Chemistry Reference
In-Depth Information
The ultrasonic cleaning bath
always necessary to agitate mechanically the reac-
tion mixture during sonication. A further complica-
tion in trying to reproduce work reported in the
literature is that all baths operate at frequencies and
powers that depend upon the transducers used, and
with bath geometries peculiar to the particular
manufacturer.
Water (containing a little surfactant) normally is
used as the coupling medium in the bath and this
will limit the upper temperatures of operation to a
maximum of just below 100°C. Factory-fitted ther-
mostatic control in baths normally is available for
temperatures above that reached under its normal
working conditions (generally about 40°C). Below
this temperature a cooling system (cooling coil or
circulating bath liquid) is required, although this
can interfere with the clear passage of ultrasound
through the bath. It is important to record the tem-
perature inside the reaction vessel because this will
be always a few degrees above that of the bath itself
due to localised ultrasonic heating.
The ultrasonic cleaning bath is by far the most widely
available and cheapest source of ultrasonic irradia-
tion for the chemical laboratory. Although it is pos-
sible to use the bath itself as a reaction vessel, this is
seldom done because of the problems involved with
chemical attack of the bath walls and with the con-
tainment of any evolved vapours and gases. Normal
usage therefore involves the immersion of standard
glass reaction vessels into the bath (Fig. 16.5). This
is important because conventional apparatus can be
transferred directly into the bath and so an inert
atmosphere or a static pressure can be achieved
readily and maintained throughout a sonochemical
reaction.
It is important to establish the optimum position
for the reaction vessel in the bath both vertically
(due to the discrete wavelength of sound in water)
and horizontally (in terms of the position of the
vessel with respect to the transducers on the base).
The simplest method of establishing this point is to
locate the vessel such that maximum disturbance is
observed on the surface of the liquid contained in
that vessel. Once this position is found, a template
can be used to ensure that future reactions are
carried out in the same region of the bath. Naturally,
in order to attain reproducible results, the same glass
vessel should be used each time because differences
in the thickness of the glass bases of vessels will affect
ultrasonic power transfer into the reacting system.
The amount of energy that reaches the reaction is
low—normally between 1 and 5 W cm
-2
. This is so
low that, for heterogeneous systems, it is almost
The ultrasonic probe
In order to increase the amount of ultrasonic power
available to a reaction it is desirable to introduce the
energy directly into the system rather than rely on
its transfer through the water of a tank and the reac-
tion vessel walls. The simplest method to achieve this
is to introduce the ultrasonically vibrating tip of a
sonic probe into the reaction itself (Fig. 16.6). The
reaction mixture
water + detergent
stainless
steel tank
optional
heater
transducers bonded
to base
Fig. 16.5
The ultrasonic cleaning bath for sonochemistry.
Fig. 16.6
The ultrasonic probe system for sonochemistry.
