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DISTILLATION
Benzene and Toluene have fixed
boiling points. Presence of any impurities results in change in boiling points. The
impurities may be praffinic, cyclo-paraffinic, and olefinic type of hydrocarbons etc.
Hence distillation is carried out on Benzene and Toluene to determine their boiling points
which in turn reflects their purity.
CRYSTALLIZING POINT
Being a pure chemical compound
Benzene exhibits characteristic crystallizing point. Trace amounts of impurities will
result in lowering of crystallizing point. This is one of the ways to determine the purity
of Benzene. Hence, there is minimum limit on this property in the specification. A
knowledge of this also helps in determining suitability of its use as reagent chemical or
for conversion to other chemical intermediates or finished products.
Crystallizing point is not
specified in the specification of Toluene as it is below -90 deg C which is difficult to
measure. Crystallizing point is also referred a solidification point.
ACID WASH COLOUR
The presence of olefinic
compounds in Benzene or Toluene can be easily detected by carrying out "Acid wash
colour" test. The principle of this test is the ability of olefines to react with
concentrated sulphuric acid and discolour the acid.
SULPHUR AND SULPHUR COMPOUNDS
Sulphur can be present either as Thiophene sulphur or as
Hydrogen Sulphide and Sulphurdioxide in Benzene or Toluene. The presence of Sulphur can
adversely affect the chemical process where Benzene / Toluene are used as a raw material.
Also presence of sulphur, beyond specified limit, causes corrosion of process equipment
and storage tanks. They also emit objectionable odour. Hence, there is a limit on presence
of sulphur.
COPPER CORROSION
The Benzene sample should not give any discoloration to
copper strip. Discoloration of copper strip indicates presence of reactive sulphur or
hydrogen sulphide as an impurity.
SAFETY AND HANDLING
Benzene and Toluene are
classified as " Class A" petroleum products, and all safety precautions, as laid
down in Petroleum Act and Rules, are to be observed in their handling. Because of their
flammability, volatility and toxicity, they are handled under strict safety regulations.
They are stored in steel containers. Adequate ventilation, labeling and electrical
grounding are required. Fire must be extinguished by using CO2 or dry chemical fire
extinguishers. They should be handled only in closed systems.
TOXICITY
Benzene, particularly, is a poisonous substance with
chronic toxic effects. The permissible exposure limits in air is 10 ppm and shor term
exposure limit is 25 ppm. The immediate dangerous to life or health is 2000 ppm. Exposure
to liquid and vapour may produce primary irritation to skin, eyes and upper respiratory
tract. Scaling dermatitis may also develop from defattening of skin.
Acute exposure to Benzene results in central nervous
system depression, headache, dizziness, nausea, convulsions, coma and death. The short
term effects of the inhalation, ingestion, or skin contact of benzene in high dosages, are
immediately apparent but, the effects of chronic exposure to lower levels of Benzene are
not. Periodic blood and unine tests are required for those who are regularly exposed to
Benzene.
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