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3.6A: Single Solvent Crystallization

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    Figure 3.49: a) An old sample of \(\ce{N}\)-bromosuccinimide (NBS), b) Crystallization of NBS using hot water, c) Crystallized NBS.

    The crystallization pictured in this section shows purification of a roughly \(1 \: \text{g}\) sample of old \(\ce{N}\)-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.

    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.

    Figure 3.50: 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

    1. Transfer the impure solid to be crystallized into an appropriately sized Erlenmeyer flask (Figure 3.50a). If the solid is granular, first pulverize with a glass stirring rod.
      It is not recommended to perform crystallizations 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 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 will contain solvent to a height less than \(1 \: \text{cm}\), 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.
    2. 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 (Figure 3.50b).

    Add the Minimum Amount of Hot Solvent

    1. When the solvent is boiling, grasp the beaker with a hot hand protector (Figure 3.50d), cotton gloves, or a paper towel holder made by rolling a sheet of paper towel into a long rectangle (Figure 3.50c). 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 \(50 \: \text{mL}\) 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.
    Figure 3.51: Dissolving NBS in the minimum amount of hot water (boiling stones are also present).
    1. 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 or stir bar if you don't), and bring the solution to a gentle boil (Figure 3.51a).
    2. Add solvent in portions (Figure 3.51b), swirling to aid in dissolution, until the solid just dissolves (Figure 3.51d). For \(100 \: \text{mg}\)-\(1 \text{g}\) of compound, add \(0.5\)-\(2 \: \text{mL}\) 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 (Figure 3.52). 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.
    Figure 3.52: Examples of samples "oiling out" during dissolving: a) Oily droplets of acetanilide in a solution decolorized with charcoal, b+c) Oily droplets containing methyl red and acetanilide in water.
    Figure 3.53: a-c) Cooling and crystallization of NBS, d) Further cooling in an ice bath along with some solvent (in the bottle) for rinsing.

    Allow the Solution to Slowly Cool

    1. 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 (Figure 3.53a). Allow the solution to slowly come to room temperature.
    2. As the solution cools, eventually solid crystals should form (Figure 3.53b). 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 (see Troubleshooting section for advice on how to slow it down).
    3. 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 (Figure 3.53d). Also place a portion of solvent in the ice bath, to be used later for rinsing during suction filtration.
    4. Use suction filtration to recover the solid from the mixture.

    This page titled 3.6A: Single Solvent Crystallization is shared under a CC BY-NC-ND 4.0 license and was authored, remixed, and/or curated by Lisa Nichols via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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