Dear year 11 Students,
The following are the main points from Unit -1. This may help you revise the unit while preparing for your final examination. Please read and research more about each point using your text book or web. Please also practice answering the past IGCSE exam papers.
Experimental Techniques – Main Points
A locating agent is used when the method is used for separation of colourless substances. (example- in the separation of amino acids, ninhydrine is used as a locating agent.)
Rf value( retention factor)- It is used to identify the components separated by chromatography. Rf = Distance moved by the solute front from the base line/ Distance moved by the solvent front from the base line.
Notes from chemguide: Chromatography is used to separate mixtures of substances into their components. All forms of chromatography work on the same principle.
They all have a stationary phase (a solid, or a liquid supported on a solid) and a mobile phase (a liquid or a gas). The mobile phase flows through the stationary phase and carries the components of the mixture with it. Different components travel at different rates. We'll look at the reasons for this further down the page.
In paper chromatography, the stationary phase is a very uniform absorbent paper. The mobile phase is a suitable liquid solvent or mixture of solvents. Rf values
Some compounds in a mixture travel almost as far as the solvent does; some stay much closer to the base line. The distance travelled relative to the solvent is a constant for a particular compound as long as you keep everything else constant - the type of paper and the exact composition of the solvent, for example.
The distance travelled relative to the solvent is called the Rf value. For each compound it can be worked out using the formula:
If a sample gives just one spot in the chromatogram that means the sample contains only one substance or it is pure.