CINCH NucWik

Center for Radiochemistry and Nuclear Materials
Department of Chemistry
Loughborough University

The students will learn how to use isotopic labeling of an insoluble compound and ion exchange reaction between solid and solution phases in determination of a surface area of the compound. Furthermore, the use of a GM counter as well as preparation of unsealed samples will be familiarized.

Here we introduce the theory of ion exchange on insoluble crystal surfaces.

Basic Theory

If crystals of a relatively insoluble substance are placed in contact with a saturated solution of the substance, an exchange of ions between the solution and surface will take place.
If the ions in solution are labeled with a radioactive isotope, this exchange, which depends upon the surface area of the crystals, can be determined, since at equilibrium:

If the molecules are assumed to be cubic:

where ρ is the density (6.2 g/cm³ PbSO₄).

Since there will be 1 radioactive atom per molecule, the specific surface area (i.e. the surface area per gram of sample) can readily be calculated.

1. Carefully weigh out 0.1 g of lead sulphate into each of 2 plastic scintillation vials.
2. Add 2 cm³ of ³⁵S-labelled lead sulphate solution into each.
3. Place caps on vials and shake vigorously for 15 min.
4. Filter using a Buchner funnel and filter paper.
5. Pipette 0.5 cm³ of each filtrate onto 2 aluminium planchettes.
6. Carefully transfer the planchettes using a tray lined with absorbent tissue to the wooden block beneath the infrared lamp in the fume hood.
   Dry under the IR lamp taking care not to leave them too long in case of fire.
7. Transfer the dried planchettes to the Geiger counter on a tray.
8. Count both on shelf 2 of the Geiger counter – count for 1 min, repeat this 3 times and average the counts obtained.
9. Pipette 0.5 cm³ of original ³⁵S-labelled lead sulphate solution (without the addition of lead sulphate powder) onto a 3^rd planchette , dry and count as above.
10. Calculate the surface area, which should give a value of 104 – 105 cm²/g.

• A lab coat, gloves and safety glasses 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 red waste buckets provided.
• Lead sulphate is toxic by ingestion, inhalation and skin contact.
  When weighing out the powder, great care must be taken to avoid creating air-borne dust.

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, and finally the calculated results.

• IR lamp
• Geiger counter in a lead castle
• Scaler timer
• Clamp stand + pressure tubing
• 1 mL autopipette

• 0.2 g lead sulphate
• Filter paper
• Al planchette x 3
• 1 mL tips
• Scintillation vials x 2
• Buchner funnel
• Flask
• Tray for transfer

• 4.5 mL ³⁵S saturated lead sulphate (0.37 kBq/mL).