Filtration Techniques

Hope College: General Chemistry Laboratories
Fall: Chem 127 & Chem 132 - Spring: Chem 128

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Table of Contents

1. Introduction
2. Gravity Filtration
3. Vacuum Filtration

Introduction

Filtration is commonly used in research and chemical manufacturing to separate solids from liquids. You will employ two filtration techniques (gravity filtration and vacuum filtration) from time to time in this course.

Gravity Filtration

Gravity filtration usually utilizes the long stem funnel with normal quarter-folded 11 cm diameter filter paper (Figure PAGENUM), when the filtration is done at room temperature or below. It is convenient to support the filter funnel by a utility clamp.

Optimum filtration performance is achieved by the appropriate selection of filter paper porosity, by proper folding and positioning of the filter paper, and by following a few simple technique guidelines described below:

1. Filter paper porosity: Filtration operations in General Chemistry involve crystalline precipitates of relatively large crystal size so that coarse or medium porosity filter paper is used. Sargent Welch S-32915, Whatman 1, and E-D 617 have appropriate pore size for work in this course.
2. Filter paper folding and positioning: Maximum advantage of capillary attraction must be taken to speed the passage of the filtrate through the paper. Much of this depends upon correct folding and positioning of the paper. The folding steps are illustrated in Figure PAGENUM.
   • Place the folded paper in the funnel so as to give a snug fit.
   • Tear off the corner of the paper to enable a closed seal to be made across the folded portion of the paper.
   • Moisten the paper with the solvent to be used in the filtration and press the top of the paper against the funnel all around its circumference to seal the space between the funnel wall and the paper.
   • This method of folding and positioning the paper insures that most of the paper does not make direct contact with the funnel wall thereby permitting liquid to flow between paper and glass below an air tight seal.
   • Place the tip of the funnel against the side of the vessel which receives the filtrate to eliminate spattering and to provide an unbroken stream of liquid from the undersurface of the paper to the bottom of the collection container.

• Transferring the mixture: If it is important to quantitatively (completely) transfer the solid to the filter paper, a beaker should be used as a transfer vessel rather than a flask.
  • Before filtering the mixture, allow the solid to settle to the bottom of the beaker.
  • Then carefully decant (pour off liquid leaving solid in container) the supernatant liquid through the filter. This is done to prevent early accumulation of precipitate on the filter paper which would slow the filtration process.
  • Transfer the bulk of the solid during the last stages of the filtration process.
  • If the precipitate needs to be washed, the washing is most efficiently done while the solid is still in the beaker. The major transfer is done with the last wash.
  • Use a stirring rod to direct the flow of liquid into the funnel and prevent small amounts of liquid from running down the outside of the beaker when the transfer process is interrupted.
  • Employ a stream of solvent from a wash bottle to transfer the final portion of precipitate if quantitative transfer is required.
  • Utilize a rubber policeman on the end of a stirring rod to get the last grains of precipitate out of the beaker in a quantitative transfer.
  • Try to keep the filter paper about 3/4 full of liquid during filtration to provide maximum filtra- tion speed.
  • Never fill the cone completely, since many precipitates tend to creep upward and may be par- tially lost.
  • Also for many purposes it is convenient to have a small area of paper free of precipitate near the top so that the filter paper can be removed without getting precipitate on the fingers.

 

Vacuum Filtration

Vacuum filtration of crystalline precipitates has the advantage of increased speed over gravity filtration because of the greater pressure differential between the top surface of the liquid and the bottom side of the filter paper or glass frit provided by the vacuum. In this course you will do vacuum filtrations with Büchner funnels equipped with filter paper when less than 100 percent recovery of precipitate is required and with glass-fritted crucibles when 100 percent recovery of precipitate is required. A typical vacuum filtration set-up is shown in Figure PAGENUM.

 

The vacuum is provided by an aspirator on the water faucet. The trap is used to prevent water from backing up from the faucet to contaminate the filtrate. The pinch clamp provides a convenient valve to produce or break vacuum in the system.

 

To do a vacuum filtration:

• Select a piece of coarse or medium porosity filter paper that fits flat on the perforated floor of the funnel so that all of the holes are covered.

• Moisten the paper with solvent and apply vacuum to seal the paper to the funnel floor.

• Still applying vacuum pour the supernatant liquid into the funnel followed by the solid, spreading it

  out evenly over the filter paper surface.

• To wash the crystals break vacuum by removing the pinch clamp. Add wash liquid to just cover the crystals and carefully break up clumps of crystals with a spatula without perforating the filter paper.

• Using the pinch clamp, reapply the vacuum to draw off the wash liquid.

• Always break vacuum before turning off the water flow to prevent backup of water into the trap.

• To recover the crystals invert the funnel on a large watch glass.