Lab Exercise - Determination of ²¹⁰Po from Vegetation

Developed By

Laboratory of Radiochemistry
Department of Chemistry
University of Helsinki
Finland

Learning Goals

Explanation and Exercise Guide

Theory

• ²¹⁰Po and ²¹⁰Pb (read)

Experimental Procedure

• Wet Digestion of Plant Samples

1. 2-5 g of dry plant sample material is weighed into a Teflon-tube (2a) or Kjeldahl's flask (2b). The exact mass of the sample is recorded.

2a. The sample is digested with MARS5TM-microwave machine according the introductions given in the manual. 20 mL of aqua regia (3:1 conc. HCl - HNO₃) and 0.2-0.5 Bq of ²⁰⁹Po tracer solution (T_1/2 = 103 a) is added to the tube and left to stand for overnight. Write down the activity concentration and the volume of the tracer. After digestion the sample is moved into a decanter glass and the teflon-tube is rinsed with 3 x 2 mL of conc. HCl. Add 15 mL of conc. HCl and evaporate to 2-3 mL and leave cool down.

2b. Alternatively the digestion can be carried out in 500 mL Kjeldahl's flask in concentrated HNO₃ and HCl. First add 50 mL of conc. HNO₃ and start heating carefully, because vigorous foaming may appear in the beginning. The digestion is continued until the HNO₃ solution is evaporated to 10 mL. (If there still is unburned fat etc. HNO₃ can be added and the digestion continued.) Let the flask cool well and add 20 mL of conc. HCl and evaporate HNO₃ away (brown gases disappear). The sample is evaporated to 2-3 mL and left to cool down.

• Polonium Deposition on Silver Disc

1. 10 mL of 0.5 M HCl is added to the digested sample and possible precipitate (silicates) is removed by filtration. The precipitate is washed with 2 x 5 mL of 0.5 M HCl and the washing solution is combined with the filtrate.

2. 3 mL of saturated hydrazine monohydrochloride solution is added. The solution is heated close to the boiling point, let to cool and another 0.5 mL of          hydrazine monohydrochloride solution and 1 mL of 10% ascorbic acid is added.

3. A silver disc is washed with alcohol to remove fats. The sample solution is poured into the deposition vessel. The beaker is washed with small amount of    distilled water and the rinse is poured to the deposition vessel. Water is added so that the total volume in deposition vessel is 50 mL. The sample is              deposited at 60 – 65 ^OC (water bath) for 2 hours using mechanical stirrer.

4. The solution is removed from the deposition vessel. Wash the vessel once with a small amount of 0.5 M HCl. Remove the silver disc from deposition          vessel and rinse well with water and ethyl alcohol. When the disc is dry, the radioactivity of ²¹⁰Po is determined by alpha spectrometry.

Questions for the Students

1. Describe in detail the whole separation procedure and explain why each step was made.
2. Draw a separation procedure scheme.
3. Draw the alpha spectrum and determine the number of counts in both alpha peaks.
4. Since the amount of ²⁰⁹Po added is known, the concentration of ²¹⁰Po in the sample can be calculated. Give the result as Bq/g dry weight, and calculate the uncertainty of the uncertainty. Calculate also the chemical yield of the separation process.

Safety Aspects

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Equipment

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