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Recrystallization and hot filtration


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Jul 5, 2021
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In chemistry, recrystallization is a technique used to purify chemicals. By dissolving both impurities and a compound in an appropriate solvent, either the desired compound or impurities can be removed from the solution, leaving the other behind. It is named for the crystals often formed when the compound precipitates out. Alternatively, recrystallization can refer to the natural growth of larger ice crystals at the expense of smaller ones.

The method of purification is based on the principle that the solubility of most solids increases with increased temperature. This means that as temperature increases, the amount of solute that can be dissolved in a solvent increases.

An impure compound is dissolved (the impurities must also be soluble in the solvent), to prepare a highly concentrated solution at a high temperature. The solution is cooled. Decreasing the temperature causes the solubility of the impurities in the solution and the substance being purified to decrease. The impure substance then crystallizes before the impurities- assuming that there was more impure substance than there were impurities. The impure substance will crystallize in a purer form because the impurities won't crystallize yet, therefore leaving the impurities behind in the solution. A filtration process must be used to separate the more pure crystals at this point. The procedure can be repeated. Solubility curves can be used to predict the outcome of a recrystallization procedure.

Recrystallization works best when:​
  1. The quantity of impurities is small.
  2. The solubility curve of the desired solute rises rapidly with temperature.

Single solvent crystallization, example with explanations.

The crystallization pictured in this section shows purification of a roughly 1g sample of old NBS - bromosuccinimide (NBS), which was found in its reagent bottle as an orange powder. The crystallization uses water as the solvent, which has no flammability issues, and so a hotplate is used.
a) An old sample of N-bromosuccinimide (NBS), b) Crystallization of NBS using hot water, c) Crystallized NBS
If a crystallization is to be performed using flammable organic solvents, a steam bath is recommended and in some situations necessary (when using diethyl ether, acetone, or low-boiling petroleum ether). The following procedure should be used as a guideline for the process, and some key differences between using water and organic solvents are discussed in a future section.​
a) Impure NBS added to the flask, b) Heating water on a hotplate, c) Addition of hot water to the solid, using a paper towel holder to hold the beaker, d) Addition of hot water to the solid (not NBS, a different system) using a silicone hot hand protector.​

Prepare the Setup:
  • Transfer the impure solid to be crystallized into an appropriately sized Erlenmeyer flask (Fig.2 a). If the solid is granular, first pulverize with a glass stirring rod.​
  • It is not recommended performing crystallization in a beaker. The narrow mouth of an Erlenmeyer flask allows for easier swirling and minimized evaporation during the process, as solvent vapors instead condense on the walls of the flask (they "reflux" on the sides of the flask). The narrow mouth of an Erlenmeyer also allows for a flask to be more easily covered during the cooling stage, or even potentially stoppered for long crystallizations. A round-bottomed flask is also not ideal for crystallization, as the shape of the flask makes it difficult to recover a solid at the end of the process.​
  • It is important that the flask be not too full or too empty during the crystallization. If the flask will be greater than half-full with hot solvent, it will be difficult to prevent the flask from boiling over. If the flask contain solvent to a height less than 1 cm, the solution will cool too quickly. It is common to use between 10-50 times as much solvent as sample, and a rough guide is to use a flask, where the sample just covers the bottom in a thin layer.​
  • Place some solvent in a beaker or Erlenmeyer flask along with a few boiling stones on the heat source, and bring to a gentle boil. Use a beaker if the solvent will be poured and an Erlenmeyer flask if the solvent will be pipetted. If a hot filtration step is anticipated for later in the procedure, also prepare a ring clamp containing a funnel with fluted filter paper (Fig.2 b).​
Add the Minimum Amount of Hot Solvent.
  • When the solvent is boiling, grasp the beaker with a hot hand protector (Fig.2 d), cotton gloves, or a paper towel holder made by rolling a sheet of paper towel into a long rectangle (Fig.2 c). To the side of the heat source, pour a small portion of boiling solvent into the flask containing the impure solid, to coat the bottom of the flask. If the crystallization is being performed on a small scale (using a 50 ml Erlenmeyer flask or smaller), it may be easier to use a pipette to transfer portions of the solvent to the flask.​
  • It's customary to not place the dry solid atop the heat source before adding solvent, or the solid may decompose. When the solid is dispersed in a small amount of solvent, it can then be placed on the heat source.​
  • Place the flask containing the impure solid and solvent atop the heat source. Use some method to prevent bumping (boiling stones if you plan to "hot filter", a boiling stick), and bring the solution to a gentle boil (Fig.3 a).​
  • Add solvent in portions (Fig.3 b), swirling to aid in dissolution, until the solid just dissolves (Fig.3 d). For 100 mg-1g of compound, add 0.5-2 ml portions at a time. Note that it may take time for a solid to completely dissolve, as there is a kinetic aspect to dissolution. Each addition should be allowed to come completely to a boil before adding more solvent, and some time should be allowed between additions. Not allowing time for dissolution and consequently adding too much solvent is a main source of error in crystallization.
    It is not uncommon for droplets of liquid to be seen during the heating process (Fig.4). This is when the material "oils out", or melts before it dissolves. If this happens, the liquid droplets are now the compound you are crystallizing, so continue adding solvent in portions until the liquid droplets fully dissolve as well.​
  • Watch the solution carefully to judge whether the size of the solid pieces (or liquid droplets) change with additional solvent: if they don't, they may be an insoluble impurity. Addition of excess solvent in an attempt to dissolve insoluble impurities will negatively affect the recovery. If insoluble solid impurities are present, the solution should be filtered (insert a hot filtration step at this point). Colored impurities can also be removed at this point with charcoal.​
a) Oily droplets of acetanilide in a solution decolorized with charcoal, b and c) Oily droplets containing methyl red and acetanilide in water.​

