paper chromatography

• chromatography has...


  two stages
• chromatography


  an analytical technique that separates components in a mixture between a mobile phase and a stationary phase
• mobile phase


  where the molecules can move - this is always a liquid or a gas
• stationary phase


  where the molecules can't move - this can be a solid or a really thick liquid
• chromatography process


  - during the chromatography experiment, the substances in the sample constantly move between the mobile and the stationary phases: an equilibrium is formed between the two phases

  - the mobile phase moves through the stationary phase, and anything dissolved in the mobile phase moves with it

  - how quickly a chemical moves depends on how it's 'distributed' between the two phases: whether it spends more time in the mobile phase or the stationary phase

  - the chemicals that spend more time in the mobile phase than the stationary phase will move further through the stationary phase

  - the components in a mixture will normally separate through the stationary phase, so long as all the components spend different amounts of time in the mobile phase

  - the separated components form spots: the number of spots formed may change in different solvents as the distribution of the chemical will change depending on the solvent

  - a pure substance will only ever form one spot in any solvent, since there is only one substance in the sample
• paper chromatography process


  1. start by drawing a baseline on the chromatography paper using a pencil. the baseline is insoluble, so it won't move with the solvent

  2. place a small spot of the sample mixture on the baseline

  3. pour the solvent into a beaker. place a glass rod over the beaker, and tape or clip the paper to it, ensuring that the paper's base just touches the solvent without being submerged

  4. allow the solvent to slowly travel upwards through the paper, taking with it a few soluble pigments from the sample mixture. this will create different spots along the paper

  5. remove the paper from the beaker before the solvent reaches the top.

  you now have a chromatogram

  6. analyse the chromatogram to identify the different components of the sample mixture. each spot on the chromatogram represents a different component of the mixture.

  7. finally, mark the distance travelled by the solvent (and calculate the Rf value)
• what is the stationary phase in paper chromatography?

  chromatography paper, which is often filter paper
• what is the mobile phase in paper chromatography?

  the solvent, which is often water or ethanol
• the amount of time the molecules spend in each phase depends on...

  - how soluble they are in the solvent

  - how attracted they are to the paper
• solute


  a substance that is dissolved in a solvent
• solvent


  the substance in which the solute dissolves
• solution


  a homogeneous mixture consisting of a solute dissolved into a solvent
• molecules with higher solubility in the solvent and which are less attracted to the paper will...

  spend more time in the mobile phase and be carried further up the paper
• the result of chromatography analysis is called a...

  chromatogram
• chromatogram


  a visible record showing the result of separation of the components of a mixture by chromatography
• Rf value


  the ratio between the distance travelled by the dissolved substance (the solute) and the distance travelled by the solvent
• Rf value formula


  distance travelled by substance / distance travelled by solvent
• the further through the stationary phase a substance moves, the ... the Rf value

  larger
• how is chromatography carried out to see if a certain substance is present in a mixture?

  - run a pure sample of that substance (a reference) alongside the unknown mixture

  - if the Rf values of the reference and one of the spots in the mixture match, the substance may be present (although you haven't yet proven they're the same)
• the Rf value is affected by the...

  solvent - if you change the solvent, the Rf value for the substance will change
• by testing the mixture and the reference in a number of solvents...

  you can determine whether or not the reference is in the mixture - if the Rf value of the reference matched the Rf value of one of the spots in the mixture in all of the solvents, then it's likely the reference compound is present in the mixture