3. Gravimetric Analysis
Gravimetric methods of analysis, based upon
the measurement of mass (weight).
Two major types:
1- In precipitation methods, the analyte is
converted to a sparingly شحيح soluble precipitate that
is filtered, washed free of impurities, and
converted to a product of known composition by
suitable heat treatment.This product is then
weighed .
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4. In a precipitation method for determining Ca in
natural water recommended by the (Association
of Official Analytical Chemist),
an excess of oxalic acid H2C2O4, is added to a
carefully measured volume of the sample.
The addition of ammonia then causes essentially
all the calcium in the sample to precipitate as Ca
oxalate.
The reaction :
Ca+2(aq.) + C2O4
2-(aq.) CaC2O4 (s)
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5. The ppt. is collected in a weighed filtering crucible, dried ,
and then ignited at red heat. The process converts the ppt.
entirely to calcium oxide :
CaC2O4 (s) CaO (s) + CO (g )+ CO2(g)
The crucible and ppt. are cooled, weighed, and the weight
of calcium oxide is det. By subtraction of the known
weight of the crucible.
The calcium content of the sample is then computed from
the stoichiometry of the process.
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6. 2- In volatilization methods, the analyte or
its decomposition products are volatilized
at a suitable temperature.
The volatile is then collected and weighed,
or alternatively the weight of the product is
determined indirectly from the weight loss
of the sample .
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7. Example:
is the det. of the NaHCO3 content of antacid tablets
A weighed quantity of the finely ground sample is treated
with dilute H2SO4 to convert the NaHCO3 to
CO2
NaHCO3(aq) + H2SO4(aq) NaHSO4(aq)
H2O(l) + NaHSO4(aq)
This reaction is carried out in a flask that is connected to a
weighed absorption tube containing an absorbent that
retains the carbon dioxide selectively as it is evolved as
the solution is heated.
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8. The difference in weight of the tube before and after
absorption is used to calculated the amount of sodium
.
hydrogen carbonate (NaHCO3)
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9. The result of a gravimetric analysis is
generally computed from two experimental
measurements:
---The weight of sample,
--- the weight of a product of known
composition. When the product is the analyte ,
the concentration is given by the
( Analyte) Weight of sample
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CALCULATION OF RESULTS
FROM GRAVIMETRIC DATA
Percent A % = x 100 %
Weight of A
Example page 159,160 (177- 181)
11. Homogeneous Precipitation
In homogeneous precipitation, the precipitant is
generated slowly by a chemical reaction.
Digestion
Following precipitation , most procedures call
for a period of standing in the presence of the
hot mother liquor. This treatment called
digestion, promotes حعحزز slow recrystallization of
the precipitate.
Particle size increases and impurities tend to be
expelled طرد from the crystal.
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12. Purity
Absorbed impurities are bound to the surface
of a crystal. Adsorbed impurities (within the
crystal) are classified as inclusions or
occlusions.
Inclusions إ
يتحاء are impurity ions that randomly
occupy sites in the crystal lattice.
Occlusions إقفحل are pockets of impurity that are
trapped inside the growing crystal.
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13. Adsorbed, occluded, and included impurities
are said to be
co-precipitated i.e.,
The impurity is precipitated along with the
desired مطلوب product, even though the solubility
of the impurity has not been exceeded.
Some impurities can be treated with a masking agent
to prevent them from reacting with the precipitant.
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14. many procedures call for washing away the mother liquor االصلى المحلبل,
redissolving the precipitate, and reprecipitating the product.
During the second precipitation ,the concentration of impurities in
the solution is lower than during the first precipitation, and the
degree of co-precipitation therefore tends to be lower.
Ca2+
+ 2 RH CaR2(s) + 2 H+
Analyte N-p-chlorophenyl- Precipitate
cinnamohydroxamc acid
Mn2+
+ 6 CN Mn(CN)6
Impurity Masking agent Stays in solution
4-
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15. Peptization
Some precipitates can be washed with water, but many
require electrolyte to maintain coherence تملسك حبقى .
For these precipitates, the ionic atmosphere is required
to neutralize the surface charge of the tiny particles.
If electrolyte is washed away with water, the charged
solid particles repel صحد each other and the product breaks
up حتكسر .
This breaking up, called peptization, results in loss of
product through the filter.
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post-precipitation
impurities collected on the product while a precipitate is
standing in the mother liquor
16. Example:
AgCl will peptize if washed with water, so it is washed
with dilute HNO3 instead .
Electrolyte used for washing must be volatile, so it will be
lost during drying.
Volatile electrolytes include HNO3, HCl, NH4NO3, NH4Cl,
and (NH4)2CO3
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17. Product Composition
The final product must have a known, stable
composition.
A hygroscopic حلححللمامحمي substance is one that picks up
water from the air and is therefore difficult to weight
accurately.
Many precipitates contain a variable quantity of water
and must be dried under conditions that give a known
(possibly zero) stoichiometry of H2O.
Ignition (strong heating) is used to change the
chemical form of some precipitates.
For example, igniting Fe(HCO2)3.nH2O at 850°C for 1 h
gives Fe2O3, and igniting Mg(NH4)PO4.6H2O at 1100°C
gives Mg2P2O7.
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19. Properties of Precipitates and Precipitating
Reagents
The ideal precipitating reagent should react with the
analyte to give a product that is:
1. readily filtered and washed free of contaminants.
2. of sufficiently low solubility so that no significant
loss of the analyte occurs during filtration and washing.
3. unreactive with constituents of the atmosphere.
4. of known composition after it is dried or, if
necessary, ignited.
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Properties of Precipitates and
Precipitating Reagents
20. Colloidal Precipitates
Colloidal suspensions are too small particles to be
readily filtered and are not usable for gravimetric analysis.
The stability of most such suspensions can be decreased
by heating, by stirring, and by adding an electrolyte to bind
together to form an amorphous mass اللشحكاح that settles out
of solution and is filterable.
The process of converting a colloidal suspension into a
filterable solid is called coagulation or
agglomeration تكت.
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21.
22. A general equation for calculating the
results of a gravimetric analysis is
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Example page (177- 181)
23. Example:
An iron ore was analyzed by dissolving a 1.1324 g sample
in concentrated HC1. The resulting solution was diluted
with water, and the iron(III) was precipitated as the hydrous
oxide Fe2O3.xH2O by the addition of NH3.
After filtration and washing, the residue was ignited at a
high temperature to give 0.5394 g of pure Fe2O3(FW =
159.69). Calculate
(a) the percent Fe (FW = 55.847) and
(b) the percent Fe3O4 (FW = 231.54) in the sample.
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