KJ 3055-
Chapter 10, Appendix 1
Microwave
digestion bombs
The aggressive digestion action
produced at the higher temperatures and pressures generated in these bombs
result in remarkably short digestion times, with many materials requiring
exposures of less than one minute to obtain complete dissolution. And because
of their unique, high strength design, they provide a much more vigorous action
that can be obtained with open-cup microwave digestion systems that are
restricted to lower temperatures and pressures. In addition, there is no loss
of volatile matter from these sealed vessels, and the sensitive parts of a
microwave oven are not subjected to corrosive acid fumes.
Heating energy is transferred to
the reaction mixture in the form of radio waves with the wavelength in the
micrometer range. Microwaves enhance the rotation kinetic energy of water
molecules (and other polar molecules, such as mineral acids) and therefore rise
the temperature.
Fig. 1. A microwave digestion bomb:
schematics (left) and real appearance (right). Source>>
Temperature-pressure correlation
for acid digestion reagents
The
MARS 5 microwave digestion system is designed to accelerate chemical reactions
in high-temperature, high-pressure reinforced Teflon vessels. Fig. 2 shows typical operating conditions using water and
common mineral acids.
Fig. 2. The Mars 5 microwave digestion
system with XP-1500 vessels is restricted to conditions below and to the left
of the heavy black line. Maximum temperature is 300°C, maximum pressure is 800
psi (54.4 bar; 10 psi = 0.68948 atm). Experimentally determined curves are
shown for pure water, reagent grade HNO3, reagent grade HF, and a 1:1 mixture
of HF and HNO3. The curve for water-saturated water vapor curve (from steam
tables) is also shown for reference. Operating times at high temperatures
should not exceed 30 minutes. Source>>
References
Pressure Digestion Systems for Closed Acid Digestion Kingston, H. M., Haswell, S. J. (Eds.), Microwave-enhanced chemistry: fundamentals, sample preparation,and applications, American Chemical Society, Washington, D.C., 1997.
H.M. Kingston, L.B. Jassie, (Eds.), Introduction to microwave sample preparation: theory and practice). American Chemical Society, Washington, D.C., 1988.
F.G. Banica, 2009-04-12