Laboratory Glass-Working for Scientists - Sciencemadness Dot Org

GENERAL CHEMICAL PROPERTIES OF GLASS
is proportional to the square root of time. The process has an
activation energy. For a soda~lime~silica glass over 98 per cent of
the evolved gas is water. B. J. TODD (1955) has studied these effects
in detail. The adsorbed water on glass can be troublesome in gaseous
manipulation, as R. W. Bunsen first appreciated.
At high temperatures glass loses its more volatile components. The
loss of silica, lime, magnesia and alumina is negligible, but boric
oxide, lead oxide, sodium oxide and potassium oxide can also be lost.
When the glass is heated in a flame, reaction may occur with some of
the flame gases; sulphur dioxide can react with soda glass and lead
glass to form sodium sulphate and lead sulphate respectively, and of
these only the former can be washed off. An account of these effects
is given by W. E. S. TURNER (1945). The loss of weight of vitreous
silica on ignition is negligible; crucibles can be heated to 1050°C, and
precipitates can be ignited at 100G°C in crucibles with a porous base
of vitreous silica.
Diffusion through Glass
The mobility of the sodium ions in a soda-lime-silica glass at elevated
temperatures is fairly high; if an evacuated bulb of such a glass
is dipped into molten sodium nitrate and electrolysis is brought about
by bombarding the inside of the bulb with electrons, the circuit being
completed with an electrode in the sodium nitrate, then metallic
sodium appears in the bulb. By immersing the bulb in other molten
salts the sodium ions can be replaced by ions of silver, copper,
thallium and vanadium. These ions also diffuse into glass from their
molten salts in the absence of an electric field. When potassium is
distilled in a borosilicate glass vessel it becomes slightly contaminated
with sodium which diffuses from the glass and is replaced by potassium
(D. K. C. MACDONALD and J. E. STANWORTH, 1950). Vitreous
silica allows helium, hydrogen, neon, nitrogen, oxygen and argon to
diffuse through it, with the permeability decreasing in the order given.
The permeability of silica becomes greater if the glass devitrifies. The
permeability to helium of soda-lime-silica glass is 10 5 (or more)
times less than that of vitreous silica. For practical vacuum purposes
soda and borosilicate glasses can be regarded as impermeable to
gases at ordinary temperatures, except in work at extremely low
pressures when the diffusion of atmospheric helium through the glass
may become significant.
The permeability of glass at high temperatures seems to have been
discovered by R. Boyle. In his collected works published in 1744
there is a paper in Volume III 'A discovery of the perviousness of
glass to ponderable parts of flame' in which he writes \ .. it is plain
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