Page 121 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

GLIISSWARE. CERAMICS, PLASTIC WARE 3.19

filling burettes and transferring solids to graduated flasks, a short-stem,
wide-necked funnel is useful.
Porcelain apparatus. Porcelain is generally employed for operations in which
hot liquids are to remain in contact with the vesse1 for prolonged periods. It is
usually considered to be more resistant to solutions, particularly alkaline
solutions, than glass, although this will depend primarily upon the quality of
the glaze. Shallow porcelain basins with lips are employed for evaporations.
Casseroles are lipped, flat-bottomed porcelain dishes provided with handles;
they are more convenient to use than dishes.
Porcelain crucibles are very frequently utilised for igniting precipitates and
heating small quantities of solids because of their cheapness and their ability
to withstand high temperatures without appreciable change. Some reactions,
such as fusion with sodium carbonate or other alkaline substances, and also
evaporations with hydrofluoric acid, cannot be carried out in porcelain crucibles
owing to the resultant chemical attack. A slight attack of the porcelain also
takes place with pyrosulphate fusions.
Fused-silica apparatus. Two varieties of silica apparatus are available
commercially, the translucent and the transparent grades. The former is much
cheaper and can usually be employed instead of the transparent variety. The
advantages of silica ware are: (a) its great resistance to heat shock because of
its very small coefficient of expansion, (b) it is not attacked by acids at a high
temperature, except by hydrofluoric acid and phosphoric acid, and (c) it is more
resistant to pyrosulphate fusions than is porcelain. The chief disadvantages of
silica are: (a) it is attacked by alkaline solutions and particularly by fused alkalis
and carbonates, (b) it is more brittle than ordinary glass, and (c) it requires a
much longer time for heating and cooling than does, Say, platinum apparatus.
Corning Vycor apparatus (96 per cent silica glass) possesses most of the merits
of fused silica and is transparent.
Plastic apparatus. Plastic materials are widely used for a variety of items of
common laboratory equipment such as aspirators, beakers, bottles, Buchner

Table 34 Plastics used for laboratory apparatus
Material Appearancet Highest Chemical reagents f Attacking
temperature Acids Alkalis organic
(OC) solvents§
Weak Strong Weak Strong
Polythene (L.D.) TL 1, 2
Polythene (H.D.) TL-O 2
Polypropylene T-TL 2
TPX (Polymethylpentene) T 1, 2
Polystyrene T Most
PTFE (Teflon) O v
Polycarbonate T Most
PVC [Poly(vinyl chloride)] T-O 2, 3, 4
Nylon TL-O v
t O = opaque; T = transparent; TL = translucent.
$A = attacked; F = fairly resistant; R = resistant; R* = generally resistant but attacked by oxidising
mixtures; V = very resistant.
5 1 = hydrocarbons; 2 = chlorohydrocarbons; 3 = ketones; 4 = cyclic ethers; V = very resistant.