Ch 12 Gravimetric Method of Analysis Flashcards
are quantitative methods that are based on determining the mass of a pure compound to which
the analyte is chemically related.
Gravimetric methods
the analyte is separated from a solution of the sample as a precipitate and is converted to a
compound of known composition that can be weighed.
precipitation gravimetry
the analyte is separated from other constituents of a sample by converting it to a gas of known chemical composition.
volatilization gravimetry
the analyte is
separated by deposition on an electrode by an electrical current.
electrogravimetry
the mass of a reagent of known concentration required to
react completely with the analyte provides the information needed to determine the analyte
concentration.
gravimetric titrimetry,
consists of solid particles with
diameters that are less than 10^-4
cm
A colloid
may be perfectly clear and appear to
contain no solid. The presence of the
second phase can be detected, however,
by shining the beam of a flashlight
into the solution
colloidal suspensions
Because particles
of colloidal dimensions scatter visible radiation, the path of the beam
through the solution can be seen by
the eye.
Tyndall effect
is an unstable solution that contains a
higher solute concentration than a
saturated solution. As excess solute
precipitates with time, supersaturation
decreases to zero
A supersaturated solution
The effect of relative supersaturation on particle size can be explained if we assume
that precipitates form in two ways:
nucleation and particle growth
To increase the particle size
of a precipitate, _______ the
relative supersaturation during
precipitate formation
decreased
is a process in which a
minimum number of atoms, ions,
or molecules join together to give a
stable solid.
Nucleation
Individual colloidal particles are so small that they are not retained by ordinary filters. Moreover, the ____________ prevents their settling out of solution under the
influence of gravity. F
Brownian motion
A type of precipitation process wherein , a large
number of very fine particles
is produced
Nucleation
when a precipitate is formed at high
relative supersaturation, __________ is the major precipitation mechanism, and a large
number of small particles is formed
nucleation
Low relative supersaturation
produces _______________
crystalline suspensions.
What law describe colloids?
Brownian motions
This prevents colloids from settling out of the solution under the
influence of gravity. Fortunately, however, we can coagulate, or agglomerate, this individual particles to give a filterable, amorphous mass that will settle
out of solution.
Brownian motion
Coagulation can be hastened by
heating, by stirring, and by adding an electrolyte to the medium.
Colloidal suspensions are stable because of
the repulsion force from the particles of the colloid that are either
positively or negatively charged
the double layer that exerts an electrostatic repulsive force that prevents particles from colliding
and adhering
electric double layer
is the process by which a coagulated colloid reverts to its original dispersed state
Peptization
is a process in which a
precipitate is heated in the solution
from which it was formed (the mother
liquor) and allowed to stand in contact
with the solution.
Digestion
is the solution from
which a precipitate was formed.
Mother liquor
is a process in which
normally soluble compounds are carried
out of solution by a precipitate.
Coprecipitation
four types of coprecipitation:
surface adsorption, mixed-crystal
formation, occlusion, and mechanical entrapment
From the four coprecipitation, which are product of equilibrium processes, and kinetics of crystal growth.
Surface adsorption and
mixed-crystal formation are equilibrium processes, and occlusion and mechanical entrapment arise from the kinetics of crystal growth.
is often the major
source of contamination in
coagulated colloids but of no
significance in crystalline
precipitates.
Surface adsorption
The purity of many coagulated
colloids is improved by .
digestion
is defined as the surface area per unit mass of solid and usually has
the units of square centimeters per gram.
Specific surface area
For a given mass of solid, the specific surface
area increases dramatically as particle size _______, and it becomes enormous for colloids
decreases
A drastic but effective way to minimize the effects of adsorption
Reprecipitation
is a type
of coprecipitation in which a contaminant ion replaces an ion in the lattice
of a crystal.
Mixed-crystal formation
is a type of coprecipitation
in which a compound is trapped within
a pocket formed during rapid crystal
growth.
Occlusion
Mixed-crystal formation may
occur in both
colloidal and
crystalline precipitates
occurs when crystals lie close together during growth. Several crystals grow together and in so doing trap a portion of the solution in a tiny pocket.
Mechanical entrapment
occlusion and mechanical
entrapment are confined to
crystalline precipitates.
is a
process in which a precipitate is
formed by slow generation of a
precipitating reagent homogeneously
throughout a solution.
Homogeneous precipitation
After precipitation, the precipitate is usually _____ to remove excess solvent
oven dry
Some precipitates are also ignited to decompose the solid and form a compound
of known composition. This new compound is often called the .
weighing form
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.
chelating agents
Some organic reagent can precipitate slightly soluble nonionic products called
coordination compounds
are cyclical metal-organic
compounds in which the metal is a part
of one or more five- or six-membered
rings.
Chelates
are relatively nonpolar and, as a consequence, have solubility that are low in water but high in organic liquids.
chelates
Name some chelating or precipitating agent
8-Hydroxyquinoline (oxine)
Dimethylglyoxime
Sodium Tetraphenylborate
is an organic precipitating agent that produce precipitate that are so bulky
Dimethylglyoxime
is an important example of an organic precipitating reagent that forms salt-like precipitates.
Sodium tetraphenylborate
The two most common gravimetric methods based on volatilization are those for
determining
water and carbon dioxide
