Analytical Chemistry Chapter 4 Flashcards
Are based on mass measurement.
Are quantitative methods that are based on
determining the mass of a pure compound to which the analyte is chemically related.
Are based on mass
measurements with an analytical balance, an instrument that yields highly accurate and precise data.
Gravimetric methods
Two major types of gravimetric methods
Precipitation methods and Volatilization methods
The analyte is converted to a soluble precipitate, then filtered, wash and converted to a product of known composition by suitable heat treatment that can be weighed.
Precipitation Method
The analyte or its decomposition products are volatilized at a suitable temperature. The volatile product is then collected and weighed, or the mass of the product is determined indirectly from the loss in mass of the sample.
Volatilization Method
Developed or refined many of the techniques of gravimetric analysis involving silver and chlorine.
The techniques were used to determine the atomic weights of 25 of the elements
Became the first American to receive Nobel prize in chemistry.
Theodore W. Richards
(and his students at Harvard University)
Ideally, a gravimetric precipitating agent should react ______ or at least _____ with the analyte.
specifically
selectively
_____, which are rare, react only with a single chemical species.
_____, which are more common, react with a limited number of
species.
Specific reagents
Selective reagents
In addition to specificity and selectivity, the ideal precipitating reagent would react with the analyte to give a product that is:
- 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 composition after it is dried or, if necessary, ignited
Precipitates consisting of large particles are generally desirable for gravimetric
work. Why?
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.
The particle size of solids formed by precipitation varies enormously. At one
extreme are ________, whose tiny particles are invisible to the naked eye.
Colloidal particles 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.
This phenomenon is called the _____. At the other extreme
are particles with dimensions on the order of tenths of a millimeter or greater.
The temporary dispersion of such particles in the liquid phase is called a _________. The particles of a crystalline suspension tend to settle spontaneously and
are easily filtered.
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 these variables can be accounted for, at least qualitatively, by assuming
that the particle size is related to a single property of the system called_______, where:
RS = (Q-S)/S
This equation is also known as: __________
(Q-S)/S is large the precipitate is?
(Q-S)/S is small the precipitate is?
colloidal suspensions
tyndall effect
crystalline
suspension
relative
supersaturation
Q = concentration of the solute at any instant
S = equilibrium solubility
Von Weimarn Equation
Colloidal
Crystalline Solid
A ______ consists of solid particles with diameters that are less than
10^-4 cm.
In _____, colloidal
suspensions may be perfectly ____ and appear to contain _____. The
presence of the second phase can be
detected, however, by shining the
________ into the solution.
colloid
diffuse light
clear and no solid
beam of flashlight
an unstable solution that contains a higher solute concentration than a saturated solution. With time, supersaturation is relieved by precipitation of excess solute.
Supersaturated solution
The effect of relative supersaturation on particle size can be explained if we assume
that precipitates form in two ways: by _______
nucleation
particle growth
Is a process in which a minimum number of atoms, ions,
or molecules join together to give a stable solid.
Precipitates form by nucleation and by particle growth. If nucleation predominates, a _____
number of very fine particles is produced. If particle growth predominates, a _____ number of larger particles is obtained.
Nucleation
large
smaller
Precipitates that have very low solubilities, such as many sulfides and hydrous oxides, generally form as ______.
colloids
Individual colloidal particles are so small that they are not retained by ordinary filters. Moreover, _____ prevents their settling out of solution under the influence of gravity.
Brownian motion
Coagulation can be hastened by:
heating
stirring
adding an electrolyte
is a process in which a substance (gas, liquid, or solid) is held on the surface of a solid.
is retention of a substance within the pores of a solid.
Adsorption
The charge on a colloidal
particle formed in a gravimetric analysis is determined by the
_____________ that is in excess when the precipitation is complete.
charge of the lattice ion
Attached directly to the solid surface is the ________, which consists mainly of adsorbed silver ions.
Surrounding the charged particle is a layer of solution, called the _______, which
contains sufficient excess of negative ions principally nitrate) to just balance the charge on the surface of the particle.
The primarily adsorbed silver ions and the negative counter-ion layer constitute an _______ that imparts stability to the colloidal suspension.
