Gravimetric 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 then converted
to a compound of known composition that can be weighed.
Precipitation Gravimetry
the analyte is separated from
other constituents of a sample by conversion to a gas of
known chemical composition. The weight of this gas then
serves as a measure of the analyte concentration
Volatilization Gravimetry
the analyte is separated by deposition of
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 process of precipitation involves three steps which are
supersaturation, nucleation, and particle growth
________ occurs upon addition of the first drops of the precipitating agent
supersaturation
then starts to occur
wherein a minimum number of atoms, ions, or molecules
aggregate together to form a stable solid.
Nucleation
then starts to occur
wherein a minimum number of atoms, ions, or molecules
aggregate together to form a stable solid.
Nucleation
Further
precipitation then involves a competition between additional
nucleation and growth on existing nuclei
particle growth
[Factors That Determine the Particle Size of Precipitates]
are invisible to
the naked eye (10^-7 to 10^-4 cm in diameter).
show no tendency to settle from
solution and are difficult to filter
Colloidal suspensions
[Factors That Determine the Particle Size of Precipitates] The temporary dispersion of particles in the liquid
phase
tend to settle
spontaneously and are easily filtered
Crystalline suspension
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
states that the particle size of
precipitates is inversely proportional to
the relative saturation of the solution
during the precipitation process
Relative Supersaturation / Von Weirman Equation
a minimum
number of atoms, ions, or molecules aggregate together to form a
stable solid.
nucleation
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 and
growth of existing nuclei
particle growth
Control of Particle size
Increasing the solubility of the precipitate S
Precipitation using dilute solutions to minimize Q
Slow addition of the precipitating agent with good stirring to
keep the concentration of solute Q low
Brownian motion prevents their settling out of solution under the influence of gravity. However, we can coagulate, or agglomerate, the individual particles of most colloids to give a filterable, amorphous mass that will settle out of solution
Colloidal Precipates
Coagulation can be hastened by:
Heating
Stirring
Adding an electrolyte to the medium
Coagulation can be hastened by:
Heating
Stirring
Adding an electrolyte to the medium
a substance (gas, liquid, or solid) is held on the
surface of a solid.
Absorption
is the process by which a coagulated colloid reverts to its original dispersed state
Peptization
is the process by which a coagulated colloid reverts to its original dispersed state
Peptization
are more easily filtered and purified than
coagulated colloids.
Crystalline precipitates
The particle size of crystalline solids can often be
improved significantly by
minimizing Q or maximizing S, or
both.
of crystalline precipitates (without stirring) for
some time after formation often yields a purer, more
filterable product.
Digestion
The amount of impurities depends on nature of
precipitate and condition of precipitation
It may be due to
-coprecipitation
-post-precipitation
is the process in which normally soluble
compounds are carried out of the solution. It leads to an increase
in the mass of precipitate
Coprecipitation
four types of coprecipitation
surface absorption
mixed-crystal formation
occlusion
mechanical entrapment
Is likely to cause significant contamination of
precipitates with large specific surface areas, that is,
coagulated colloids.
Surface Absorption
How to minimize absorbed impurities on colloids
Washing of colloidal particles
is a drastic but an effective way to minimize the effects of absorption.
Reprecipitation
one of the ions in the crystal lattice
of a solid is replaced by an ion of another element
Mixed Crystal Formation
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. Several crystals grow together and in so doing
trap a portion of the solution in a tiny pocket
Mechanical entrapment
________may occur in both colloidal and
crystalline precipitates, but _________ are confined to crystalline precipitates.
Mixed-crystal formation; occlusion and mechanical
entrapment
may cause either negative or
positive errors in an analysis. If the contaminant is not a compound of the ion being determined, a positive error will always result. In contrast, when the contaminant does contain the ion being determined, either positive or negative errors may occur
Coprecipated Impurities
is a process in which a
precipitate is formed by slow generation of a
precipitating reagent homogeneously throughout a
solution
Homogeneous precipitation
Solids formed by homogeneous precipitation are?
generally purer and more easily filtered
Steps in gravimetric analysis
Preparation of the Solution
Precipitation
Digestion
Filtration
Washing
Drying or Ignition
Weighing
Calculation
Preliminary separation of potential
interferences before precipitating the
analyte
Adjustment of solution condition (pH /
temperature / volume / concentration of
test substance) to maintain low solubility of
precipitate and maximum precipitate
formation
Preparation of the Solution
The precipitating reagent is added at a
concentration that favors the formation of a
“good” precipitate
This may require low concentration, extensive
heating (often described as“digestion”), or careful
control of the pH
Precipitation
Also known as Ostwald ripening, the small
particles tend to dissolve and precipitate on
the surfaces of the larger crystals
Digestion
Precipitate is separated from mother liquor (the
solution from which a precipitate was formed.
Choice depends on the nature of precipitate, cost of
media, and heating temperature required for
drying
Filtration
Coprecipitated impurities, especially those
on surface, are removed by
Washing
its purpose is to remove the
remaining moisture
It is done by heating about 120-150℃ for 1-2
hours
Drying
Its purpose in a
muffle furnace at temperatures ranging from
600-1200℃ is to get the material with exactly
known chemical structure so that the amount
of analyte can be determined accurately.
