Gravimetric and Combustion Analysis

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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
  initial moles of analyte.
• There are 2 kinds of gravimetric analysis
  precipitation, and volatilization

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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.

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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
  biodegradation reactions.
• It is easily collected via a second reaction and
  massed.

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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.

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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.

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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.

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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.

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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.

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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.

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Factors in Particle Size

• solubility of the ppt
• temperature
• reactant concentrations
• electrolyte concentrations
• how quickly the reactants are mixed together to
  form the ppt

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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
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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.

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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.

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

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PPT Formation

• There are two ways that ppts form
• nucleation
• particle growth

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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.

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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.

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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

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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)

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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.

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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.

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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?

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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.

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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.

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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.

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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.

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Structure of a Colloid

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

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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

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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.

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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.

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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.

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Masking Agents

• 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.

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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.

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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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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.

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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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Gravimetric Calculations
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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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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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