Chapter 10: Gravimetric Methods 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
In this type of gravimetry, the analyte is separated from a solution of the sample as a precipitate and is converted to 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. The mass of the gas then serves as a measure of the
analyte concentration.
volatilization gravimetry
the analyte is separated by deposition on an electrode by an electrical current. The mass of this product then provides a measure of the analyte concentration
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
uses a mass spectrometer to separate the gaseous ions formed from the elements making up a sample of matter. The concentration of the resulting ions is then determined by measuring the electrical current produced when they fall on the surface of an ion detector.
atomic mass spectrometry
the analyte is converted to a sparingly soluble precipitate, which is then filtered, washed free of impurities, converted to a product of known composition by suitable heat treatment, and weighed.
precipitation gravimetry
is a process in which a solid is cycled through heating to high temperature, cooling, and
weighing steps until its mass becomes constant to within 0.2 to 0.3 mg.
drying or ignition to constant mass
it should react specifically or at least selectively with the analyte
gravimetric precipitating agent
rare reagents that react only with a single chemical species
specific reagents
common reagents that react with a limited number of species
selective reagents
properties of precipitates and precipitating reagents
- easily filtered and washed free of contaminants
- of sufficiently low solubility that no significant loss of the analyte occurs during filtration and washing
- unreactive with constituents of the atmosphere
- of known chemical composition after it is dried or, if necessary, ignited
TRUE or FALSE
precipitates consisting of large particles are generally desirable for gravimetric work because these particles are easy to filter and wash free of impurities. In addition, precipitates of this type are usually purer than are precipitates made up of fine particles.
TRUE
consists of solid particles
with diameters that are less than
10^-4 cm.
colloid
in diffuse light, it may be perfectly clear and appear to contain no solid. The presence of the second phase can be detected, however, by shining a beam of flashlight into the solution
colloidal suspension
TRUE or FALSE
It is very difficult to filter the particles of a colloidal suspension. To trap these particles, the pore size of the filtering medium must be so small that filtration take a very long time.
TRUE
TRUE or FALSE
With suitable treatment, however, the individual colloidal particles can be made to stick together, or coagulate, to produce large particles that are easy to filter.
TRUE
whose tiny particles are invisibble to the naked eye and range from 10^-7 to 10^-4 cm in diameter.
colloidal suspensions
TRUE or FALSE
Colloidal particles show no tendency to settle from solution and ar difficult to filter
TRUE
TRUE or FALSE
particles of colloidal dimensions scatter visible radiation, thus, the path of the beam through the solution can be seen by the eye.
TRUE
the effect of colloidal particles that show no tendency to settle from solution and are difficult to filter. because particles of colloidal dimensions scatter visible radiation, the path of the beam through the solution can be seen by the eye.
Tyndall effect
the temporary dispersion of such particles (with dimensions on the order of tenths of a millimeter or greater) in the liquid phase is called a
crystalline suspension
TRUE or FALSE
the particle size of a precipitate is not influenced by precipitate solubility, temperature, reactant concentrations, and the rate at which reactants are mixed.
FALSE
the particle size of a precipitate is influenced by precipitate solubility, temperature, reactant concentrations, and the rate at which reactants are mixed.
the net effect of the precipitate solubility, temperature, reactant concentrations, and the rate at which reactants can be accounted for, at least qualitatively, by assuming that the particle size is related to a singe property of the system called
relative supersaturation
the formula of relative supersaturation
relative supersaturation= Q - S/ S
Q- conc. of the solute at any instant
S- equilibrium solubility
TRUE or FALSE
It is very difficult to filter the particles of a colloidal suspension To trap these particles, the pore size of the filtering medium must be so small that filtrations take a very long time. With suitable treatment, however, the individual colloidal particles
can be made to stick together, or coagulate, to produce large particles that are easy to filter.
TRUE
TRUE or FALSE
Experimental evidence indicates that the particle size of a precipitate varies inversely with the average relative supersaturation during the time when the reagent is being introduced
TRUE
TRUE or FALSE
When (Q-S)/S is large, the precipitate tends to be colloidal
TRUE
TRUE or FALSE
when relative supersaturation is small, a crystalline solid is more likely.
TRUE
The effect of relative supersaturation on particle size can be explained if we assume
that precipitates form in two ways through
nucleation
particle growth
a few ions, atoms, or molecules (perhaps as few as four or five) come together to form a stable solid
nucleation
What happens when nucleation predominates
a precipitate containing a large number of small particles results
what happens when particle growth predominates?
a smaller number of larger particles is produced.
