Chapter 10: Gravimetric Methods of Analysis Flashcards by Clint Anthony Arcilla

are quantitative methods that are based on determining the mass of a pure compound to which the analyte is chemically related.

Gravimetric methods

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

it should react specifically or at least selectively with the analyte

gravimetric precipitating agent

How well did you know this?

Not at all

Perfectly

rare reagents that react only with a single chemical species

specific reagents

How well did you know this?

Not at all

Perfectly

common reagents that react with a limited number of species

selective reagents

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

consists of solid particles
with diameters that are less than
10^-4 cm.

colloid

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

whose tiny particles are invisibble to the naked eye and range from 10^-7 to 10^-4 cm in diameter.

colloidal suspensions

How well did you know this?

Not at all

Perfectly

TRUE or FALSE
Colloidal particles show no tendency to settle from solution and ar difficult to filter

TRUE

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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.

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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