Chapter: Modern Analytical Chemistry: Gravimetric Methods of Analysis
Particulate Gravimetry: Quantitative Applications
Quantitative Applications
Particulate gravimetry is commonly encountered in the environmental analysis of water, air,
and soil samples. The analysis for
suspended solids in water samples, for example, is accomplished by filtering an appropriate volume
of a well-mixed sample
through a glass fiber filter
and drying the
filter to constant weight at 103–105
°C.
Microbiological testing of water also
is accomplished by particulate gravimetry. For example, in the analysis for coliform bacteria
an appropriate volume of sample is
passed through a sterilized 0.45-μm
membrane filter. The filter is then placed
on a sterilized absorbent pad saturated with a culturing medium and incubated for 22–24 h at 35 ±0.5 °C. Coliform
bacteria are identified by the presence
of individual bacterial colonies
that form during the incubation period. As with qualitative appli- cations of precipitation gravimetry, the signal in this case
is a visual observation
rather than a measurement of mass.
Total airborne particulates are determined using
a high-volume air sampler equipped
with either cellulose fiber or glass
fiber filters. Samples
taken from urban environments require approximately 1 h of sampling time,
but samples from rural
environments require substantially longer times.
Grain
size
distributions for sediments
and soils are used to determine
the amount of sand, silt,
and clay present
in a sample. For example, a grain size
of 2 mm serves as a boundary between
gravel and sand. Grain size boundaries for sand–silt
and silt–clay are given as 1/16 mm and 1/256
mm, respectively.
Several standard methods
for the quantitative analysis of food samples are based on measuring the sample’s mass following a selective solvent
extraction. For example, the
crude fat content
in chocolate can
be determined by extracting with ether for 16 h in a Soxhlet extractor. After the extraction is complete, the ether is al-
lowed to evaporate, and the residue is weighed after
drying at 100 °C. This analysis
has also been accomplished indirectly by weighing a sample before
and after ex- tracting with supercritical CO2.
Quartz crystal microbalances equipped with thin-film polymer or chemical coatings have found numerous
quantitative applications in environmental analysis. Methods have been reported
for the analysis
of a variety of gaseous
pollutants, in- cluding ammonia, hydrogen sulfide, ozone,
sulfur dioxide, and
mercury. Bio- chemical
particulate gravimetric sensors
also have been developed. For example, a piezoelectric immunosensor has been
developed that shows a high selectivity for human serum albumin and
is capable of detecting microgram quantities.
Quantitative Calculations
The result of a quantitative analysis by particulate gravimetry is just the ratio, using
appropriate units, of the amount
of analyte to the
amount of sample.
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