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## Gravimetric and Combustion Analysis

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### Gravimetric Analysis In gravimetric analysis, the analyte is reacted and the product is collected, massed, and then the mass of product is used to back calculate the ... – PowerPoint PPT presentation

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Title: Gravimetric and Combustion Analysis

1
Gravimetric and Combustion Analysis
2
Gravimetric Analysis
• In gravimetric analysis, the analyte is reacted
and the product is collected, massed, and then
the mass of product is used to back calculate the
initial moles of analyte.
• There are 2 kinds of gravimetric analysis
precipitation, and volatilization

3
Gravimetric Analysis
• Precipitation is the common gravimetric analysis
that all students conduct. Here a slightly
soluble or insoluble product is precipitated out,
then dried and massed.
• The mass of product is then used to calculate the
quantity of analyte in the original sample.

4
Gravimetric Analysis
• Volatilization occurs when the product is a gas,
which is typically collected and massed.
• Carbon dioxide is the common volatilization
product in acid/base reactions or in
• It is easily collected via a second reaction and
massed.

5
Gravimetric Analysis
• Gravimetric analysis is still used to produce
standards as well as for some special reactions.
• Although highly accurate, it can be time and
labor consuming.

6
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7
Precipitation Reactions
• If you conduct a gravimetric analysis, one of the
most important things is to pick the right
precipitate to form.
• This precipitate should be fairly insoluble, form
nice crystals that are easily filtered with no
impurities, have a known composition, and be
stable upon drying.
• That is actually a tall order, so the conditions
are carefully controlled to maximize the
precipitate yield.

8
Precipitation Crystals
• One of the most important aspects of a good
gravimetric analysis is the particle size of the
precipitate.
• Ideally, it forms nice, large crystals which are
easily collected, dont clog the filter, and
dont collect as many impurities due to their
smaller surface area.

9
Precipitation Crystals
• Large crystals dispersed in a solution are called
a crystalline suspension, and the particles will
settle easily.
• Large crystals may have diameters of 0.1 mm or
more.
• Small, fine crystals and colloids are the most
difficult to collect.

10
Precipitation Crystals
• Colloids are particles so small, with diameters
of less than 10-4 cm, that they cant be seen
until you shine a flashlight through them, and so
small they go right through most filters.
• Colloidal particles disperse throughout a
solution to form colloidal suspensions.
• As stated, they are too small to be seen, and
they must be treated to force the colloid
particles to form filterable crystals.

11
Precipitation Crystals
• What is interesting in what we see as a mature
science, is that the mechanism of precipitate
formation and crystal size is not truly
understood.
• However, there are several factors which help
determine the particle size of a ppt.

12
Factors in Particle Size
• solubility of the ppt
• temperature
• reactant concentrations
• electrolyte concentrations
• how quickly the reactants are mixed together to
form the ppt

13
Relative Supersaturation
• There is an equation which relates the particle
size to the relative supersaturation of a
solution

where S is the solubility of the ppt and Q is
the actual concentration of the solute in sln
14
Relative Supersaturation
• If Q is higher than S, then the solution is
supersaturated.
• As two reagents are mixed, it is actually typical
to have supersaturation, even if it is just
localized.

15
Relative Supersaturation
• But the higher the relative supersaturation, the
more likely it is to have colloidal particles,
whereas the lower the relative supersaturation,
the more likely it is to have solid crystals.
• So the trick during ppt reactions is to keep the
relative supersaturation low to minimize colloids.

16
• How does the relative supersaturation affect the
particle size of a ppt?

17
PPT Formation
• There are two ways that ppts form
• nucleation
• particle growth

18
PPT Formation
• In nucleation, a very small number of particles
stick together to form a stable solid.
• It may be as few as 4 particles that form this
stable solid.
• They may form spontaneously or they may form
around a small foreign particle such as dust.
• If the ppt forms mostly by nucleation, either a
colloid or very fine crystals will result.

19
PPT Formation
• In particle growth, more solute particles add to
a solid.
• If enough add to a nucleated solid, then it
becomes a crystal.
• As more particle growth occurs, the larger the
crystals become.
• Obviously, we want to maximize the process of
particle growth, as it forms large crystals.

