Method

1. Accurately weigh 0.5 0.7 g of the sample into a 500 ml beaker. The sample must be

dried in an oven at 105 C for at least two (2) hours.

2. Add 200 250 ml of distilled water to dissolve the sample and then add 0.5 ml conc.

HCl.

3. Heat the solution to near boiling and then add 0.25M BaCl2 in a slow, continuous stream

from a burette while stirring the solution in the beaker constantly. Allow the precipitate

to settle at periodic intervals to determine if precipitation is complete. DO NOT

REMOVE THE GLASS ROD FROM THE BEAKER!! NB. Precipitation is complete when

BaCl2 added to the unstirred solution fails to produce any more precipitate.

4. When there is no indication of further precipitate being formed, add an extra 10% of the

volume of BaCl2 used to the contents of the beaker. Leave the glass rod in the beaker,

cover the beaker with a watch glass and digest on a steam bath until the supernatant

liquid is clear.

5. During this time, accurately weigh a covered porcelain crucible which has been heated

on a Bunsen burner for fifteen (15) minutes prior to being cooled in a desiccator.

6. Filter the contents of the beaker. ENSURE QUANTITATIVE TRANSFER OF ALL THE

PRECIPITATE FROM THE BEAKER TO THE FILTER PAPER.

7. Wash the precipitate with hot distilled water until no turbidity appears when a few ml

of washings, acidified with a few drops of dilute HNO3, are reacted with AgNO3. This is

the test for residual chloride.

8. Carefully fold the filter paper into a package enclosing the precipitate and place it into

the pre-weighed porcelain crucible.

9. Place the crucible in an inclined position on a pipe-clay triangle, with the cover slightly

displaced, over a Bunsen burner. Heat gently with a small flame until all the moisture is

driven off and the paper begins to smoke and char. Adjust the burner so that the paper

continues to char without catching fire. IF THE PAPER INFLAMES, COVER THE CRUCIBLE

TO SMOTHER THE FIRE, AND LOWER THE BUNSEN FLAME.

10. When the paper is completely carbonised and no more smoke is given off, gradually

raise the Bunsen flame enough to burn off the carbon completely. NB. The precipitate

should finally be white with no black particles. Allow the crucible and contents to cool.

11. Place the crucible in a vertical position in the pipe-clay triangle, and moisten the

precipitate with four (4) drops of dil. H2SO4, heat very gently until acid has fumed off.

Cover the crucible and heat to dull redness for fifteen (15) minutes.

12. Cool the covered crucible in a desiccator for one (1) hour and then weigh the crucible

and contents. Repeat the heating, cooling and weighing cycles until three successive

mass readings agree to within 0.5 mg.