Allow the Solution to Slowly Cool.
  • When the solid is just dissolved, remove the flask from the heat source using a hot hand protector, paper towel holder, or glove, and set it aside to cool. Remove the boiling stick or stir bar if used for bump protection (boiling stones can be picked out of the solid at a later point if used).​
  • To encourage slow cooling, set the flask atop a surface that does not conduct heat well, such as a folded paper towel. Cover the mouth of the Erlenmeyer flask with a watch glass to retain heat and solvent (Fig.5 a). Allow the solution to slowly come to room temperature.​
  • As the solution cools, eventually solid crystals should form (Fig.5 b). If the solution is only warm to the touch or cloudy and no crystals have formed, use a glass stirring rod to scratch the glass and initiate crystallization.​
  • After crystallization has begun, the crystals should slowly grow as the temperature decreases. An ideal crystallization takes between 5-20 minutes to fully crystallize, depending on the scale. Complete crystallization in less than 5 minutes is too quick.​
  • When the solution is at room temperature, place the flask into an ice bath (ice-water slurry) for 10-20 minutes to lower the compound's solubility even more and maximize crystal formation (Fig.5 d). Also place a portion of solvent in the ice bath, to be used later for rinsing during suction filtration.​
  • Use suction filtration to recover the solid from the mixture.​
a -c) Cooling and crystallization of NBS, d) Further cooling in an ice bath along with some solvent (in the bottle) for rinsing.​

Mixed solvent crystallization, example with explanations.

The crystallization pictured in this section shows purification of a roughly 1 g sample of trans-cinnamic acid. Trans-cinnamic acid is soluble in methanol and insoluble in water, and this crystallization uses a mixed solvent of methanol and water to give a 74% recovery.​
a) Initial sample of trans-cinnamic acid, b) Crystallization using methanol/water, c) Crystallized trans-cinnamic acid.

