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Lecture 3
L t
Gravimetry
G
i t

Gravimetric Methods of Analysis
are quantitative methods based on
determining the mass of a pure
compound to which the analyte is
chemically related
calculations are based on stoichiometry

Types of Gravimetric Methods
1
...
Volatilization Methods
3
...
Precipitation Methods
p

Types of Gravimetric Methods
Particulate Methods
the analyte is determined following
its removal from the sample matrix
by filtration or extraction

Types of Gravimetric Methods
Volatilization Methods
the analyte ( its decomposition p
y (or
p
product)
)
are volatilized at a suitable temperature

the volatile product is then collected and
p
weighed

Types of Gravimetric Methods
Electrogravimetric Methods
the analyte is deposited as a solid film
on one electrode in an electrochemical
cell

Types of Gravimetric Methods
Precipitation Methods
based on the formation of an insoluble
compound following the addition of a
p
precipitating reagent (or p
p
g
g
( precipitant)
p
)
to a solution of analyte
the analyte is converted to a sparingly
soluble salt

Steps Common to All Gravimetric
Precipitation M th d
P i it ti Methods
1
...
Precipitation
3
...

4 Filtration
5
...
Drying or ignition
7
...
Calculation

Properties of Precipitates and
Precipitants
Specific – rare; react only with one chemical
species

Selective – more common; react with a
limited number of species
p

Properties of Precipitates and
Precipitants
Give a product with the f ll
d
h h following
characteristics:
• readily filtered and washed free of
contaminants
• low solubility
• unreactive with constituents of the atm
• high purity and known composition after
g p
y
p
drying or ignition

Controlling Particle Size
Success and ease of filtration is dependent upon
the size of the precipitate’s particle:
• Ions in solution – 10-8 cm
7
4
• Colloidal particles – 10-7 to 10-4 cm in diameter

- difficult to filter
- particles invisible to the
naked eye
k d

Controlling Particle Size
• Colloidal particles – no tendency to settle down
• C t lli particles – >10-4 cm
Crystalline
ti l
10 4
- generally desirable in
gravimetric work
- easy to filter and wash
- usually purer than ppts
made up of fine particles
- tends to settle
spontaneously

What Factors Determine Particle
Size?
mechanism of ppt formation is still not fully
pp
f y
understood
influenced by experimental variables such as pp
y p
ppt
solubility, temperature, reactant concentrations,
and rate at which reactants are mixed
d
hi h
i d

What Factors Determine Particle
Size?
Von W i
V Weimarn Equation
E
ti

Q −S
RSS =
S
particle size is related to a single property
called relative supersaturation (RSS)
measure of the extent to which a solution (or
localized region in solution) contains more
g
)
dissolved solute than that expected at
equilibrium

Von Weimarn Equation
Q −S
RSS =
S
•Q

solute’s actual concentration

• S

solute’s equilibrium concentration
l t ’
ilib i
t ti

• Q–S

measure of solute’s supersaturation
when precipitation begins

What Factors Determine Particle
Size?
RSS – precipitate tends to be colloidal
RSS – crystalline solid is more likely

∴ to increase particle size, minimize RSS during
size
precipitation

Ways to Minimize RSS
To minimize RSS
Q
conduct pptn in
pp
dilute solution of
analyte
add precipitant
slowly with vigorous
y
g
stirring

S
usually increases
y
at high temperature
manipulate the pH

How Do Precipitates Form?
precipitate formation involves physical and
chemical process
Physical Process Consists of Two Events:
h
C
fT
• Nucleation
• Particle growth
further precipitation involves a competition
between the two physical processes

Nucleation
when few ions, atoms, molecules come
together to form a stable solid/particle

Two types:
• Spontaneous – no external factors
• I d
Induced – provide site f nucleation b adding
d
id it for
l ti by ddi
seed of crystal

Nucleation

Particle Growth

Summary
RSS Nucleation predominates larger
number of small particles ( g surface area)
p
(high
)

RSS Growth predominates smaller
number of large particles (low surface area)

Colloidal Precipitates
particles are so small they are not retained
by individual filters

to make it filterable
agglomerate

coagulate or

Coagulation

process when smaller particles of precipitates
h
ll
ti l
f
i it t
clump together to form larger particles
(flocculation)

