Difficulty: Intermediate | Prerequisites: Part 1 study notes (theory and solvent selection), basic lab safety.
This companion set of notes covers the hands-on techniques you will use during the recrystallization experiment: gravity filtration, vacuum filtration, solvent testing, and melting point measurement. Knowing why each step is done a certain way (hot versus cold, gravity versus vacuum) is what separates a clean result from a failed purification. If you understand the theory from Part 1, this document tells you how to execute it at the bench.
Gravity filtration removes insoluble impurities from a hot solution. Vacuum filtration collects pure crystals from a cold slurry. Always wet the filter paper and wash the crystals with cold solvent, never hot, or you will redissolve your product and lose it.
Hot filtration (gravity filtration)
Filtering a solution while it is hot, using gravity alone, to remove insoluble solid impurities. The solution passes through filter paper in a funnel and drips into an Erlenmeyer flask below. Think of it as: skimming the junk out while everything you want is still dissolved.
Cold filtration (vacuum filtration)
Filtering a cold slurry under vacuum to collect recrystallised crystals. The liquid is pulled through the filter paper by reduced pressure, leaving the solid on top. In simple terms, this is how you harvest your purified crystals.
Buchner funnel
A flat-bottomed porcelain funnel with perforations, sized to hold a circular piece of filter paper. It sits on top of a vacuum flask and is sealed with a rubber adapter.
Vacuum flask (filter flask / side-arm flask)
A thick-walled Erlenmeyer flask with a side arm that connects to a vacuum line via rubber tubing. The vacuum pulls solvent through the Buchner funnel.
Glass wool
A plug of fine glass fibres placed in a funnel during gravity filtration. It acts as an additional filter to catch fine insoluble particles.
Erlenmeyer flask
A conical flask used to collect the filtrate during gravity filtration and to hold the solution during cooling and crystallisation. Its tapered shape reduces evaporation.
Ice bath
A beaker or container filled with ice and water, used to cool the filtered solution and promote crystal formation. Faster cooling generally produces smaller crystals.
Hot water bath
A beaker of hot water on a hot plate, used to heat test tubes (during solvent testing) and to keep solvents warm. Safer than direct heating for flammable solvents.
Capillary tube (melting point tube)
A narrow, sealed glass tube packed with a small amount of dried solid and placed into a Mel-Temp apparatus for melting point measurement.
Filter paper
A circular piece of porous paper placed in the Buchner funnel. It must be wetted with cold solvent before use so that it seals flat against the funnel and does not shift during filtration.
Hot filtration is performed while the solution is hot. Its purpose is to remove insoluble impurities (sand, sawdust) from the dissolved compound. You must filter quickly to prevent the compound from crystallising in the funnel and being lost with the impurities.
Cold filtration is performed after cooling, once the compound has recrystallised. Its purpose is to separate the pure crystals from the liquid solvent (which still contains dissolved impurities). The vacuum speeds this up and leaves the crystals drier.
Rule of thumb: hot filtration removes what you do not want (insoluble junk). Cold filtration collects what you do want (pure crystals).
Equipment: stemmed funnel, filter paper (folded), ring stand and ring clamp, Erlenmeyer flask to catch the filtrate, glass wool (optional, for finer filtration).
Wet the filter paper with hot solvent so it sticks to the funnel.
Pour the hot solution through the filter. The insoluble impurities stay on the paper. The filtrate (your dissolved compound) collects in the flask below.
Rinse the beaker with a small amount of additional hot solvent and pour through to minimise sample loss.
Equipment: Buchner funnel, rubber adapter, vacuum flask (side-arm flask), rubber tubing, vacuum source, filter paper.
Place the filter paper in the Buchner funnel and wet it with a small amount of cold recrystallisation solvent. Turn on the vacuum so the paper seals flat.
Pour the cold crystal slurry onto the funnel. The vacuum pulls the solvent through, leaving the crystals on the paper.
Wash the crystals twice with small portions of cold solvent to remove surface impurities.
Keep the vacuum running for several minutes to dry the solid.
Two reasons. First, the wet paper sticks to the funnel and stays flat, preventing crystals from slipping under the edges and being lost. Second, cold solvent does not dissolve the crystals, so wetting does not cost you product. Hot solvent would dissolve crystals on contact.
The crystals sit in residual mother liquor that contains dissolved impurities. A small wash with cold solvent rinses those impurities away without dissolving the crystals themselves. Use as little as possible, as even cold solvent dissolves a small amount of product.
The crystals redissolve. You wash your purified product straight through the filter paper and into the waste flask, destroying your yield and defeating the purpose of the entire recrystallization.
Set up a hot water bath (beaker on a hot plate).
Calibrate a pipette to measure approximate solvent volumes.
Obtain a vial of the impure unknown solid.
Test the solid's solubility in several solvents: add a small amount of solid to a test tube with a few drops of cold solvent. If it does not dissolve, heat the test tube in the hot water bath. Note whether the solid dissolves when hot. Let the solution cool and check whether crystals reform. A suitable solvent dissolves hot, recrystallises cold.
Weigh a small amount of the unknown sample (0.1 to 0.5 g).
Add small portions of the chosen hot solvent to the solid until it just dissolves.
Decant the hot solution into a clean beaker, leaving any visible insoluble impurities behind.
Weigh the remaining unknown solid (to calculate how much you used).
Heat a calculated amount of solvent in a flask.
Add the hot solvent to the solid in a separate flask until most of it dissolves.
Perform gravity filtration: pour the hot solution through a funnel containing glass wool to collect the purified filtrate.
