Laboratory Exercise - procedure to Isotopic Exchange Reactons

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Written and developed by Prof. Tor Bjørnstad (IFE/UiO) 

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General

In this exercise a solution of NaI containing 131I is mixed with RI-solutions (ethyl iodide and butyl iodide) of different concentrations. The exchange from NaI to RI is monitored using a solvent extraction method. The NaI and RI are contacted in acetone as both compounds are soluble in this solvent.

At various intervals a sample of acetone mixture is added to the test tube containing toluene and water in order to separate the two compounds by extracting the RI into toluene and the NaI into water. The rate constant is obtained from the rate of 131I exchange for various RI-concentrations.

NB! Although the amounts of radioactive material in this experiment are small, the ALARA rule applies and the radioactive solutions should be treated as hazardous. Usual safety precautions such as the use of protective equipment (gloves, lab coat, eye protection) are therefore mandatory. Since organic iodides are volatile, all treatment of radioactive solutions shall be carried out in a fume hood. Keep the stopper on the reaction flask whenever possible.

Detailed Procedure
  1. Fill a large beaker with crushed ice.
  2. To 15 test tubes add 2 ml of aqueous 0.08 M NaI and 2 mL toluene. Cork them and place them in the ice.
  3. Obtain two 25 mL Erlenmeyer flasks. Pipette into each one 1 ml of 0.08 M NaI in acetone and have the laboratory assistant add 1 mL of very dilute Na131I solution in acetone. Place the flasks in an ice bath.
  4. Obtain and test a stop watch. When everything is ready proceed to the next step.
  5. To one flask add 1 mL of 0.15 M ethyl iodide (RI) solution in acetone. As soon as the RI is added start the stop watch (t = 0). At this moment the exchange reactions begins. After approximately 7 minutes record the time and add 1 mL of a 0.45 M RI-solution in acetone to the next flask.
  6. At regular time intervals after adding the RI (7-10 min) record the time and remove a 0.1 mL sample from a flask using a micropipette. Quickly transfer the sample into one of the test tubes containing aqueous NaI and toluene. To avoid heating the solution during sample-taking, hold the flask by the neck rather than by the base.
  7. Shake the test tube vigorously for 60 s, and centrifuge the mixture for 30 s at maximum speed. Remember to balance the centrifuge by another test tube filled with pure water to the same weight as the tube in the experiment.
  8. Remove the test tube from the centrifuge and transfer 1.5 mL with a pipette of the organic phase (upper layer) to a counting vial. At the outset the radioactivity in the organic phase is far below that in the aqueous phase. Therefore, take care not to contaminate the organic phase with drops of water.
  9. Count the samples with the gamma spectrometer (NaI or HpGe).
Reporting Schemes and Results
Table 1 Background counting
Number of counts Sb                                                                      
Counting time (min)
Counting rate Rb (cpm)
Table 2 Counting series No. 1
Time Number of counts S Corr. counting rate Rn=RT-Rf (CPM) Uncertainty σ
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Number of counts Sr of rest activity Counting time (min) Counting rate Rf (cpm) uncertanity σ
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Table 2 Counting series No. 2
Time Number of counts S Corr. counting rate Rn=RT-Rf (CPM) Uncertainty σ
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Number of counts Sr of rest activity Counting time (min) Counting rate Rf (cpm) uncertanity σ
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  • Plot the corrected counting rate as a function of time for the two series
  • Determine the reaction rate r from the slopes of the plots using eqn. 13.
  • Plot r as a function of concentration of RI (two points) and determine k from the slope using eqn. 15. in Theoretical Bakground to Isotopic Exchange Reactons