Transcription
[Music] Hi everyone. In today's tutorial, I'm going to show you how to turn Epsom salt into 98% concentrated sulfuric acid.
For this tutorial, you will need some Epsom salt. Make sure that it is pure and unscented. You will also need some lead dioxide electrode. If you don't have one, simply watch my tutorial in which I explain how I made mine. You will also need a copper electrode. This is just some standard plumbing tube, and uh, you will have to make sure that these two electrodes are very clean. Don't touch them with your fingers; you don't want to add some grease to them. You will need a glass dish and finally a clay pot with no hole in it.
All right, so I have here 2062 g of Epsom salt. I'm going to explain in a moment why I chose this uh amount of Epsom salt; it's basically to simplify the calculations that I'm going to show. And I'm going to put some distilled water into the clay pot, so all right, distilled water. And then I'm going to fill the uh the glass dish with also distilled water and make sure that there is at least 1 liter so that all the Epsom salt can be dissolved. Now make sure that the copper electrode is clean and use it in order to dissolve the Epsom salt. You don't need to dissolve all the Epsom salt right now, as we do the electrolysis it will keep dissolving into the solution. Connect the copper electrode to a wire. [Music] Right. And then put the lead dioxide electrode into the clay pot and uh connect the lead dioxide electrode to the positive side of a power supply. Once again, this can be batteries, but I like to use uh um an adjustable voltage supply. Right now I am applying 10 volts, and I can see that I had 0.1 amp flowing. When I just turned it on, there was a zero current; that's perfectly normal because the distilled water in which the lead dioxide electrode is is almost a perfect insulator. It's not perfectly insulating because since the temperature sure is not the absolute zero, there is still a slight ionization of the distilled water due to thermal activity, and that uh that is what let the current flow into the the the distilled water. But now the current has increased to 0.1 amp because it started to form some sulfuric acid, which is conductive. Okay, so this current is going to increase as time passes, so we are going to keep track of this.
So now all we have to do now is to wait, and while we are waiting, let us do some calculations. I see many tutorials that limit the explanations to simply saying that sulfuric acid is produced at the anode while magnesium hydroxide is produced at the cathode. This is satisfactory for many people, but not for me, so let me try to explain this this a little bit better. Initially, we have only water molecules, magnesium, and sulfate ions. In the cathode half cell, there are very few water molecules that dissociate into protons and hydroxide ions due to thermal energy. In the anode half cell, only water molecules are present, again with a few of them dissociating into protons and hydroxide ions. Now at a cathode, we have several possibilities for reactions, and in principle the one with the highest reduction potentials are the ones that are more likely. However, the three possibilities with the highest potentials involve protons, but only a few of them are available, so these reactions don't significantly happen. Among the two remaining reactions, the last one is the most likely because it has the greatest reduction potential. Therefore, we conclude that water is reduced to produce hydrogen gas that bubbles at and hydroxide ions. At the anode, we also have several possibilities, and in principle the first one with the highest oxidation potential is more likely, but this reaction requires hydroxide ions, which initially are almost inexistent in this half cell, so this reaction does not significantly happen, at least not initially. Thus, we have to look for the reaction with the next highest oxidation potential, which is the one in which water is oxidized to produce oxygen that bubbles at and protons that make the solution acidic. Now the role of the porous membrane, the clay, is to allow the two half cells to remain electrically neutral by letting ions flow from one half cell to the other while preventing the movement of large volumes of solution, so the two solutions don't mix up significantly. But because the ions can pass through the membrane, some reactions that initially were not likely can happen. Now, for example, in the cathode half cell, the reduction of protons is initially unlikely because protons are not available. However, protons are produced in the anode half cell and are very mobile, so they can easily move to the cathode half cell. But this doesn't generate any pollution of the solution since either these protons are now reduced into hydrogen gas that bubbles out or they form with the sulfate ions some sulfuric acid that neutralizes the magnesium hydroxide to form again some Epsom salt. So these unwanted reactions don't generate any pollution; they simply slow down the overall production of sulfuric acid and magnesium hydroxide. To summarize, water is reduced at the cathode, and the solution turns into magnesium hydroxide, and water is oxidized at the anode, and the solution turns into sulfuric acid. The membrane mainly lets the sulfate ions move from the cathode to the anode. The cell potential is the sum of the reduction potential at the cathode and the oxidation potential at the anode and equal to -2.05 V. Thus, the power supply must apply a voltage at least equal to positive 2.057 volts in order to force the reaction to occur.
Now why did I use 246 g of Epsom salt? Simply in order to simplify the following calculations. The formula of Epsom salt is MgSO4·7H2O, and from there we can easily find that its molar mass is 246 g per mole. This means that I used precisely one mole of Epsom salt, and this is also the number of sulfate ions. As a result, if all the sulfate ions could be turned into sulfuric acid, then we would end up with one mole of sulfuric acid. So this gives us an upper limit of the yield that we can expect. We will obtain dilute sulfuric acid that we can concentrate up to 98% simply by boiling the water off. Such concentrated sulfuric acid is known to have a molarity of 18.4 mol per liter. Since we have 1 mole of acid, we conclude that this corresponds to a volume of 54.4 mL of concentrated sulfuric acid. Once again, this is an upper limit; we will obtain much less, but hopefully a few milliliters.
