A Simple Semi-Micro
Combustion/Pyrolysis Apparatus
For Use With Any
Stereomicroscope
One of the typical unit operations in
classical microchemical (and also semi-microchemical) work, as founded
by Behrens, Emich and others, is the exposure of the test specimen to heat, if possible
under different boundary conditions (e.g., in the presence or absence of air,
other gases, liquids like water, solvents or strong mineral acids). For me it was
desirable to watch the different effects during these operations with the aid
of a stereomicroscope. With such an instrumental setup, you can study
temperature dependent effects under a much broader range of conditions than,
for instance, on the classical hotstages, attached to or part of a compound
microscope. The major limitations are that magnifications are not that high,
and that image distortions occur because of the convex surface(s) of the glass
tube(s) of the combustion/pyrolysis oven.
Because, to my knowledge, no commercial
equipment is currently available for this technique, I tried to build it myself
from easily obtainable, inexpensive parts. Literature that I found most useful
in the proper selection of materials and in the optimum design of the
construction is cited in the general references section at the end of this
paper.
The temperature range that is easily accessible
with the described setup extends from room temperature up to approx. 600°C. That is sufficient for most of the usual
microchemical work, including controlled ashing and pyrolysis. Higher
temperatures are possible if some additional care is taken with a further
optimized design of the little oven/incinerator.
The stereomicroscope that I use in my home
shop is a “TECHNIVAL”, from the seventies of the last century, made by Carl
Zeiss, Jena; see Figure 1 for a front view of this instrument, with a LOMO ring
light illuminator attached to the front lens. Nice-to-have’s are: a transmitted
light stand and a double-gooseneck fiber optic illuminator attached to the stereomicroscope
(not shown in Figure 1).
click image to enlarge (68K)  |
FIGURE 1
One advantage of this instrument is that its base
plate is heavy, and sturdy enough to counterbalance the weight of the microscope
even if it is swung out around the vertical axis to its “180 degrees opposite”
resting position, enabling larger specimens and extra equipment to be placed
under the front lens (see pictures below). This can be used to the great advantage
of the investigator in the following experiments.
Things that are necessary in addition to the
stereomicroscope are:
- A pencil, circular cross section
- A laboratory power supply, capable of
delivering direct current, 0-24 Volts, up to 3 or 4 Amps, together with
some cables, banana plugs and alligator clips
- Kanthal® wire, 0.5 mm
diameter (or equivalent, like Nichrome®-wire)
- Pasteur pipettes, made of ordinary
(“soft”) glass or made of borosilicate glass
- Fused silica (“quartz glass”) tubing,
different diameters and lengths (to fit your needs), if you want to reach
temperatures above 600°C
- Platinum wire, approx. 2 mm diameter
- A little horizontal resting stand for
Pasteur pipettes, home-made from wooden latches and slats
- If you want: a thermocouple and
electronic thermometer to measure the temperature characteristics of your home-made
micro incinerator
