METHODS AND MANIPULATION 51 however, the extent to which a particular solute has moved, relative to the extent to which the solvent has moved, known as the R F of the solute, will be a function of the partition coefficient of the solute between the solvent and the film of retained water. When the ascending solvent-front has reached a convenient height, the strip is removed, the position of the solvent-front marked, and the paper strip dried. The positions of the various solutes, if they are coloured compounds, now appear as clear separate spots. Frequently however, the solutes are colourless, and the position of their spots must be determined by indirect methods, such as their fluorescence in ultraviolet light, or their absorption in such light (when the spots appear almost black), or by spraying the paper with a dilute solution of a reagent which will give a coloured insoluble derivative with the solutes. The first of the two experiments given below illustrates the separation of
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