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Posted on Thu, Apr 09, 2020

Hot gravity filtration and vacuum filtration in recrystallization

Recrystallization is the process of obtaining pure crystals of a compound from a solution containing impurities. Hot gravity filtration is commonly used to remove these impurities from a solution prior to recrystallization. Hot filtration is necessary for recrystallization when impurities exist in solution. Firstly, recrystallization requires a hot solution because the solution needs to be super saturation in order for crystals to form upon cooling. Secondly, the impurity may have different solubility in certain solvents than the compound to be crystallized. The idea is to choose a solvent that dissolves the compound to be crystallized upon heating, but the impurity doesn’t dissolve in the solvent at high temperatures. The impurity is then filtered out during the hot gravity filtration process.

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Hot gravity filtration: how to do it

Hot gravity filtration requires an Erlenmeyer flask, a stemless funnel, filter paper, and a hot plate. The flask and the funnel need to be stabilized and held in place by clamps in order to prevent anything from tipping over. The filter paper is often folded into a fluted shape in order to prevent pressure from building up from the hot filtrate in the Erlenmeyer flask. If the paper wasn’t fluted, then it would stick to the sides of the funnel and pressure would build up underneath, which would reduce the filtration efficiency. The solution is heated on a hot plate and poured little-by-little into the filter-lined funnel. Some solutions will crystallized immediately upon touching the room-temperature filter paper. This problem can be avoided by heating the whole assembly prior to pouring the hot solution into it. Any crystals that form in the filter paper can be dissolved by adding a little solvent. This is also why a stemless funnel is used. A funnel stem is very small and the solution will cool rapidly while moving through it; this could cause unwanted crystallization within the stem, and could clog up the filtration system. When the whole system is working properly, the solution with the dissolved compound will move through the filter and into the Erlenmeyer flask, while the insoluble impurities are left behind on the filter paper. After the hot gravity filtration is completed, the filtrate in the Erlenmeyer flask will slowly cool and crystals will form and grow larger. These crystals are then filtered with vacuum filtration in a second step. 

Vacuum filtration is used to collect crystals

Vacuum filtration can be a fast process because the vacuum helps to rapidly pull the solution through the filter. When collecting crystals, a buchner funnel is used to collect the crystals while the solution is pulled out by the vacuum. An additional benefit of the vacuum is that the crystals can be dried to a certain degree just by maintaining the vacuum after the solution has been pulled out. A lot of remaining solvent can be removed from the crystals this way. 

Hot gravity filtration is not limited to recrystallization procedures. It can be used in any situation where impurities need to be removed from solution, and a solvent can be chosen that will dissolve one compound at high temperatures, but will not dissolve the other compound. Solubility tables are very useful for choosing an appropriate solvent for this purpose. 

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