Chemistry ToolsFree Chemistry ToolsChemistry calculators & study toolsA chart-based organic chemistry guide to solvent selection, single-solvent and mixed-solvent recrystallization, hot filtration, charcoal treatment, seeding, scratching, washing, drying, yield, and purity.
| Method | What It Does | When to Use |
|---|---|---|
| Single-solvent recrystallization | One solvent dissolves the crude solid hot and poorly dissolves it cold. | Most routine solid purification when a suitable solvent exists. |
| Mixed-solvent recrystallization | Good solvent dissolves product; miscible poor solvent lowers solubility. | When no single solvent gives the right hot-cold solubility difference. |
| Hot gravity filtration | Hot solution passes through warmed gravity filtration setup. | Removes insoluble impurities, charcoal, dust, or boiling chips before cooling. |
| Activated charcoal treatment | Small amount of charcoal adsorbs colored impurities from hot solution. | Useful for colored by-products; excessive charcoal lowers yield. |
| Seeding | Tiny pure crystal initiates nucleation in a supersaturated solution. | Useful when a clear cooled solution refuses to crystallize. |
| Scratching | Glass rod creates rough nucleation sites on flask wall. | Useful after cooling when supersaturation is likely. |
| Vacuum filtration | Cold crystals are collected and washed under suction. | Final isolation step after crystals have fully formed. |
| Factor | Target Behavior | Why It Matters |
|---|---|---|
| Ideal solvent behavior | Low cold solubility, high hot solubility | Maximizes recovery and purification. |
| Too good | Dissolves product at room temperature | Crystals may not form; recovery is low. |
| Too poor | Does not dissolve product even when hot | Purification cannot occur. |
| Reactive solvent | Solvent reacts with compound | Reject even if solubility looks good. |
| Very high boiling solvent | Difficult to remove; may cause oiling out | Use only with a clear reason. |
| Mixed-solvent pair | Good and poor solvents must be miscible | Examples: ethanol-water, ethyl acetate-hexane, acetone-water. |
| Green/safety screen | Prefer lower hazard solvents when performance is similar | Consider ethanol, ethyl acetate, water, heptane, or 2-MeTHF where suitable. |
| Step | Action | Purpose |
|---|---|---|
| 1 | Screen solvents on microscale | Find hot-soluble/cold-insoluble behavior before risking full sample. |
| 2 | Dissolve crude solid hot | Use the minimum amount of hot solvent. |
| 3 | Decolorize if needed | Use a tiny amount of activated charcoal only for colored impurities. |
| 4 | Hot-filter if needed | Remove insoluble impurities while keeping product dissolved. |
| 5 | Cool slowly | Let crystals grow before using an ice bath. |
| 6 | Induce crystallization if needed | Seed, scratch, or evaporate a little solvent if no crystals appear. |
| 7 | Collect and wash | Vacuum filter and rinse with minimal ice-cold solvent. |
| 8 | Dry and assess purity | Record recovery, melting point, appearance, TLC/NMR if available. |
| Problem | Likely Cause | Fix |
|---|---|---|
| No crystals appear | Too much solvent or undersaturation | Evaporate solvent, cool further, scratch, or seed. |
| Crystals form in funnel | Hot filtration setup cooled too much | Warm funnel/flask, add a little hot solvent, filter quickly. |
| Oiling out | Product separates as liquid before crystallizing | Redissolve, change solvent, use lower-boiling solvent, cool slowly. |
| Low recovery | Too much solvent, excessive washing, transfer losses | Use less hot solvent, colder wash, more careful transfers. |
| Colored crystals | Colored impurity co-crystallized or charcoal not used effectively | Repeat with small charcoal treatment and hot filtration. |
| Broad melting point | Impurities trapped in product | Repeat recrystallization with better solvent or slower cooling. |
| Powdery precipitate | Cooling or anti-solvent addition too fast | Redissolve and cool more slowly. |
Recrystallization is a purification method for solid compounds. The crude solid is dissolved in a hot solvent or hot solvent mixture, insoluble impurities are removed if needed, and the solution is cooled so the desired compound crystallizes in a more ordered and purer form. The method works because most solids are more soluble in hot solvent than in cold solvent. When the hot solution cools, the desired compound becomes less soluble and leaves solution as crystals, while many soluble impurities remain dissolved in the mother liquor.
The method is especially important in organic chemistry because many reactions produce solid products contaminated with colored by-products, unreacted starting material, salts, side products, or mechanical impurities. Chromatography can often purify small amounts quickly, but recrystallization is still valuable because it is scalable, inexpensive, and gentle when the solvent system is chosen well. A well-executed recrystallization can improve melting point sharpness, remove color, and give product that is easier to characterize by NMR, IR, and elemental analysis.
