Gravimetric analysis, which by definition is based upon the measurement
of mass, can be generalized into two types; precipitation and volatilization.
The quantitative determination of a substance by the precipitation method
of gravimetric analysis involves isolation of an ion in solution by a
precipitation reaction, filtering, washing the precipitate free of contaminants,
conversion of the precipitate to a product of known composition, and finally
weighing the precipitate and determining its mass by difference.
From the mass and known composition of the precipitate, the amount of
the original ion can be determined.
For successful determinations the following criteria must be met: The
desired substance must be completely precipitated. In most determinations
the precipitate is of such low solubility that losses from dissolution
are negligible. An additional factor is the "common ion" effect, this
further reduces the solubility of the precipitate. When Cl-
is precipitated out by addition of Ag+
+ Cl- AgCl(s)
The (low) solubility of AgCl is reduced still further by the excess
of Ag+ which is
added, pushing the equilibrium to the right. We can further decrease
the solubility by decreasing the temperature of the solution by using
an ice bath. The weighed form of the product should be of known
composition. The product should be "pure" and easily filtered. It is
usually difficult to obtain a product which is "pure", i.e. one which
is free from impurities but careful precipitation and sufficient washing
helps reduce the level of impurity.
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|Gravimetric methods are
quantitative methods that are
based upon determining the
mass of a pure compound to
which the analyte is related
Theodore W. Richards (1868 -
1928) and his graduate students at
Harvard developed or refined many
of the techniques
analysis of silver and chlorine.
These techniques were used to
determine the atomic weights
25 of the elements by preparing
pure samples of the chlorides
of the elements, decomposing
known weights of the compounds,
and determining the chloride
content by gravimetric methods.
From this work Richards
the first American to receive
Nobel Prize in Chemistry in 1914.
After the addition of the precipitating agent to the solution of the ion
under analysis there is an initial induction period before nucleation
occurs. This induction period may range from a very short time period
to one which is relatively long, ranging from almost instantaneous to
After induction, nucleation occurs, here small aggregates or nuclei
of atoms form and it is from these "clumps" of atoms that the crystals
which form the filtrate will grow. These nuclei may be composed of just
a few atoms each so there may be up to 1010
of the nuclei per mole of precipitating product. As these nuclei form
ions from the solution (which at this point are in excess) congregate
around them. For example if hydrochloric acid were added very slowly to
a solution of silver nitrate, silver chloride nuclei would form and silver
ions (which would be in excess relative to Cl-
ions) would congregate around them.
In addition to the primary adsorbed silver ion, there are some
nitrate ions aggregating further from the AgCl nucleus. These are counter
ions and tend to aggregate around the [AgCl:Ag]+
center because these centers have a net positive charge (excess Ag+)
and additional negative charge is required to maintain electrical neutrality.
The counter ions are less tightly held than the primary adsorbed ions
and the counter ion layer is somewhat diffuse and contains ions other
than those of the counter ions. These layers of charges are known as the
electric double layer.
|Nucleation is a process in
a minimum number of atoms, ions,
or molecules join together to give
a stable solid.
|Adsorption is a process
in which a
substance (gas, liquid, or solid) is
held on the surface of a solid. In
contrast, absorbtion involves the
retention of a substance within the
pores of a solid.
|The electric double layer
of a colloid
consists of a layer of charge absorbed
on the surface of the particles and a
layer with a net opposite charge in the
solution surrounding the particles
|A colloid is a solid made
particles having diameters less
than 10-4 cm.
After nucleation growth occurs, large nuclei grow
at the expense of smaller nuclei which dissolve. This process helps produce
more easily filtered crystals (since it produces larger crystals).
Growth of larger nuclei or crystallites can be encouraged
by digestion, a process which involves heating the solid and
mother liquor for a certain period of time. During digestion, small
particles dissolve and larger ones grow. Digestion of the product is
an important practical process and you will find that most if not all
gravimetric analysis involve a digestion period.
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for analytical precipitation
In an ideal world, an analytical precipitate for gravimetric
analysis should consist of perfect crystals large enough to be easily
washed and filtered. The perfect crystal would be free from impurities
and be large enough so that it presented a minimum surface area onto which
foreign ions could be adsorbed. The precipitate should also be "insoluble"
(i.e. be of such slight solubility that loses from dissolution would be
Without going into detail, it has been shown (Von Weimarn)
that the particle size of precipitates is inversely proportional to
the relative supersaturation of the solution during precipitation;
relative supersaturation = (Q-S)/S
For the best possible results, conditions need to be
adjusted such that Q will be as low as possible and S will be relatively
|A supersaturated solution
unstable solution that contains
more solutes than a saturated
solution. with time, supersatur-
ation is relieved by precipitation
of the excess solute. To increase
the particle size of a precipitate,
minimize the relative supersatur-
ation during the precipitate
relative supersaturation = (Q-S)/S
where Q is the concentration at
any instant and S is the equilibrium
The following methods are used to approach these criteria,
(we will employ the first);
Precipitation from hot solution. The solubility
S of precipitates increases with temperature and so an increase in S
decreases the supersaturation.
