Paper chromatography
Chromatography can be used to separate mixtures of coloured A substance formed by the chemical union of two or more elements.. Mixtures that are suitable for separation by chromatography include inks, dyes and colouring agents in food.
Simple chromatography is carried out on paper. A spot of the mixture is placed on a pencil line near the bottom of a piece of chromatography paper 鈥 the line must be in pencil because pencil is insoluble in water and so will not move as the chromatography progresses. The paper is then placed upright in a suitable The liquid in which the solute dissolves to form a solution., such as water.
As the solvent soaks up the paper, it carries the mixtures with it. Different components of the mixture will move at different rates. This separates the mixture out.
Image caption, 1. Spots of ink or plant dye are placed on a pencil line
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Rf values
Different The result of separating mixtures by chromatography. and the separated components of the mixtures can be identified by calculating the Also called 鈥榬etention factor鈥 and abbreviated to Rf. In chromatography, it is the distance moved by a substance divided by the distance moved by the solvent. (Rf). The Rf value is worked out by using this equation:
The Rf value is a measure of how soluble a particular substance is in a given solvent. The further the substance moves, the larger the value of Rf for that substance, and so the more soluble it is in the solvent that you are using.
The Rf value of a particular compound is always the same if the chromatography has been carried out in the same way. This allows industry to use chromatography to identify compounds in mixtures.
Chromatography can also be done when the different substances in the mixture are colourless. The chromatogram can be exposed to a A substance used to detect another substance by making it visible., which reacts with the invisible chemicals so that they can be seen.