The key idea is controlled solubility. A successful recrystallization does not dissolve the product in a large excess of hot solvent and then hope crystals appear. It uses the minimum amount of hot solvent necessary to dissolve the solid. This creates a saturated or nearly saturated hot solution. As the solution cools, it becomes supersaturated and crystal growth becomes favorable. Too much solvent lowers recovery because product remains dissolved even when cold. Too little solvent leaves product undissolved and traps impurities.
Solvent selection is the most important decision. A good solvent dissolves the target compound well when hot and poorly when cold. It should dissolve impurities either very well at all temperatures, so they remain in solution, or poorly at all temperatures, so they can be removed by hot filtration. It should not react with the compound. It should have a boiling point below the compound's melting point when possible, so the product does not oil out. It should be reasonably safe, affordable, and easy to remove from the crystals.
Students often test several solvents on a microscale before committing the entire sample. A small amount of crude solid is placed in separate test tubes, and a few drops of solvent are added. If the solid dissolves cold, the solvent is usually too good. If it does not dissolve even at boiling, the solvent is too poor. The best candidate dissolves the solid only when hot and gives crystals when cooled. This small screening step saves material and prevents the classic error of drowning the sample in a poor solvent system.
Common single solvents include water, ethanol, methanol, isopropanol, ethyl acetate, acetone, toluene, and hexane or heptane for nonpolar compounds. Mixed-solvent recrystallization is used when no single solvent has the ideal hot-cold solubility profile. The compound is dissolved in a minimum amount of a good hot solvent, then a miscible poor solvent is added until the solution approaches cloudiness. The mixture is warmed until clear and then cooled slowly. Ethanol-water and ethyl acetate-hexane are common mixed systems.
The single-solvent method is the simplest recrystallization workflow. The crude solid is placed in an Erlenmeyer flask, and a small amount of solvent is added. The mixture is heated to boiling or near-boiling, and additional hot solvent is added dropwise until the solid just dissolves. The solution should be clear or nearly clear. If insoluble material remains and is clearly not product, hot gravity filtration can remove it. The solution is then allowed to cool slowly without disturbance.
Slow cooling matters because crystal growth is a molecular ordering process. Rapid cooling can trap impurities inside crystal lattices or create tiny crystals with high surface area that hold mother liquor. A good sequence is to remove the flask from heat, let it cool undisturbed to room temperature, then place it in an ice bath only after crystals have begun to form or the solution is fully room-temperature. Ice-bath cooling too early can give a powdery precipitate rather than well-formed crystals.
After crystallization is complete, crystals are collected by vacuum filtration. The crystals are washed with a small amount of ice-cold solvent. The wash removes mother liquor clinging to the crystal surface without dissolving much product. Washing with warm solvent or too much solvent lowers yield. The product is then dried on the funnel, between filter paper, in a desiccator, or by gentle warming if the compound is stable.
Mixed-solvent recrystallization is useful when the compound is too soluble in one solvent and too insoluble in another. The first solvent is the good solvent; it dissolves the compound when hot. The second solvent is the poor solvent or anti-solvent; it reduces solubility and encourages crystallization. The two solvents must be miscible with each other. If they form layers, the mixture will not behave as a controlled recrystallization medium.
The technique begins by dissolving the crude solid in a minimum amount of hot good solvent. The poor solvent is added slowly, often dropwise, until the hot solution becomes faintly cloudy. The cloudiness means the solution is near saturation. A little more hot good solvent is added until the solution becomes clear again. Then the solution is allowed to cool slowly. This careful cloud-clear endpoint gives a better chance of balancing purity and yield.
Ethanol-water is common for polar organic solids. Ethyl acetate-hexane is common for moderately nonpolar molecules. Acetone-water can be useful for compounds soluble in acetone but not water. Toluene-hexane can work for nonpolar aromatic compounds. The exact choice depends on compound polarity, thermal stability, safety, and downstream drying. Mixed-solvent recrystallization is powerful, but it requires patience because adding too much poor solvent can cause oiling out or rapid precipitation.
Hot gravity filtration removes insoluble impurities from a hot recrystallization solution. It is not always needed. If the hot solution is already clear and no solid impurities are present, skipping hot filtration avoids unnecessary product loss. If charcoal has been used, or if dust, boiling chips, insoluble salts, or mechanical impurities remain, hot filtration is useful. The goal is to remove insoluble material while keeping the desired product dissolved.
