PRECIPITATED SILICA 267the name of " water glass." Several grades of water glass of differ-
ent ratios Na2O:SiO2 are sold, but perhaps the most common
grade approximates the composition Na 2 Si4O 9 (= Na 2 O-4SiO2 =
Na 2 Si0 3 -3Si0 2 ).
The addition of an acid to a sodium silicate solution causes a
separation of silicic acid which appears as a jelly-like substance.
Orthosilicic acid has the composition H^SiC^, metasilicic acid,
H 2 Si0 3 ; the acid corresponding to the sodium salt of the above
formula, H 2 Si4O 9. Suspended in water these different silicic acids
are more or less easily interchangeable one into another, but,
if silicic acid is heated, it loses all its water and becomes the an-
hydride. The anhydride practically will not take on water again
to form acids.
The very finely divided anhydride prepared by precipitating
and drying silicic acid is more reactive than the most finely
powdered quartz, and it finds use as a reagent in certain analytical
tests.
Materials: water-glass, 25 cc.
6.ZVHC1.
12ATHC1.
Apparatus: 8-inch porcelain dish.
600-cc. beaker,
suction filter and trap bottle,
hot plate.
Procedure: To 25 cc. of water glass in a porcelain dish add
25 cc. of water and slowly stir in 6N HC1 until the acid is in excess.
The liquid first coagulates to a jelly, then the jelly hardens and
breaks up on stirring into seemingly dry lumps, and later after
an excess of acid is added (about 30 cc. in all) the mass grows
partially fluid again. Place the dish on a water bath or a hot
plate to evaporate to complete dryness. If a hot closet or hot
plate at 130° is available the dish and contents should be baked for
1 hour at this temperature. Otherwise heat the dish over a flame
for 15 minutes, avoiding, if possible, letting any part of the con-
tents get above 150° as this would render traces of iron oxide
very difficult to redissolve in acid. While the dish is still warm
moisten the contents completely with 12 N HC1. Let it digest
for 15 minutes. Wash the contents of the dish into a 600-cc.
beaker and let the silica settle. Wash by decantation several times