Precipitation from dilute solution. (We
will employ this method next semester for the crystallization of a metal
oxalate) This keeps Q low. Slow addition of precipitating reagent
with effective stirring. This also keeps Q low, stirring prevents local
high concentrations of the precipitating agent.
Precipitation at a pH near the acidic end of the
pH range in which the precipitate is quantitative.
Many precipitates are more soluble at the lower (more acidic) pH values
and so the rate of precipitation is slower.
Digestion of the precipitate.
(Also the digestion period results in some improvement in the internal
perfection of the crystal structure [sometimes called ripening], here
some internal foreign atoms may be expelled).
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in Precipitates No discussion of gravimetric
analysis would be complete without some discussion of the impurities
which may be present in the precipitates.
This is anything unwanted which precipitates with
the thing you do want. Coprecipitation occurs to some degree in
every gravimetric analysis (especially barium sulfate and those
involving hydrous oxides). You cannot avoid it - all you can do
is minimize it by careful precipitation and thorough washing.
Here unwanted material is adsorbed onto the surface
of the precipitate. Digestion of a precipitate reduces the amount
of surface area and hence the area available for surface adsorption.
Washing can also remove surface material.
This is a type of coprecipitation in which impurities
are trapped within the growing crystal.
Sometimes a precipitate standing in contact with the
mother liquor becomes contaminated by the precipitation of an impurity
on top of the desired precipitate.
Problems with coprecipitation and
surface adsorption may be reduced
by careful washing of the precipitate.
With many precipitates, peptization
occurs during washing. Here part of
the precipitate reverts to the colloidal
This results in the loss of part of the
precipitate because the colloidal form
may pass through on filtration. By
washing with ice cold water this can be minimized
Generally the solids are dried at
about 120oC but conditions for
drying can vary considerably. To
determine the correct drying regime,
a thermogravimetric balance may be used.
Jacob Berzelius (1779 - 1848), considered the leading chemist
of his time, developed much of the apparatus and many of the
techniques of 19th century analytical chemistry. Examples
include the use of ashless filter paper in gravimetry, the
use of hydrofluoric acid to decompose silicates, and the use
of the metric system in weight determinations. He performed
thousands of analyses of pure compounds to determine the atomic
weights of most of the elements known then. Berzelius
also developed our present system of symbols for elements
You may find reference to the gravimetric factor in some
texts - this is the ratio of RMM of substance sought to that of substance
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Worked Examples and Problems
A certain barium halide exists as the hydrated salt BaX2.2H2O,
where X is the halogen. The barium content of the salt can be determined
by gravimetric methods. A sample of the halide (0.2650 g) was dissolved
in water (200 cm3) and excess sulfuric acid added. The mixture
was then heated and held at boiling for 45 minutes. The precipitate
(barium sulfate) was filtered off, washed and dried. Mass of precipitate
obtained = 0.2533 g. Determine the identity of X.
The precipitate is barium sulfate. The first stage is to
determine the number of moles of barium sulfate produced, this will, in
turn give us the number of moles of barium in the original sample.
Relative Molecular Mass of barium sulfate
= 137.34 (Ba) + 32.06 (S) + (4 x 16.00) (4 x O)
Number of moles
= mass / RMM
= 0.2533 / 233.40
= 1.09 x 10 -3
This is the number of moles of barium present in the
precipitate and, therefore, the number of moles of barium in the original
sample. Given the formula of the halide, (i.e. it contains one barium
per formula unit), this must also be the number of moles of the halide.
From this information we can deduce the relative molecular mass of the
original halide salt:
RMM = mass / number of moles
= 0.2650 / 1.09 x 10-3
The relative atomic mass of 2 X will be given by the
RMM of the whole salt - that of the remaining components; So RAM of
2 X = 244.18 - 173.37 = 70.81
2 X = 70.81, so X = 35.41.
The RAM of chlorine is 35.45 which is in good agreement
with the result obtained and hence the halide salt is hydrated barium
chloride and X = Chlorine
1. A sample (0.203 g) of hydrated magnesium chloride (MgClm.nH2O)
was dissolved in water and titrated with silver nitrate solution (0.100
mol dm-3), 20.0 cm3 being required. Another sample of the hydrated
chloride lost 53.2 % of its mass when heated in a stream of hydrogen chloride,
leaving a residue of anhydrous magnesium chloride. Calculate the values
of m and n
(Answer: m = 2, n = 6)
2. When an sample of impure potassium chloride (0.4500g)
was dissolved in water and treated with an excess of silver nitrate,
0.8402 g of silver chloride was precipitated. Calculate the percentage
KCl in the original sample.
(Answer: 97.12 %)