Recrystallization is a purification technique that is very frequently used for organic compounds.  When solid reaction products are isolated, they are seldom “pure” but are usually contaminated with small amounts of impurities such as reaction by-products, unreacted starting material, etc. The process of recrystallization is used to “purify,” or separate, the desired compound from the impurities.

Definitions and Terms:

Pure: All molecules in the sample are identical (an impossible goal!)

Impurities: Other undesired compounds present in the sample of the desired compound. The impurities are usually unreacted starting materials or by-products from the reaction.

Solvent: A liquid, either a single compound or a mixture, which will dissolve the solid to be purified.

Solute: The material (impure) that is dissolved in the solvent.

Mother liquor: The solution remaining after the “pure” product has been removed by suction filtration.  This solution contains some desired product that did not crystallize but now has a higher proportion of impurities.

Saturated solution: A solution that has the maximum possible concentration of solute that can exist at equilibrium with the solid substance, at a given temperature.

Solubility: The concentration (in g/100 mL, mol L^-1, etc.) of solute in a saturated solution. Solubility usually increases with increasing temperature.

Crystallization: The process of cooling, without the loss of solvent, a hot, nearly saturated solution of a compound so that as the temperature falls, the solubility of the solute decreases and crystalline material is deposited from the solution. These crystals can be collected by filtration and will be of higher purity than the starting solute.

Recrystallization: Repetition of the above process using the crystalline product that came out of solution, to achieve further purification.

Flow sheet for crystallization:

Suppose one has a compound, A, that is contaminated by small portions of B & C (impurities). A solvent (or sometimes a solvent mixture) is chosen in which A is very soluble when the liquid is hot, but much less soluble when the liquid is cold. Suppose that B is less soluble in this particular solvent system than A and that C is more soluble than A, i.e. solubility B<A<C (B and C are impurities). The process whereby A may be purified from the impurities B and C is outlined in the following flow chart.  The numbers in parenthesis refer to the numbered detailed explanations on the following pages.

General:

Unless an impurity is very insoluble in the chosen solvent (e.g. compound B), it may remain in solution through the whole process. Since it is only a minor component, there may not be enough of it to form a saturated solution, even after cooling. A very soluble component (e.g. compound C) in small proportions will certainly not come out of solution on cooling. The crystallized sample will only contain any of the soluble impurities if any remaining mother liquor evaporates from the crystals at the end of the cold suction filtration step. The purified product will probably still contain some of the impurities, but at a much lower level. A “2^nd crop”, if obtained, will tend to be less pure than the original crop as it is obtained from the “mother liquor”, a solution which has a higher concentration of impurities (B & C).

