Separation Techniques

What is a separation technique?

A separation technique is where you use a method to separate a substance from another substance, you can use chromotography, distillation, filtering ect.

Solid from liquid

There are four methods to separate a solid from a liquid. Decanting is used to separate a dense, insoluble solid from a liquid. Filtration is used to separate a mixture from a solid and a liquid. Substances that do not decompose on strong heating can be purified by evaporation to dryness. Crystallisation is used to purify crystals and substances that decompose on strong heating. A saturated solution can be produced by evaporation.

Filtration method can also be used to separate two solids but only if one of them is soluble in a solvent. A magnet can be used to separate magnetic substances, such as iron, nickel, cobalt and steel, from non- magnetic ones.

Filtration

Filtration is commonly the mechanical or physical operation which is used for the separation of solids from fluids (liquids or gases) by interposing a filter through which only the fluid can pass. Upon filtration, the solid that remains on the filter paper is called the residue, while the liquid that passes through the filter paper is called the filtrate.

How to set up a filtration (experiment)?

1. Prepare a filter cone:

2. Gravity filtration with a filter cone:

The following video will show the filtration experiment as seen in the above procedures:

Distillation

Simple distillation is used to obtain a pure solvent from a solution. 

The following video shows and explains about simple distillation:

Fractional distillation is used to separate a mixture of miscible liquids. The liquid with the lowest boiling point distils over first.  

The following video shows fractional distillation:

Crystallisation

Crystallisation is a separation technique that is used to separate a solid that has dissolved in a liquid and made a solution. The solution is warmed in an open container, allowing the solvent to evaporate, leaving a saturated solution. A solution that has much solid dissolved in it as it can possibly contain is called a saturated solution. As the saturated solution is allowed to cool, the solid will come out of the solution and crystals will start to grow. The crystals can then be collected and allowed to dry. 

-       Making a substance pure

-       Solution should be saturated

-       Change the state of substance from aqueous (aq) to solid.

Chromatography

Chromatography is used to separate or analyse the components in a sample. Chromatography is also used to determine the purity of a sample. A pure sample gives only one spot on a chromatogram. Rf value is the distance travelled by the substance divided by the distance travelled by the solvent. A locating agent is used on a chromatogram to help us see the colourless substances.

Chromatography: "A method, often used in laboratories, which enables the easy and efficient separation of mixtures of chemical compounds using the phenomenon of adsorption." The word comes from the Greek chromatos (color) and graphein (to write).  So, as one might guess, chromatography involves separating chemicals and identifying them by color.

The process relies on the fact that different molecules will behave in different ways when they are dissolved in a solvent and moved across an absorbent medium. In a very simple example, one could take ink and make a mark on a piece of paper. The paper could be dipped into water, and the capillary action of the water would pull the ink through the paper. As the ink moved, its ingredients would separate out, revealing a distinctive pattern which could be used to determine the components of the ink.

The following video shows the procedure in paper chromatography and how to find the Rf values:

Extra:

In 1910, Tswett first used this method in the separation of leaf pigments, and the technique achieved rapid growth later when it was applied to the separation of carotinoid pigments, among other uses.

The separation method called 'column chromatography' uses a glass column filled with adsorbent though which passes a composite liquid mixture. The technique operates such that each component will form in a different section of the column arranged by color according to the adsorption affinity of each material.

In performing column chromatography, a vertical glass column is filled evenly with the proper amount of adsorbent. Next, a liquid mixture is poured into the column and the liquid passes through the adsorbent. Each compound is absorbed into beds at different heights depending on the component's individual adsorption affinity.

At this stage, the sections into which each of the components have been absorbed are still not completely separated. However, if the appropriate desorbent is poured into the column, the components which have been adsorbed on the adsorbent dissolve into the desorbent and start moving downwards in the column. Each component moves towards the lower part of the column, but the migration rates again differ according to each component's adsorption affinity. The components in the lower layers move faster, and in the end, each components will be clearly separated. This stage of the column chromatography process is called 'development.'

If the material being separated is a mixture of pigments, there will be colored zones at different heights in the column filled with adsorbent. These colored zones are called a chromatogram.

When the development stage is over and the pigments appear, the adsorbent is pushed out of the column, divided into each zone and the absorbates are extracted separately using a desorbent. This stage of the process is called 'elution.' Alternately, without removing the adsorbent from the column, a desorbent may be successively poured in from above, and each zone preferentially eluted by the desorbent will dissolve into it and trickle down the column one at a time. The liquids can then be collected from the bottom of the column as they drip out."

-       To separate a mixture

-       Find out the number of components

-       Depends on solubility of compounds/ substances in a solvent

-       When the solvent comes into contact with the ink, the compounds in the ink can “choose” to stay at its place or get absorbed and follow the water up the chromatography paper. Thus, the higher the solubility of the compounds, the lesser the distance travelled.