Why Are Colloids Stable?
all particles present are either (+) or (-)
charged
g

the charge structures repel each other since
they are similarly charged

Colloidal Precipitates
Consider the following precipitation reaction:
AgNO
A NO3 (aq) + N Cl(aq)
NaCl

AgCl
A Cl(s) + N NO3(aq)
NaNO

NaCl acts as the precipitating reagent
AgCl tends to form colloidal precipitates

Start of Precipitation
initially chloride concentration is very low
due to the excess Ag+
g
on th surfaces th
the
f
there are i
ions with partially
ith
ti ll
unsatisfied bonding capacity which tend to
attract additional Ag+ to the surface
primary adsorbed layer

Start of Precipitation

counter-ion layer (NO3-) is present to just

balance the charge on the surface
g

Colloidal Precipitates
Start of Precipitation
AgCl particle
Primary adsorbed layer
Counter-ion layer
Water

Colloidal Precipitates
When enough or excess NaCl has been added:
AgCl particle
Na+

Na+
Cl-

Na+
Na+ ClNa+

Na+
Na+

AgCl(s) Cl- Na+
Na+

Na+

Primary adsorbed layer

ClNa+
Na+

Counter ion
Counter-ion layer

Colloidal Precipitates
Electrical Double Layer – imparts stability of
colloidal particles
- exerts electrostatic
repulsive forces that
prevent particles from
colliding and adhering
- ∴ coagulation cannot
occur

Rules on Selectivity of Ion
Adsorption
1
...

2 Concentration Effect
other factors being equal, the ion which is present in
greater concentration will be adsorbed
t
t ti
ill b d b d
preferentially

Rules on Selectivity of Ion
Adsorption
3
...
Ion Charge Effect
a multi-charged ion will be adsorbed more readily
than a singly-charged ion
5
...
5 F Na2SO4 solution
...
Using the rules,
PREFERENCE
y
Solubility

Ba2+, SO42‐
,

Concentration

Na+ > SO42‐

Common ion

Ba2+, SO42‐

Charge

2+
2‐
Ba2 , SO42

Size

Ba2+, SO42‐

Example 1

The preferred ion is SO42-
...
The colloid has the formula

BaSO4:SO42‐::Na+

Exercise 1
A d
drop of K2C O4 solution i added to 0 5 F A NO3
f
CrO
l i
is dd d
0
...
If the solubility of Ag2CrO4 is surpassed and
the conditions are favorable for colloid formation
formation,
what is the formula for the colloid?

Ans: Ag2CrO4:Ag+::NO3‐

Types of Colloids
1
...
Suspensoids
hydrophobic colloids
example: AgCl:Ag+::NO3-

How to Effect Coagulation
e- charge on surfaces must either b
h
f
t ith be
removed or neutralized

Can b i d d i t
C be induced in two ways:
1
...
Heating the solution (with stirring)
g
(
g)

Increase the Concentration of Ions
add inert electrolyte which [ ] of counter
ions in the vicinity of each particle
more ions
thickness
particles can approach and coagulate
amount of electrolyte needed to cause
spontaneous coagulation is called critical
coagulation
concentration
(flocculation
value)

Heating the Solution

[ ] of ions in primary layer

kinetic energy may be sufficient to
overcome electrostatic repulsion
∴ colloidal suspensions can often be coagulated
by h ti
b heating, stirring, and adding an electrolyte
ti i
d ddi
l t l t

Peptization of Colloids

process by which a coagulated colloid returns
to its dispersed state
happens during washing with H2O when the
concentration of the surrounding electrolyte
falls below its flocculation value

Peptization of Colloids
Problem:
Washing – minimize contamination
- risk of losing ppt from peptization
if pure H2O is used
Solution:
- wash ppt with a solution containing an
electrolyte that volatilizes when ppt is dried or
ignited
i it d

Summary
Precipitation of Colloidal
Precipitates
best from hot, stirred solutions
solutions containing sufficient electrolyte
filterability is i
fil
bili i improved if it i allowed to
d i is ll
d
stand in contact with the hot solution from
which it was formed , i e ≥ 1 hour (digestion
i
...

or Ostwald Ripening)

