Second Exam Flashcards
What are the two types of gravimetric methods of analysis
precipitation and volatilization methods
Precipitation methods steps
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.
Ideally, a gravimetric precipitating agent should react ________ or if not
________ with the analyte, and would give a product that is:
1. readily filtered and washed free with contaminants.
2. of sufficiently low solubility so that no significant loss of the analyte
occurs during filtration and washing.
3. unreactive with the constituents of the atmosphere.
4. of known composition after it is dried, or if necessary ignited.
specifically; selectively
Precipitates of _____________ size are desirable in gravimetric work because it is easy to filter and washed free of impurities.
large particle
Factors that Affect the Particle Size of a Precipitate
colloidal and 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.
relative supersaturation is also known as
Von Weimarn’s ratio
Formula for relative supersaturation
(Q-S)/S
where Q is the concentration of a species at any instant and S is its
equilibrium solubility.
– an unstable solution that contains a higher solute
concentration than a saturated solution. With time, supersaturation is relieved by
precipitation of the excess solute.
Supersaturated solution
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
In ________, a few ions, atoms or molecules come together to form a
stable solid
nucleation
Further precipitation then involves a
competition between additional nucleation and growth on existing nuclei
(_______).
particle growth
If _________ predominates, a precipitate containing a large number of
small particles results; if ______ predominates, a smaller number of
larger particles results.
nucleation; growth
Rate of nucleation _______ enormously with _______ relative supersaturation
increase; increase
Conditions of Analytical Precipitation
Use dilute solutions (low Q)
Add reagents slowly with effective stirring (low Q)
Precipitate from hot solution (high S since precipitates have higher solubility at
higher temperature)
For precipitates whose ions come from weak acids, employ as low a pH as possible
for complete precipitation (low Q). e.g. CaC2O4 (C2O4
2– is the anion of HC2O4–
,
which in turn is an anion of H2C2O4. Protonation of C2O4
2– prevents it from forming
precipitate with Ca2+)
Use complexing agents (high S)
The process of converting the colloidal suspension into filterable solid is called
coagulation or agglomeration or flocculation.
The stability of most colloidal suspensions can be decreased by:
heating, by stirring and by adding an electrolyte.
According to ________, 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
Attached directly to the solid surface is the __________.
Surrounding the charged particle is a layer of solution called
________ which contains sufficient excess of ions to just
balance the charge on the surface of the particle.
primary adsorbed layer; counter-ion layer
The _____________ and ______________
constitute an electric double layer that imparts stability to the
colloidal suspension.
primary adsorbed layer and the counter-ion layer
can be brought about by a short period of heating particularly if
accompanied by stirring.
Coagulation of a colloidal suspension
_____________ decreases the number of adsorbed ions and thus, the
thickness of the double layer.
Heating
refers to the process by which a coagulated colloid reverts to its
original dispersed state.
Peptization of Colloids
– the process in which a precipitate is heated for an hour or
more in the solution from which it was formed (the mother liquor).
Digestion
– the solution from which a precipitate was formed.
Mother liquor
are generally more easily filtered and
purified than coagulated colloids, and the
size of individual crystalline particles, and
their filterability can be controlled to some
extent
Crystalline Precipitates
____________ is accomplished by using dilute solutions and
adding the precipitating reagent slowly and with good mixing.
Minimizing Q
____________ can be achieved by precipitating from hot
solution or by adjusting the pH of the precipitation medium.
Maximizing S
a phenomenon in which otherwise soluble compounds are
removed from solution during precipitate formation.
coprecipitation
There are four types of coprecipitation:
-surface adsorption
-mixed-crystal formation
-occlusion
-mechanical entrapment
can cause significant contamination of
precipitates with large specific surface areas, that is coagulated colloids.
Surface adsorption
– 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.
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
here, one of the ions in the crystal lattice
of a solid is replaced by an ion of another element.
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 ________________ (rate of precipitation formation is low), both
occlusion and mechanical entrapment are at a minimum and digestion is helpful in reducing these types of coprecipitation.
low supersaturation
_____________ and _____________ are equilibrium processes whereas occlusion and mechanical entrapment arise from the kinetics of crystal growth.
Surface adsorption and mixed crystal formation
_______________ may cause either negative or positive errors in an analysis.
Coprecipitation impurities
