(a) Criteria of purity
Paper chromatography: (To separate substances) a drop of the substance is placed at the centre of a piece of filter paper and allowed to dry. Three or four more drops are added to it. Water is
dripped on, drip by drip, so the ink spreads creating different coloured circles. Paper + rings = chromatogram. Rings are created because different substances travel at different rates. (To identify substances) Spots of substances placed onto a pencilled line (as ink would separate) which is called the origin, and labelled. Paper goes in solvent, and solvent travels up paper, then paper is taken out. There are spots which have travelled different distances.
–Interpreting simple chromatograms:
- Number of rings/dots = number of substances
- If two dots travel the same distance up the paper they are the same
- You can calculate the Rf value to identify a substance, given by the formula:
Rf value = distance moved by substance / distance moved by solvent
To make colourless substances visible you use a locating agent: 1. Dry paper in oven 2. Spray it with locating agent 3. Heat it for 10 minutes in oven.
The stationary phase is the material on which the separation takes place (e.g. the paper). The mobile phase consists of the mixture you want to separate, dissolved in a solvent.
Measuring Purity:
Pure substances have a definite, sharp meting/boiling point; a substance + impurity has lower melting point and higher boiling point, at a range of temperatures; more impurity means bigger change. This is why salt is used on roads to prevent the formation of ice or to melt ice.
Purity is important in drugs and foodstuffs, they cannot contain harmful substances.