Gravimetric method

 NAME MUHAMMAD MOHSIN KHAN
 R.NO 14EN04
 COURSE ENV.POLLUTION MONITORING-I
 TOPIC GRAVIMETRIC METHOD USED
FOR ANALYSIS OF WATER
 SEMESTER BS-ENV SCIENCES (V)
 SUBMITT TO SIR NAEEM

INTRODUCTION
 Gravimetric analysis describes a set of methods
in analytical chemistry for the quantitative
determination of an analyte based on the mass of a
solid. A simple example is the measurement of solids
suspended in a water sample: A known volume of
water is filtered, and the collected solids are
weighed.
In most cases, the analyte must first be converted to
a solid by precipitation with an appropriate reagent.
The precipitate can then be collected by filtration,
washed, dried to remove traces of moisture from the
solution, and weighed. The amount of analyte in the
original sample can then be calculated from the
mass of the precipitate and its chemical composition.

Dry to a constant weight
 All solids have a certain affinity for water, and may absorb
moisture from the laboratory air. Reagents that readily pick up
water are termed hygroscopic. Those that absorb so much
water that they will dissolve in it and form a concentrated
solution are called deliquescent (e.g., sodium hydroxide,
trichloroacetic acid). These types of substances will continually
increase in weight while exposed to the air. For this reason,
many types of laboratory procedures require that a sample be
dried to a constant, reproducible weight (i.e., absorbed moisture
removed to some standard, low level). This is especially
important for the gravimetric methods. Generally, the sample is
dried in a 103° C to 110° C oven for about 1 hour and allowed
to cool to room temperature in a desiccator. It is then weighed,
and heated again for about 30 minutes. The sample is cooled
and weighed a second time. The procedure is repeated until
successive weighings agree to within 0.3 mg.

Use of Analytical Balance
 The analytical balance is the most accurate and precise
instrument in an environmental laboratory. Objects of up
to 100 grams may be weighed to 6 significant figures.
Volumetric glassware is accurate to no more than 4
significant figures, and the accuracy of complex analytical
methods rarely justifies more than 2 significant figures.
Analytical balances are generally used for gravimetric
analyses, and for the preparation of standard solutions.
Less demanding tasks, such as the weighing of sodium
phosphate for the preparation of a pH buffer, should be
done using the less accurate and more robust, toploading
balance.
 Early analytical balances were entirely mechanical with
two weighing pans, one for the chemical, and one for the
counterweights. Now, most analytical balances are hybrid
mechanical and electronic with a single pan for the
substance to be weighed.

Environmental analysis of
Gravimetric Method (Physical,
Type and Pretreatment)
 Total Solids
Evaporation
 Suspended Solids
Filtration
 Dissolved Solids
Filtration + Evaporation
 Oil & Grease
extraction with C2Cl3F3 + distillation of solvent
 Surfactants
extraction into ethyl acetate + evaporation

Environmental analysis of
Gravimetric Method of
(Thermal, Types &
Pretreatment)
 Volatile Solids
Evaporation + 550`C for 15 min
 Volatile Suspended Solids
Filtration + 550`C for 15 min

Environmental analysis of Gravimetric
Method(Precipitation, Types and
Pretreatment)
 Mg
with Diammonium hydrogen phosphate
and final pyrolysis
 Na
with zinc uranyl acetate
 Silica
precipitation/ ignition/ volatilization (with
HF)
 SO4
with Ba

Total, Dissolved & Suspended
Solids
 Total solids (TS) is generally defined as all matter in a water or
wastewater sample that is not water. Because solids are not a specific
chemical compound, but rather a diverse collection of dissolved and
particulate matter, their concentration cannot be determined in an
unambiguous way. Instead, they must be defined by the procedure
used to estimated their concentration. This is referred to as
an operational definition. Depending on a number of factors, the
operationally defined solids concentration may be greater than, less
than, or about the same as the true solids concentration. The
operational definition that has been adopted through years of use is: all
matter that remains as residue upon evaporation and drying at 180oC
for one hour.
 Total solids may be differentiated according to size into total dissolved
solids (TDS) and total suspended solids . Once again, this is an
operational distinction, whereby all solids passing through filter paper of
a certain pore size are called dissolved, and those retained are termed
suspended. Therefore, the overall definition of total dissolved solids is:
all matter, and is not lost by evaporation and drying at 180oC for one
hour.

Advantages
 Gravimetric analysis, if methods are followed
carefully, provides for exceedingly precise
analysis. In fact, gravimetric analysis was used
to determine the atomic masses of many
elements to six figure accuracy. Gravimetry
provides very little room for instrumental error
and does not require a series of standards for
calculation of an unknown. Also, methods
often do not require expensive equipment.
Gravimetric analysis, due to its high degree of
accuracy, when performed correctly, can also
be used to calibrate other instruments in lieu of
reference standards.

Disatvantages
 Gravimetric analysis usually only provides for the
analysis of a single element, or a limited group of
elements, at a time. Comparing modern dynamic
flash combustion coupled with gas chromatography
with traditional combustion analysis will show that
the former is both faster and allows for simultaneous
determination of multiple elements while traditional
determination allowed only for the determination of
carbon and hydrogen. Methods are often convoluted
and a slight mis-step in a procedure can often mean
disaster for the analysis (colloid formation in
precipitation gravimetry, for example). Compare this
with hardy methods such as spectrophotometry and
one will find that analysis by these methods is much
more efficient.

Why Gravimetric Method is
important…………?
 Except for particulate gravimetry, which is the most trivial
form of gravimetry, you probably will not use gravimetry
after you complete this course. Why, then, is familiarity
with gravimetry still important? The answer is that
gravimetry is one of only a small number of definitive
techniques whose measurements require only base SI
units, such as mass or the mole, and defined constants,
such as Avogadro’s number and the mass of 12C.
Ultimately, we must be able to trace the result of an
analysis to a definitive technique, such as gravimetry, that
we can relate to fundamental physical
properties.2 Although most analysts never use gravimetry
to validate their results, they often verifying an analytical
method by analyzing a standard reference material whose
composition is traceable to a definitive technique.

Gravimetric method

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