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258 Modern Analytical Chemistry Method 8.2 Determination of Si in Ores and Alloys 9

Representative Methods Description of Method. Silicon is determined by dissolving the sample in acid.

Dehydration of the resulting solution precipitates silicon as SiO 2 . Because a variety of
other insoluble oxides also form, the precipitate’s mass does not provide a direct
measure of the amount of silicon in the sample. Treating the solid residue with HF
results in the formation of volatile SiF 4 . The decrease in mass following the loss of
SiF 4 provides an indirect measure of the amount of silicon in the original sample.

Procedure.
Transfer a sample of between 0.5 and 5 g to a platinum crucible along
with an excess of Na 2 CO 3 , and heat until a melt is formed. After cooling, dissolve the
residue in dilute HCl. Dehydrating silicon to SiO 2 is accomplished by evaporating the
solution to dryness on a steam bath and heating the residue for 1 hour at 110 °C.
Moisten the residue with HCl, and repeat the dehydration. Remove any acid-soluble
materials from the residue by adding 50 mL of water and 5 mL of concentrated HCl.
Bring to a boil, and filter through #40 filter paper. Wash the residue with hot 2% v/v
HCl followed by hot water. Evaporate the filtrate to dryness twice, and, following
the same procedure, treat to remove any acid–soluble materials. Combine the two
precipitates, and dry and ignite to a constant weight at 1200 °C. After cooling, add 2
drops of 50% v/v H 2 SO 4 and 10 mL of HF. Remove the volatile SiF 4 by evaporating to
dryness on a hot plate. Finally, bring the residue to constant weight by igniting at
1200 °C.
Questions
1. According to the procedure, the sample should weigh between 0.5 and 5 g. On
what basis should a decision on the amount of sample be made?
In this procedure the critical measurement is the decrease in mass following the
volatilization of SiF 4 . The reaction that occurs is
SiO 2 (s) + 4HF(aq) ® SiF 4 (g)+2H 2 O(l)
The water and any excess HF are removed during the final ignition and do not
contribute to the change in mass. The loss in weight, therefore, is equal to the
grams of SiO 2 present after the dehydration step. For every 0.1 g of Si in the
original sample, a weight loss of 0.21 g is expected. The amount of sample used
is determined by how much Si is present. If a sample is 50% w/w Si, a 0.5-g
sample will give a respectable weight loss of 0.53 g. A 0.5-g sample that is only
5% w/w Si, however, will give a weight loss of only 0.053 g. In this case, a larger
sample is needed.

2. Why are acid-soluble materials removed before the dehydrated residue is
treated with HF?
Any acid-soluble materials present in the sample will react with HF or H 2 SO 4 . If
the products of these reactions are volatile or decompose at the ignition
temperature of 1200 °C, then the change in weight will not be due solely to the
volatilization of SiF 4 . The result is a positive determinate error.
3. Why is H 2 SO 4 added with the HF?
Many samples containing silicon also contain aluminum and iron. After
dehydration, these metals are present as Al 2 O 3 and Fe 2 O 3 . These oxides are
potential interferents since they also are capable of forming volatile fluorides.
In the presence of H 2 SO 4 , however, aluminum and iron form nonvolatile
sulfates. These sulfates decompose back to their respective oxides when ignited
to 1200 °C. As a result, the change in weight after treating with HF and H 2 SO 4 is
due only to the loss of SiF 4 .