Often, these nuclei form on the surface of suspended solid contaminants, such as dust particles. Further precipitation then is governed by the competition between
additional nucleation
particle growth
TRUE or FALSE
The rate of nucleation is believed to increase enormously with increasing relative supersaturation
TRUE
TRUE or FALSE
the rate of particle growth is only moderately enhanced by high relative supersaturations
TRUE
is a process in which a minimum number of atoms, ions, or molecules join together to give a stable solid
nucleation
What will you do to increase the particle size of a precipitate
minimize the relative supersaturation during precipitate formation
Experimental variables that minimize supersaturation and produce crystalline precipitates include
elevated temperature (increase solubility)
dilute solutions (minimize Q)
slow addition of the precipitating agent with good stirring (minimize Q at any given instant)
Precipitates that have very low solubilities, such as many sulfides and hydrous oxides,
generally form as
colloids
TRUE or FALSE
If the solubility of the precipitate depends on pH, larger particles can also be produced by controlling pH.
TRUE
is a process in which a substance (gas, liquid, or solid) is held on the surface of a solid
adsorption
retention of a substance within the pores of a solid
absorption
it can be hastened by heating, by stirring, and by adding an electrolyte to the medium.
coagulation
TRUE or FALSE
Colloidal suspensions are stable because all the particles of the colloid are either positively
or negatively charged and thus repel one another
TRUE
Attached directly to the solid surface is what layer
(e.g. which consists mainly of adsorbed silver ions)
primary adsorption layer
Surrounding the charged particle is a layer of solution, called what layer
(e.g. contains sufficient excess of negative ions, principally nitrate, to just balance the charge on the surface of the particles )
counter ion layer
The primarily adsorbed silver ions and
the negative counter-ion layer constitutes an ___________ that imparts stability to the colloidal suspension.
electric double layer
TRUE or FALSE
As colloidal particles approach one another,
this double layer exerts an electrostatic repulsive force that prevents particles from
colliding and adhering
TRUE
TRUE or FALSE
Colloidal suspensions can often be coagulated by heating, stirring, and adding an electrolyte
TRUE
TRUE or FALSE
Coagulation of a colloidal suspension can often be brought about by a short period
of heating, particularly if accompanied by stirring.
TRUE
Heating decreases the number of
adsorbed ions and thus the thickness, di, of the double layer. The particles may also
gain enough kinetic energy at the higher temperature to overcome the barrier to close
approach imposed by the double layer.
TRUE or FALSE
An even more effective way to coagulate a colloid is to increase the electrolyte
concentration of the solution
TRUE
If we add a suitable ionic compound to a colloidal suspension, the concentration of counter-ions increases in the vicinity of each particle. As a result, the volume of solution that contains sufficient counter-ions to balance
the charge of the primary adsorption layer decreases. The net effect of adding an electrolyte is thus a shrinkage of the counter-ion layer
is the process by which a coagulated colloid reverts to its original dispersed
state.
Peptization
is an unstable solution that contains a
higher solute concentration than a saturated solution. As excess solute precipitates with time, supersaturation decreases to zero
supersaturated solution
prevents the settling out of colloidal particles in solution under the influence of gravity
brownian motion
TRUE or FALSE
we can coagulate, or agglomerate, the
individual particles of most colloids to give a filterable, amorphous mass that will
settle out of solution
TRUE
TRUE or FALSE
as the concentration of silver nitrate is further decreased, the distance between particles becomes small enough for the forces of agglomeration to take effect and a coagulated precipitate to appear.
TRUE
TRUE or FALSE
Coagulation of a colloidal suspension can often be brought about by a short period
of heating, particularly if accompanied by stirring. Heating decreases the number of
adsorbed ions and thus the thickness, di, of the double layer. The particles may also
gain enough kinetic energy at the higher temperature to overcome the barrier to close
approach imposed by the double layer.
TRUE
TRUE or FALSE
increasing electrolyte concentration is an even more effective way to coagulate a colloid in a solution. Adding a suitable compound to a colloid suspension, the concentration of counter-ions increases in the vicinity of each particle, which as a result, the volume of solution that contains sufficient counter-ions to balance the charge of the primary adsorption layer decreases. Thus, the net effect of adding an electrolyte causes a shrinkage of the counter-ion layer and allow particles to approach one another more closely and agglomerate
TRUE
is the process by which a coagulated colloid reverts to its original dispersed
state.
peptization
TRUE or FALSE
When a coagulated colloid is washed, some of the electrolyte responsible for its coagulation is leached from the internal liquid in contact with the solid particles. Removal of this electrolyte has the effect of increasing the volume of the counter-ion layer. The repulsive forces responsible for the original colloidal state are then reestablished, and particles detach themselves from the coagulated mass. The washings become cloudy as the freshly dispersed particles pass through the filter. The problem is usually solved by washing the precipitate with a solution containing an electrolyte that volatilizes when the precipitate is dried or ignited
TRUE
TRUE or FALSE
Colloids are best precipitated from hot, stirred solutions containing sufficient electrolyte
to ensure coagulation.