The filter funnel and receiving flask should be warm. If the solution cools during filtration, crystals can form in the filter paper or funnel stem, blocking the flow and lowering recovery. Fluted filter paper is often used because it increases surface area and speeds filtration. The receiving flask may contain a small amount of hot solvent to reduce premature crystallization. The solution should be filtered quickly but carefully.
Students often confuse hot gravity filtration with vacuum filtration. Vacuum filtration is used at the end to collect cold crystals. Hot vacuum filtration can cause rapid solvent boiling, crystallization in the funnel, and product loss unless a procedure specifically calls for a heated vacuum setup. In most basic organic labs, hot gravity filtration is the standard method for removing insoluble impurities before crystallization.
Activated charcoal is used to remove colored impurities. It has a large surface area and can adsorb conjugated colored by-products, tarry material, and trace impurities. It should be used sparingly. Too much charcoal can adsorb the desired compound and reduce yield. A small spatula tip is often enough for a student-scale recrystallization. The charcoal is added to the hot solution, the mixture is boiled briefly, and then the charcoal is removed by hot filtration.
Charcoal must not be added to a superheated solution or a solution boiling violently. It can cause sudden bumping or foaming. The safer approach is to remove the solution from heat briefly, add a small amount of charcoal carefully, then resume gentle heating. If the solution remains strongly colored after treatment, a second small treatment may be better than one excessive dose. The goal is decolorization without sacrificing product.
Charcoal treatment is not a universal purification tool. If the impurity is colorless and chemically similar to the product, charcoal may not help. If the product itself is colored, charcoal may remove product. If the solution contains fine charcoal after filtration, the final crystals may be contaminated with black particles. This is why hot filtration after charcoal treatment should be done thoroughly, often with a small plug of filter aid if the procedure allows it.
Sometimes a supersaturated solution refuses to crystallize. Seeding and scratching help initiate nucleation. Seeding means adding a tiny crystal of pure product to the cooled supersaturated solution. The seed provides a template for crystal growth. Scratching means rubbing the inside wall of the flask with a glass rod to create microscopic rough spots that promote nucleation. Both methods are useful when the solution is clear, cool, and likely supersaturated.
Seeding works best when the solution is not too hot. If the seed dissolves immediately, the solution is still too warm or too unsaturated. More cooling or slight evaporation may be needed. Scratching should be gentle; the goal is to create a nucleation site, not grind the glass. If neither method works, the solution may contain too much solvent. Careful evaporation of some solvent followed by cooling can restore supersaturation.
Students should distinguish crystallization from precipitation. Crystallization is controlled and tends to improve purity. Rapid precipitation can trap impurities. If a solution suddenly forms a cloudy solid mass after aggressive scratching or too much poor solvent, the material may need to be redissolved and cooled more slowly. Beautiful crystals are not guaranteed, but controlled nucleation usually gives better purification than crashing the compound out of solution.
Oiling out occurs when the product separates as a liquid instead of crystals. This often happens when the compound's melting point is below the boiling point of the solvent or when the solution becomes supersaturated at a temperature where the compound is still liquid. The oil can trap impurities and later solidify as an impure mass. Oiling out is a common beginner frustration because it looks like the product separated, but purification is poor.
The first response is usually to reheat until the oil dissolves and then adjust the solvent system. Adding a little more solvent may prevent premature separation. Using a lower-boiling solvent, changing to a mixed-solvent system, or cooling more slowly can help. Scratching an oil is usually not useful until the material begins to solidify. If oiling out continues, a different solvent system should be screened on a small scale.
Oiling out teaches the importance of boiling point and melting point. A recrystallization solvent should ideally boil below the compound's melting point, although this is not always possible. The solvent must also create a strong enough solubility difference between hot and cold conditions. When students record recrystallization results, noting oiling out is valuable because it identifies solvent systems to avoid later.
After crystals are collected by vacuum filtration, they are washed with a small amount of cold solvent. The wash solvent is usually the same recrystallization solvent or the poor solvent from a mixed-solvent system. It must be cold because warm solvent dissolves product. The wash should be small because even cold solvent dissolves some material. The purpose is to rinse away mother liquor and soluble impurities on the crystal surface.
Drying is part of purification. Wet crystals can give inaccurate mass, broad melting ranges, extra NMR solvent peaks, and poor combustion or elemental analysis. Crystals may be dried by continued suction, pressing gently between filter paper, air drying, desiccation, or gentle heating. The method depends on volatility of the solvent and stability of the compound. High-boiling solvents require longer drying and may make the final product look deceptively heavy.