1. Solvent: The choice of a solvent is an art! In this course, you will be told what solvents to use. In general, one follows the adage “like dissolves like” (e.g. polar solids tend to dissolve in polar solvents, non-polar solids in non-polar solvents), but the choice of a good solvent is much more complex. Some factors to consider for a solvent follow: It must have a reasonably low boiling point, to aid in easy removal from the isolated “pure” product. There should be a wide range between the boiling point of the solvent and the melting point of the product. The solvent should boil well below the melting point of the solute, otherwise the product will not crystallize but separate as an oil as the saturated solution cools. The solvent must not react with the product and must be able to dissolve the product when hot, but not dissolve much of it at 0°C. Frequently, mixtures of miscible solvents (e.g. ethanol and water) are employed. If a solid is very soluble in hot ethanol and still rather soluble in cold ethanol, most of the desired product will remain in solution on cooling and yields will be low. If this solid is insoluble in water, an ethanol-water mixture can be found that has the desired properties, e.g. high solute solubility when hot, low solubility when cold. Mixed solvents are used in two ways: either the mixture is made up before the crystallization then used as if it were a pure solvent, or, more frequently, the solute is dissolved in the hot solvent in which it is very soluble and then after the hot filtration step, the second solvent is added, slowly, to the hot solution to force the solute out of solution.
2. Boiling the solvent: The solvent might be boiled on a steam bath if its boiling point is less than ~85°C or with a hot plate if the solvent has a higher boiling point. Boiling stones are required in either case and solvents are always boiled in Erlenmeyer flasks.
3. Dissolving the solute: The solute is placed in an Erlenmeyer flask, boiling stones are added and the MINIMUM amount of boiling solvent is added to dissolve the sample. The solution is boiled and if all the solid does not dissolve, more solvent is added and the solution boiled again. This procedure is continued carefully until all the solute is dissolved or until no more remaining solid will dissolve. The remaining solid could be insoluble impurities. Use of too much solvent will cause the desired product to remain in solution on cooling, rather than crystallizing. Use of too little solvent will cause premature crystallization in the flask or in the filter paper during the hot gravity filtration. When all of the solute has been dissolved, a few mL of extra solvent (maybe 10% of the solution volume used) should be added to compensate for solvent loss that occurs during subsequent heating / boiling.
4. Charcoal: If the solution (solvent + solute) is coloured (usually due to high molecular weight impurities) activated charcoal may be added, as it tends to adsorb these impurities and thus remove the colour. The charcoal is removed during the hot gravity filtration. Charcoal particles cause boiling solvents to foam, so remove the solution from the heat and when the boiling stops (~ 45 seconds – if crystals form in the solution during this time period, more solvent needs to be added, refer back to step 3.), add the charcoal (~0.1g). The solution should be boiled gently for a minute or two before filtering.
5. Hot filtration: This procedure removes insoluble impurities from the solution. Hot gravity filtration employs a fluted #4 (615 or P8) filter paper in a glass funnel. The funnel and flask are preheated on the steam bath and are supported in or on the steam bath (in the bath for large flasks or high boiling solvents, on the bath for small flasks or low boiling solvents) during the filtration. The heated glassware keeps the solution from cooling and thereby prevents premature crystallization in the filter paper, funnel or flask. When the solvent is non-aqueous, it is advisable to dry the glass funnel, with a towel, just before use. The filter paper should also be kept as full as possible during the filtration by continually adding hot (boiling) solution. It is best not to fill the paper more than about 3/4 full as this will keep charcoal from washing over the top of the paper during the pouring. This filtration is not done with suction as the vacuum will cause the hot solution, which is to be retained, to boil vigorously which could cause loss of or contamination of the product.
6. Cooling: The hot filtered solution is cooled to cause the crystallization of the product. First the solution is cooled slowly to room temperature (10–20 minutes) and then chilled in an ice bath for 10–15 minutes. Leaving the solution to stand undisturbed, so that slow steady cooling occurs, allows the formation of large crystals, while swirling the solution causes more rapid cooling, resulting in smaller crystals.
7. Ice bath: For efficient cooling of a flask and its contents, an ice bath should contain enough water to form a slurry of ice cold (0°C) water and ice around the flask. Using only ice results in inefficient cooling as the flask is cooled only at the points of ice contact rather than over the whole immersed surface.
8. Cold filtration: This is used to collect the purified crystals on a flat filter paper (#5 or 610 or P2, hardened) in a Buchner funnel placed on a thick walled suction flask that is connected to a water aspirator. The water aspirator, when turned on full, achieves a partial vacuum that pulls the solvent from the crystals quickly. Before pouring the cold crystals / solvent mixture into the funnel, the filter paper, wetted with solvent (you might still have to use a small amount of water to “wet” the filter paper if the solvent is non-aqueous), is sealed onto the funnel by turning on the vacuum. The crystals/solvent mixture should be swirled then poured carefully, but quickly, into the funnel. The “mother liquor” can be used to rinse any remaining crystals out of the crystallizing flask and it may also be concentrated to obtain a second crop of product, so the suction flask must be clean before it is used. A good habit to form is to clean out the suction flask before each If necessary, pressing the crystals with a clean spatula will speed up removal of solvent.
9. Washing crystals: Washing is necessary to remove mother liquor from the solid product after suction filtration. Washing is best done with small volumes of fresh, ice-cold crystallization solvent, but note that this will cause some loss of crystals as the product will still have some solubility in the cold solvent. It is useless simply to pour solvent through the crystals under suction as the solvent goes through so quickly that little is achieved. The proper procedure is to release the vacuum, add the ice-cold, fresh solvent to the crystals in the funnel, carefully swirl so the solvent mixes with the crystals but does not disturb the filter paper, then reapply the vacuum to remove the washing solvent. Consult an instructor before washing crystals.
10. Product: If the solvent is volatile the product crystals can be dried on the Buchner funnel under suction for 15–20 minutes. If the solvent is less volatile (e.g. water), it is best to scrape the crystals onto a clean dry watch glass and allow them to dry in your locker over the week. Note: Some solid products will sublime if left open in this way, so if you are unsure, check with an instructor. When the crystals are dry, weights and melting points can be determined.

