2nd Exam- GRAVIMETRIC Flashcards
the analyte is converted to a sparingly soluble precipitate which then filtered, washed free of impurities, and converted to a product of known composition by suitable heat treatment, and weighed.
Precipitation methods
analyte is separated from other constituents by conversion to a gas of known chemical composition. The volatile product is then collected and weighed or alternatively the mass of the product is determined indirectly from the loss of mass in the sample.
Volatilization Methods
Precipitation Methods STEPS WMCFWDW
Steps:
1. Weighing the sample
2. Moisture determination
3. Conversion of the analyte to
a sparingly soluble salt (precipitate)
4. Filtration of the precipitate
5. Washing the precipitate
6. Drying or ignition of the precipitate (conversion to a form suitable for weighing)
7. Weighing of the converted form of the precipitate
is formed when an aggregate of atoms, ions or molecules is greater than 10–4 cm.
precipitate
ions in soln < colloidal particles < precipitate
10^-8 < 10^-7 to 10^-4 < 10^-4 cm
are electrically charged and resist combination to form larger particles which will settle down the solution.
colloidal particles
gravimetric precipitating agent should react specifically or if not selectively with the analyte, and would give a product that is:
- readilyfilteredandwashedfreewithcontaminants.
- of sufficiently low solubility so that no significant loss of the analyte occurs during filtration and washing.
- unreactivewiththeconstituentsoftheatmosphere.
- of known composition after it is dried, or if necessary ignited.
Factors that Affect the Particle Size of a Precipitate
colloidal suspension
crystalline suspension
are tiny particles that are invisible to the naked eye (10-7 to 10-4 cm in diameter); shows no tendency to settle from the solution nor are they easily filtered.
colloidal suspension
the temporary dispersion of such particles in the liquid phase, dimensions are in order of tenths of a millimeter or greater.
crystalline suspension
The particle size of a precipitate is influenced by such experimental variables such as
precipitate solubility, temperature, reactant concentration, and rate at which reactants are mixed.
The particle size of a precipitate is influenced by such experimental variables such as precipitate solubility, temperature, reactant concentration, and rate at which reactants are mixed. The net effect of these variables can be accounted by assuming that the particle size is related to a single property of a system called
relative supersaturation or Von Weimarn’s ratio.
= Q-S / S
Q is conc of a species at any instant
S is its equilibrium solubility
an unstable solution that contains a higher solute concentration than a saturated solution. With time, _______ is relieved by precipitation of the excess solute.
Supersaturated solution; supersaturaturation
when relative supersaturation is large, precipitate tends to be .
colloidal
when relative supersaturation is small (Q is low and S is high), a _________ is more likely to form.
crystalline solid
Precipitates form by two different pathways
- Nucleation 2. Particle Growth
a few ions, atoms or molecules come together to form a 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 (particle growth).
In nucleation
If _________ predominates, a precipitate containing a large number of small particles results; if __________ predominates, a smaller number of small particles results
nucleation, growth
To increase the particle size of a precipitate, minimize the __________ during the precipitation formation.
relative supersaturation
The
process of converting the colloidal suspension into
filterable solid is called
coagulation or
agglomeration or flocculation
explains the reason why the ions
that are preferentially adsorbed on the surface of
the crystal lattice are those which are common to
the lattice and are in excess
Paneth-Fajans-Hahn Rule
Paneth-Fajans-Hahn Rule layers
counter layer (counter-ion)
primary adsorptive layer
crystal
refers to the process by which a coagulated colloid
reverts to its original dispersed state.
when a coagulated colloid is washed, some of the
electrolyte is leached from the internal liquid in contact
with the solid particles.
peptization of colloids
Colloids are best precipitated from _________ (3) to ensure coagulation
hot,
stirred solutions
containing sufficient electrolyte
– the process in which a precipitate is heated
for an hour or more in the solution from which it was
formed (the mother liquor).
- weakly bound
water appears to be lost from precipitate; the result
is a denser mass that is easier to filter.
digestion
are generally more easily filtered and purified than
coagulated colloids, and the size of individual
crystalline particles, and their filterability can be
controlled to a degree.
