<Li> If R value = 1 then the solute has no affinity for the stationary phase and travels with the solvent front . </Li> <P> For example, if a compound travels 9.9 cm and the solvent front travels 12.7 cm, the R value = (9.9 / 12.7) = 0.779 or 0.78 . R value depends on temperature and the solvent used in experiment, so several solvents offer several R values for the same mixture of compound . A solvent in chromatography is the liquid the paper is placed in, and the solute is the ink which is being separated . </P> <P> Paper chromatography is one method for testing the purity of compounds and identifying substances . Paper chromatography is a useful technique because it is relatively quick and requires only small quantities of material . Separations in paper chromatography involve the same principles as those in thin layer chromatography, as it is a type of thin layer chromatography . In paper chromatography, substances are distributed between a stationary phase and a mobile phase . The stationary phase is the water trapped between the cellulose fibers of the paper . The mobile phase is a developing solution that travels up the stationary phase, carrying the samples with it . Components of the sample will separate readily according to how strongly they adsorb onto the stationary phase versus how readily they dissolve in the mobile phase . </P> <P> When a colored chemical sample is placed on a filter paper, the colors separate from the sample by placing one end of the paper in a solvent . The solvent diffuses up the paper, dissolving the various molecules in the sample according to the polarities of the molecules and the solvent . If the sample contains more than one color, that means it must have more than one kind of molecule . Because of the different chemical structures of each kind of molecule, the chances are very high that each molecule will have at least a slightly different polarity, giving each molecule a different solubility in the solvent . The unequal solubility causes the various color molecules to leave solution at different places as the solvent continues to move up the paper . The more soluble a molecule is, the higher it will migrate up the paper . If a chemical is very non-polar it will not dissolve at all in a very polar solvent . This is the same for a very polar chemical and a very non-polar solvent . </P>

Why is whatman filter paper used in chromatography