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. In other cases, it may be easier to
remove the analyte by evaporation. The analyte might be collected—perhaps in a
cryogenic trap or on some absorbent material such as activated carbon—and
measured directly. Or, the sample can be weighed before and after it is dried;
the difference between the two masses gives the mass of analyte lost. This is
especially useful in determining the water content of complex materials such
as foodstuffs. Procedure
The sample is dissolved, if it is not already in solution.
The solution may be treated to adjust the pH. If it is known that species are
present which interfere, the sample might require treatment with a different
reagent to remove these interferents. The precipitating reagent is added at a
concentration that favors the formation of a "good" precipitate. This may
require low concentration, extensive heating, or careful control of the pH.
Digestion can help reduce the amount of coprecipitation.
After the precipitate has formed and been allowed to "digest", the solution
is carefully filtered. The filter is chosen to trap the precipitate; smaller
particles are more difficult to filter. Depending on the procedure followed, the
filter might be a piece of ashless filter paper in a fluted funnel, or a
filter crucible. Filter paper is convenient because it does not typically
require cleaning before use; however, filter paper can be chemically attacked
by some solutions, and may tear during the filtration of large volumes of
solution. The alternative is a crucible whose
bottom is made of some porous material, such as sintered glass, porcelain or
sometimes metal. These are chemically inert and mechanically stable, even at
elevated temperatures. However, they must be carefully cleaned to minimize
contamination or carryover(cross-contamination).
Crucibles are often used with a mat of glass or asbestos fibers to trap small
particles. After the solution has been filtered, it
should be tested to make sure that the analyte has been completely
precipitated. This is easily done by adding a few drops of the precipitating
reagent; if a precipitate is observed, the precipitation is incomplete.
After filtration, the precipitate – including the filter paper or crucible –
is heated. This achieves three purposes: The remaining moisture is removed.
Secondly, the precipitate is converted to a more chemically stable form. For
instance, calcium ion might be precipitated using oxalate ion, to
produce calcium oxalate; it might then be heated to convert it into the oxide.
It is vital that the empirical formula of the weighed precipitate be known, and
that the precipitate be pure; if two forms are present, the results will be
inaccurate. The precipitate cannot be weighed with
the necessary accuracy in place on the filter paper; nor can the precipitate be
completely removed from the filter paper in order to weigh it. The precipitate
can be carefully heated in a crucible until the filter paper has burned away;
this leaves only the precipitate. After the precipitate is allowed to
cool, it is weighed. The mass of the crucible is subtracted from the combined
mass, giving the mass of the precipitated analyte. Since the
composition of the precipitate is known, it is simple to calculate the mass of
analyte in the original sample. Example
A chunk of ore is to be analyzed for sulfur content. It is treated with
concentrated nitric acid and potassium chlorate to convert all of the sulfur to
sulfate. The nitrate and chlorate are removed by treating the solution with
concentrated HCl. The sulfate is precipitated with barium and weighed as
BaSO4. 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.
Disadvantages 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. Compare this with hardy
methods such as spectrophotometry and one will find that analysis by these
methods is much more efficient. Steps in a Gravimetric Analysis
After appropriate dissolution of the sample the following steps should be
followed for successful gravimetric procedure:
1. Preparation of the Solution: This may involve several steps including
adjustment of the pH of the solution in order for the precipitate to occur
quantitatively and get a precipitate of desired properties, removing
interferences, adjusting the volume of the sample to suit the amount of
precipitating agent to be added. 2.Precipitation: This requires addition
of a precipitating agent solution to the sample solution. Upon addition of the
first drops of the precipitating agent, supersaturation occurs, then nucleation
starts to occur where every few molecules of precipitate aggregate
together forming a nucleous. At this point, addition of extra precipitating
agent will either form new nuclei or will build up on existing nuclei to give
a precipitate. This can be predicted by Von Weymarn ratio where, according to
this relation the particle size is inversely proportional to a quantity
called the relative supersaturation where
Relative Supersaturation =/S The Q is the concentration of reactants
before precipitation, S is the solubility of precipitate in the medium
from which it is being precipitated. Therefore, in order to get particle
growth instead of further nucleation we need to make the relative
supersaturation ratio as small as possible. The optimum conditions for
precipitation which make the supersaturation low are:
a. Precipitation using dilute solutions to decrease Q b. Slow addition of
precipitating agent to keep Q as low as possible c. Stirring the solution during
addition of precipitating agent to avoid concentration sites and keep Q low d.