TRUE
TRUE or FALSE
The filterability of a coagulated colloid often improves if it is allowed to stand for an hour or more in contact with the hot solution from which it was formed. During this process, which is known as digestion, weakly
bound water appears to be lost from the precipitate. The result is a denser mass that
is easier to filter
TRUE
are generally more easily filtered and purified than are coagulated
colloids.
crystalline precipitates
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
TRUE or FALSE
the size of individual crystalline particles, and thus their filterability, can be controlled to some extent
TRUE
TRUE or FALSE
The particle size of crystalline solids can often be improved significantly by minimizing Q or maximizing S, or both
TRUE
Digestion of crystalline precipitates (without stirring) for some time after formation
often yields a
purer, more filterable product
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
are generally more easily filtered and purified than are coagulated colloids.
crystalline precipitates
The particle size of crystalline solids can often be improved significantly by
minimizing Q (concentration of solute)
maximizing S (equilibrium solubility)
or both
Minimizing Q (conc of solute) can be done by
using dilute solutions
adding precipitating reagent slowly with good mixing
Maximizing S is increased by
precipitating from hot solution
adjusting the pH of the precipitation medium
TRUE or FALSE
Digestion of crystalline precipitates (without stirring) for some time after formation
often yields a purer, more filterable product
TRUE
results in bridging between adjacent particles, a process that yields larger and more easily filtered crystalline aggregates
recrystallization
is a process in which normally soluble compounds are carried out of solution by a precipitate.
coprecipitation
TRUE or FALSE
Contamination of a precipitate by a second substance whose solubility product has been exceeded is not coprecipitation
TRUE
four types of coprecipitation
surface adsorption
mixed-crystal formation
occlusion
mechanical entrapment
surface adsorption and mixed crystal formation are
equilibrium processes
occlusion and mechanical entrapment arise from the
kinetics of crystal growth
TRUE or FALSE
Adsorption is a common source of coprecipitation and is likely to cause significant contamination of precipitates with large specific surface areas
TRUE
TRUE or FALSE
Although adsorption does occur in crystalline solids, its effects on purity are usually undetectable because of the relatively small specific surface area of these solids
TRUE
TRUE or FALSE
The net effect of surface adsorption is, therefore, the carrying down of an otherwise soluble compound as a surface contaminant
TRUE
In minimizing adsorbed impurities on colloids, 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
A drastic but effective way to minimize the effects of adsorption, which the first precipitate usually carries down only a fraction of the contaminant present in the original solvent. Thus, the solution containing the redissolved precipitate has a significantly lower contaminant concentration than the original, and even less adsorption occurs during the second precipitation.
reprecipitation
one of the ions in the crystal lattice of a solid is replaced by an ion of another element
mixed-crystal formation
types of coprecipitation
reprecipitation
mixed-crystal precipitation
occlusion and mechanical entrapment
is a type of coprecipitation in which a compound is trapped within a pocket formed during rapid crystal growth.
occlusion
occurs when crystals lie close together during growth
mechanical entrapment
TRUE or FALSE
Both occlusion and mechanical entrapment are at a minimum when the rate of precipitate formation is low, that is, under conditions of low supersaturation.
TRUE
TRUE or FALSE
Mixed-crystal formation may
occur in both colloidal and
crystalline precipitates, but
occlusion and mechanical
entrapment are confined to
crystalline precipitates.
TRUE
2 types of coprecipitated errors
positive errors
negative errors
If the contaminant is not a compound of the ion being determined, _______ always results
positive errors
when the contaminant does contain the ion being determined, either _______________ may occur
positive or negative errors
is a process in which a precipitate is formed by slow generation of a precipitating reagent homogeneously throughout a solution. of crystalline precipitates also results in marked increases in crystal size as well as improvements in purity
homogeneous precipitation
Some precipitates are also ignited to decompose the solid and form a compound of known composition, which is often called
weighing form
Recording thermal decomposition
curves are called
thermogravimetric analysis
the mass versus temperature curves are termed
thermograms
Organic reagents that yield sparingly soluble coordination compounds typicallycontain at least two functional groups. Each of these groups is capable of bonding with a cation by donating a pair of electrons, which reagents that form compounds of this type are called ______ and products are called ________
chelating agents
chelates
are relatively nonpolar and, as a consequence, have solubilities that are low in water but high in organic liquids. Usually, these compounds possess low densities and are often intensely colored
metal chelates
Approximately two dozen cations form sparingly soluble chelates with ___________. solubility vary widely from cation to cation and are pH dependent because of instant deprotonation during chelation reaction
8-hydroxyquinoline (oxine)
is an organic precipitating agent of unparalleled specificity with nickel
Dimethylglyoxime
is an important example of an organic precipitating reagent that forms salt-like precipitates. near specific precipitating agent for potassium and ammonium ions . however, Hg, Rb, Cs must be removed by prior treatment
Sodium Tetraphenylborate
water vapor is collected on any of several solid desiccants, and its mass is determined from the mass gain of the desiccant
direct determination
the amount of water is determined by the loss of mass of the sample during heating is less satisfactory because it must be assumed that water is the only component that is volatilized
indirect determination
