 In a previous video we had a bit of a look at the pH scale, started to introduce some of the reasons why this is such an important tool for chemists. The thing that I want to just draw your attention to before we slip past this particular one is the fact that as the scale as we move up or down through the scale, the changes that we see are changing by a factor of 10. So this is not a numerical scale, this is actually a logarithmic scale where each single number change on the pH scale actually represent a change in the concentration of the ions of a factor of 10. Now if we're looking at the concentration of the hydrogen ions then we can relate the concentration directly to the pH through a mathematical formula and this is something that we're going to be looking at in a little bit more detail. The only other thing that I want to I guess look at at this stage is this value of pH 7 and this is the value that we have for neutral solutions. Solutions where there is an equal number of hydrogen ions and hydroxide ions. In order to understand that we need to have a little bit more look at water in particular and also the pH calculation. So let's just briefly look at water. Water itself is capable of behaving as a Bronsted-Larry acid and base. Water molecules can both donate and accept a proton from one another. When this happens, we'll draw it as an equilibrium, the proton may go from one water molecule to another. As it does that, the one that it leaves behind becomes a hydroxide and the one that it ends up with becomes the hydronium ion and it's actually more accurate to think of the pH scale as actually working on the concentration of these hydronium ions. So when you see H+, you can actually think in water this actually manifests itself as the hydronium ion as an H3O+. At 25 degrees, the concentration of the hydrogen ions is equal to the concentration of the hydroxide ions. So you can see to whatever degree this occurs, because the ratio is one to one to one to one, the proportion of each of these in pure water is going to be the same. They're going to have a one-to-one ratio. Our pH calculation, as we said previously, is a log scale. Therefore, if we convert the concentration through a calculation based on log 10, we can get a value for the pH. And this is how we get our pH of 7 for a neutral solution. The concentration of the hydrogen ions is equal to the concentration of the hydroxide ions and they're both equal to 10 to the minus 7. The reverse, of course, of the pH equation is that we can find the concentration of the hydrogen ions by moving the negative sign to the other side and then inverting a log, which means raising something to the power of 10. So it becomes 10 to the power of minus pH. So this is an alternate way of looking at this particular equation just to give you a bit of a way of converting between either a known pH and an unknown concentration or a known concentration and an unknown pH. To go with our pH, we have another concept called POH. I hope it should be relatively obvious to you that if we're looking at a pH calculation that's based on a log scale and related to the concentration of hydrogen or hydronium ions in solution, that the POH would be related to the concentration of hydroxide ions in solution. And as before, when we said that for pure water, the pH of 7, the concentration of the hydrogen ions is the same for the concentration of hydroxide ions in pure water. So therefore the POH will also be 7, 7 and 7 are 14. As we look at different examples, we'll see that as we add acids, the pH is actually going to go down as the concentration of hydrogen ions goes up, but this is also going to mean that the POH is going to go up. So we want to have a look at how each of these things is going to work in practice by looking at some specific examples.