Experiment 2 Liquid Liquid Extraction Theory Flashcards Table View

     

What is liquid liquid extraction? A widely used method to separate mixtures (mixture can be solid or liquids)
What is liquid liquid extraction based upon? It is based on the differences in solubility of the components in two immiscible solvents
When choosing the solvents for liquid-liquid extractions, what characteristics must they have? 1) First solvent would preferably completely dissolve both compounds in the mixture
When choosing the solvents for liquid-liquid extractions, what characteristics must they have? 2) Second solvent selected must be immiscible with the first solvent
When choosing the solvents for liquid-liquid extractions, what characteristics must they have? 3) Second solvent should preferentially dissolve only one component of the original mixture
How does liquid liquid extraction work? Mixture is dissolved or suspended in the first solvent and then this solution is introduced to the second solvent.
How does liquid liquid extraction work? By shaking the solvent together in an apparatus called a separatory funnel, one component of the mixture will be transferred into the second solvent
How does liquid liquid extraction work? This would leave the other component in the first solvent.
How does liquid liquid extraction work? Final separation of two solvents is accomplished using separatory funnel - followed by evaporation of solvents, produces the separated components
But if all we did was the following procedure, we would not have totally separated the two compounds. How do we increase our success? Compound that we are extracting with 2nd solvent, even though is more soluble in 2nd solvent than 1st but can still dissolve in 1st solvent
What is the extent of the compound dissolve in the less soluble 1st solvent governed by? The partition coefficient of extraction. This forces us to repeat the extraction step many times
What happens when we repeat the extracting step many times? We add a fresh portion of the solvent to separatory funnel containing original solution of mixture, to ensure that we get best possible separation
Draw the diagram for successive extractions Now compare to the one in the manual
Solutions should never occupy more than what amount of volume of the separatory funnel? Solutions should never occupy more than 2/3 of the volume of the funnel or else proper mixture will be difficult
What do we rest the stopper of the funnel before use? On a clean watch glass
How do we properly shake the separatory funnel? -One hand on the stopper, the other on the stopcock -Point stem away from yourself and neighbors
How do we properly shake the separatory funnel? Gently shake the funnel and open the stopcock to release the initial pressure -Then shake more vigorously and release
How do we properly shake the separatory funnel? -Repeat until no more pressure develops -This is to enhance the separation of a compounds
When cleaning separatory funnels, how should you approach it? Do not hit fragile step against edge. Since we use aqueous solutions, it is not necessary to dry it before use
What are the two-phase systems in liquid-liquid extractions in chem 235? Aqueous phase and an immiscible organic phase. Compound isolated is extracted into organic phase
What organic solvents can we use for the solvents in liquid liquid extraction? diethyl ether, chloroform, petroleum ether
When doing liquid liquid solvents, what volume of 1st solvent should we use to dissolve original mixture? We want to dissolve original mixture in a minimum amount of first solvent.
When doing liquid liquid solvents, how much of the second solvent volume should we use? For a given amount of volume of extraction solvent, we should divide that volume up to do several extractions
When doing liquid liquid solvents, how much of the second solvent volume should we use? We get better results than if we were to use the entire amount for one extraction
When doing liquid liquid solvents, how much of the second solvent volume should we use? Total volume of the extraction solvent used for the successive extractions usually approximates the total volume of the other solvent
Where do we see emulsions in the liquid-liquid extraction procedure? -Two solvents do not cleanly separate; an emulsion (foamy layer) forms between them
Why do emulsions form in extractions? It is a finely dispersed suspension of one liquid layer in another
Why do emulsions form in extractions? and happens when dissolved materials in the extraction cause the mutual solubility of the two layers to increase
Why are emulsions a bad thing? Until the layers become distinct again, a separation is not possible.
How do we get rid of emulsions? Wait 10-20 minutes and swirl the flask gently and the emulsion will disappear
How do we get rid of emulsions? If water is one of the solvents, we could add some saturated NaCl solution. This breaks up emulsion.
How do we get rid of emulsions? Because Saturated NaCl is less soluble in organic solvent than water and hence will pull the water out of organic layer
Why do we need a dying procedure after an extraction? After extraction, organic layer is usually saturated with water, and the latter must be removed before organic solvent is evaporated away
Why do we need a dying procedure after an extraction? leaving behind your solid compound. Drying agents are used to absorb this small amount of water
What are commonly used drying agents? Magnesium Sulfate Sodium sulfate Calcium chloride for hydrocarbons or alkyl halides
How much drying agent do we add? In general amount does not exceed 1/10th of liquid volume
How can we check? -Swirl the corked flask, if all drying agent remains clumped on bottom and does not swirl freely, not enough has been added
How can we check? But as long as some of the agent will move freely as individual grains, you added enough. -Get to see if its swirling again in 5 minutes
How long do we dry our solvent for? For at least 10 minutes with occasional swirling. We remove the salt by gravity filtration through fluted filter paper
How long do we dry our solvent for? We just removed water, so glassware used should be dry
What methods do we use to remove the organic solvent from desired compound? 1) evaporation on a steam bath or hot water bath (in fumehood) 2) evaporation at reduced pressure
What methods do we use to remove the organic solvent from desired compound? 3) distillation (if solute is high boiling liquid or solid) 4)Evaporation by means of rotary evaporator