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