Crystalline Precipitates
Particle Size
Q
conduct pptn in
pp
dilute solution of
analyte
add precipitant
slowly with vigorous
y
g
stirring

S
usually increases
y
at high temperature
manipulate the pH

Summary
Types of Precipitates
1
...
Crystalline Precipitates

Contamination of Gravimetric
Precipitates
P i it t
1
...
Post-precipitation
impurity is a insoluble compound that ppts
pu ty s an soluble co pou d t at
after all or major part of the analytical ppt
has formed
minimize by removing the interfering ions
y
g
g

Types of Coprecipitation
1
...
Mixed-Crystal
2 Mixed Crystal Formation
3
...
Mechanical Entrapment

Surface Adsorption
normally soluble compound is carried out of
solution on the surface of colloid
usually consists of the 1o and the counter-ion
y
layer
can be minimized by digestion or
reprecipitation

Mixed-Crystal Formation
contaminant ion replaces an ion in the
lattice of a crystal
the 2 ions must have:
- same charge
- sizes that differ by no more than 5%
- the two salts must belong to the same
crystal type
can be minimized by removing the interfering
ion or using a different precipitant

Occlusion and Mechanical
Entrapment
compound is trapped within a pocket formed
during rapid crystal growth
confined to crystalline precipitates
can be minimized by lowering rate of
precipitation and digestion

Homogeneous Precipitation
techniques in which a ppt is formed by slow
generation of the precipitant homogeneously
throughout the solution
relative supersaturation
adjust pH to allow precipitant to act
Example: precipitation of Ba with sulfamic acid
SO42- is generated homogeneously by heating
HSO3NH2 + H2O

H+ + SO42- + NH4+

Drying and Ignition

the purpose of drying (120 – 150 oC in an oven) or
ignition in a muffle furnace (600 – 1200 oC) is to
get a material with exactly known chemical
structure so that the amount of analyte can be
accurately determined

Calculations: Gravimetric Factor
Concerned with 2 Measurements

1
...
Mass of product of known composition
p
p
stoichiometrically related to the analyte

Calculations
Calculate the mass of Fe contained in 0
...
70,
55
...
70 g Fe2O3 contains 2 x 55
...
500 g Fe2O3 x (2 x 55
...
70 g Fe2O3

= 0
...
1324 g sample in
concentrated HCl
...
Aft filt ti
id Fe
H by the dditi
f
After filtration
and washing, the residue was ignited at a high temperature
to give 0
...
Calculate (a) the % Fe (FM =
55
...
54) in the sample
...
2356 g sample containing only NaCl (FM = 58
...
23) yielded 0
...
32)
...


Problems
1
...
7406 g sample of impure magnesite, MgCO3, was decomposed
with HCl; the liberated CO2 was collected on calcium oxide and
found to weigh 0 1881 g Calculate the % Mg in the sample
0
...

sample
...
Ammoniacal nitrogen can be determined by treatment of the
sample with chloroplatinic acid; the product is slightly soluble
ammonium chloroplatinate:
H2PtCl6 +  2NH4+
6 

(
(NH4)2PtCl6 +  2H+

The precipitate decomposes on ignition, yielding metallic platinum
and gaseous products:
(NH4)2PtCl6

Pt(s) +  2Cl2(g) +  2NH3(g) +  2HCl(g)

Calculate the percentage of ammonia in a sample if 0
...
5881 g of platinum
...
A 0
...
4430 g
...
3181 g
...


4
...
The amount of Fe in the sample is to be determined
g
gravimetrically by isolating it as Fe2O3
...
A 0
...
The aluminum and
g
p
p
y
yq
magnesium were precipitated with 8‐hydroxyquinoline
...
8154 g
...
The weight of this mixed solid was found to be 1
...

Calculate the % w/w Al and % w/w Mg in the alloy
...
The earliest determinations of elemental atomic weights were
accomplished gravimetrically
...
16539
g was dissolved and the Br− precipitated as AgBr, yielding 12
...
What is the atomic weight for Mn if the atomic weights for Ag
and Br are taken to be 107
...
904 respectively
...
What weight of sulfur (FM = 32
...
40) precipitate will be equal to half of
the percentage sulfur in the sample

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