Increase solubility by precipitation from hot solution e. Adjust the pH in
order to increase S but not a too much increase np as we do not want to loose
precipitate by dissolution f. Usually add a little excess of the precipitating
agent for quantitative precipitation and check for completeness of the
precipitation 3. Digestion of the Precipitate: The
precipitate is left hot for 30 min to 1 hour in order for the particles to be
digested. Digestion involves dissolution of small particles and reprecipitation
on larger ones resulting in particle growth and better precipitate
characteristics. This process is called Ostwald ripening. An important advantage
of digestion is observed for colloidal precipitates where large amounts of
adsorbed ions cover the huge area of the precipitate. Digestion forces the small
colloidal particles to agglomerate which decreases their surface area and thus
adsorption. You should know that adsorption is a major problem in
gravimetry in case of colloidal precipitate since a precipitate tends to
adsorb its own ions present in excess, Therefore forming what is called a
primary ion layer which attracts ions from solution forming a secondary or
counter ion layer. Individual particles repel each other keeping the colloidal
properties of the precipitate. Particle coagulation can be forced by either
digestion or addition of a high concentration of a diverse ions strong
electrolytic solution in order to shield the charges on colloidal particles and
force agglomeration. Usually, coagulated particles return to the colloidal state
if washed with water, a process called peptization.
4. Washing and Filtering the Precipitate: It is crucial to wash the
precipitate very well in order to remove all adsorbed species which will add to
weight of precipitate. One should be careful nor to use too much water since
part of the precipitate may be lost. Also, in case of colloidal precipitates
we should not use water as a washing solution since peptization would occur.
In such situations dilute nitric acid, ammonium nitrate, or dilute acetic acid
may be used. Usually, it is a good practice to check for the presence of
precipitating agent in the filtrate of the final washing solution. The presence
of precipitating agent means that extra washing is required. Filtration should
be done in appropriate sized Goosh or ignition filter paper.
5. Drying and Ignition: The purpose of drying or ignition in a muffle furnace
at temperatures ranging from 600-1200 oC is to get a material with exactly known
chemical structure so that the amount of analyte can be accurately determined.
6. Precipitation from Homogeneous Solution: In order to make Q minimum we
can, in some situations, generate the precipitating agent in the precipitation
medium rather than adding it. For example, in order to precipitate iron as
the hydroxide, we dissolve urea in the sample. Heating of the solution
generates hydroxide ions from the hydrolysis of urea. Hydroxide ions are
generated at all points in solution and thus there are no sites of
concentration. We can also adjust the rate of urea hydrolysis and thus control
the hydroxide generation rate. This type of procedure can be very advantageous in
case of colloidal precipitates. Solubility in Presence of Diverse Ions
As expected from previous information, diverse ions have a screening effect on
dissociated ions which leads to extra dissociation. Solubility will show a
clear increase in presence of diverse ions as the solubility product will
increase. Look at the following example: Find the solubility of AgCl in 0.1 M
NaNO3. The activity coefficients for silver and chloride are 0.75 and 0.76,
respectively. AgCl(s) = Ag+ + Cl-
We can no longer use the thermodynamic equilibrium constant and we have to
consider the concentration equilibrium constant or use activities instead of
concentration if we use Kth: Ksp = aAg+ aCl-
Ksp = [Ag+] fAg+ [Cl-] fCl- 1.0 x 10-10 = s x 0.75 x s x 0.76
s = 1.3 x 10-5 M We have calculated the solubility of
AgCl in pure water to be 1.0 x 10-5 M, if we compare this value to that
obtained in presence of diverse ions we see % increase in solubility = {(1.3 x
10-5 – 1.0 x 10-5) / 1.0 x 10-5} x 100 = 30% Therefore, once again we have an
evidence for an increase in dissociation or a shift of equilibrium to right in
presence of diverse ions. References
^ Holler, F. James; Skoog, Douglas A.; West, Donald M.. Fundamentals of
analytical chemistry. Philadelphia: Saunders College Pub. ISBN
0-03-005938-0. ^ Hulanicki A.. Reactions of Acids and
Bases in Analytical Chemistry. Horwood. ISBN 0-85312-330-6.
External links Gravimetric Quimociac Technique