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Gravimetric and Combustion Analysis


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

Gravimetric and Combustion Analysis
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

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.

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

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.

Precipitation Crystals
  • One of the most important aspects of a good
    gravimetric analysis is the particle size of the
  • 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.

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
  • Small, fine crystals and colloids are the most
    difficult to collect.

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.

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
  • However, there are several factors which help
    determine the particle size of a ppt.

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

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

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

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.

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

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

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.

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.

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

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)

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
  • The pH is then adjusted to maximize the ppt
    formation once the large crystals have begun to

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.

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?

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.

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

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.

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.

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

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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
  • So colloid particles are stable and resist
    crystal formation.

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

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.

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.

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

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.

  • 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.

  • 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.

  • 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.

  • 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.

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

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

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

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

  • 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

Gathering Agents
  • Occasionally, reprecipitation is intentionally
    used to gather a trace component that
  • 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.

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

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
  • The precipitating reagent then reacts with the
    solute ion to form the desired solid precipitate.

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
  • The temperature must be carefully decided as many
    compounds will decompose if the heat is too high.

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
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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