 As we've looked at previously, there's a number of the tasks or the outcomes, learning outcomes that are required of you that actually involve the carrying out of experimental work, not just the application of your knowledge. This is another area where it's important that you are involved practically in identifying the different pH of a range of acid base and neutral substances. So again, this video just gives you a little bit of background, hopefully, just to remind you of some important things about the pH scale. pH is an idea that's over 100 years old, pretty much originated from the work of the Danish chemist Søren Sørensen and his biochemical application of the effect of hydrogen ion concentration on enzyme activity. You remember in a previous video, we looked at the fact that in biochemical systems, in body systems, the pH is very important in a number of different types of places in the body and for a large number of chemical reactions which occur within the body. So the idea that hydrogen ion concentration could actually impact on enzyme function was a pretty important idea and one that really needed to be explored in a lot of detail and one about which we are still learning even 100 years later. It was Sørensen who created the pH scale. Now people get kind of caught up with what the P in the pH scale, so you can talk about this as a probability of hydrogen ions as a proportion of hydrogen ions. The most important thing we need to be aware of is the fact that pH is a negative log scale. It's a log base 10 scale. What that means is the higher the value of the hydrogen ions, the lower the pH. So a pH of one is stronger acid than a pH of three. So this is how this scale works. You get smaller and smaller and you get stronger and stronger acid solutions. It took a little while for us to be able to measure or at least to build technology that was sufficient for us to conveniently measure pH. In the 1930s we were able to develop, or scientists were, I didn't, were able to develop sensitive millivolters which were able to use electrochemical means to measure pH. So we had this pH scale.