Annex IV
189
Annex IV
Determination of silica in rocks
Learning objective:
To illustrate an analytical process based on the gravimetric technique (specifically, on the
weight difference between the sample before and after treatment with hydrofluoric acid). The
method relies on the volatilization of the silica in the following reaction:
Si02 + 4HF ~ F4Sii + 2H20
Reference:
P.G. Jeffery
Chemical Methods of Rock Analysis
Pergamon Press, Oxford, 1975, pp 406,407
Method
Reagents
Hydrofluoric acid, concentrated.
Sulphuric acid, 20 N.
Procedure .
Ignite a clean platinum crucible of about
30-mL capacity over the full flame of a Meker
burner, allow to cool and weigh empty.
Transfer to it approximately 1 g of the finely
ground quartzitic material and reweigh to
give the weight of the sample.! Again ignite
the crucible over the Meker burner, gently at
first and then finally over the full flame of the
burner for a period of 1 h. Allow to cool and
reweigh the crucible and contents. Repeat the
ignition until no further loss in weight
occurs. Report the change in weight as the
"loss on ignition". This loss is due to the
oxidation of ferrous iron and organic matter,
and to the volatilization of water, carbon
dioxide and other gases.
Add 4 drops of 20 N sulphuric acid to the
residue, followed by 20 mL of concentrated
hydrofluoric acid. Transfer the crucible to a
hot plate or sand bath and remove the silica
and excess hydrofluoric acid by volatilization
! Tsubaki describes a modification to this
procedure in which 3 g of boric acid is
added to a 0.5 g sample portion prior to
igniting at 1000°C. The determination is
then completed as described above.
in the usual way. Gently heat the crucible on
the hot plate to remove the excess sulphuric
acid and then ignite over the full flame of the
Meker burner for 10 min. Allow to cool and
reweigh the crucible. Repeat the ignition
until no further loss in weight occurs. Determine a reagent blank by volatilization in the
same way, but omitting the sample material.
The blank value obtained with analytical
grade hydrofluoric acid will probably amount
to between 0.3 and 1.0 mg. This should be
added to the loss on volatilization to give the
total silica content of the sample.
In a very few samples some silicate mineral grains may remain undecomposed. These
can usually be attacked by a second evaporation with smaller amounts of hydrofluoric
and sulphuric acid.
189
Annex IV
Determination of silica in rocks
Learning objective:
To illustrate an analytical process based on the gravimetric technique (specifically, on the
weight difference between the sample before and after treatment with hydrofluoric acid). The
method relies on the volatilization of the silica in the following reaction:
Si02 + 4HF ~ F4Sii + 2H20
Reference:
P.G. Jeffery
Chemical Methods of Rock Analysis
Pergamon Press, Oxford, 1975, pp 406,407
Method
Reagents
Hydrofluoric acid, concentrated.
Sulphuric acid, 20 N.
Procedure .
Ignite a clean platinum crucible of about
30-mL capacity over the full flame of a Meker
burner, allow to cool and weigh empty.
Transfer to it approximately 1 g of the finely
ground quartzitic material and reweigh to
give the weight of the sample.! Again ignite
the crucible over the Meker burner, gently at
first and then finally over the full flame of the
burner for a period of 1 h. Allow to cool and
reweigh the crucible and contents. Repeat the
ignition until no further loss in weight
occurs. Report the change in weight as the
"loss on ignition". This loss is due to the
oxidation of ferrous iron and organic matter,
and to the volatilization of water, carbon
dioxide and other gases.
Add 4 drops of 20 N sulphuric acid to the
residue, followed by 20 mL of concentrated
hydrofluoric acid. Transfer the crucible to a
hot plate or sand bath and remove the silica
and excess hydrofluoric acid by volatilization
! Tsubaki describes a modification to this
procedure in which 3 g of boric acid is
added to a 0.5 g sample portion prior to
igniting at 1000°C. The determination is
then completed as described above.
in the usual way. Gently heat the crucible on
the hot plate to remove the excess sulphuric
acid and then ignite over the full flame of the
Meker burner for 10 min. Allow to cool and
reweigh the crucible. Repeat the ignition
until no further loss in weight occurs. Determine a reagent blank by volatilization in the
same way, but omitting the sample material.
The blank value obtained with analytical
grade hydrofluoric acid will probably amount
to between 0.3 and 1.0 mg. This should be
added to the loss on volatilization to give the
total silica content of the sample.
In a very few samples some silicate mineral grains may remain undecomposed. These
can usually be attacked by a second evaporation with smaller amounts of hydrofluoric
and sulphuric acid.