 

EXPeriment

Here is a video to illustrate the technique of recrystallization.

https://ocw.mit.edu/resources/res-5-0001-digital-lab-techniques-manual-spring-2007/videos/recrystallization/

Kimberly Berkowski, Sarah Tabacco, Aayesha Siddiqui, and Eileen Huang. RES.5-0001 Digital Lab Techniques Manual. Spring 2007. Massachusetts Institute of Technology: MIT OpenCourseWare, https://ocw.mit.edu. License: Creative Commons BY-NC-SA.

This virtual lab uses open source content. Any online content is the intellectual property of the respective owners and protected under copyright.

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Recrystallization of benzoic acid

Approximately 2.0 g of impure benzoic acid was weighed and transferred to a 250 mL Erlenmeyer flask (boiling stones!). Record the actual mass used (from spreadsheet) in the Mass data. Approximately 100 mL of water was boiled (with boiling stones) using a hot plate (set to ~260). Some boiling water was added slowly to the crude sample (taking care to not add too much) and continued to heat (boil) until the sample just dissolved then an additional couple of mL of water was added. The stem-less funnel was heated in a hot water bath on the hot plate while dissolving the solid. The solution was removed from the heat and let it stand for 30–45 seconds (to check that no crystals form at this point), then 0.1 g of activated charcoal was added slowly. The mixture was boiled gently for 2 minutes, on the hot plate to keep the solution from boiling over.

The hot solution was filtered through a fluted filter paper (#4) set in the pre-heated stemless funnel supported over a second Erlenmeyer flask placed in the hot water bath. This prevents premature cooling and crystallization during the filtration. Should premature crystallization occur and clog the filter paper, place the filter paper (and solid) in the original Erlenmeyer (containing remaining solute and solvent), add an appropriate amount of boiling water to redissolve the benzoic acid and filter as before using a new fluted filter paper.

The filtrate was cooled, undisturbed while crystals formed, then chilled thoroughly in an ice bath for 10–15 minutes. After the sample was chilled, the precipitate was collected by suction filtration. The product was washed several times with small amounts of ice-cold water and the solid was transferred to a pre-weighed watch glass and left to dry for a week.  Record the actual mass of the isolated benzoic acid after dying (provided in spreadsheet) in the Mass data.

Characterization of the products by melting point

After the sample has dried for a week, the melting point (range) was determined.

Record the experimental melting points of the purified benzoic acid after dying (provided in spreadsheet) in the Melting point data.

Characterization of the products by TLC

The following samples were analyzed by TLC:

• the crude benzoic acid
• the recrystallized benzoic acid
• the stock sample of benzoic acid

A small sample of each from the above list were placed in 3 different test tubes. To each tube about 1 mL of acetone was added to dissolve the solids. Each of these samples were spotted onto a TLC plate in the same manner as in the TLC lab, using the small capillaries (spot each sample once or twice). This plate was eluted in a jar containing one of the following solvent systems (either 40/60 ethyl acetate/ hexane, 45/55 ethyl acetate/ hexane or 50/50 ethyl acetate/ hexane)

 

LAB DATA TABLES

 

COMPLETE THE FOLLOWING TABLES USING THE DATA PROVIDED TO YOU IN THE SPREADSHEET ON BRIGHTSPACE. INCLUDE ALL APPROPRIATE UNITS.

 

MAss data

mass of crude Benzoic acid	2.10
mass of recrystallized Benzoic acid	1.15

                       

Melting point data

 

	Literature Melting Point	Experimental Melting Point
Benzoic acid	121-125 °C	121.3

Reference: https://www.sigmaaldrich.com/catalog/product/sial/76170?lang=en&region=CA

 

 

 

• TLC Plate: Copy your plate from the spreadsheet and paste it below. The photograph has two images showing the plate under UV and with the UV light off. It may be easier to use the plate without the UV to make the measurements. In table 1, clearly identify the 3 samples (the crude benzoic acid, recrystallized benzoic acid and the stock sample of benzoic acid)

applied to the plate (see the information given under “sample preparation”) In table 2 below, fill in the spot labels (if the sample produced more than one spot label them as A1, A2, A3, A4 or 1A, 1B, 1C, etc from top to bottom) and heights. Report the R_f values for each of the spots. If the sample produced more than one spot, the R_f of each spot in the sample should be calculated. Note that one of the samples produced a second (very faint) spot that was seen under UV light, but did not photograph well. The spot is circled and should be included in table 2 below.

