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What Are The Common Techniques Used For Detecting Colourless Spots?

Answer»

It is easy to distinguish coloured spots visually but for colourless COMPOUNDS alternate techniques need to be adopted which can be specific or non-specific.

  • A common non-specific method is suspension of developed chromatogram in iodine vapour. Most organic compounds show up as BROWN spots.
  • The sheet is viewed in a UV VIEWING cabinet under 366 nm and 254 nm wavelength lamp ILLUMINATION. On observation the spots need to be carefully marked with a pencil for Rf calculations.
  • Under specific methods amines and amino acids are observed by spraying heated paper on development with 0.2% hydrazine. Deep blue or purple spots begin to appear.
  • Alkaloids – Dragendroff’s reagent spray results in orange or orange yellow spots.
  • Aldehydes&Ketones – 2,4-DNPH spray in methanol and sulphuric acid results in orange or yellow spots.

It is easy to distinguish coloured spots visually but for colourless compounds alternate techniques need to be adopted which can be specific or non-specific.


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