Environmental Engineering Reference
In-Depth Information
extreme case, improper loading and balancing of rotors can cause the rotors to break
loose while spinning, resulting in total damage of centrifuge, flying metal fragments,
and shock wave.
Distillation
Distillation is the procedure needed for ammonium, cyanide, sulfide, and phenol
analysis. The distillation device consists of a distilling flask, a cooling water
condenser, and a receiving flask to collect distillate. When the distillation flask
containing volatile chemical(s) is heated, the most volatile component vaporizes at
the lower temperature, and the vapor passes through a cooled tube (condenser),
where it condenses back into its liquid state in the receiving flask. Use vacuum
distillation if the compound will decompose at atmospheric pressure before its
boiling point is reached. Cautions include the following: (a) Make sure all
connections in the apparatus are tight. A leakage during distillation can be the major
error of analysis. (b) Use proper tubing for the cooling water and clamp the tubing so
it will not be in contact with the heating device. Do not leave distillation unattended.
(c) Adding a few boiling stones always ease the bumping.
Reflux
Reflux is used in COD analysis where organic compounds are oxidized by a strong
oxidizing agent under heated and acidic conditions. A reflux apparatus allows a
reaction mixture to be heated for an extended period of time without a loss of water
or solvent. The water or solvent vapor is refluxed back to the reaction mixture by a
condenser that is vertically attached to the heated flask. A reflux device is similar to
distillation in a sense, hence operational and safety rules described above also apply
to reflux.
Concentration
Concentration apparatus is common in all laboratories performing trace analysis.
The apparatus can be as simple as a mild N 2 stream from a gas cylinder, or an all-
glass apparatus such as Kuderna-Danish (K-D) evaporative concentrator or a rotary
evaporator (rotavap) (Fig. 6.2). Their purpose is to remove excess solvent from the
mixture of analyte and solvent so that
the concentration of analytes will be
sufficiently high to be detected.
In a K-D concentrator, boiling chips should be added and the assembly set over
a vigorously boiling water bath. The water level should be maintained just below the
lower joint and the apparatus mounted so that the lower rounded surface of the flask
is bathed in steam. Volatile solvent are allowed to escape under the hood, and the
analyte refluxes until
the final concentrate is collected in the lower tube of
approximately 1-2 mL.
In a rotavap, the rotating distilling flask is lowered into a water bath where the
water temperature should not exceed the boiling point of the solvent. The solvent is
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