Lab Exercise - Determination of ⁵⁵Fe in Nuclear Waste

Developed By

Laboratory of Radiochemistry
Department of Chemistry
University of Helsinki
Finland

Learning Goals

• Solvent-extraction
• Liquid Scintillation Counting with electron capture
• Using Quench Correction

Explanation and Exercise Guide

Theory

Production and Creation of ⁵⁵Fe (read)
Theory behind Extraction of ⁵⁵Fe (read)

Experimental Procedure

Separation of ⁵⁵Fe

1. Elute 2 g of ion exchange resin with 8 mL of 8 M HNO₃ overnight. Separate the resin by filtration.
2. Take 1 mL of the solution into a centrifuge tube and add 10 mg Fe-carrier (FeCl₃) and dilute with water to total volume of 10 mL. Take a 100 µL sample for AAS/ICP-MS/ICP-OES measurement.
3. Add drop by drop concentrated ammonia until Fe(OH)₃ precipitates (pH 8-9 ). Centrifuge the sample for a couple of minutes and discharge the supernatant.
4. Dissolve the precipitate in 8 M HCl and transfer the solution into 100 mL extraction funnel. Extract the sample two times with 25 mL di-isopropyl ether. Discharge the aqueous phase.
5. Combine the ether fractions and back-extract the iron into aqueous phase by extracting it twice with 15 mL of distilled water. Transfer the water solution into a centrifuge tube and discharge the ether fraction.
6. Add ammonia to precipitate the iron (pH 8-9). Use a warm water bath to facilitate the precipitation. Centrifuge and discharge the supernatant. Wash the precipitate with small amount of water, centrifuge and discharge the rinse water.
7. Dissolve the precipitate in 5 mL 0.2 M HCl in a warm water bath.
8. Pour the solution into an ion exchange column (Dowex 50 w x 4; 50/100, pretreated with 20 mL of 0.2 M HCl). Wash the column with 10-20 mL of 0.2 M HCl.
9. Elute the iron from the column with 40 mL of 0.5 % oxalic acid into a centrifuge tube.
10. Add a drop of 30 % hydrogen peroxide. Precipitate the iron with concentrated ammonia on a warm water bath. Centrifuge and discharge the solution.
11. Dissolve the residue into a small amount of strong HF. Transfer the sample into a liquid scintillation vial (weighed before addition). Add 1 mL of distilled water, weigh the sample.
12. Take a 100 µL sample for the iron measurement with AAS/ICP-MS/ICP-OES.
13. Measure the activity of ⁵⁵Fe from the rest of the sample by liquid scintillation counting.

Measurement of ⁵⁵Fe by Liquid-Scintillation Counting

Add 9.5 mL of scintillation cocktail Optiphase ’Hisafe’3 (LKB, Wallac). Shake well. Measure after ½ hour with a liquid scintillation counter.

To be able to calculate ⁵⁵Fe radioactivity, counting efficiency need to be determined (efficiency calibration). Counting efficiency is determined from a ⁵⁵Fe standard series using Fluorescein (C₂₀H₁₂O₅) as a quenching agent.

A quenching standard series
Add reagents into plastic scintillation vials as follows:

Shake well. After ½ hour measure the samples in a liquid scintillation counter.

Draw a quenching standard quenching curve. Determine the counting efficiency for ⁵⁵Fe from the curve.

Determination of Chemical Yield

Chemical yield of the analysis is determined by iron measurement of the 100 μL aliquot with AAS/ICP-MS/ICP-OES. Prepare iron standard (1, 2, 3, 5, 10 ppm) and measure them. Draw a standard curve. Measure your unknown sample and determine its iron content (ppm) from your standard curve. Calculate the yield (%). Take into account the dilution of the samples.

Other

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

Calculate the chemical yield of ⁵⁵Fe separation.

Calculate the activity concentration of ⁵⁵Fe in the resin and its uncertainty.


Equipment

Safety Aspects