As colloidal particles approach one another,
this double layer exerts an electrostatic repulsive force that prevents particles from
colliding and adhering.
primary adsorption layer
counter-ion layer
electric double layer
Is a process by which
a coagulated colloid returns to its dispersed state.
Is the process by which a coagulated colloid reverts to its original dispersed state.
Peptization
Colloids are best precipitated from hot, stirred solutions containing sufficient electrolyte to ensure coagulation. 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 ________, weakly
bound water appears to be lost from the precipitate. The result is a denser mass that
is easier to filter.
digestion
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
are generally more easily filtered and purified than are coagulated colloids.
Crystalline precipitates
The value of Q can often
be minimized by using ____________, with good mixing.
Often, S is increased by ____________.
dilute solutions and adding the precipitating reagent slowly
precipitating from hot solution or by adjusting the pH of the precipitation medium
Is the solution from
which a precipitate was formed.
Mother liquor
Improves the purity
and filterability of both colloidal and crystalline precipitates.
Digestion
is a process in
which normally soluble compounds are carried out of solution by a
precipitate
Coprecipitation
When otherwise soluble compounds are removed from solution during precipitate formation, we refer to the process as ________. Contamination of a precipitate by
a second substance whose solubility product has been exceeded is _________.
coprecipitation
not coprecipitation
There are four types of coprecipitation:
_______ formation are equilibrium processes, and _______ arise from the kinetics of crystal growth.
surface adsorption
mixed-crystal formation
occlusion
mechanical entrapment
Surface adsorption and
mixed-crystal
occlusion and mechanical entrapment
Is a common source of coprecipitation and is likely to cause significant contamination of precipitates with large specific surface areas, that is, coagulated colloids.
Adsorption
Is often the major
source of contamination in coagulated colloids but of no significance in crystalline precipitates.
In adsorption, a normally
soluble compound is carried out of solution on the surface of a coagulated colloid.
This compound consists of the ______ and an ______ from the counter-ion layer.
Adsorption
primarily adsorbed ion
ion of opposite charge
_______ 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 is _______. In this process, the filtered solid is redissolved and reprecipitated.
The first precipitate usually carries down only a fraction of the contaminant present
in the original solvent.
reprecipitation or double precipitation
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
Can cause either
negative or positive errors.
Coprecipitation
Is a process in which a precipitate is formed by slow generation of a
precipitating reagent homogeneously
throughout a solution.
Homogeneous precipitation
After filtration, a ________ is heated until its mass becomes constant.
______ removes the solvent and any volatile species carried down with the precipitate. Some precipitates are also ignited to decompose the solid and form a compound
of known composition. This new compound is often called the_____.
gravimetric precipitate
Heating
weighing form.
The temperature required
to dehydrate a precipitate
completely may be as low as ____ or as high as _____.
100°C
1000°C
Recording thermal decomposition
curves is called ____
The mass versus
temperature curves are termed______.
thermogravimetry or thermal gravimetric analysis
thermograms
There are two types of organic reagents:
(1) one forms slightly soluble nonionic products called ________,
(2) forms products in which the bonding
between the______.
coordination compounds
inorganic species and the reagent is largely ionic
Approximately two dozen cations form sparingly soluble chelates with
8-hydroxyquinoline.
Is an organic precipitating agent of unparalleled specificity. Only
nickel(II) is precipitated from a weakly alkaline solution.
Is an important example of an organic
precipitating reagent that forms salt-like precipitates.
Hydroxyquinoline
Dimethylglyoxime
Sodium Tetraphenylborate
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.
Reagents that form compounds of this type are called ______, and their products are called ______.
Are cyclical metal-organic
compounds in which the metal is a part of one or more five- or six-membered rings. Ex: Heme
chelating agents
chelates
The two most common gravimetric methods based on volatilization are those for determining:
_____is quantitatively distilled from many
materials by heating.
In _____, water vapor is collected on any of several solid desiccants, and its mass is determined from the mass gain of the desiccant.
The _______ in which 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.
Widely used to determine water in items of commerce.
water and carbon dioxide
Water
direct determination
indirect method
classical method for the determination of carbon and hydrogen in organic compounds is a _____ in which the combustion product H2O and CO2 are collected selectively on weighed absorbents.
The ______serves as the analytical parameter.
Gravimetric Procedure
increase in mass
