Difficulty: Introductory lab technique | Prerequisites: Part 1 of these study notes (recrystallization concepts), basic lab safety
This document covers the hands-on side of the recrystallization experiment: the step-by-step procedure, the two filtration techniques (gravity and vacuum), the equipment involved, and the practical decisions you make at the bench. If Part 1 explained why recrystallization works, this part explains how to do it and what can go wrong.
You test solvents, dissolve your crude solid in the best one while hot, gravity-filter to remove insoluble junk, cool the filtrate to crystallise the pure compound, vacuum-filter to collect the crystals, dry, weigh, and measure the melting point. Each step has a specific reason, and the common mistakes are about temperature and solvent volume.
Buchner funnel
A flat-bottomed porcelain or plastic funnel with small holes, designed to sit on top of a vacuum flask. Filter paper is placed over the holes. Used for vacuum (cold) filtration to collect recrystallised crystals.
Vacuum flask (filter flask, side-arm flask)
A thick-walled Erlenmeyer flask with a side-arm hose connector. The side arm connects to a vacuum source, which pulls solvent through the Buchner funnel and filter paper sitting on top.
Erlenmeyer flask
A conical flask with a narrow neck, used to hold the hot solution during recrystallization. The narrow neck reduces solvent evaporation.
Glass wool
Loosely packed glass fibres placed inside a funnel stem during gravity filtration. Acts as an additional barrier to trap fine insoluble particles.
Rubber adapter / seal
A gasket that creates an airtight seal between the Buchner funnel and the vacuum flask, ensuring the vacuum pulls liquid downward through the filter paper rather than leaking around the edges.
Hot water bath
A beaker of hot water on a hot plate, used to heat the solvent and the solution indirectly. Safer than direct flame heating, especially with flammable organic solvents.
Ice bath / cold bath
A container of ice and water used to cool the solution rapidly after gravity filtration, prompting crystallisation.
Capillary tube
A thin glass tube packed with a small amount of the dried solid and placed in the Mel-Temp apparatus for melting point measurement.
Decanting
Carefully pouring liquid off the top of a settled solid. Used when a soluble impurity must be separated from an insoluble one without filtration.
Before you begin the full-scale recrystallization, you need to identify which solvent works for your unknown.
Add a small amount of the solid (0.1 to 0.5 g) to a test tube.
Add a few drops of cold solvent and observe. If it dissolves in the cold, that solvent is not suitable.
If it does not dissolve cold, heat the test tube in the hot water bath. If the solid dissolves when hot, that solvent is a candidate.
Let the solution cool. If crystals form on cooling, the solvent is suitable for recrystallization.
Repeat with different solvents until you find one that meets both criteria.
Weigh the remaining unknown solid and record the mass.
Heat a calculated amount of the chosen solvent in a flask on the hot water bath.
Add the hot solvent to the solid in a separate flask, a small amount at a time, until most of the solid dissolves. Do not add excess.
The sand and sawdust impurities will not dissolve regardless of temperature, so undissolved granular material at this stage is expected.
Purpose: remove insoluble impurities (sand, sawdust) while keeping the target compound in solution.
Set up a funnel with filter paper (or glass wool in the stem) over a clean Erlenmeyer flask.
Wet the filter paper with hot solvent so it adheres to the funnel.
Pour the hot solution through the funnel. The insoluble particles stay on the paper; the dissolved compound passes through.
Rinse with a small amount of additional hot solvent to recover any compound clinging to the filter.
Speed matters here. If the solution cools during filtration, the compound will start to crystallise on the filter paper and you will lose product. Pre-heating the funnel and working quickly both help.
Allow the filtrate to cool to room temperature first. Slow cooling encourages larger, purer crystals.
Then place the flask in an ice bath to drive crystallisation to completion.
If no crystals form, the solution may be too dilute (too much solvent was used). Gently boil off some solvent and cool again.
Purpose: collect the recrystallised crystals and separate them from the mother liquor (the liquid containing dissolved impurities).
Set up the Buchner funnel on the vacuum flask with a rubber adapter to seal the connection.
Place filter paper in the Buchner funnel and wet it with a small amount of cold solvent. This holds the paper flat and prevents crystals from dissolving.
Turn on the vacuum.
Pour the crystal slurry onto the filter. 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 pull air through the crystals and start drying them.
Cold solvent is essential at this stage. Hot solvent would redissolve the crystals and wash them into the filtrate.
Transfer the crystals to a pre-weighed clean, dry beaker.
Store the beaker in your drawer to allow the solid to dry thoroughly (typically overnight or until the next lab session).
Weigh the dried solid and calculate percent recovery.
Pack a small amount of the dried, purified solid into a capillary tube.
Place the capillary tube in the Mel-Temp apparatus and heat slowly.
Record the temperature range over which the solid melts.
Compare to literature values for the candidate compounds to confirm identity and purity.
