Experimental+procedure+for+Isotopic+dilution

Back to main __**Experimental**__ 1. You have been given a solution containing phosphate & chloride, which also contains 3.7 kBq per 1cm 3 of 32 P

2. Pipette 0.5 cm 3 to an aluminium planchette and evaporate under the infrared lamp in the fume hood. Handle the planchette with tweezers and transfer to the fume hood on a tray.

3. Once completely evaporated add another 0.5 cm 3 and evaporate

4. Repeat until a total of 2 cm 3 have been evaporated

5. Cover the planchette with sellotape and count on shelf 2 of a Geiger counter for 100 sec

6. Remove the planchette and measure the background count for 100 sec

__**Precipitation Methods**__ 1. Take 2.5 cm 3 of the phosphate/chloride solution and place in a boiling tube

2. Add 1 cm 3 concentrated nitric acid and 5 cm 3 of distilled water

3. Warm to approximately 80 °C in the dry bath in the fume hood

4. Measure out 3.8 cm 3 of 10 % neutral ammonium molybdate into a 15 mL centrifuge tube and add 1 g ammonium chloride

5. Once the phosphate/chloride soln is warmed, transfer to the 15 mL centrifuge tube

6. Leave to stand at room temp. A precipitate of ammonium molybdophosphate will form after about 30 minutes

7. Centrifuge at 5000 rpm for 5 minutes

8. Pour off supernatant into a clean tube and retain

9. Transfer approximately 20 mg of this precipitate onto an aluminium planchette and dry under the infrared lamp. Do not do this for no more than 30 seconds at a time due to the risk of it catching fire.

10. Reweigh the tray and if the precipitate has reached a constant weight, cover with sellotape and count in the Geiger counter. If the weight is reducing, repeat the drying step until it is constant.

11. Calculate the weight of disodium hydrogen phosphate in the original solution assuming the precipitate to be (NH 4 ) 3 PO 4 .12MoO 3 as described previously

12. Evaporate 2 cm 3 of the supernatant on another planchette in 0.5 increments, cover with sellotape and count