Let the filtrate cool to room temperature undisturbed.
Place the flask in an ice bath to promote further crystallisation.
Wait until crystallisation appears complete.
Set up the vacuum filtration apparatus with a Buchner funnel.
Wet the filter paper with a small amount of cold solvent and turn on the vacuum.
Carefully pour the crystallised solid and solvent onto the funnel.
Wash the crystals twice with small portions of cold solvent.
Continue running the vacuum for several minutes to dry the solid.
Weigh a clean, dry beaker.
Transfer the purified solid to the beaker.
Store in your drawer to allow thorough drying.
Once dry, pack a small amount into a capillary tube and measure the melting point using the Mel-Temp apparatus.
Compare the measured melting point to the reference table to identify your unknown.
Calculate percent recovery.
Item | Sizes / Specs | Location |
|---|---|---|
Beakers | 50, 100, 250, 600 mL | Drawer |
Hot plate | - | Fume hood |
Graduated cylinder | 10 mL | Drawer |
Test tubes | 3" and 6" | Drawer |
Erlenmeyer flasks | 50, 125, 250 mL | Drawer / shelf |
Plastic powder funnel | - | Drawer |
Buchner funnel and vacuum flask | - | Common equipment shelf |
Filter paper | - | TA / front hood |
Solvents (water, methanol, ethanol, ethyl acetate, acetone, n-hexane) | - | Front hood |
Glass wool | - | TA / front hood |
Students sometimes think wetting the filter paper is optional. It is not. An unwetted paper can shift or fold during vacuum filtration, letting crystals slip past into the filtrate.
Students often wash their crystals with generous amounts of cold solvent, thinking more washing means higher purity. Even cold solvent dissolves a small amount of product. Use the minimum needed to rinse away surface impurities.
A frequent error is filtering too slowly during hot (gravity) filtration. If the solution cools in the funnel, the compound crystallises on the filter paper and is lost with the impurities.
Students sometimes assume vacuum filtration is always better than gravity filtration. Each serves a different purpose. Vacuum filtration on a hot solution would cool it prematurely and crystallise product in the funnel, mixing it with the impurities you are trying to remove.
⚠️ Know which filtration type is used at which stage and why. Hot/gravity filtration removes insoluble impurities. Cold/vacuum filtration collects purified crystals.
⚠️ Be able to explain two reasons for wetting the filter paper with cold solvent: it holds the paper in place and prevents crystals from dissolving.
⚠️ Understand the consequence of using hot solvent to wash crystals: you redissolve your product and lose yield.
⚠️ Be able to label the components of both filtration setups: funnel, filter paper, flask, vacuum line, rubber adapter.
⚠️ Know why gravity filtration must be done quickly while the solution is hot: if the solution cools, the compound crystallises in the funnel and is lost.
True or False: Vacuum filtration is used to remove insoluble impurities from a hot solution. (False. Gravity filtration is used for that. Vacuum filtration collects crystals from a cold slurry.)
Fill in the blank: The filter paper in a Buchner funnel should be wetted with __________ solvent before filtration. (cold)
True or False: Washing recrystallised crystals with hot solvent will improve their purity. (False. Hot solvent redissolves the crystals.)
Fill in the blank: During gravity filtration, the liquid that passes through the filter is called the __________. (filtrate)
True or False: Glass wool is placed in the funnel during gravity filtration to catch fine insoluble particles. (True.)
Q: Explain why gravity filtration must be performed while the solution is hot, not after cooling.
A: If the solution cools, the desired compound will crystallise in the funnel and be trapped alongside the insoluble impurities (sand, sawdust). Filtering hot keeps the compound dissolved in the filtrate.
Q: Give two reasons why the filter paper in a Buchner funnel is wetted with cold solvent before vacuum filtration.
A: First, the wet paper adheres to the funnel surface and stays flat, preventing crystals from slipping underneath. Second, cold solvent does not dissolve the crystals, so wetting does not cause product loss.
Q: A student washes their recrystallised crystals with warm solvent instead of cold. What happens to their percent recovery, and why?
A: Percent recovery drops significantly. The warm solvent redissolves the purified crystals, which are then pulled through the filter into the waste flask.
Q: Describe the key difference between the purpose of gravity filtration and vacuum filtration in a recrystallization experiment.
A: Gravity filtration removes insoluble impurities from the hot solution (you keep the filtrate). Vacuum filtration collects the purified solid from the cold slurry (you keep the solid on the filter).
Q: Why is an ice bath used after gravity filtration rather than simply leaving the solution at room temperature?
A: The ice bath lowers the temperature further, reducing the solubility of the compound and driving more of it out of solution as crystals. This increases the mass of crystals recovered.
Q: A student's crystals appear wet after vacuum filtration. What should they do, and why is thorough drying important before measuring melting point?
A: Continue running the vacuum for several more minutes, then store the solid in a beaker to air-dry in the drawer. Residual solvent depresses and broadens the measured melting point, making the sample appear less pure than it is and potentially leading to a misidentification.
The filtration techniques here reappear throughout the organic chemistry lab sequence. Gravity filtration is used in any reaction workup where you need to remove an insoluble drying agent or byproduct. Vacuum filtration comes back whenever you isolate a crystalline product from a reaction mixture.
Melting point determination connects to mixed melting point analysis, where you co-melt your unknown with a known compound. If the mixture melts at the same range as the known, they are the same substance. This is a standard identification technique covered later in the course.
The principle of using cold solvent to wash crystals without dissolving them is the same logic behind washing precipitates in quantitative analysis (analytical chemistry).
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