Finally, something important is how long should we let the electrolysis go? For each sulfate ion, two protons must be produced to form one molecule of sulfuric acid, and we saw that this requires two electrons. Thus, we need a total charge of two moles of electrons. Hence, we calculate two times the elementary charge times Avogadro's number, which gives a total charge of 192,500 coulombs. This corresponds to 53.6 amp hours. In other words, if we maintain a current of 1 amp, then it will take 53.6 hours for the process to complete. In practice, the process will never complete because some secondary reactions take place and some protons leak out of the anode half cell, but this gives us the minimum time that we should wait.
It has been 1 hour, and there is now a current of 120 milliamps. We can see some reaction happening in the cathode, and almost all the Epsom salt has been dissolved. There's still a little bit left, so we can stir a little bit. We don't see any change in the clay pot, but it should start to have some uh sulfuric acid in it; it should be a little bit acidic.
It has been 24 hours, and now the current is about 1 amp. So we can see that some um magnesium hydroxide forms; it's not fully dissolved, and here I had to add water because it was almost empty. So why was the water gone? Um, I have two hypotheses about this. Well, first, we know that the water is being oxidized at the anode, and some uh oxygen is escaping, so that means that the amount of water is being uh reduced. And also, since we are forming sulfuric acid, which is a powerful dehydrant, I guess that it attracts the water molecule very tightly, so maybe this also contributes to decrease the volume because the the density increases, the mass density increases. So uh these are the possible explanations that I have.
It has been 72 hours with a current greater than 1 amp for at least 52 hours, so I think this is the best we can do. So here we can see that the cathode has been covered with uh magnesium hydroxide, and there is some undissolved one here. Um, it's definitely not pure because it's yellowish, and it's supposed to be white. So um it can also be mixed up with some Epsom salt because some secondary reaction happens when some sulfuric acid from the anode compartment leaked into the cathode compartment, and it may react with the magnesium hydroxide in order to produce again some some salt. So I have here over 100 mL of supposedly sulfuric acid. As you can see, it's pink, so I suspect that these are clay particles. Um, I tried to filter, but uh it didn't work, so uh my filter is probably not fine enough. Um, so well, anyway, that's what we have so far. So first thing, let us check that it is acidic. So let me put a drop on this paper. Yes, we have here a nice red color, nice red color. And and let's see, not sure if it's pH 1, 2, 3, 4; it's hard to tell, but definitely it is acidic.
Now before concentrating the acid, let us first estimate its current concentration. So I'm going to make a very rough estimate, uh rough estimate, because I cannot measure volume very precisely, and also since our acid is polluted uh it won't be completely accurate. Okay, so uh here I'm going to measure the mass of 5 mL of this sulfuric acid. So now my scale is zero; I don't know if you can see it on the camera, and I'm going to put 5 mL. So I'm going to adjust the amount with a pipette. So yeah, all right, a little bit more. All right, so 5 mL, and this is 5.26 g. So that means it's about 1.05 g per milliliter. And if I look at an online table, uh today's temperature is 30°, so if I look at a table of concentration, this corresponds to about 8% concentration.
All right, so now I'm going to put this acid into a flask and boil the water off in order to concentrate it. And in order to reach the maximum concentration of 98%, we need to heat the acid up to a temperature of 337° C. So that's really hot. All right, so I have my sulfuric acid here, and let me turn on the heater to the maximum. I have a thermometer, and when the temperature will reach 337° C, then the acid will have reached its maximum concentration. Right now the temperature is about 120° C, so we have some water that is boiling off, and it will take a while before the temperature rises and uh reaches 337° C. So just be patient; all the water needs to go away for the temperature to be able to rise.
So the temperature reached 337°. I let it cool off, and I end up with this. So there's a lot of pollution, but we might be able to decant it. Uh, also we can notice that the pink color went away, so maybe those impurities have been boiled away. I don't know, uh, and as expected, we don't have much. So first let me try to pour everything in this beaker, and then I will try to decant it, and this should be more or less 98% concentrated sulfuric acid. So we are going to make a test for that. All right, so I have about 5 mL of almost 98% sulfuric acid. It's not super pure, but it must be able to do the job. So let us make a test. I have here a napkin, and let me pour a little bit on it. Yeah, nice. Look at this. This look at this. We have concentrated sulfuric acid. Now, of course, this is not much of it, but we can scale everything up in order to make more. Okay, so if you like this video, please give it a thumbs up, write a comment, and if you haven't done so, subscribe to my channel. Thanks for watching.