What does pKa mean?

See the new video from Pharma Drama. Prof. Simon Gaisford explains the concept of pKA:

Welcome to Pharma Drama, the channel where we look at the science of healthcare and healthcare products. In this video I’ll explain the concept of pKa, which I know feels a bit abstract but once you know what it means you’ll understand how acids, bases and salts behave in water! And talking of water, get yourself a drink and let’s make a start.

Since pKa is a concept that describes something to do with acids, I should start by reminding you what an acid is. An acid is a molecule that is capable of donating a proton to water – we use the shorthand AH to represent an acid, with H being the proton and A being the rest of the molecule. When added to water, some of the acid molecules may donate their protons to water, forming A minus and the hydronium ion H3O plus. If you want to understand acids in a bit more detail I have a video on that, the link for which is below.

Now I think if I mention the word acid some of you might imagine dissolving bodies in the style of Breaking Bad, and it is true that some strong acids (like hydrochloric or sulphuric) can do that. But there are many more acids that won’t. No one has yet, to my knowledge at least, dissolved someone they dislike in vinegar (acetic acid) or orange juice (citric acid). That’s because acetic and citric acids are weak acids. But what is it about acids that makes them strong or weak?

The answer is that the acid itself (AH) is not reactive – it is the hydronium ion (H3O plus) that will react with things. Therefore, how strong we perceive an acid to be is really a function of how many of its protons it will donate to water to enable the formation of hydronium ions. Acetic and citric acids are ‘weak’ acids because they do not donate many protons at all, whereas hydrochloric and sulphuric acids are ‘strong’ acids because the donate nearly all of their protons. In other words, because it is the hydronium ion that is reactive, it doesn’t matter what the rest of the acid (the ‘A’) looks like – all acids chemically react with things because of the hydronium ion and whether they react strongly or weakly is simply a function of how many hydronium ions are created.

What this means is that acid dissociation will, like all chemical reactions, reach a position of equilibrium and it is the extent to which an acid will donate its protons that defines whether the position of equilibrium lies to the left-hand side (weak acid) or right-hand side (strong acid).

It is of course a bit difficult to define the strength of an acid by saying ‘well it’s quite to the right-hand side’ – we need something more quantitative. To do that we define an acid equilibrium constant, Ka, which is given by the product of the concentrations of the products divided by the product of the concentrations of the reactants (and I note here that because water is usually present in excess, because it is the solvent, we ignore its concentration). For the pedants among you, I will say that formally we should use activity coefficients in this equation, rather than concentrations, and the activity coefficient of water is taken to be one, which is why it doesn’t appear in the equation, but you don’t really need to worry about that!

So now we have a number that we can use to quantify the strength of an acid. The more protons that an acid releases, the higher the value of Ka. To keep the numbers easier to manage we usually use the negative log of the Ka value, which we term pKa. The higher the value of Ka, the lower the value of pKa. So when you see the pKa value given for an acid now you know what it is – it is a number that tells you how many protons an acid will donate. The more protons that are donated, the lower the pKa value (this is similar to pH – the more protons in solution, the lower the pH). Strong acids are defined as those with a pKa value below zero.

Now, you might say to me ‘can we define the same thing for bases Simon?’. And the answer is, of course, we certainly can! Since a base is a proton acceptor, I hope you can see that the strength of a base is effectively the opposite of an acid – the more protons that are accepted, the stronger the base. Note here also that, as in the case of acids, it is not the base itself that is reactive, it is the hydroxide ions (OH-) that are created that react with almost anything. We can define an equilibrium constant in the same way as we did for acids, which we would term Kb, but the use of acid dissociation constants is so common in the pharmaceutical world that base dissociation is always expressed in terms of a Ka, in which case we consider the dissociation reaction from right to left and we define the equilibrium constant like this. You will note it’s the same as that defined for acids, but the terms on the right-hand side are reversed (because a base is the opposite of an acid!). Taking negative logs again yields a pKa value. A strong base has a pKa value greater than 14.

And that is all we need to discuss. A pKa is simply a value that tells us to what extent an acid or base has dissociated. We need to know that because acids and bases are only reactive once dissociated – so the greater the extent of dissociation, the stronger the acid or base! I hope you found that brief description useful. If you did, please hit the ‘like’ button and consider subscribing – it really helps the channel. Otherwise, thank you so much for watching, and I’ll see you again soon.