Students should avoid overheating crystals during drying. Some compounds sublime, decompose, or lose solvent of crystallization. A constant mass check is helpful: weigh, dry longer, and weigh again until the mass changes minimally. The reported percent recovery should be based on dry product, not damp crystals. Purity should be assessed separately by melting point, TLC, spectroscopy, or another method.
Recrystallization always balances yield and purity. Using more solvent can improve purity because impurities remain dissolved, but it lowers recovery because more product stays in solution. Using less solvent can improve recovery but may trap impurities. Rapid cooling can increase the amount of solid recovered but may reduce purity. Slow cooling can improve crystal quality but sometimes leaves more product dissolved. The best procedure depends on whether the priority is maximum mass, maximum purity, or a practical compromise.
Percent recovery is not the same as percent yield. Recovery compares purified product mass with crude sample mass. Yield compares product mass with the theoretical amount from a reaction. A recrystallization recovery above 100 percent usually means the product is wet or contaminated. A very low recovery may mean too much solvent was used, the product remained dissolved, the wrong solvent was selected, or crystals were lost during transfers and filtration.
Melting point is a classic purity check for recrystallized solids. A pure compound usually has a sharp melting range close to the literature value. Impurities often lower and broaden the melting range. However, melting point alone does not prove identity. It should be combined with TLC, IR, NMR, or mixed melting point when appropriate. Good recrystallization notes should report solvent system, mass before and after purification, percent recovery, crystal appearance, melting range, and any troubleshooting steps.
If no solid dissolves when heated, the solvent may be too poor or there may be too little solvent. Add hot solvent gradually. If the compound dissolves at room temperature, the solvent is too good and recovery will likely be poor. If crystals form in the funnel during hot filtration, the apparatus was too cool or the solution was too concentrated. Reheat, add a small amount of hot solvent, and filter through a warmed setup.
If crystals do not appear on cooling, the solution may be undersaturated. Evaporate some solvent, cool again, scratch, or seed. If powdery crystals form immediately, cooling may have been too fast or too much poor solvent may have been added. Redissolve and cool slowly. If the product is colored, consider a small amount of activated charcoal followed by hot filtration. If black specks remain, filtration after charcoal was incomplete.
If yield is low but purity is high, too much solvent may have been used or the wash volume may have been excessive. If yield is high but melting point is broad, impurities may have co-crystallized. Repeat recrystallization with a better solvent system or slower cooling. If the sample oils out, change solvent, use less heat, adjust the mixed-solvent ratio, or cool more slowly. Troubleshooting is normal; recrystallization is a technique learned by observation, not just by reading a recipe.
Start by choosing candidate solvents based on polarity, safety, boiling point, and known solubility. Test a tiny amount of crude solid in each solvent. Reject solvents that dissolve the compound cold or fail to dissolve it hot. Choose the solvent with the best hot-cold solubility difference. Dissolve the full crude sample in the minimum amount of hot solvent. Add boiling chips or stir properly to avoid bumping if the setup allows it.
If insoluble impurities are present, perform hot gravity filtration through warmed equipment. If colored impurities are present, use a small amount of activated charcoal and then hot-filter. Let the clear hot solution cool slowly and undisturbed. Use scratching or seeding only after the solution has cooled and remains supersaturated. Cool in an ice bath near the end to maximize recovery. Collect crystals by vacuum filtration and wash with a minimal amount of cold solvent.
Dry the crystals thoroughly. Record the mass, percent recovery, solvent system, crystal appearance, and melting range. Compare purity before and after recrystallization if possible. If the result is poor, diagnose the failure using solvent choice, temperature, solvent volume, filtration, cooling rate, washing, and drying as separate variables. This structured approach turns recrystallization from guesswork into a repeatable purification method.
These notes synthesize university and open chemistry laboratory technique references. Always follow your lab manual, SDS, and instructor guidance for actual experiments.
| Source | Why It Was Used |
|---|---|
| Chemistry LibreTexts: Organic Chemistry Lab Techniques | Solvent choice, recrystallization theory, and purification workflow. |
| Chemistry LibreTexts: Choice of Solvent | Criteria for selecting recrystallization solvents and avoiding poor solvent choices. |
| Chemistry LibreTexts: Recrystallization | General recrystallization procedure and theory for purifying organic solids. |
| University of Richmond Organic Chemistry Lab | Practical crystallization tips including scratching, seeding, cold washing, and filtration. |
Chemistry LibreTexts: Organic Chemistry Lab Techniques source Chemistry LibreTexts: Choice of Solvent source Chemistry LibreTexts: Recrystallization source University of Richmond Organic Chemistry Lab source