YS/T 273.4-2020Methods for chemical analysis and physical properties test of cryolite - Part 4: Determination of aluminium content - EDTA titration method (English PDF)

6.1 Sample

Weigh 0.25 g of specimen (5), accurate to 0.0001 g.

6.2 Parallel test

Conduct two tests in parallel. Take the average value.

6.3 Determination

6.3.1 Place the sample (6.1) in the platinum crucible (4.1). Add 6 g of acidic potassium sulfate or potassium pyrosulfate (3.1). Put the platinum crucible into the high- temperature furnace (4.2) when the temperature is lower than 300 °C. Start timekeeping, when the furnace temperature reaches 725 °C ± 25 °C. Melt for about 30 minutes, until the fluorine is completely driven out. Take it out. Cool it to room temperature.

6.3.2 Add 100 mL of water and 5 mL of hydrochloric acid (3.3) in a 250 mL beaker.

Then put the frit (6.3.1) together with the platinum crucible, into the beaker. Heat it.

Use a glass rod to stir, to make the frit fall off. Wash out the platinum crucible. Continue heating, until the frit is completely dissolved. Remove and cool to room temperature.

Transfer the solution into a 250 mL volumetric flask. Use water to dilute it to the mark.

Mix well.

6.3.3 Pipette 100.00 mL of solution (6.3.2). Add 1 drop of phenolphthalein ethanol

solution (3.9). Dropwise add sodium hydroxide solution (3.5), until the solution turns red. Dropwise add hydrochloric acid (3.4), until the solution is colorless. Add an excess of 1.5 mL. Add 15 mL of Na2EDTA solution (3.7). Heat and boil for 2 minutes. Remove it. Add sodium hydroxide solution (3.5) dropwise, until the solution turns red. Add hydrochloric acid (3.4) dropwise, until the solution is colorless. Add 20 mL of buffer solution (3.6). Use running water to cool to room temperature.

6.3.4 Add 3 drops of xylenol orange solution (3.10). Use zinc nitrate standard titration solution (3.8) for titration, to make the solution change from yellow to rose red.

6.3.5 Add 0.5 g ~ 1 g of sodium fluoride (3.2). Heat and boil for 2 minutes. Then remove it. Use running water, to cool to room temperature. Use zinc nitrate standard titration solution (3.8) for titration, until the solution changes from yellow to rose red. Record the volume of zinc nitrate standard titration solution, which is consumed in the second

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