Step-by-Step Procedures

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.

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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).

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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.