 We have a number of ways of measuring this in the classroom and you'll have the opportunity in the laboratory hopefully to explore pH through pH meters, through data loggers, and also through universal indicator. Now it's important that you think about the relative degrees of accuracy of each of these devices, particularly when we talk about things like calibration, whilst we can just use a solution like universal, which is very convenient either as a solution or as a paper. We get a lot more accuracy out of our pH meters and data loggers, but if we don't calibrate them properly, if we don't standardize them to what we would expect for solutions of non-concentration, then the values can be very accurate, but invalid. So important that we make sure that we're aware of how to use these devices and how to get the most out of them. This is just a quick scale to give you a bit of an idea of what we talked about before the fact that battery acid, for example, is an extraordinarily strong acid. It has a pH of zero and yes pH, I guess, commonly goes from zero to fourteen, but we can have values that extend beyond that both into the negative range below zero as well as above fourteen. But this is pretty much the scale that we tend to look at and tend to think about. You'll find some common substances on this pH scale and the colors pretty much correspond to the sorts of colors you might expect from a universal indicator. And also they give you an idea of the concentration of hydrogen ions with a comparison to distilled water. So if we set an arbitrary value of zero and of course we don't have zero hydrogen ions in distilled water, so just important when we look at comparative scales that sometimes we set standards in order to make comparisons which are not necessarily the absolute ones. We'll look at how we calculate pH later in this part of the module and then you'll see obviously that the concentration of hydrogen ions into distilled water is not zero, but this just gives you a bit of an idea. So ten times as many hydrogen ions in a pH of six as there is in water, a hundred times in a pH of five and one tenth in a pH of eight. So just gives you a little bit of a comparison without looking at the absolute numbers at this stage. What's more important is to have a look at some of these examples. Cleaners, soaps, drain cleaners, ammonia all up in the base range. There's a few different things that we use commonly like toothpaste which is slightly basic counteract the acid of our saliva or at least some of the foods that we eat and that the bacteria that grow on our teeth hopefully not may produce, but also things like coffee, acid rain and acid rain can actually get well below five, hopefully it won't. Tomato juice, different types of juices, vinegar and then stomach acid that we've talked about previously and battery acid. So just a range of different substances. As I said this particular one is more about the practical, so getting the opportunity in class to look at a number of different types of substances common and otherwise and to measure and record their pHs. Thanks for watching.