Use of a Microwave Oven for Drying Samples
DONALD B. AULENBACH, Associate Professor
Environmental Engineering
Rensselaer Polytechnic Institute
Troy, New York 12181
INTRODUCTION
Microwaves are electromagnet radiations with frequencies from 300 Megahertz
(MHz) to about 300 Gigahertz (GHz) and wave lengths in the range of 1 m. to 1 mm. The
frequency most commonly used for commercial microwave heating ovens is 2450 MHz
which has a wave length of approximately 12 cm. One of the properties of microwave
radiations is that it produces a rapidly changing alternating field when it passes through
space or matter. Matter, however, is composed of both positive and negatively charged particles and their separation in some cases is pronounced, particularly in the very polar
molecule water. When microwaves pass through matter, they cause the molecule to vibrate
or even rotate in trying to align with the changing electromagnet field. The more polar the
molecule, the greater is the vibration. The vibration is in the order of 109 per second. This
vibration or molecular friction causes heat. The more polar the molecule, the greater is the
heating of that substance.
The first commercial microwave range went on the market in 1947. After a lag phase,
the market grew rapidly and by 1969, it was estimated there were over 50,000 ovens in use in
the U nited States. Some of the advantages of microwave ovens over ordinary heating ovens
include speed, ease of operation and efficiency. With modern production methods, the cost
is now compatible with other ovens (approximately $400 for a deluxe model and even less
than $300 for a lower priced version). This price is also comparable to that of a conventional
laboratory drying oven. Thus, consideration was made of the possibility of using a
microwave oven for drying of the laboratory samples.
The most common usage of a drying oven in the water and wastewater laboratory is for
the determination of total solids (now referred to as total residue) and suspended solids
(now referred to as non-filterable residue). The total solids determination consists of
primarily the evaporation of all of the water from a given sample and weighing the amount
of residue which remains. In the case of the measurement of suspended solids, the sample is
first passed through a suitable filter (glass fiber filter was used in all these experiments),
followed again by evaporation of the water from the sample. In both cases, the water may be
considered to have been completely evaporated at the oven temperature used when the sample reaches constant weight (1). In order to achieve this, drying overnight in an oven at
either 103-105°C or 179-18PC is recommended. In many instances, for example, the
determination of suspended solids to control an activated sludge system, it is desirable to
have results in less time than overnight. If the speed of evaporation by means of microwave
oven can produce the same results in less time, this will enable more rapid control of a
system dependent upon this analysis. In addition, if samples need to be drying in the oven
for a shorter period of time, a given volume of oven capacity can handle many more samples
due to the shorter time for each evaporation. Thus it was considered expedient to evaluate
the possibility of the use of microwave oven for evaporating water from water and
wastewater samples.
EQUIPMENT AND METHODS
The microwave oven used was a Model KTM8090, manufactured by Litton Precision
Products Co. for Montgomery Ward, Inc. and consumed 1.5 KW.
Total solids measurements were made in either a wide top evaporating dish or a tall
form large crucible.
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