Chromatography

(GCSE Chemistry)

Chromatography is a separation technique used to identify different solutes dissolved within a particular solvent. We use this in the analysis of substances to find out what a mixture (often ink) consists of. There are a number of required steps to ensure this practical is as successful as possible;

1. Firstly, you must draw a pencil line across the bottom of the chromatography paper so that all of the solvents start at the same line. Pen should not be used because the ink from the pen is a mixture itself so would spread up the paper and not be effective. 
2. There are two phases to chromatography; the stationary phase is the paper that does not move and remains stationary. The mobile phase is the solvents that move up the paper and are mobile (move).
3. We often use a set of pure inks (that we know are pure) to identify what's in an impure substance. Place drops of the inks across the pencil line and submerge the bottom of the paper in a solvent from a beaker (but the ink dots should not touch the solvent)
4. The solvent will travel up the paper and carry the inks with it, depending on how soluble it is. The more soluble a substance is, the further up the paper it will travel. 
5. We can use the dye marks on the paper from the pure substances to compare with an unknown substance to indicate what it is made up from. Remember this is only an indicator and not factual evidence.

(Adrien Ledoux)

We can calculate the Rf values of a substance by using the distance travelled by the solvent and the distance travelled by the ink spot. The calculation is;

Rf value= distance travelled by substance/distance travelled by the solvent.

The closer to 1 the Rf is, the more attracted to the mobile phase (solvent) the substance is and we can say the more soluble a substance is.