in what circumstances is it necessary to filter a hot recrystallization solution?

1.5E: 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 too necessary in crystallization when charcoal is used to remove highly colored impurities from a solid, as charcoal is then fine that it cannot be removed by decanting.

A hot filtration is performed past first pouring a few \(\text{mL}\) of solvent through a funnel containing a "fluted filter newspaper". A fluted filter paper has many indentations and high surface area, which allows for a fast filtration. The funnel is immune to go hot, while the mixture to be filtered is brought to a boil. The boiling mixture is so poured through the filter paper in portions (Figures 1.81b+d).

Figure 1.81: a) Pouring solvent through the funnel hot (annotation: hotplate is used in this state of affairs as the solvent is water), b) Filtering a solution containing insoluble impurities, c+d) Variations using a paper prune instead of a ring clamp to concur the funnel.

It is best to employ a ring clench to secure the filtration funnel, although the funnel could also be but placed atop the flask. If non using a ring clamp, it is recommended to identify 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 decumbent to tipping then using a ring clamp is considerably safer.

A hot filtration is used for filtering solutions that will crystallize when allowed to absurd. It is therefore important that the funnel is kept hot during filtration through contact with hot solvent vapors, or crystals may prematurely grade on the filter paper or in the stalk of the funnel (Figure one.82).

Figure 1.82: A filtration that was conducted at too depression a temperature: a) Crystallization occurred in the stalk of the funnel, b) A sheer can be seen on the filter paper, representing solid that has crystallized somewhat on the filter newspaper, c) Obvious crystals formed on the filter paper.

Crystallization on the filter paper can clog the setup and cause a loss of yield (equally the filter paper will be afterward 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 humid solvent in the filter flask helps to dissolve crystals that prematurely form in the stem of the funnel. With hot filtration, information technology is advised to use a curt-stemmed funnel (Effigy i.83a) or stemless funnel if available, instead of a long-stemmed funnel (Figure 1.83b), as material is less likely to crystallize in a curt or absent stem.

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

Equally 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 expanse and quicker filtration. The folds also create infinite between the filter paper and glass funnel, allowing for displaced air to more easily exit the flask equally liquid drains.

Effigy 1.84: a) Quadrant-folded filter paper (non recommended for hot filtration), b) A fluted filter paper after folding, c) Unfolded fluted filter paper during hot filtration.

Stride-past-Pace 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.

Effigy 1.85: a) Short-stemmed funnel, b) If not using a band clamp, place a aptitude paper clip between the funnel and Erlenmeyer flask, c) Fluted filter newspaper (unfolded), d) Pouring solvent through the funnel to get the funnel hot.

Fix the Filtration Setup

1. Obtain a stemless or brusque-stemmed funnel (Figure i.85a), and insert it into a ring clamp, attached to a band 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 non 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.

3. With a make clean Erlenmeyer flask of the correct size for the crystallization beneath the funnel and on the estrus source, pour a few \(\text{mL}\) of hot solvent into the funnel (Figure one.85d).
   1. If using a band clamp, arrange the clamp so that in that location is a small gap betwixt the mouth of the Erlenmeyer and bottom of the funnel: this allows for air to exist displaced when liquid flows into the flask. If the gap is as well large, hot vapors will escape without heating the funnel.
   2. If non using a band clench, place a bent paper clip between the flask and funnel (Figure one.85b).
4. Let the solvent to boil and get the entire setup hot. If using charcoal, insert that procedure at present.

Figure 1.87: a) Hot filtering, b) Holding a hot flask with a paper towel holder, c) Liquid wetting the as well broad newspaper towel holder.

Filter the Solution in Portions

5. When the filter flask is quite hot, and the solution to be filtered is humid, pour the boiling mixture into the filter funnel in portions. Bear upon the flask to the filter paper in the funnel as you cascade (Figure i.87a).
6. Prophylactic annotation: the flask may be quite hot, and hot vapors may scald your mitt as you cascade (cascade sideways so your mitt is not above the funnel). If the flask is likewise hot to concord with your hands, use a "newspaper towel holder" to hold the flask (Effigy 1.87b):
   1. Fold a department of newspaper towel over several times such that the resulting strip is roughly i inch wide. If desired, secure the strip together using a few pieces of tape.
   2. When belongings a flask, the paper towel holder should be beneath the lip of the flask. In this way, liquid will not wick toward the newspaper towel when pouring (towel remains dry in Effigy 1.87a), but wet with the as well wide towel in Figure 1.87c).

Figure i.88: a) Flask is returned to the heat source in between pouring, Filter papers that b) should be rinsed, c) should not exist rinsed.

7. When non pouring the mixture to be filtered, return the flask to the heat source (Effigy 1.88a).
8. When the mixture is completely filtered, set the empty flask on the benchtop ( safety notation: do not rut an empty flask, or it may cleft). Inspect the funnel: if crystals are seen on the filter paper (every bit in Effigy 1.88b), rinse with a few \(\text{mL}\) of boiling solvent to dissolve them. A rinse is non needed in Figure 1.88c.
9. Inspect the filtrate (the liquid that has gone through the filter paper). If charcoal was used and the filtrate is grey, or y'all tin run into fine black particles, then charcoal passed through the filter paper either through a hole or past using the wrong filter mesh size. If classmates exercise not take gray in their solutions, information technology was likely a hole. Repeat the hot filtration stride with a new filter paper and flask.

Hot Filtration Summary

Table 1.11: Procedural summary for hot filtration.

Contributor

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

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