Separations

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

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

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

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

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

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

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!

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

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.

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

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

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

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.