Interpreting Chromatograms (Edexcel IGCSE Chemistry (Modular))

Revision Note

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Interpreting chromatograms

  • We can use a chromatogram to compare the substances present in a mixture to known substances and make assumptions

    • Pure substances will produce only one spot on the chromatogram

    • Impure substances will produce more than one spot on the chromatogram

    • If two or more substances are the same, they will produce identical chromatograms

    • If the substance is a mixture, it will separate on the paper to show all the different components as separate spots

  • It is common practice to include a known compound as a reference spot

    • This can help match up to an unknown spot or set of spots in order to identify it

Interpreting chromatograms

Chromatogram showing the separation of brown ink

Diagram showing the analysis of a mixture and pure substances using chromatography

  • We can draw several conclusions from this chromatogram:

    • The brown ink is a mixture as there are three dots

    • Red, yellow and blue are pure as there is only one dot for each 

    • The brown ink contains red, blue and yellow as the dots are in line with one another horizontally

Examiner Tip

Chromatograms in exams will be in black and white so to identify whether a mixture contains a known sample, the dots need to be in line with one another. 

Rf Values

  • Rf values are used to identify the components of mixtures

  • The Rf value of a particular compound is always the same

    • However, it does depend on the solvent used

    • If the solvent is changed then the Rf value changes

  • Calculating the Rvalue allows chemists to identify unknown substances because it can be compared with the Rvalues of known substances under the same conditions

  • The retention factor, Rf, is calculated by the equation:

bold italic R subscript bold f bold space bold equals bold space fraction numerator bold distance bold space bold moved bold space bold by bold space bold substance over denominator bold distance bold space bold moved bold space bold by bold space bold solvent end fraction

  • The Rf value:

    • Is a ratio

    • Has no units

    • Will always be less than 

Worked Example

A student obtained the following chromatogram when carrying out chromatography. 

using-rf-values-to-identify-mixture-components

Calculate the Rf value of the substance. 

Answer:

  • The Rf value of the substances in the chromatogram above can be calculated by:

    • Rf = fraction numerator distance space moved space by space substance over denominator distance space moved space by space solvent end fraction = 3 over 6 = 0.5

Examiner Tip

When you calculate Rvalues in exams, make sure to use your ruler carefully to measure the distance moved by the solvent and the substance as mark schemes can be strict about the values accepted for these. 

For both measurements, the distance should be measured from the baseline to the centre of the dot.

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