 The first of these processes is the diaphragm process. So this is a diaphragm that actually sat in between the two cells. This was a little bit of a problem because with one of the high yields, we had to increase the size of the current. This actually involved an asbestos membrane. So that was an extra little problem. I'll just put asbestos there, which obviously not something you're expected to know, but in a description might be there. That would ring some fairly serious alarm bells both in terms of the effect on the environment and also the social cost associated with exposure of humans to asbestos. But the problem with the diaphragm process wasn't just about asbestos. It was also about the fact that you either had very high costs in order to produce high yields or you had to drop your yields. The other problem with that was that the membrane did allow the migration of some chloride ions across and what that meant that was that in the cell that should have just been sodium hydroxide, there was some chloride ions in there as well. And that is one of the additional species that that's in there in our product that is going to reduce our purity. So if we are doing a process like this, we want to make sure that we are reducing any of those. To give you just an extra little bit of background, you can see that the source of the sodium for the sodium hydroxide is sodium chloride aqueous. So this is like ocean water, a brine, very high salty content. And so therefore the sodium and the chloride ions are in solution. So as each of these is being oxidized or reduced, what we are finding is that some of those chloride ions rather than being removed as chlorine gas are actually remaining in our final solution and therefore contaminating the purity of our final product. Mercury is a much better one if we're looking at purity. So in the mercury process, mercury is actually one of our electrodes and it forms an amalgam, which basically just means that the sodium that is being produced is well, sodium is so reactive with water we can't just it won't just sit in the water. So therefore if we do produce sodium, we have to remove it from the water and forming an amalgam with mercury, which is basically just an alloy of these two metals together. Maybe we could pull that sodium out of the solution. We could move that sodium to a second container where we were able to effectively just react it with water and produce sodium hydroxide and hydrogen gas. This meant that there was much less contact with the chloride ions from the original brine and that meant we had a very high purity product. The problem, of course, with that is the fact itself that it has mercury, which has again some implications for both humans and also for the environment. And we wanted to make sure that we have all of those sodium ions being reduced. And again, that was going to increase our costs in order to increase our yield. So one of the things that was interesting about the history of development of this particular process is two very dangerous environmental substances, a heavy metal like mercury and asbestos that we've replaced now with a membrane of something that we have encountered elsewhere in this course, which is PTFE, which is polytetrafluoroethene. So this is a polymer. This polymer seriously increases the purity of our sodium hydroxide product if it is able to selectively move the sodium ions through that membrane into the second cell in order for the sodium hydroxide production. Then that is going to continue to increase the purity. Obviously, there's other ions that are there, whether we want to make sure that we sort of selecting against those so we don't have a combination of substances that will affect our purity. And also this one seriously reduced the costs. So this meant that we were able to increase our yield. Just just a little bit of background worth maybe having a look at these processes just in a little bit of detail. They've all come from industrial process that we used to cover in the old course. So therefore older textbooks and by older textbooks, ones from just two years ago will contain a lot of information on a lot of these processes to give you a little bit more background. I don't think it's important that you look at lots of these processes. I do think it's important that you look at one of these in a bit of detail with all of these factors. So you know the sort of things that you might write about for each of these different factors. Thanks for watching.