What is pH?

An explanatory video by Prof. Simon Gaisford – Pharma Drama

Welcome to Pharma Drama, the channel where we look at the science of healthcare and healthcare products. In this short video I’ll explain the concept of pH – and I should say that before watching it, it would help a lot if you’ve already watched the videos on acids and bases! So watch those if you haven’t already, then make a drink, relax and let’s make a start.

Acids are molecules that can release a proton, H+, in water and bases are molecules that can accept a proton. Protons are highly reactive and will interact with a lot of molecules causing all kinds of chemical reactions – that is why acids are so destructive. The greater the concentration of an acid dissolved in water, the greater the number of protons available to react. Similarly, the greater the concentration of a base dissolved in water, the fewer the number of protons (remember though that strong bases generate the hydroxide ion, OH-, in water and that is also very reactive!).

It follows that we can characterise the acidity or basicity of a solution by measuring the concentration of protons in solution. The greater the concentration of protons, the more acidic the solution and similarly the lower the concentration of protons the more basic the solution. Concentrations, I remind you, are usually expressed in molar units (there is a video on concentrations, you’ll be pleased to hear, if you are unclear on moles). So, we might measure a concentration of protons of, for instance, 0.001 molar.

That’s fine, but does a concentration of 0.001 molar mean anything to you? No, I thought not. In order to create values that are easier to understand, we use the term pH to define acid strength. In general, in chemistry at least, the term p is used to mean negative logarithm (pH, pKa, pOH and so on), so in this case pH simply means the negative logarithm of the concentration of protons. As we have a concentration of 0.001 molar, taking the logarithm gives -3, so the negative logarithm is 3. In other words, a concentration of protons of 0.001 molar corresponds to a pH value of 3. Similarly therefore, a pH value of 1 corresponds to a proton concentration of 0.1 molar and so on.

The pH scale ordinarily runs from 1 to 14, 1 being most acidic and 14 being most basic, although it is possible to have pH values beyond these extremes. Because it’s a logarithmic scale, a change of 1 pH unit corresponds to a 10-fold change in concentration of protons. Since acidity and basicity are opposites of each other, the mid-point of the pH scale, pH 7, corresponds to a position of neutrality. So water at neutral pH should have a pH value of 7. If we add an acid the pH value will drop and as if we add a base the pH value will rise.

Knowing the pH value of a solution is very important. For one thing you wouldn’t want to put your hand in a solution of high or low pH but also how acids, bases and salts behave is very dependent upon pH and we will look at that in separate videos. But because pH is very important to how many molecules, especially biochemical molecules, behave, when you look at the human body you will find that the pH in various regions is very specific and very tightly controlled. For instance, the pH in the stomach is low (usually between 1 and 3) because its job is to degrade food that you have eaten and also kill any bacteria that might be present, while the pH rises along the gut, rising to near 7 in the colon, because you have a large number of bacteria there helping to digest your food. Similarly, the pH of the blood is maintained at 7.4. Your body maintains these pH values with buffers and if it didn’t, you’d be in a world of trouble! Most of the drugs that we formulate are, however, weak acids and bases and so the fact that we need them to dissolve in the body means we must understand how they behave as a function of pH.

Right, that’s all we need to say about pH. It simply means the negative logarithm of the concentration of protons, and runs on a scale of 1-14, with 1 being most acidic, 7 being neutral and 14 being most basic. I hope you found that 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