It is assumed that experimenter performing this technique have previously performed or read about a single-solvent crystallization.​
  • Determine two miscible solvents that can be used for the crystallization (Fig.7 a): the desired compound should be soluble in one solvent (called the "soluble solvent") and insoluble in the other solvent (called the "insoluble solvent").​
  • Transfer the impure solid to be crystallized into an appropriately sized Erlenmeyer flask (Fig.7 b).​
  • Place some "soluble solvent" into the flask (Fig.7 c), add a boiling stick (or boiling stones if preferred), then heat atop a steam bath (Fig.7 d). A hotplate can be used cautiously if using the mixed solvents methanol/water or ethanol/water.​
a) Trans-cinnamic acid is insoluble in water and soluble in methanol, b) Adding the solid to the flask, c) Adding methanol off the heat source, d) Adding enough methanol to dissolve the solid.​
  • Add more of the "soluble solvent" in portions until the solid just dissolves (Fig.8 a). Be sure to allow time in between additions, and allow each addition to come completely to a boil before adding more.​
  • Add the "insoluble solvent" in portions, with heating until the solution becomes just cloudy (Fig.8 c).​
  • Add the "soluble solvent" dropwise with heating until the solution again clarifies (Fig.8 d).​
a) Addition of just enough soluble solvent (methanol) to dissolve the solid, b) Addition of the insoluble solvent (water) dropwise, c) Sustained cloudiness after addition of enough insoluble solvent, d) Addition of soluble solvent to clarify.​
  • Remove the flask from the heat source, remove the boiling stick and set the flask atop a paper towel folded several times. Cover the mouth of the Erlenmeyer flask with a watch glass, and allow the solution to slowly cool to room temperature (Fig.9 a).​
  • As the solution cools, eventually solid crystals should form (Fig.9 b). Use a glass stirring rod to scratch the flask and initiate crystallization, if necessary. Place the crystals in an ice-water bath for 10-20 minutes and collect the solid by suction filtration.​

Hot filtration.

A hot filtration is generally used in some crystallization, when a solid contains impurities that are insoluble in the crystallization solvent. It is also necessary in crystallization when charcoal is used to remove highly colored impurities from a solid, as charcoal is so fine that it cannot be removed by decanting.

A hot filtration is performed by first pouring a few ml of solvent through a funnel containing a "fluted filter paper". A fluted filter paper has many indentations and high surface area, which allows for a fast filtration. The funnel is allowed to get hot, while the mixture to be filtered is brought to a boil. The boiling mixture is then poured through the filter paper in portions (Fig.10 b and d).​
a) Pouring solvent through the funnel hot (note: hotplate is used in this situation as the solvent is water), b) Filtering a solution containing insoluble impurities, c and d) Variations using a paper clip instead of a ring clamp to hold the funnel.​

It is best to use a ring clamp to secure the filtration funnel, although the funnel could also be simply placed atop the flask. If not using a ring clamp, it is recommended to place a bent paper clip between the flask and funnel to allow for displaced air to escape the bottom flask as liquid drains (Fig.10 c and d). Without a ring clamp, the setup is more prone to tipping, and so using a ring clamp is considerably safer.

A hot filtration is used for filtering solutions that will crystallize when allowed to cool. It is therefore important that the funnel is kept hot during filtration through contact with hot solvent vapors, or crystals may prematurely form on the filter paper or in the stem of the funnel (Fig.11).​
a) Crystallization occurred in the stem of the funnel, b) A shear can be seen on the filter paper, representing a solid that has crystallized somewhat on the filter paper, c) Obvious crystals formed on the filter paper.​

Crystallization on the filter paper can clog the setup and cause a loss of yield (as the filter paper will be later thrown away). Crystallization in the stem hinders filtration, and can act as a plug on the bottom of the funnel. An advantage of hot filtration is that the boiling solvent in the filter flask helps to dissolve crystals that prematurely form in the stem of the funnel. With hot filtration, it is advised to use a short-stemmed funnel (Fig.12 a) or stemless funnel (Fig.12 c) if available, instead of a long-stemmed funnel (Fig.12 b), as material is less likely to crystallize in a short or absent stem.​
a) Short-stemmed funnel, b) Long-stemmed funnel (don't use for hot filtration), c) Stemless Funnel​

As it is essential that a solution filters quickly before it has a chance to cool off in the funnel, a "fluted filter paper" (Fig. 13 b and c) is commonly used instead of the quadrant-folded filter paper sometimes used with gravity filtration (Fig. 13 a). The greater number of bends on the fluted filter paper translate into increased surface area and quicker filtration. The folds also create space between the filter paper and glass funnel, allowing for displaced air to more easily exit the flask as liquid drains.​
a ) Quadrant-folded filter paper (not recommended for hot filtration), b) A fluted filter paper after folding, c) Unfolded fluted filter paper during hot filtration.​

Step-by-Step Procedures.

