KJM5911 Lab Exercise 4 - Szilard-Chalmers Reaction and Liquid-Scintillation Counting

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Learning Goals

After this exercise, you should know about and understand the following topics:

  • The difference between radioactivity with or without carrier.
  • Szilard-Chalmers method to produce carrier-free radioactive halides.


Experimental Procedure

You will get n-activated ethyliodide from your supervisor. The 128I half-life is only 25 min, so make sure you are well prepared and that you know exatly what you should do. Practice the complete procedure with "cold" reagents before starting to work with the radioactive material!

  1. Use an automatic pipette to transfer 1 mL of the n-irradiated ethyliodide to a counting vial.
  2. Transfer 2 mL ethyliodide to a test tube containing 2 mL NaHSO3 solution. The test tube must have a cap and be able to withstand centrifugation.
  3. Shake the test tube violently for 60 s.
  4. Transfer 1 mL of each phase to counting vials.
  5. Count the three counting vials on a NaI well-crystal. Count long enough to ensure that the counting uncertainty is less than 5%.
  6. Calculate the count rate (in cps or cpm) and correct for background.
  7. Correct for decay and calculate: A) the retention (amount in the organic phase) and B) the yield of the carrier free radioactivity in the inorganic phase.

Repeat the experiment, but this time add a little inorganic iodine to the NaHSO3 solution before the extraction. Does it change the yield? Of course, now the inorganic phase will not be carrier free anymore.


For this exercise you should write a report with a two-page limit explaining about what you did and your results.

Safety Aspects

  • Only the supervisor is allowed to fetch irradiated ethyliodine, no one else is authorized to handle the n source. The radiation field close to the source is rather intense and all handling is done with remote manipulation-tools. If you company your supervisor to watch, ensure that you stand at least one metre away from the source.
  • The amount of radioactivity after the n irradiation is barely measurable, so normal C-type lab rules apply. The short half-life makes it easy to handle waste and possible contaminations - it will be gone by tomorrow.
  • Normal chem. lab safety rules should of course be followed, but there are no particular dangers.
  • Make sure to ballance the centrifuge properly, and remember that the density of the ethyliodide solution is much higher than the one for water!