Separations

1. Separations

2. Laboratory Separations & Extractions • Types of Filtration • Gravity Filtration • Suction Filtration • Sinter Funnels • Methods of Separation • Centrifuges • Separating Funnels • Distillation • Rotary Evaporation • Extraction Methods • Extracting Solutes from Liquids • Solid Phase Extraction • Soxhlet Extraction

3. Why Separate? • To separate solids from liquids • Panning for gold! • Removing precipitates (ppts) • Liquids from other liquids • Removing accelerants from a fire scene from the water used to put the fire out! • Extract solutes from solvents • Removing drugs of abuse from blood • Soluble components from insoluble components • Methods differ dependent on the phases of the substances involved and the amount of material present

4. Gravity Filtration • Separates solids from liquids • Used when the liquid is retained • Filter paper is folded and placed inside the funnel • A conical flask is placed underneath to catch the filtrate • Filter paper is fluted or a cone • Fluting gives greater surface area, so filtering is quicker

5. Suction Filtration • Used when the solid precipitate is needed • A round filter paper is placed inside a Buchner funnel • The side of the Buchner flask is attached to a vacuum • Buchner flask should be clamped • Prevents toppling • Do not allow filtrate to reach the suction arm • Fine ppt can block pores and make the process slower • Let the ppt settle before turning on the suction • Hirsch funnel can be used for small quantities Buchner Funnel Buchner Flask Hirsch Funnel

6. Sinter Funnel • Used for gravimetric analysis • For small amounts of solids • Sinter funnel weighed before and after filtration • Sample + funnel dried to a “constant weight” • The amount of substance can then be determined

7. Centrifuges • Solid can be removed from a small amount of liquid • E.g. blood cells from blood plasma • Sample placed in a vial • Vial is then placed in a rotor • Balancing the rotor is very important • Vials are put opposite each other on the rotor • Rotors spins at varying thousands of rpm (revolutions per minute) • The solid is forced into the base of the vial

8. Separating Funnels • For immiscible liquids (i.e. liquids that don’t mix) • E.g. diethyl ether and water • The mixture is poured into the funnel • The layers are allowed to separate • The lower layer can then be tapped off • REMEMBER – remove the stopper when tapping off the bottom layer, otherwise a vacuum will be formed!

9. Distillation • For separating miscible liquids • Boiling points must be different by 10˚C • Mixture is heated in round-bottomed flask • All joints should be greased to ensure a good seal • First liquid reaches boiling point • Vapours travel up to the Liebig condenser, which is water cooled • Vapours condense (distillate) • Distillate is collected in a receiving flask

10. Rotary Evaporation • For removal of large amounts of low boiling solvent • Sample is contained in a large round-bottomed flask • Which rotates gently in a heated water bath • Vacuum pump reduces the pressure – which makes solvent boils at a lower temp than normal b.p. • Solvent vapours are cooled in a water cooled condenser • Solute is left in the round bottomed flask • Condensate collected in the receiving flask.

11. Extraction of Solutes from Liquids • Separating funnels can be used • Move the solute from one solvent to another immiscible solvent. • Solvent containing solvent and extracting solvent are put in separating funnel • Stopper the funnel and shake • Release the pressure • Hold the funnel upside down • Make sure funnel is held in • Open the tap • Repeat several times • Allow to settle (right side up) • Remove stopper • Tap lower liquid

12. Common Uses • Extraction of organic compounds from water into an organic solvent • E.g. extracting pesticides from drinking water into organic solvent for analysis • If upper layer is water • The extracting solvent is less dense • Run off the lower layer • Add more solvent • Extract again and add to original sample collected • If lower layer is water • Run off the water • Collect the solvent in a collection vessel • Replace the water back into the separating funnel • Repeat extraction

13. Solid Phase Extraction (SPE) • More sophisticated than the separating funnel method • Very large sample size • Sucked through a a cartridge containing an adsorbent • Organic compounds adsorb to the adsorbent • The run off water is collected in a side-arm flask • The flask is then emptied and a collection tube placed inside it. • Another organic solvent (a few mil) is passed through the sample • The solvent desorbs the sample from the solid phase and is collected in the collection tube • SPE extracts AND concentrates the organic component

14. Soxhlet Extraction • For extracting soluble species from an insoluble matrix • E.g. pesticides in soil or on the surfaces of fruit and vegetables • The sample is held in a porous thimble • Solvent is held in a reservoir below • Solvent is heated • Solvent vapour rises up the tubing, condenses and collects around the thimble • Soaking the thimble and the sample • The solvent then siphons back into the reservoir • The solvent absorbs the sample from the thimble and carries it to the reservoir • This process repeats and the extracted sample congregates in the reservoir. • Then the rotary evaporator is used to remove the excess solvent.