Matter #4 Separation and Purification Methods
Separation Methods are different physical changes applied to mixtures that allow to obtain one or more pure substances.
Depending on the properties of the substances to be separated, different methods are used.
The following is a list of the main methods used in the chemistry lab and industry.
| Separation Method | What to Separate |
|---|---|
|
FILTRATION |
A solid from a Liquid |
|
CRYSTALLIZATION |
A solute from its solution |
|
EVAPORATION |
A solute from its solution |
|
SIMPLE DISTILLATION |
A solvent from its solution |
|
FRACTIONAL DISTILLATION |
Two miscible liquids with similar boiling points |
|
PAPER CHROMATOGRAPHY |
Different colorful substances from a solution |
Using a separating funnel
Separation of two immiscible liquids. We use a separating funnel to separate them. One of the liquids has more density and different IMF.
Filtration
Separation of insoluble/dispersed solid particles from a fluid (gas or liquid) by the use of a filter medium that permits the fluid to pass through but retains the solid particles.
When separating a mixture, you need to know the properties of each substance present in the mixture. If one of them is soluble in water and the other is not, you can use FILTERING. The substance left in the filter paper is the RESIDUE and the liquid that passes the filter is the FILTRATE
What do we want to isolate?
After the filtration process, the substance that we want to purify will determine the steps.
If we need only the substance in the filtrate, we will evaporate the filtrate to obtain the crystals of the solute.
We can do it through EVAPORATION and then CRYSTALLIZATION
EVAPORATION
CRYSTALLIZATION
If instead, we need to purify the substance in the residue (in the filter paper) we would need to wash it with distilled water to get rid of the soluble particles and then dry
Distillation
- The process of separating the components of soluble solid/liquid mixture by selective evaporation and condensation.
- This is possible since the components have different boiling points. (more than 25 degrees in difference at least)
- Types: Simple Distillation & Fractional Distillation
Simple Distillation
This method can only be used to separate mixtures where the components differ widely in boiling point (by approx. 25°C).
Example: Acetone- Water Mixture
When acetone-water mixture is heated to the boiling point of acetone; acetone boils , vaporizes , condenses and separates.
Fractional Distillation
This special distillation method is used to separate mixtures where the components have only slight difference in boiling point
Example: Ethanol – Water Mixture
Fractionating Column:
- Establishes a temperature gradient
- Allows repeated condensation and evaporation
- Enriches the vapour with only one component
If we have a mixture of alcohol and water, we can use fractional distillation to separate them.
- Alcohol will distill first since the Boiling Point is around 78°C.
- The temperature in the column will stay at 78°C while alcohol is distilling.
- Water will not distill since the column will be at a lower temperature than 100°C and it will fall down back to the round bottom flask
- After all the alcohol has passed through, the temperature of the thermometer will rise again until it reaches 100°C.
Fractional Distillation in Industry
CRUDE OIL:
Fractional distillation is largely used in to separate the components of crude oil which are compounds made of only carbon and hydrogen (hydrocarbons). The bigger and more complex the molecules, the higher the melting and boiling point.
ETHANOL:
Alcohol is produced using fermentation of sugary plants. The fermentation takes place in vessels where water and plants are mixed and let to “ferment”. This process produces alcohol which has to be distilled to separate it from the mixture.
AIR:
Air is cooled until liquid and then separated by warming it up slowly to separate the gases with different boiling points.
Crude Oil
Ethanol
ETHANOL: Ethanol is made in two ways, one biological and one chemical. We will cover the biological only for now.
By fermentation – the biological way : Ethanol is made from glucose using yeast, in the absence of air
Yeast is a mass of living cells. The enzymes in it catalyse the reaction.
The process is called fermentation, and it is exothermic.
Ethanol can be made in this way from any substance that contains sugar, starch, or cellulose. (These break down to glucose.) For example it can be made from sugarcane, maize, potatoes, and wood.
The yeast stops working when the % of ethanol reaches a certain level, or if the mixture gets too warm.
Theethanol is separated from the final mixture by fractional distillation.
Chromatography
Chromatography: is used to separate a mixture of substances with different solubility. One of the most used by us is paper chromatography which uses pigments (colors) to recognize pure substances. It can be performed in different ways.
#1 placing a drop of the mixture on a paper and then add a solvent which can push the soluble substances as the water runs on the paper through capillarity.
Facts about Chromatography
Chromatography: In the lab we will perform the paper chromatography as follows.
The paper is called the Static Phase
The solvent (water in this case)is called the Moving Phase
The solvent, is also called Eluent or Elution. Eluent front is the maximum level the solvent reaches
Substances that run the same distance correspond to the same compound.
In the example, we can say that the sample contains substances w, x and z but it does not contain y
Retardation Factor (Rf)
Retardation factor Is the ratio between the distance covered by one pure substance and the distance covered by the solvent
Retardation factor is an intensive property. We can use it to identify substances.
Rf red = dr/dw = around 0.5 (no units)
Rf yellow = dy/dw= around 0.9
Rf blue = db/dw = around 0.3
Yellow is most soluble blue is the least soluble
If the pigment travels with the water front, the Rf value will be equals to 1, because both distances are the same.
If the pigment does not move at all, the pigment distance will be zero, so the Rf value will be zero.
Those are the maximum and minimum values posibles of Rf.
Rf does not have any unit. It is only a number.
Some pigments are "colorless" - Separating Aminoacids
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Paper chromatography is also used to separate aminoacids, which are colorless.
After the separation, Ninhydrin, a Locating Agent must be applied to be able to “see” the distance covered by each one.
| AMINO ACID | Rf value |
|---|---|
|
CYSTEINE |
0.08 |
|
LYSINE |
0.14 |
|
GLYCINE |
026 |
|
SERINE |
0.27 |
|
ALANINE |
0.38 |
|
PROLINE |
0.43 |
|
VALINE |
0.60 |
|
LEUCINE |
0.73 |