Classical Method of Analysis

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Classical Method of Analysis

• Chapter 12-Gravimetric Method of Analysis
• Chapter 13- Titrimetric Methods Precipitation
  Titrimetry

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Chapter 12-Gravimetric Method of Analysis

• Quantitative methods that are based on
  determining the mass of a pure compound to the
  analyte which is chemically related.
• Based on mass measurements made with an
  analytical balance that produces highly accurate
  and precise data.
• There are several types of gravimetric analysis
  precipitation gravimetry, volatilization
  gravimetry, and electrogravimetry. The other two
  methods are gravimetric titrimetry and atomic
  mass spectrometry.

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Precipitation Gravimetry

• The analyte is converted to a sparingly soluble
  precipitate.
• For example Recommended Ca determination method
  in natural waters by Association of Offical
  Analytical Chemist.

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Properties of Precipitates and Precipitating
Reagents

• Ideally, a gravimetric precipitating agent should
  react specifically or at least selectively with
  the analyte.
• In addition, a precipitating reagent should have
  following properties
• Easily filtered and washed free of impurities
• No significant loss of the analyte occurs during
  filtration and washing
• Stable under atmospheric conditions
• After drying and ignition, known chemical
  composition
• Only few reagents posses these properties.

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Particle Size and Filterability of Precipitates

• Factors Determining the Particle Size of
  Precipitates
• The particle size of solids formed by
  precipitation varies widely.
• At one extreme, colloidal suspensions carry tiny
  particles in the range of 10-7 to 10-4 cm in
  diameter and they have no tendency to settle down
  from a solution and are not filtered easily.
• At anther extreme, dimensions of particles are on
  the order of a tenths of a millimeter or greater
  and the temporary dispersion of such particles in
  the liquid phase called a crystalline suspension.
  Such a suspension has a tendency to settle down
  spontaneously and are easily filtered.

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The Use of Relative Supersaturation

• Several parameters such as precipitate
  solubility, temperature, reactant concentration,
  and rate of reactant mixing can influence the
  particle size. The overall effect of these
  factors can be qualitatively combined into a
  single property called relative super saturation,
  (Q-S)/S, where Q is the concentration of the
  solute at any instant and S is its equilibrium
  solubility.
• Experimental evidence indicates that the particle
  size of a precipitate varies inversely with the
  average relative super saturation during the time
  when reagent being introduced. Thus, (Q-S)/S is
  large, the precipitate tends to be colloidal
  when (Q-S)/S is small, a crystalline solid is
  more likely.

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Mechanism of Precipitate Formation

• Precipitates form by nucleation and by particle
  growth. If nucleation predominates, a large
  number of very fine particles results if
  particle growth predominates, a smaller number of
  larger particles is obtained.
• In nucleation, a few ions, atoms, or molecules
  come together to form stable solid. Often these
  nuclei form on the surface of suspended solid
  contaminants, such as dust particles.
• Further precipitation then involves a competition
  between additional nucleation and growth on
  existing nuclei.
• If nucleation predominates, a precipitate
  containing a large number of small particles
  results if growth predominates, a smaller number
  of larger particles is produced.
• The rate of nucleation is believed to increase
  vastly with increasing relative super saturation.
  On the contrary, the rate of particle growth is
  only moderately enhanced by high relative super
  saturations.

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Experimental Control of Particle Size

• Experimental variables that minimize
  supersaturation and thus produce crystalline
  precipitates include
• Elevated temperature to increase the solubility
• Dilute solutions
• Slow addition of precipitating agent with good
  stirring
• Controlling pH

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Colloidal Precipitates

• Colloids are so small to hold on a filter and
  they can settle down in solution as well due to
  Brwonian motion (stochastic process, random
  fluctuation). In order to have the colloids
  filterable, we can coagulate or agglomerate.
• Coagulation of Colloids It can be hastened by
  heating, stirring and adding an electrolyte

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Colloidal AgCl particle in a solution containing
excess silver nitrate.
Primary adsorption layer contains adsorbed silver
ions. A counter-ion layer surrounds primary
adsorption layer with sufficient excess negative
ions (nitrate, NO3- ions) to balance the charge
on the surface of the particle.
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The presence of an electric double layer imparts
the stability to the colloidal suspension

• As colloidal particle approach one another, this
  electric double layer, exerts an electrostatic
  repulsive force that prevents particles from
  colliding and adhering.

Effect of AgNO3 and electrolyte concentration on
the thickness of the double layer surrounding a
colloidal AgCl particle in a solution containing
excess AgNO3.
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• Increasing the electrolyte concentration has the
  effect of decreasing the volume of the
  counter-ion layer, thereby increasing the chances
  for coagulation.
• Colloidal suspension can often be coagulated by
  heating, stirring, and adding an electrolyte.

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Peptization of Colloids

• It is a process that a coagulated colloid returns
  to its dispersed state.

Crystalline Precipitates

• Crystalline precipitates are generally more
  easily filtered and
• purified than that coagulated colloids are.

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Coprecipitation

• A phenomenon in which otherwise soluble compounds
  are removed from solution during precipitate
  formation.
• There are four types of co-precipitation surface
  adsorption, mixed-crystal formation, occlusion,
  and mechanical entrapment.
• Mixed-crystal formation a contaminant ion
  replaces an ion in the lattice of a crystal.
• Occlusion a compound is trapped within a pocket
  formed during rapid crystal growth.
• Mechanical entrapment occurs when crystals lie
  close together during growth.

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Drying and Ignition of Precipitates

• After filtration, a gravimetric precipitate is
  heated until its mass becomes constant.
• Ignition decomposes the solid and form a compound
  of known composition.

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Applications of Gravimetric Methods

• Gravimetric methods have been developed for most
  inorganic anions and cations as well as neutral
  species such as water, sulfur dioxide, carbon
  dioxide, and iodine.
• A variety of organic substances can also be
  easily determined gravimetrically. Examples
  lactose in milk products, salicylates in drug
  formulations, nicotine in pesticides etc.

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• Inorganic Precipitating Agents Table 12-2 in the
  textbook.
• Reducing Agents Table 12-3 shows the reagents
  can convert the an analyte to its elemental form.
• Organic Precipitating Agents There are numerous
  examples of this type. In one form of organic
  reagents, they produce sparingly soluble
  non-ionic coordination compounds In the other
  forms products with largely ionic bonds.
• Organic Functional Group Analysis See Table 12-4
  in the textbook
• Volatilization Gravimetry Based on
  volatilization such as water and carbon dioxide
  determination.

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Chelating Agents
Organic reagents that yield sparingly soluble
coordination compounds typically contain at
least two functional groups. Each of these groups
is capable of bonding with a cation by donating
a pair of electrons. The functional groups are
located in the molecule such that a five- or six-
membered ring results from the reaction.
Reagents that form compounds of this type are
called chelating reagents, and their products are
called chalets.
8-Hydroxquinoline (oxine)