Why is suction filtration preferable

As water flows through the faucet and the aspirator, suction is created in the flask. A water aspirator creates suction through the Bernoulli Principle technically, the Venturi Effect , for liquids.

Water coming from the faucet is constricted inside the aspirator Figure 1. As the water flow must be the same going into the aspirator as it is going out, the water speed must increase in the constricted area in the direction of flow. A similar phenomenon can be seen in creeks and rivers where the water flows the fastest at the narrowest portions of streams. When the water increases its velocity in the direction of the water flow, conservation of energy dictates that its velocity in perpendicular directions must decrease.

The result is a lowered pressure adjacent to the fast-moving liquid. In other words, the gain in velocity of the constricted liquid is balanced by a reduction in pressure on the surrounding material the gas. For this reason, the speed at which the water flows through the faucet is correlated with the amount of suction experienced in the connected flask.

A strong flow of water will have the fastest speeds through the aspirator and the greatest reduction in pressure. Connect thick-walled hosing from the side arm to a vacuum trap and the water aspirator.

Place a vacuum sleeve on the Buchner or Hirsch funnel, then filter paper on the funnel so it arches downward. The solvent should drain with suction.

Swirl the mixture to be filtered to dislodge the solid from the sides of the flask. Apply suction again for a few minutes repeat the rinse step if necessary. Dry the solid on a watch glass along with the filter paper, overnight if possible.

The solid will flake off the paper when dried. Table 1. Lisa Nichols Butte Community College. Complete text is available online. Suction Filtration Overview Suction filtration vacuum filtration is the standard technique used for separating a solid-liquid mixture when the goal is to retain the solid for example in crystallization.

Figure 1. Rinsing As the goal of suction filtration is to fully separate a solid from its surrounding liquid, rinsing the solid is necessary if the liquid cannot easily evaporate. Water Aspirator A vacuum source is necessary for suction filtration and vacuum distillation. Step-by-Step Procedures Figure 1. The arrows show the direction of suction.

Assemble the suction filtration flask Clamp a side-arm Erlenmeyer flask to a ring stand or latticework and attach a thick-walled rubber hose to its side arm. Connect this thick tubing to a " vacuum trap " Figure 1. It is best to not bend or strain the tubing as much as is practical, as this may cause poor suction. A vacuum trap is necessary when connecting apparatuses to a vacuum source as changes in pressure can cause back-suction. Vacuum filtration is used to collect a desired solid too.

But compare with gravity filtration, it is much faster in the result of the solvent and air being forced through the filter paper by the application of reduced pressure.

Remember, never use vacuum filtration to filter a solid from a liquid if you want to use the liquid, and the solvent boiling at about degrees or lower will boil off in the vacuum flask while the pressure is reduced. Gravity filtration can be used to collect solid product, although generally vacuum filtration is used for this purpose because it is faster. Hot gravity filtration is commonly used to remove these impurities from a solution prior to recrystallization.

Hot filtration is necessary for recrystallization when impurities exist in solution. Secondly, the impurity may have different solubility in certain solvents than the compound to be crystallized. Why is the final product from the recrystallization process isolated by vacuum filtration rather than gravity filtration? Vacuum filtration removes product faster and more completely than gravity filtration. It also helps the product dry. What problems might arise if a hot solution is filtered by vacuum filtration?

A hot solution could cause premature crystallization leading to some impurities remaining in the product and lessening the purity of the sample. It is used when the volume of liquid is greater than to separate out the unwanted solid substances impurities from the solution which contains the wanted material.