Analía Bellizzi – Chemistry Classes

Ronald Reagan Senior High School

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

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