acidic form (HA) and 50% is in the basic form (A^-). This equilibrium point is significant because it represents the pH at which the acid and its conjugate base coexist in equal concentrations, providing a balanced state between the two forms.

Can the concept of pH and pKa be applied to diprotic and polyprotic acids?

Yes, the concept of pH and pKa can be extended to diprotic and polyprotic acids. These acids have multiple dissociation steps, each with its own pKa value. For each dissociation step, the same principles apply: if the pH is less than the pKa, the acidic form predominates; if the pH is greater than the pKa, the basic form predominates. When the pH equals a specific pKa, the corresponding acidic and basic forms are present in equal amounts. Understanding these multiple equilibria is essential for analyzing the behavior of complex acids in solution.

How can you use pKa to predict the predominant form of a monoprotic acid in a solution?

To predict the predominant form of a monoprotic acid in a solution, compare the pH of the solution to the pKa of the acid. If the pH is less than the pKa, the acidic form (HA) will predominate. If the pH is greater than the pKa, the basic form (A^-) will predominate. This comparison allows you to determine whether the acid will primarily exist in its protonated (acidic) or deprotonated (basic) form, which is crucial for understanding the acid's behavior in different pH environments.