Ch 27 Gravimetric Analysis
Quantitative Analysis
Classical
• Gravimetric – mass of analyte
• volumetric (or titrimetric) – volume of solution
containing sufficient reagent to react completely with
analyte
Instrumental
• Electroanalytical – properties resulting from Ox./Red.
behavior of analyte
• Spectroscopic – measures electromagnetic radiation
absorbed or emitted by analyte
• Chromatographic – separates a mixture into its
components
gravi – metric
(weighing - measure)
Chemical analysis based on the determination of
weight of a substance of known composition (the final
product) that is chemically related to the analyte.
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Gravimetric Analysis
Precipitation Method
• precipitation method: Dissolved analyte
converted to sparingly soluble precipitate
Ag+ + Cl- à AgCl(s)
• volatilization method: Analyte is volatilized at
suitable temperature; the volatile product is
collected and weighted
NaHCO3(aq)+H2SO4(aq)àCO2(g)+H2 O(l)+NaHSO4(aq)
(1) The desired substance: completely precipitated.
"common ion" effect can be utilized:
Ag+ + ClAgCl(s)
excess of Cl- which is added
(2) The weighed form: known composition.
(3) The product: "pure", easily filtered.
CO2(g)+2NaOH(s)àNa2CO3(s)+H2 O(l)
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Filtration
Steps in Gravimetric Analysis
(precipitation)
Dry and weigh sample
Dissolve sample
Add precipitating reagent in excess
Coagulate precipitate usually by heating & wait for
some time (Aging)
5) Filtration-separate precipitate from mother liquor
6) Wash precipitate
7) Dry and weigh to constant weight (0.2-0.3 mg)
avoid colloidal suspension, ideally,
produce crystals
1)
2)
3)
4)
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Mother liquor
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Particle Size & Filterability - Precipitates
Colloids – (d = 10-6 to 10 -4 mm)
-invisible to naked eye
-don’t settle out of solution
-difficult or impossible to filter
Particles – (d = 0.10 mm or greater)
-spontaneously settle out of solution
-readily filtered and washed free of impurities
-more desirable (typically of higher purity than
colloids)
Mechanisms of Precipitation
Two competing processes:
(1) Nucleation
When a small number of
ions, atoms, molecules
initially unite.
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(2) Particle growth
The 3-D growth of a
particle nucleus into a
larger crystal
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Particle Size & Filterability - Control
Particle Size & Filterability - Control
Relative supersaturation (RSS)
RSS = (Q-S)/Q
Where Q = concentration of solute; S =
equilibrium solubility of solute
RSS can be used estimate/control the type of
precipitate that is formed:
large: nucleation, small particles (colloids)
small: particle growth, crystalline solid likely
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Techniques to promote crystal growth
Particle Size & Filterability - Control
(1) Raising the temperature (increase S).
(2) Adding precipitant slowly with vigorous
mixing (decrease Q).
(3) Keeping the volume of solution large
(decrease Q).
The goal is to form crystalline precipitates so RSS
must be minimized.
Recall:
RSS = (Q-S)/Q = 1-S/Q
Q = [solute]
S = solute’s Equil. Sol.
This can be done by:
Increasing S
Decreasing Q
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Treatment of colloidal Precipitates
pH control of precipitation
(1) Increasing the electrolyte concentration
Ca2+ + C2O42- D CaC2O4 (s)
H2C2O4 D 2 H + + C2O42-
•Decreasing the vol. of the counter-ion
layer
•Increasing the chance for coagulation
Homogeneous Precipitation
The precipitant is generated slowly by a chemical
reaction.
Fe3+ + 3 HCO2- D Fe(HCO2)3⋅nH2O(s)
HCOOH+OH- D HCO2-+H2O
(NH2)CO + 3 H2O + heat D OH- + CO2(g)+ 2NH4+
Colloidal Particle of AgCl
Boundary of ionic
atmosphere
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Treatment of colloidal Precipitates
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Treatment of colloidal Precipitates
(2) Using a volatile electrolyte
(3) Digestion and aging
Avoid peptization
Ex. AgCl, wash with HCl. Drying precipitate at
110°C will remove HCl.
This displace the less volatile, excess counter ion.
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Digestion: Heating the solution for about an hour after
precipirate formation. This helps to remove weakly
bound water
Aging: Storing the solution, unheated, overnight. This
allows trapped contaminates time to “work their way
out”.
Both can result in a denser precipitate that is easier to
filter.
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Representative Gravimetric Analyses
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Advantages/Disadvantages
•
•
•
•
Experimentally simple and elegant
Accurate
Precise (0.1-0.3 %)
Macroscopic technique-requires at least
10 mg ppt to collect and weigh properly
• Time-consuming (1/2 day?)
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Calculation
Calculation
• Design of experiment
• Content Calculation
• Evaluation of the results
• % of analyte, % A
• %A = weight of analyte
weight of sample
x 100
• weight of ppt directly obtained à%A
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Gravimetric Factor
How Do We Get %A from ppt?
• G.F. =
• % A = weight of ppt x gravimetric factor (G.F.) x 100
weight of sample
G.F. =
a (FW of analyte)
b (FW of precipitate)
• Analyte
CaO
FeS
UO2(NO3)2.6H2O
Cr2O3
a (FW of analyte)
b (FW of precipitate)
• G.F. = # gms of analyte per 1 gm ppt
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ppt
G.F.
CaCO3
BaSO4
U3O8
Ag2CrO4
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Gravimetric Factor
• Analyte
CaO
FeS
UO2(NO3)2
Cr2O3
ppt
CaCO3
BaSO4
U3O8
Ag2CrO4
Exercise
G.F.
CaO/CaCO3
FeS/BaSO4
3UO2(NO3)2/U3O8
Cr2O3/2Ag2CrO4
• Consider a 1.0000 g sample containing 75%
potassium sulfate (FW 174.25) and 25%
MSO4. The sample is dissolved and the
sulfate is precipitated as BaSO4 (FW
233.39). If the BaSO4 ppt weighs 1.4900 g,
what is the atomic weight of M 2+ in MSO4?
• ANS: Mg2+
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