Equipment | Purpose | Location |
|---|---|---|
Beakers (50, 100, 250, 600 mL) | Holding solutions, water baths, collecting product | Drawer |
Hot plate | Heating the water bath and boiling solvent | Hood |
Graduated cylinder (10 mL) | Measuring solvent volumes | Drawer |
Test tubes (3 inch and 6 inch) | Solvent testing with small samples | Drawer |
Erlenmeyer flasks (50, 125, 250 mL) | Dissolving crude solid, collecting filtrate | Drawer / shelf |
Plastic powder funnel | Gravity filtration setup | Drawer |
Buchner funnel and vacuum flask | Vacuum filtration to collect crystals | Common equipment shelf |
Filter paper | Lines the funnel to trap solids | TA / front hood |
Glass wool | Plugs the funnel stem during gravity filtration | TA / front hood |
Solvents (water, methanol, ethanol, ethyl acetate, acetone, n-hexane) | Dissolving and recrystallising the unknown | Front hood |
Mel-Temp apparatus | Measuring melting point of purified product | Shared equipment |
Students often think gravity filtration and vacuum filtration are interchangeable. They are not. Gravity filtration is done hot to keep the product dissolved. Vacuum filtration is done cold to keep the product solid. Swapping the temperature defeats the purpose of each technique.
Students sometimes skip wetting the filter paper before vacuum filtration, thinking it is an optional step. It is not. The wet paper seals against the funnel so the vacuum pulls liquid through the paper, not around it. Without the seal, crystals slip past and product is lost.
Students frequently wash collected crystals with hot solvent instead of cold, and then wonder why their percent recovery drops. Hot solvent redissolves the very crystals you are trying to keep.
Students sometimes assume that if crystals do not form on cooling, the experiment has failed. More often, it means too much solvent was used. Concentrating the solution by gentle boiling usually fixes this.
Be able to draw and label both the gravity filtration apparatus (funnel, filter paper, Erlenmeyer flask) and the vacuum filtration apparatus (Buchner funnel, filter paper, rubber adapter, vacuum flask, rubber tubing to vacuum source). This has appeared as a lab practical question.
Know why the filter paper is wetted with cold solvent (not hot) before vacuum filtration: it seals the paper to the funnel and prevents crystal loss.
Expect a question on what happens at each stage if you use the wrong temperature. Hot solvent during vacuum filtration = crystals redissolve. Cold solution during gravity filtration = product crystallises on the filter.
The procedural flowchart (solvent test, dissolve, gravity filter, cool, vacuum filter, dry, measure melting point) is a frequent "put these steps in order" question.
True or false: During gravity filtration, you should rinse the filter paper with cold solvent. (False: use hot solvent to prevent the product from crystallising on the paper.)
Fill in the blank: The rubber adapter between the Buchner funnel and the vacuum flask creates a ______ so the vacuum functions properly. (seal / airtight seal)
True or false: If no crystals appear after cooling, you should add more solvent. (False: the problem is usually too much solvent already. Boil off some solvent, then cool again.)
Fill in the blank: After vacuum filtration, you wash the crystals with small portions of ______ solvent. (cold)
True or false: The Mel-Temp apparatus measures percent recovery. (False: it measures melting point, which indicates purity.)
Q: During gravity filtration, you notice crystals forming on the filter paper. What went wrong and how do you fix it?
A: The solution cooled too much during filtration, causing premature crystallisation. Rinse the filter paper with additional hot solvent to redissolve the crystals and push them through into the collection flask. Working faster and pre-heating the funnel help prevent this.
Q: You set up vacuum filtration but the solvent does not pull through. What are two likely causes?
A: The rubber adapter is not sealed properly, so the vacuum is leaking around the funnel rather than pulling through the paper. Alternatively, the filter paper may not be seated flat against the funnel, allowing air to bypass the paper.
Q: Why is it important to let the solution cool to room temperature before placing it in the ice bath?
A: Slow initial cooling promotes the growth of larger, more ordered crystals that exclude impurities more effectively. Plunging a hot solution directly into an ice bath can produce very small crystals that trap impurities within the crystal lattice.
Q: A classmate washes their collected crystals with hot ethanol during vacuum filtration. What will happen to their percent recovery?
A: Their percent recovery will be significantly lower. The hot ethanol will redissolve the purified crystals, and the dissolved compound will be pulled through the filter and discarded with the filtrate.
Q: Put the following steps in the correct order: vacuum filtration, dissolving in hot solvent, melting point measurement, solvent testing, gravity filtration, cooling in ice bath, drying.
A: Solvent testing, dissolving in hot solvent, gravity filtration, cooling in ice bath, vacuum filtration, drying, melting point measurement.
Q: Why does the gravity filtration apparatus use a regular funnel and Erlenmeyer flask, while the vacuum filtration apparatus uses a Buchner funnel and a thick-walled vacuum flask?
A: Gravity filtration needs only gravity to move liquid through the filter, so standard glassware is sufficient. Vacuum filtration uses reduced pressure, which requires a thick-walled flask to withstand the pressure difference without cracking, and a flat-bottomed Buchner funnel that seats against the flask with a seal so the vacuum pulls evenly through the filter paper.
The gravity and vacuum filtration setups you learn here are used in virtually every organic chemistry lab for the rest of the course, including extraction, column chromatography fraction collection, and product isolation after synthesis reactions.
Melting point measurement connects forward to mixed melting point tests, where you combine your unknown with a known compound to confirm identity: if the melting point stays sharp and unchanged, the two samples are the same substance.
The procedural logic of recrystallization (dissolve, remove junk, precipitate the pure compound) mirrors the logic of extraction and washing in liquid-liquid separations, just in a different phase.
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