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What is liquid liquid extraction? |
A widely used method to separate mixtures (mixture can be solid or liquids) |
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What is liquid liquid extraction based upon? |
It is based on the differences in solubility of the components in two immiscible solvents |
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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 |
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When choosing the solvents for liquid-liquid extractions, what characteristics must they have? |
2) Second solvent selected must be immiscible with the first solvent |
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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 |
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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. |
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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 |
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How does liquid liquid extraction work? |
This would leave the other component in the first solvent. |
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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 |
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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 |
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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 |
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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 |
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Draw the diagram for successive extractions |
Now compare to the one in the manual |
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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 |
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What do we rest the stopper of the funnel before use? |
On a clean watch glass |
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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 |
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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 |
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How do we properly shake the separatory funnel? |
-Repeat until no more pressure develops
-This is to enhance the separation of a compounds |
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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 |
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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 |
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What organic solvents can we use for the solvents in liquid liquid extraction? |
diethyl ether, chloroform, petroleum ether |
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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. |
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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 |
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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 |
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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 |
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Where do we see emulsions in the liquid-liquid extraction procedure? |
-Two solvents do not cleanly separate; an emulsion (foamy layer) forms between them |
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Why do emulsions form in extractions? |
It is a finely dispersed suspension of one liquid layer in another |
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Why do emulsions form in extractions? |
and happens when dissolved materials in the extraction cause the mutual solubility of the two layers to increase |
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Why are emulsions a bad thing? |
Until the layers become distinct again, a separation is not possible. |
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How do we get rid of emulsions? |
Wait 10-20 minutes and swirl the flask gently and the emulsion will disappear |
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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. |
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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 |
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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 |
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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 |
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What are commonly used drying agents? |
Magnesium Sulfate
Sodium sulfate
Calcium chloride for hydrocarbons or alkyl halides |
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How much drying agent do we add? |
In general amount does not exceed 1/10th of liquid volume |
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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 |
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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 |
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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 |
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How long do we dry our solvent for? |
We just removed water, so glassware used should be dry |
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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 |
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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 |