20
PPT Formation and Supersaturation
• In highly supersaturated solutions, nucleation
occurs much faster than particle growth
• So colloids are quite common as well as very fine
crystals in these solutions

21
PPT Formation and Supersaturation
• To try to minimize the supersaturation
• have dilute solutions (lower Q),
• mix reagents together very slowly with vigorous
mixing to lower localized supersaturation
• mix at higher temperatures where the solubility
is higher (higher S)

22
PPT Formation and Supersaturation
• Depending on the ppt formed, we can also adjust
the pH to one where the solute is moderately
soluble (higher S) to try to get large crystal
growth.
• The pH is then adjusted to maximize the ppt
formation once the large crystals have begun to
grow.

23
PPT Formation and Supersaturation
• However, the solubilities of many compounds, like
sulfides and hydroxides, are so low that they
usually form colloids.
• Some compounds, like silver chloride, tend to
form colloids or very fine crystals even though
it is not that insoluble.

24
Coagulating a Colloid
• So we have a colloid. What is its structure
like, what keeps it from forming crystals, and
how can we overcome this?

25
Structure of a Colloid
• A colloid has two layers around a solid core.
• At the center is the colloidal solid with its
positive cations electrostatically bound to the
negative anions.
• This could be called a crystallite and it is the
core, not a layer.

26
Structure of a Colloid
• Then at the surface of the crystallite, there are
positive and negative charges due to the cations
and anions of the solute.
• So excess solute ions adsorb loosely to the
surface.

27
Structure of a Colloid
• This is the primary adsorption layer, and it will
have a positive or negative charge, depending on
the excess reagent.
• If the excess reagent is the cation of the
solute, then the overall charge will be positive.
• Example excess silver nitrate added to sodium
chloride. The primary adsorption layer will be
predominately silver ions.

28
Structure of a Colloid
• Because of the overall charge of the adsorption
layer, a second layer called the counter-ion
layer forms.
• This is also composed of the excess reagent along
with other ions in solution.
• It will impart the opposite overall charge to the
entire colloidal particle.
• So if the adsorption layer is positive, the
counter-ion layer will be negative.

29
Structure of a Colloid
• Together, the two layers comprise what is called
an electrical double layer surrounding a solid
core.

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31
Structure of a Colloid
• Why do two colloid particles resist aggregating
to form a crystal?
• If two colloid particles, each with a negative
charge, come close to one another, they will
repel!
• So colloid particles are stable and resist
crystal formation.

32
Coagulating a Colloid
• What can be done to overcome this colloid
stability and force crystals to form?
• High heat, stirring, only a slight excess of the
excess reagent, and the addition of an
electrolyte can force a colloid to coagulate into
crystals.

33
Coagulating a Colloid
• High heat, initially with stirring, is thought to
lower the thickness of the double layer, thus
making it easier for two colloid particles to
collide and coagulate.
• The higher kinetic energy will also help them
gain enough energy to overcome the repulsion.

34
Coagulating a Colloid
• If too much of the excess reagent is added, then
the double layer increases in volume as more of
the excess solute ions will be adsorbed to the
surface, which in turn requires a larger
counter-ion layer.

35
Coagulating a Colloid
• So it is important to make sure that there is
only a slight excess of the excess reagent.
• Thus the diameter of the double layer will be
minimized, enabling neighboring colloids to
coagulate.

36
Coagulating a Colloid
• On the other hand, the addition of a suitable
electrolyte like nitric acid or hydrochloric acid
may also lower the diameter of the double layer.
• Now the high concentration of the appropriate ion
will make it easier to form the counter-ion layer
and its thickness will be reduced.
• Again, two neighboring colloids can get closer
together, making it easier to coagulate.

37
Digesting
• Once a colloid starts to coagulate, it is best to
digest the solution.
• Digestion is when the heated solution with the
coagulating crystals sits undisturbed for an hour
or more.

38
Digesting
• Typically, the colloidal suspension is stirred
with heating until crystals start to coagulate.
• Then stirring is stopped, and the solution is
heated to almost boiling for at least 10 minutes.
• Finally, the solution is allowed to cool slowly
and sit undisturbed for several hours.
• Digestion results in larger, purer crystals which
are easier to filter.