Table 1: Legend
Label Letter/ number	Identity  (See the lab manual for identification)
1
2
3

Plate 1:

 

 

Table 2
Spot Label	Height (cm)	Rf

Solvent system:40/60 ethyl acetate hexane

Solvent height:4.7cm

 

Introduction:  The introduction for this report should summarize the theory for the recrystallization procedure. Specifically, it should explain how the three major steps result in the removal of impurities from the sample and can be done by simply answering the three questions outlined below:

• What is the reason for using activated charcoal?

Type Response Here

• What type of impurity is removed by the hot gravity filtration? Explain.

Type Response Here

• What type of impurity is removed by the cold suction filtration? Explain.
  Type Response Here

Calculations:  Calculate the percentage recovery (note that this is not a percentage yield, since there was no reaction and thus no “theoretical yield”).

Type Response Here

 

Discussion:

Comment on the percent recovery.  Was any sample lost during the procedure?  Where could sample potentially be lost during the procedure?

Type Response Here

Comment on the results of the melting point analysis.

Type Response Here

TLC Comments:

Comment on the purity of the three samples.

Type Response Here

Compare the Rf of the isolated product to the stock sample.

Type Response Here

Comment on the relative polarity of the spots.

Type Response Here

Comment on the suitability of the solvent.

Type Response Here

Does it appear that the recrystallization was successful in increasing the purity of the sample?

Type Response Here

QUESTIONS

1. An impure sample of compound A is contaminated with two impurities B and C. The sample is to be purified by recrystallization using ethanol as the solvent. The solubility properties of the three components are summarized below.

	Solubility in ethanol at ~78 °C	Solubility in ethanol at ~ 0°C	Solubility in 80 mL ethanol at ~78 °C (g)	Solubility in 80 mL ethanol at ~ 0°C (g)
Compound A	0.12 g/mL	0.02 g/mL	Click or tap here to enter text.	Click or tap here to enter text.
Impurity B	0.58 g/mL	0.04 g/mL	Click or tap here to enter text.	Click or tap here to enter text.
Impurity C	0.004 g/mL	0.001 g/mL	Click or tap here to enter text.	Click or tap here to enter text.

The impure 10.0 g sample contains 8.0 g of compound A, 1.0 g of B and 1.0 g of C and is recrystallized using 80 mL of ethanol. The sample is boiled with 80 mL of ethanol, filtered by gravity and then cooled in ice and filtered by suction.

1. How much compound A should be obtained as the final product? Will the sample be contaminated with any of the impurities? Explain (using calculations to support your answer-fill in the missing masses in the table above). Hint: For this question you should calculate the mass of each component present in the solid and solution phases at each step of the process. Fill in the missing data in the table above. How much of each component will dissolve in the hot solvent? How much of each component will be removed from the system during the hot gravity filtration?  How much of each component will crystallize when cooled? How much of each component will be obtained as the solid portion during the suction filtration?

1. Will the sample be contaminated with any of the impurities?

• Calculate the percent purity of the initial sample (in terms of compound A) – ( i.e. express the amount of compound A in the initial sample as a percentage of the mass of initial sample).

• Calculate the percent purity of the final sample (in terms of compound A) – ( i.e. express the amount of compound A in the final sample as a percentage of the mass of final sample).

• Did the recrystallization procedure result in an increase of the percent purity of compound A in the sample? Comment on the results.

2. An impure sample of compound D is contaminated with a number of impurities. The sample is to be purified by recrystallization. The solubility properties of three potential solvents are summarized below. Choose the best solvent and explain your answer. Include comments on each of the three solvents in terms of why it is either acceptable or unacceptable as a solvent for this procedure.

	Solubility of D at the boiling point of the solvent	Solubility of D at ~ 0°C
Solvent X	0.86 g/mL	0.75 g/mL
Solvent Y	0.95 g/mL	0.15 g/mL
Solvent Z	0.12 g/mL	0.08 g/mL