Center for Radiochemistry and Nuclear Materials
Department of Chemistry
Loughborough University

In this exercise, the students learn how to determine solubility of strontium sulphate by means of LSC and find out possible uncertainty sources in this determination method.

Tracers and Solubility

Radioactive tracers can be used in what may be regarded as micro-chemical determinations such as the evaluation of very low solubilities, or very low vapour pressures. Suppose we wish to determine the solubility of barium sulphate, then ¹³⁹Ba may be selected as a tracer. The activity (A), associated with a given mass of barium sulphate (W) must first of all be determined. A small quantity of the active sulphate is then agitated with water at a known temperature, until equilibrium is reached when the activity (A*) of a given volume (V cm³) of the solution is determined.

An activity A is produced by W g of BaSO₄.

Therefore, an A* activity is produced by:

and the solubility is:

Certain errors may arise in this type of determination.
For example, it may be necessary to correct for the self-absorption of the active solid, or to arrange the experimental procedure so that this may be avoided.
Care must also be taken to avoid errors due to absorption on apparatus.

In this experiment the solubility of strontium sulphate is determined and at the same time errors likely to arise are investigated.
³⁵S (T1/2 = 87 days) will be used. Alternatively ⁸⁹Sr (T1/2 = 54 days) would also be suitable.

1. Take two 2 cm³ aliquots of ³⁵S-spiked strontium nitrate solution and place in separate 15 mL centrifuge tubes, add excess 2.5 M sulphuric acid (approx. 4-5 cm³). A white precipitate will rapidly form.
2. Centrifuge at 5000 rpm for 5 min and then pour off supernatant into the waste beaker provided.
3. Wash repeatedly with distilled water using at least 100 cm³. Carefully pour off wash water into the waste beaker leaving the precipitate in the centrifuge tube each time.
4. Add 10 cm³ of distilled water to the washed precipitate and leave to stand for 10-30 min, shaking periodically. This allows time for precipitate and water to equilibrate (see step 9).
5. Meanwhile dilute the strontium nitrate solution by a factor of 100 in the volumetric flask with distilled water– use 0.25 cm³ in 25 cm³.
6. Add 10 cm³ of scintillation fluid to 5 plastic scintillation vials.
7. Add 1 cm³ of original undiluted solution to the 1^st vial.
8. Add 1 mL of the 100x diluted solution to a 2^nd vial.
9. Once the strontium sulphate has equilibrated, centrifuge off the precipitate and add 1 cm³ of the supernatant from 1^st precipitated sample to a 3^rd vial.
10. Add 1 cm³ of the supernatant from the 2^nd precipitated sample to a 4^th vial.
11. The 5^th vial has no activity added and is the background sample.
12. Count all 5 vials on liquid scintillation counter using ¹⁴C channel (Protocol 2).
    Press button F2 to start the run.

As the majority of the activity is in the precipitate, it is clear that a minute amount of this in the supernatant may cause a gross error in the determination.

From the results obtained the solubility of strontium sulphate may be determined as follows:

• Let count of 1 cm³ original solution diluted 1/100th=z cpm
• Let count of 1 cm³ of SrSO₄ = y cpm
• Thus, 100 z cpm is equivalent to 0.1 g Sr(NO₃)₂:
• y cpm is equivalent to:
• Solution of SrSO₄=

1. What are the sources of error?
2. Highlight the potential problems of using ⁹⁰Sr for this experiment.

• A lab coat, gloves and safety spectacles must be worn.
  ³⁵S is a soft beta emitter which means that it is not readily detected.
  When checking the bench, hands, lab coat, etc for contamination it is necessary to use a sensitive instrument.
• DO NOT dispose of any liquid or solid waste down sinks or in waste bins.
  Place in the waste buckets provided.

The work report about this exercise should contain clear description of the performed work presenting the original theory, used equipment with settings, radionuclides with their activities and reference dates, answers to the two questions above, and finally the calculated results.

• Centrifuge
  
• Liquid scintillation counter
  
• 1 mL autopipette
  

• 4-5 mL of 2 M sulphuric acid
• 15 mL centrifuge tubes x 2
• > 100 mL of H₂O
• Scintillation vials x 5
• 50 mL of scintillation cocktail
• 1 mL tips
• Waste beaker
  
• Glass beaker x 2 for H₂O
  
• Polystyrene or other type of base to stand centrifuge tubes in 25 mL volumetric flask
  

4 mL of ³⁵S strontium nitrate 0.28 kBq/mL