Determination of Uranium Concentration and the 234-U 238-U Isotopic Ratio in a Solution

Lab Exercise - Determination of Uranium Concentration and the 234U 238U Isotopic Ratio in a Solution

Developed By

Laboratory of Radiochemistry
Department of Chemistry
University of Helsinki
Finland

Learning Goals

Here the learning goals and purpose of the exercise should be presented in a reasonable way.

Explanation and Exercise Guide

Theory

Isotopic Ratio of Natural Uranium (read)

Experimental Procedure

The purpose of the work is to determine the activities of 238U and 234U in a water sample and calculate the 234U/238U activity ratio. The sample solution has been obtained by dissolution of uranophane mineral (Ca[(UO2)SiO3(OH)]2×6H2O]) originating from Zaire. First, the 233U tracer is added to an aliquot of the stock solution and the solution is evaporated to dryness. After dissolution into 9 M HCl interfering radionuclides are separated by anion exchange chromatography. Finally, a uranium alpha counting source is prepared by CeF3 coprecipitation and the uranium alpha spectrum is measured by alpha spectrometry.

Performance of the work

  1. Take 2 mL of the uranophane solution into a beaker and add 4.5 dpm of 232U tracer.
  2. Evaporate to dryness
  3. Prepare the anion exchange column (instructions below)
  4. Dissolve the evaporation residue in 3 ml of 9 M HCl
  5. Pour the solution into the ion exchange column. Uranium is retained, Th and Ra are eluted, Po is retained here but will be separated from uranium in step 7
  6. Rinse the column with 30 mL of 9 M HCl
  7. Elute uranium with 20 mL of 0.1 M HCl (Po is still retained in the ion exchanger)
  8. Evaporate the solution into dryness
  9. Dissolve the residue in 2.5 mL of 1 M HCl by warming on a hot plate
  10. Transfer the solution with a disposable pipette into a Daniel tube
  11. Rinse the beaker with 2 mL of 1 M HCl and add to Daniel tube
  12. Add 200 μL of Ce carrier (Ce3+ concentration 100 μg/mL, prepared by dissolving Ce2O3 or CeCl3×7H2O in 1 M HCl)
  13. Add four drops of 15 % TiCl3 solution to reduce all uranium to +IV state
  14. Add 0.5 mL of concentrated (40%) HF to precipitate Ce(U)F3. Close the tube, shake gently and let it stay for one hour in refrigerator.
  15. Wet the 0.1 μm filter with an ethanol/water mixture and place the filter into a syringe filter.
  16. Filter 5 mL of deionized water through the filter and check that the filter system does not leak.
  17. Filter 5 mL of Ce-substrate solution (prepared from Ce-stock solution containing 860 μg CeCl3×7H2O in1 mL of 1 M HCl by pipetting 10 mL stock solution to 908 mL of 1 M HCl + 82 mL 40% HF) through the filter.
  18. Filter the CeF3 precipitate on the filter.
  19. Rinse the Daniel tube with 5 mL of deionized water and filter the solution through the syringe filter.
  20. Let the filter dry and glue it on plastic holder
  21. Determine the alpha spectrum from the source

Preparation of the ion exchange column

  1. Test the column with deionized water to check possible leakages
  2. Fill the column with Dowex 1×4 anion exchange resin (stored in MilliQ water) with the help of deionized water and check that there are no air bubbles in the column
  3. Add three bed volumes of deionized water
  4. Pretreat the column with three bed volumes of 9 M HCl

Questions for the Students

Reporting

Describe in detail the whole separation procedure and explain why each step was made. Draw a separation procedure scheme.

Draw the alpha spectrum and identify all uranium peaks.

Calculate the chemical yield of U separation.

Calculate the activity concentrations of 234U and 238U and their uncertainties. Calculate the 234U/238U ratio and its uncertainty.

Safety Aspects

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Equipment

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Preparation for the lab Supervisor

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