FE 211
ANALYTICAL CHEMISTRY
EXPERIMENT-1
THE GRAVIMETRIC
DETERMINATION OF SULFATE IN
A SOLUBLE SAMPLE
Ugur ASİT
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PURPOSE:
In this experiment, we will determinate the value of SO4-2 in the
sample solution. By this way we will become familiar with basic
processes of gravimetric analysis.
THEORY:
Gravimetric method is one of the three subgroups of quantitative
analytical method. It is based on measuring the weight of a known
substance which is related to the analyte and, by using the relation,
determining the quantity.
Gravimetric analyses divides into three subgroups; precipitation,
volatilization, electro deposition. In this experiment we are going to use
precipitation method.
As we all know solubility varies for all compounds. There are such
compounds that nearly insoluble. Precipitation method is based on this
principle that precipitating a soluble analyte as insoluble compound of it
and weighing the precipitation.
In this experiment we are going to determine the SO4- quantity by
precipitating it as BaSO4 for that BaSO4 has a solubility of 0.4 mg/100 g
water. We are going to use BaCl2 and as barium source.
Ba+2+SO4-2  BaSO4(s)
We will separate BaSO4 crystalline precipitate by filtering and wash
it. After drying and ignition we’ll weigh the pure 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,
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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 (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.
MATERIALS:
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BaCl2 solution (5 g / L).
HCl solution.
AgNO3 solution.
Ashless filtering paper.
Flask.
Porcelain crucible.
Beaker.
Water.
Bunsen burner.
PROCEDURE:
 10 ml of sample was taken in a beaker.
 50 ml water was added.
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 Add 4 ml 6 M HCL
 This solution was heated until boiling.
 And 100 ml of BaCI2.2H2O was heated until boiling an other
beaker.
 Then BaCI2.2H2O was added quickly into hot sample.
 Solution was waited and cooled. Until the top of solution became
clean.
 After that solution was decanted.
 Solution was filtered and washed with hot water three times.
 Filteted solution was tested with AgNO3 solution. If color does not
change.solution was waited one week to dry.
 One week later, filter paper and solid substance were put in
porcelain crucible. and they were chared off about one hour in
bunsen burner.
 Then porcelain crucible was transfered at oven and ignated at 800
0
C.
 finally,we had only solid (Ba2SO4 (s) ) and weighted.
 Results were recorded.
RESULTS:
Weight of sample and porcelain crucible =
20.8282 g
Weight of porcelain crucible
= 20.1516 g
Weight of (Ba2SO4 (s) )
=
0.6766
DISCUSSION:
Gravimetric analysis is one of the oldest analytical techniques and
for this reason is referred to as a "classical method." Gravimetric
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procedures are usually very accurate, but more tedious than other
methods. The only major equipment needed for gravimetric analysis
is an accurate balance.
Sulfur can be determined gravimetrically. Sulfur is found in
minerals such as iron pyrite, gypsum and coal. Coal may contain as
much as 4% sulfur in various forms: pyritic sulfur , sulfur in
sulfates and in a variety of organic forms, such as
thiophenols and thiophenes. When coal with high sulfur content is
burned sulfur dioxide is formed which is a major primary air
pollutant. Obviously the value of coal as a fuel decreases as the
sulfur content increases. For this reason, the analysis of coal for
sulfur is extremely important.
In any precipitation gravimetric analysis, the species to be
deter-mined is reacted with a reagent to yield a product that is:
a) of known composition,
b) is relatively pure, and
c) of low solubility. Knowing
the mass of the dried product and the mass of the original sample
and their formula weights (to create a gravimetric factor), the percent
of the species to be determined can be calculated..
In the analysis for sulfate, the sample to be analyzed is brought into
solution and reacted with a solution of BaCl2 , a source of Ba 2+ , to
yield BaSO4 as a white precipitate. In the procedure, the solution is
first acidified with HCl (aq) to:
a) prevent the precipitation of BaCO3
and Ba(OH)2 , and
b) aid the formation of larger precipitate crystals because fewer crystallite
nuclei are formed. Also, the BaCl2 must be added quickly to minimize
occlusion of foreign ions in the precipi-tate crystals. The crystalline BaSO
4 precipitate is then purified by digestion: a process in which the solution
from which the precipi-tate came (the mother liquor) and the precipitate
are heated to dissolve smaller precipitate particles which subsequently
reprecipi-tate on larger particles. The digestion process improves
filterability because of the larger crystalline particles, and the precipitate
is isolated by gravity filtration through paper.
If paper is used to isolate the precipitate, it must be removed
completely without losing any precipitate. This can be done by
using an filter paper which can be burned away. Care must
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be taken in this process to avoid flaming and to prevent conversion of
BaSO4 to BaS, a yellow precipitate of lower mass than BaSO4 . The
final ignition removes the carbon residue and any volatile impurities
carried down with the barium sulfate precipitate. Ignition is continued
until the mass of the crucible plus precipitate is constant.
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