Crystalline Precipitates
(without stirring)
for some time after formation frequently yields a purer,
more filterable product resulting from the dissolution
and recrystallization that occur continuously and at a
enhanced rate at elevated temperature
Digestion of crystalline precipitates (without stirring)
Methods of Improving Particle Size and
Filterablity for crystalline solids
minimizing Q and/or maximizing S
Minimizing Q
- using dilute solution
- adding the precipitating reagent slowly
- good mixing
Maximizing S
- precipitating from hot solution
- adjusting the pH of the precipitation medium
a phenomenon in which otherwise soluble
compounds are removed from solution during
precipitate formation.
coprecipitation
4 types of coprecipitation
surface adsorption
mixed-crystal formation
occlusion
mechanical entrapment
Methods for Minimizing Adsorbed Impurities on
Colloids
digestion
washing
reprecipitation
during this process, water is expelled from
the solid to give a denser mass that has a smaller
specific surface area for adsorption.
digestion
a coagulated colloid with a solution that
contains volatile electrolyte – exchange between
existing counter-ions and ions in the wash liquid.
or example, in the determination of silver by precipitation with chloride ion, the primary adsorbed species is chloride. ______ with an acidic solution converts the counter-ion layer largely to H+ so that both Cl- and H+ are retained by the solid. Volatile HCl is then given off when the precipitate
is dried.
washing
a drastic but effective way to
minimize the effects of adsorption also known as
double precipitaton. The filtered solid is redissolved
and reprecipitated.
reprecipitation
can cause significant
contamination of precipitates with large specific
surface areas, that is coagulated colloids. The net
effect is therefore the carrying down of an otherwise soluble compound as a surface
contaminant.
surface adsorption
here, one of the ions
in the crystal lattice of a solid is replaced by an
ion of another element. For exchange to occur, the
two ions must have the same charge and their sizes
differ no more than about 5 %. The two salts must
belong to the same crystal class. Examples include PbSO4 in BaSO4, MgKPO4 in MgNH4PO4 , SrSO4 in BaSO4 and MnS in CdS
mixed-crystal formation
when a crystal is growing rapidly
during precipitate formation, and foreign ions in
the counter-ion layer may become trapped or
occluded with the growing crystal.
occlusion
occurs when crystals lie
close together during growth, here, several crystals
grow together and in so doing trap a portion of
the solution in a tiny pocket.
mechanical entrapment
At low supersaturation (rate of precipitation
formation is low), both occlusion and mechanical
entrapment are at a ______ (minimum/maximum) and ______ is helpful in reducing these types of coprecipitation.
minimum and digestion
Surface adsorption and mixed crystal formation
are _________ whereas occlusion and
mechanical entrapment arise from the ______
equilibrium processes;
kinetics ofcrystal growth.
Coprecipitation error
is observed whenever colloidal silver
chloride adsorbs silver nitrate during a chloride
analysis.
positive error
Coprecipitation error
When the contaminant does not contain the ion
being determined, __________
may be observed.
either positive or negative errors
technique in which a precipitation agent is generated
in a solution of the analyte by a slow chemical
reaction.
The relative supersaturation is kept low
during the entire precipitation because the
precipitating reagent appears gradually and
homogenously throughout the solution and reacts
immediately with the analyte.
Precipitation from Homogeneous solution
_________formed
precipitates, both colloidal and crystalline are
better suited to analysis that are solids formed
by direct addition of a precipitating reagent.
homogeneously
removes the
solvent and any volatile species carried down with
the precipitate
Heating (until its mass becomes constant)
some precipitates are also _____ to decompose
the solids and form a compound of known
composition
ignited
COLLOIDAL
Relative supersaturation: _____
Mechanism: _____
Conditions: _____
High
nucleation
high Q, low S
CRYSTALLINE
Relative supersaturation: _____
Mechanism: _____
Conditions: _____
low
particle growth
low Q, high S
SI UNITS
Mass
Length
Time
Temp
Amount of Substance
Electric Current
Luminous Intensity
kg
m
s
K
mol
A
cd (candela)
the quantitative relationship between among
reacting chemical species
stoichiometry
gives the simplest whole number ratio of
atoms in a chemical compound
emperical formula
specifies the number of atoms in a molecule
molecular formula
Volatile Methods Steps
1) Weighing the sample
2) Moisture Determination
3) Analyte or its decomposition products are
volatilized at a suitable temperature.
4) Volatile product is then collected and weighed or
alternatively the mass of the product is determined
directly from the loss of the sample.
qualitatively eliminated from many inorganic samples by ignition. In direct determination, it is collected on any of the several solid desiccants and its mass is determined from mass gain of the desiccant. The indirect method determines the amount of water from the loss of mass of the sample during heating (less satisfactory because it must be assumed that water is the only component volatilized.
water
are ordinarily decomposed by acids to give carbon dioxide, which is readily evolved from solution by heat. Direct determination of the mass of carbon dioxide is established from the increase in the mass of a solid adsorbent.
carbonates
can also be determined by
volatilization the hydrogen sulfide or sulfur dioxide
evolved from the sample after treatment with acid
is collected in a suitable absorbent.
sulfides and sulfites
The classical method for the determination of
_____ and ______ in organic compounds is a
gravimetric procedure in which the combustion
products (H2O and CO2) are collected selectively
and weighed the increase in mass serves as the
analytical parameter.
carbon and hydrogen
