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1.4E: Hot Filtration

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  • Hot Filtration Overview

    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 \(\text{mL}\) 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 (Figures 1.81b+d).

    Figure 1.81: 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+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 (Figure 1.81c+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 (Figure 1.82).

    Figure 1.82: A filtration that was conducted at too low a temperature: a) Crystallization occurred in the stem of the funnel, b) A sheer can be seen on the filter paper, representing 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 (Figure 1.83a) or stemless funnel if available, instead of a long-stemmed funnel (Figure 1.83b), as material is less likely to crystallize in a short or absent stem.

    Figure 1.83: a) Short-stemmed funnel, b) Long-stemmed funnel (don't use for hot filtration).

    As it is essential that a solution filters quickly before it has a chance to cool off in the funnel, a "fluted filter paper" (Figure 1.84b+c) is commonly used instead of the quadrant-folded filter paper sometimes used with gravity filtration (Figure 1.84a). 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.

    Figure 1.84: 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.

    Figure 1.85: 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

    1. Obtain a stemless or short-stemmed funnel (Figure 1.85a), 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 Figure 1.85b).
    2. Flute a filter paper of the correct size for your funnel into an accordion shape (instructions are in Figure 1.86 and the resulting accordion is in Figure 1.85c). 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.
    Figure 1.86: Creating a fluted filter paper. The dotted lines represent locations to crease and fold the filter paper. The arrows show the direction of folding.
    1. With a clean Erlenmeyer flask of the correct size for the crystallization beneath the funnel and on the heat source, pour a few \(\text{mL}\) of hot solvent into the funnel (Figure 1.85d).
      1. 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.
      2. If not using a ring clamp, place a bent paper clip between the flask and funnel (Figure 1.85b).
    2. Allow the solvent to boil and get the entire setup hot. If using charcoal, insert that procedure now.
    Figure 1.87: 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

    1. 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 (Figure 1.87a).
    2. 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 (Figure 1.87b):
      1. 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.
      2. 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 Figure 1.87a), but wet with the too wide towel in Figure 1.87c).
    Figure 1.88: a) Flask is returned to the heat source in between pouring, Filter papers that b) should be rinsed, c) should not be rinsed.
    1. When not pouring the mixture to be filtered, return the flask to the heat source (Figure 1.88a).
    2. When the mixture is completely filtered, set the empty flask on the benchtop (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 Figure 1.88b), rinse with a few \(\text{mL}\) of boiling solvent to dissolve them. A rinse is not needed in Figure 1.88c.
    3. 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.

    Hot Filtration Summary

    Prepare a fluted filter paper and stemless or short-stemmed funnel clamped above the filter flask. Pour a few \(\text{mL}\) of solvent through the funnel, and allow the solvent to boil and get the funnel hot. When the mixture to be filtered is boiling, pour the mixture into the funnel in portions, returning it to the heat source in between additions. Rinse the filter paper with hot solvent if crystals are seen on the paper.

    Table 1.11: Procedural summary for hot filtration.


    Lisa Nichols (Butte Community College). Organic Chemistry Laboratory Techniques is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. Complete text is available online.