Once again, anytime you are doing a hot filtration, you need to add a little more solvent on so that you minimize the amount that precipitates out, as you cool it. For the recrystallization process to be successful, there must be only a small quantity of impurities in the chemical compound, and they must be soluble. First a finely powered compound is packed into a melting point capillary tube to a depth of 1-2 mm. Note that slow crystallization gives larger crystals than fast crystallization. What is the theory of crystallization that works here. We want a slow growth of our crystals. Instead of crystallizing out, it can crash out as an oil.
You might even see it getting turbid. That is because if you try to pour a hot solution through this stem, it would most definitely cool. The goal of a recrystallization is to first dissolve that solid in a hot solvent and that is going to get us a hot solution. A lot of time we put traps in between. Sometimes perfectly pure crystals are tiny little needles. Now we have our crystals here. We are going to be starting with a much cruder solid and unlikely to get very much back.
The range is taken from the first instant melting starts, to the time that the sample is completely liquid. And that way, none of our crystals can sneak underneath the filter paper and snick through to the bottom. This is going to go to a water aspirator or vacuum pump, or whatever you have for a source of vacuum. It maybe crystallizing back up, that it is supposed to be doing. It can become very highly depending on whether it has some solvent, if it is wet, or if there is more water initially than there was at the end, and so on.
You want to be as patient as possible because the more time you give it, the more crystals you are going to have. You can either scratch the glass or you can maybe add a seed crystal. When we add it into that boiling solvent, when we add the boiling solvent to our crystals, we want it to dissolve and we want to get a solution. . Samples are purified through recrystallization.
But the goal is that even though we have less products now, it is going to be more pure. The other important thing that we are doing is we are cooling. Now we have a solvent that is mixed with solid, that is mixed with our crystals. When that is nice and hot and boiling, we are going to add a very small amount of that. Some compounds decompose at or near their melting points. We can do all those steps and watch the crystals grow as it cools, how long do we let them crystallize? About 25 mL of the solvent was heated on the hot plate as well if needed to further dissolve the remaining solute after the sample was boiling. If not, what do you need to do? One thing that we do is we use a special kind of funnel.
The Mel-Temp apparatus, shown in Figure 3 c , consists of an aluminum block that is heated electrically. And also with a narrow neck is going to minimize evaporation of our solvent. But unlike a boiling chip, it is very easy to remove at the end of the day, when we want to do our crystallization and we are cooling. The solubility of the compound in the solvent used for recrystallization is important. We could still effectively remove them from your compound.
Swirling might also help dissolving. Experimental Procedure First, for part 1, 50 mg of the unknown was mixed with 3 mL of each of the four solvents in four separate small test tubes. Gravity filtration was performed on the dissolved sample by pouring it into a stemless funnel lined with fluted filter paper and into a second Erlenmeyer flask. The unknown compound was dissolved and filtered, resulting in a clear solution. Be careful when you look at it and see if it really looks like it could be your product.
We can add in a little more of solvent again, just a tiny amount in small portions. This melting point was further confirmed by comparing it to other known melting points and the identity of the unknown was determined to be cinnamic acid. What is going on there, what can you do with that point? In addition, I have not given aid to another student on this assignment. What you can do at this point? By crushing up the large chunks, you will have smaller chunks and it makes it dissolve much more readily. The problem here is that because now our crude solid has a larger amount of impurities than it had initially, our first crude product had some small amount of impurities, but we have taken most of that pure product out. Sometimes in a mixture the two compounds are mixed very tightly together. How can you get all of the sodium chloride out of this mixture? Again, cool it slowly, eventually put it in the ice bath, and then you should get a nice batch of crystals.
Or they can be insoluble at all temperatures and that would lead to a successful crystallization as well. The solvent should not react with the compound being purified and it should have the boiling point below the melting point of the compound being purified because solid melts before dissolves oiling out. If we put this on a rotovap and strip off the solvent, we obtain a crude solid again. If you added too much solvent, then when it was hot and then when you cool it, it is still too much solvent, you still have plenty of solvent to dissolve all of your compound so it is not supersaturated. In part 4, an impure sample of p-Dibromobenzene was successfully recrystallized using a mixed solvent system of ethanol and water. As opposed to heating something in a beaker, it just evaporates way too quickly.