Locating Agents & Rf Values (Cambridge O Level Chemistry)

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Alexandra Brennan

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Locating Agents

  • For chromatography to be useful the chemist needs to be able to see the components move up the paper, which is not the case for colourless substances such as amino acids or sugars
  • Locating agents are substances which react with the sample and produce a coloured product which is then visible
  • The chromatogram is treated with the agent after the chromatography run has been carried out, making the sample runs visible to the naked eye

Retention Factor (Rf) Values

  • These values are used to identify the components of mixtures
  • The Rf value of a particular compound is always the same
  • Calculating the Rf value allows chemists to identify unknown substances because it can be compared with Rf values of known substances under the same conditions

Calculation

  • The formula to calculate the retention factor is:


                       Retention space factor space equals space fraction numerator Distance space travelled space by space substance over denominator Distance space travelled space by space solvent end fraction

  • The Rf value is a ratio and therefore has no units

Using Rf values to identify mixture components, IGCSE & GCSE Chemistry revision notesUsing Rf values to identify components of a mixture

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Alexandra Brennan

Author: Alexandra Brennan

Expertise: Chemistry

Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.