Hot filtration is often used with crystallization, and this procedure should be inserted after the dissolution step, but before setting aside the solution to slowly cool.​
a) Short-stemmed funnel, b) If not using a ring clamp, place a bent paper clip between the funnel and Erlenmeyer flask, c) Fluted filter paper (unfolded), d) Pouring solvent through the funnel to get the funnel hot.​

Prepare the Filtration Setup
  • Obtain a stemless or short-stemmed funnel (Fig.14 a), and insert it into a ring clamp, attached to a ring stand or latticework (or alternatively, obtain a bent paper clip for the purpose shown in Fig.14 b).​
  • Flute a filter paper of the correct size for your funnel into an accordion shape (instructions are in Fig. 15 and the resulting accordion is in Fig.14 a). When placed in the funnel, the paper should not be shorter than the top of the funnel, or the solution might slip past the filter paper when poured.​
  • With a clean Erlenmeyer flask of the correct size for the crystallization beneath the funnel and on the heat source, pour a few ml of hot solvent into the funnel (Fig.14 d).
    a) If using a ring clamp, adjust the clamp so that there is a small gap between the mouth of the Erlenmeyer and bottom of the funnel: this allows for air to be displaced when liquid flows into the flask. If the gap is too large, hot vapors will escape without heating the funnel.
    b) If not using a ring clamp, place a bent paper clip between the flask and funnel (Fig.14 b).​
  • Allow the solvent to boil and get the entire setup hot. If using charcoal, insert that procedure now.​
a) Hot filtering, b) Holding a hot flask with a paper towel holder, c) Liquid wetting the too wide paper towel holder.​

Filter the Solution in Portions.
  • When the filter flask is quite hot, and the solution to be filtered is boiling, pour the boiling mixture into the filter funnel in portions. Touch the flask to the filter paper in the funnel as you pour (Fig. 16 a).​
  • Safety note: the flask may be quite hot, and hot vapors may scald your hand as you pour (pour sideways , so your hand is not above the funnel). If the flask is too hot to hold with your hands, use a "paper towel holder" to hold the flask (Fig. 16 a):
    a) Fold a section of paper towel over several times such that the resulting strip is roughly one inch wide. If desired, secure the strip together using a few pieces of tape.
    b) When holding a flask, the paper towel holder should be below the lip of the flask. In this way, liquid will not wick toward the paper towel when pouring (towel remains dry in Fig. 16 a), but wet with the too wide towel in Figure Fig. 16 c).​
a) Flask is returned to the heat source in between pouring, filter papers that b) should be rinsed, c) should not be rinsed.​
  • When not pouring the mixture to be filtered, return the flask to the heat source (Fig. 17 a).​
  • When the mixture is completely filtered, set the empty flask on the bench top (safety note: do not heat an empty flask, or it may crack). Inspect the funnel: if crystals are seen on the filter paper (as in Fig. 17 b), rinse with a few ml of boiling solvent to dissolve them. A rinse is not needed in Fig. 17 c.​
  • Inspect the filtrate (the liquid that has gone through the filter paper). If charcoal was used and the filtrate is grey, or you can see fine black particles, then charcoal passed through the filter paper either through a hole or by using the wrong filter mesh size. If classmates do not have grey in their solutions, it was likely a hole. Repeat the hot filtration step with a new filter paper and flask.​
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Nov 6, 2021
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What a nice "summary"! I love the step by step procedures (eg. a noob like myself doesn't know how to make filter paper, we would probably just use a coffee filter without preparation).