39
Filtration
• Once the crystals have formed and digested, they
need to be filtered.
• The washing step can be a problem, as peptization
of the coagulated colloid may occur.
• This means that the coagulated colloid reverts to
a smaller colloidal particle.

40
Filtration
• Washing with pure water often causes this problem
as this lowers the concentration of counter-ions,
which then causes the double layer to increase in
volume, and the coagulated solid may break back
into smaller colloids.
• These colloids will then go right through the
filter, and the filtrate may look cloudy.

41
Filtration
• Typically, the wash solvent is a dilute solution
of the electrolyte.
• This keeps the double layer intact, minimizing
peptization.
• This electrolyte will then volatilize during the
drying step.
• The filtered and washed crystals are then dried
to constant mass.

42
Coprecipitation of Impurities
• During the precipitation process, other soluble
compounds may also be removed from the solution
phase.
• These other compounds are carried out of solution
by the desired crystals.
• They are impurities and they are said to have
coprecipitated.
• These are NOT other insoluble compounds, but by
several mechanisms, have been carried out of
solution.

43
Coprecipitation of Impurities
• Coprecipitation occurs in several ways
• adsorption onto the surface of the crystals,
• inclusions (absorption into crystal)
• occlusions (absorption)
• Inclusions occur when ions of the impurity occupy
lattice sites in the crystal, while occlusions
are just particles which are physically trapped
inside the crystal

44
Reprecipitation
• If coprecipitation occurs or is known to be a
common occurrence with this solute, then
reprecipitation of the solute should be
conducted.
• In reprecipitation, the filtered precipitate
containing impurities is redissolved and then the
crystals are reprecipitated.

45
Reprecipitation
• This technique effectively lowers the
concentration of impurities, so the second
precipitation will contain fewer impurities.
• This is a common technique for iron and aluminum
hydroxides which coprecipitate other more soluble
hydroxides.

46
Gathering Agents
• Occasionally, reprecipitation is intentionally
used to gather a trace component that
coprecipitates.
• When the precipitate is redissolved in a very
small amount of solvent, the trace component has
been effectively concentrated.
• In this case, the precipitate used to gather the
trace component is called the gathering agent.

47
• Masking agents can also be used to prevent
coprecipitation.
• The masking agents react with the impurities to
from highly soluble complexes to keep them in
solution.

48
Homogeneous Precipitation
• In homogeneous precipitation, the precipitate is
formed through a second chemical reaction.
• First, a reagent is treated in a manner so that
it forms what is called a precipitating agent or
reagent.
• The precipitating reagent then reacts with the
solute ion to form the desired solid precipitate.

49
Homogeneous Precipitation
• As the precipitating reagent is generated in the
solution gradually, this limits the relative
supersaturation of the precipitate.
• So crystals are more likely to form, be larger,
and be more pure.
• This is relatively common for the precipitation
of hydroxide salts where urea is used to generate
the precipitating agent hydroxide.

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51
Drying a Precipitate
• Drying a precipitate seems easy.
• Many compounds can be easily dried at around
110C to remove any water which is adsorbed.
• Other compounds need much higher heat to remove
water.
• The temperature must be carefully decided as many
compounds will decompose if the heat is too high.

52
Igniting a Precipitate
• Yet other precipitates have a variable
composition and must be further treated to form a
compound of uniform composition.
• One common way to treat variable composition
compounds is through ignition high heating.
• This is common with iron analysis. Variable
composition ferric bicarbonate hydrates are
ignited at around 850C to produce anhydrous
ferric oxide.

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54
Gravimetric Calculations
55
Combustion Analysis
• Combustion analysis is still used to determine
the amount of C, H, N, O, S, and halogens in an
unknown sample.
• In the classic freshman combustion problem, a
hydrocarbon is combusted in excess oxygen gas to
produce water vapor and carbon dioxide gas.
• The water and carbon dioxide are trapped and the
mass of these products is obtained.
• Then calculations begin.

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57
Combustion Analysis
• Today, elemental combustion analyzers measure C,
N, H, and S at the same time.
• Oxygen analysis is done through pyrolysis with no
oxygen gas and halogen analysis occurs through an
automated titration.

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