[So, I am going to purify benzoic acid.] So, I am going to purify benzoic acid by recrystallization. The key to recrystallization is choosing a solvent that dissolves the target compound at high temperatures, but not low temperatures. The impurities present should either be soluble in the solvent at low temperatures, in which case they can just be washed away, or they should not be soluble at all, in which case they can be filtered off. Fifty six point three grams of benzoic acid dissolves in one liter of water at one hundred degrees Celcius.
One point seven grams of benzoic acid dissolves in one liter of water at zero degrees Celcius. Here I have two hundred and sixty milligrams of impure benzoic acid along with a few boiling chips. I am going to place ten milliliters of distilled water into the flask and set it on the hotplate.
I am also going to place ten milliliters of distilled water in another flask, which will be used to help transfer the benzoic acid. This also needs to be hot. Here is a large piece of filter paper that I will place in the glass funnel. I am going to fold it twice and then open it up and place it in the funnel. I am also going to add ten milliliters of distilled water to this flask. So the idea is once everything is boiling, I will use gravity to filter the benzoic acid solution through this setup. Everything has be hot, otherwise the benzoic acid will crystallize out and not make it through the funnel. Once everything starts to boil, I will remove the filtration apparatus and I will immediately attempt to pour all of the benzoic acid solution through it.
You can see some of the shiny benzoic acid is already crystallizing out. You can also see some of it is left in this flask. I am going to use about half of the other hot water to try to re-dissolve the benzoic acid and pour it through the funnel. Gravity filtration is slow, but if we were to use vacuum filtration it would cause much more cooling of the flask and cause more of the benzoic acid to crash out. Once all of the water gets through, I'm going to rinse the filter one last time with the remaining hot water. And it looks like all of the benzoic acid has made it through, leaving behind this black residue.
I'm going to remove the funnel and gently move the flask with the solution to the bench here so I can record the recrystallization. Ideally, the solution should not be moved at all since any agitation can disrupt the otherwise highly organized pattern in which the crystals form. It is also best to have the solution slowly cool to room temperature otherwise the crystals can form too fast and trap impurities within them. This was filmed over seventeen minutes. So you can see decent sheets forming on the top of the solution and nicer looking needles forming at the bottom of the solution.
Now that the solution is approximately at room temperature, I can place it in an ice-water bath for further cooling. To help with the transfer, I'm also going to cool ten milliliters of distilled water. While the solution is cooling, you could hook up the vacuum filtration apparatus. Just consists of a ring stand, a clamp, a Buchner funnel. This Buchner funnel is just a plastic one. It's two pieces, comes apart so you can clean it. If you can see it has little holes in there, which will allow the solution to go through and a piece of filter paper to make sure the solid doesn't go through, as the solution is getting sucked through. Common mistake is to use a piece of filter paper that's too big.
If it's too big then it won't produce the right seal for the vacuum to work properly. If it's too small, well obviously then it's just going to go around it and fall into the holes and come out with the solution. Here's a universal adapter, just a rubber adapter to complete the seal between the plastic and the glass. Here's another adapter, which is just a rubber stopper. This works as well, but it's not universal so you have to make sure it matches with the filter flask. The last thing here is the actual vacuum hose. Another common mistake is to use a condenser hose. This is for water.
We used it for the distillation. Water flows through here fine. The problem is when you try to hook a vacuum up to it, this is such thin-walled it will collapse. That way the vacuum will not get all the way to the flask itself.
So make sure you use the red hose, which is thicker-walled. You can feel the difference. You can always check to make sure your vacuum is working well. Before you turn it on, notice that it just comes apart here. But after I turn it on, if I have a good seal, I can pick up the whole thing. I could even stick my glove in there. The solution has been sitting in the ice-water bath for about 15 minutes, so I am now going to collect the crystals using the vacuum filtration apparatus. The first thing I am going to do is wet the filter paper so that it gets sucked down.
Now I am going to try to get as much of the crystals to the Buchner funnel as possible by swirling the solution and then quickly pouring. The water gets sucked through much faster than with gravity alone. So I still have some crystals left in the flask. I will use about half of the cool water and try to get them out.
And I will do this one more time with the remaining cool water. Now I'm going to leave the vacuum on for at least ten minutes so the crystals dry out. When the crystals are dry they should be somewhat flakey or fluffy and easy to move around. Well, I started with 260 milligrams of the impure benzoic acid. I ended up with 110 milligrams of the purified benzoic acid. So my percent recovery is 42 percent.
This value makes sense because some of the benzoic acid is still soluble in water even near zero degrees Celsius. I also lost some during transfers and a portion of the starting material was impurities.
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