Lab Exercise - Determination of Low Solubilities

Developed by

Center for Radiochemistry and Nuclear Materials
Department of Chemistry
Loughborough University

Learning Goals

Here the learning goals and purpose of the exercise should be presented in a reasonable way.

Explanation and Exercise Guide

Theory

Tracers and solubility (read)

Experimental Procedure

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 1st vial
8. Add 1 mL of the 100x diluted solution to a 2nd vial
9. Once the strontium sulphate has equilibrated, centrifuge off the precipitate and add 1 cm³ of the supernatant from 1st precipitated sample to a 3rd vial
10. Add 1 cm³ of the supernatant from the 2nd precipitated sample to a 4th vial
11. The 5th 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, 100z cpm is equivalent to 0.1 g Sr(NO₃)₂

y cpm is equivalent to:

Solution of SrSO₄=

Questions for the Students

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

Other

How to do calculations or other important aspects for the theory that is not directly related to exercise.

Safety Aspects

• 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.

Equipment

Centrifuge
glass beaker x 2 for H₂O
Scintillation counter
waste beaker
1 mL pipette
Polystyrene base to stand centrifuge tubes in
25 mL volumetric flask

Consumables
4-5 mL 2 M sulphuric acid
15 mL centrifuge tubes x 2
> 100 mL dH₂O
Scintillation vials x 5
50 mL Scintillation fluid
1 mL tips

Source
4 mL ³⁵S strontium nitrate @ 0.28 kBq per mL

Preparation for the lab Supervisor

The preparation that the lab Supervisor needs to do to ready the lab.

Feedback from Users and Supervisors

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