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Physical Property: Solubility
Chromatography is used to separate a mixture of liquid dyes that have different solubilities in a given solvent
Setup of Chromatography
Any part of the set-up of chromatography needs to ensure that only the dyes are soluble in water, and that no other factors can interfere with the chromatogram or affect the experiment results
A. Start Line should be drawn in pencil, not ink
Purpose: Any ink components will move up the chromatography paper and interfere with the chromatogram. Pencil lead is insoluble in solvent
Link to solubility: Pencil lead is made up of graphite, which is insoluble in all solvents. Therefore, this serves as a good material for a startline as it will not travel up the chromatography paper, but allow a marker for the ink spot (of the mixture being separated) to be placed.
B. Start Line must be above the solvent level
Purpose: This prevents the sample dot from dissolving in the solvent instead of moving up the chromatogram
Link to solubility: The ink sample is soluble in solvent and will dissolve in any given volume of solvent. Chromatography allows the solvent to "creep" (move slowly) up the chromatography paper, therefore providing a small volume of water to allow the dyes in the ink to dissolve and move up the chromatogram at different rates according to their solubilities.
C. Beaker containing chromatogram must be covered with a lid
Purpose: This prevents the evaporation of solvent in the beaker
Link to solubility: Some solvents, like ethanol, are volatile and very easily evaporated away at room temperature. This will cause the volume of solvent to decrease, and could possibly affect the results as solubility of the ink dyes is low with a low volume of solvent.
Interpretation of Chromatogram
The various ways to interpret a chromatogram can also be linked to the solubility
Solubility of individual components: Dyes that are the most soluble will travel the furthest up the chromatogram
Composition of ink sample: Dyes that have similar solubilities will travel the same distance up the chromatogram, and can be identified by matching distances.
Purity: If the sample mixture contains more than one component with different solubilities, the components will be separated based on their relative solubilities with the solvent and more than one spot will appear.
Issues with Chromatography and How to Solve Them using Solubility
When using a particular solvent (e.g., water) to run chromatography, some issues in the results can occur:
All the components are insoluble, and the sample spot remains on the start line
Some components in the mixture have similar solubilities in the given solvent, and will therefore travel the same distance up the chromatography paper. This causes the components to overlap and not be easily identified.
How to solve these two problems?
The solution is to use a different solvent (e.g., ethanol). The solubility of components in different solvents are different, therefore, using a different solvent can help to generate results.
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