The precipitate is converted to a more
chemically stable form
Ignition
After the precipitate is allowed to cool
(preferably in a desiccator to keep it from
absorbing moisture), it is
Weighed (Process: Weighing)
The combined constant factors in a
gravimetric calculation are referred to as
the
Gravimetric Factor
Volumetric Analysis is also known as
Titration
Involve measuring the volume of a solution
of known concentration that is needed to
react completely with the analyte
Volumetric Analysis / Titration
A reagent of known concentration that is used to carry out volumetric titration
Titrants or titrators
(Standard solution/ Standard Titrant)
A theoretical point reached when the amount of added titrant is chemically equivalent to the amount of analyte in the sample
Equivalence point
Is a point in titration that signifies the completion of the titration by a change in the colour or intensity of the solution
End point
Are often added to the analyte solution to produce observable physical change (which signals end point) at or near the equivalence point
Indicators
Refers to the mass difference in volume or mass between the equivalence point and the end point
Titration error
Highly purified compound that serves as a reference
material in titrations
Primary Standards
[Primary Standards] Acid-base reactions
sodium carbonate Na2CO3
- sodium tetraborate Na2B407
- potassium hydrogenphthalate KH(C8H404)
- constant boiling point hydrochloric acid
- potassium hydrogeniodate KH(IO3)2
- benzoic acid (C6H5COOH).
[Primary Standards] Complex formation reactions
silver, silver nitrate, sodium chloride, various metals and salts, depending upon the
reaction used.
[Primary Standards] Precipitation reactions
silver, silver nitrate, sodium chloride, potassium
chloride, and potassium bromide (prepared from
potassium bromate).
[Primary Standards] Oxidation-reduction reactions
- potassium dichromate K2Cr2O7
- potassium bromate KBrO3
- potassium iodate KIO3
- potassium hydrogeniodate KH(IO3)2
- sodium oxalate Na2C204
- arsenic(II) oxide As2O3
- pure iron.
are widely used to determine the amounts of acids
and bases and to monitor the progress of reactions that produce or consume
hydrogen (H+) ions.
Neutralization titrations
Standard reagents used in acid-base titrations are always strong acids or
string bases such as:
- Hydrochloric (HCL) acid;
- Perchloric acid (HCLO4) acid;
- Sulfuric (H2SO4) acid;
- Sodium hydroxide (NaOH); and
- Potassium hydroxide (KOH)
There are 3 types of indicators of Acids and Bases
- Natural
- Synthetic
- Olfactory
- these are indicators that can be organically found in nature
Natural indicators
the most
common and well-known
acid/base indicator
Litmus paper
naturally yellow in color
and turns red when it comes into
contact with a basic solution
Turmeric
indicators which are synthesized and created by the chemical process.
Synthetic indicators
Examples of synthetic indicators
Methyl Orange
Phenolphthalein
these are substances whose smell changes in acidic or basic solutions
Olfactory indicator
have a characteristic sharp smell. In a basic medium, the smell
cannot be detected, but in an acidic medium it retains its strong smell
Onions
this occurs which the indicator changes color
differs from the pH at the equivalence point
- usually minimized by choosing the indicator carefully or by making a
blank correction
Determinate error
originates from the limited ability of the
human eye to distinguish reproducibly the intermediate color of the
indicator
Indeterminate error
The pH interval over which a given indicator exhibits a color change is
influenced by:
a. Temperature
b. Ionic strength of the medium
c. Presence of organic solvents and colloidal particles
Two Sources of Hydronium (OH-) ions in an aqueous solution of strong
acids:
- Reaction of the acid with water; and
- The dissociation of water itself
at this stage we compute the concentration of the acid
from its starting concentration and the amount of base that has been added.
Pre-equivalence
hydronium and hydroxide ions are present in equal
concentrations, and the hydronium ion concentration is derived directly
from the ion-product constant for water.
Equivalence
analytical concentration of the excess base is
computed, and the hydroxide ion concentration is assumed to be equal to or
a multiple of the analytical concentration.
Post equivalence
it is the measurements of physical
properties of analytes, such as current, electrode potential, the absorption or
emission of light, and mass-to-charge ratios, and fluorescence, used for
quantitative analysis of a variety of inorganic, organic, and biochemical
analyte.
Instrumental methods of analysis
quantitative method used to measure the potential of an
electrochemical cell under static condition.
Potentiometry
in the Potentiometry is when the pair of
electrodes is placed in the sample solution, it shows the potential difference
by the addition of the titrant or by the change in the concentration of the
ions.
Principle
a device for measuring the potential of an electrochemical
cell without drawing a current or altering the cell’s composition.
Potentiometer
the electrode which contains its own potential value
and it is stable when dipped into sample solution.
Reference electrode
commonly used to establish a
reference potential for measuring other electrode’s potentials.
It is dependable but large, bulky, and affected by temperature.
Calomel Reference Electrodes or mercury sulfate electrode
and mercury oxide electrode
The most
widely marketed reference electrode system consists of a
silver electrode immersed in a solution of potassium chloride
that has been saturated with silver chloride.
- Widely used because it is simple, inexpensive, very stable
and non- toxic.
- More compact– overall better and faster
Silver- silver Chloride Reference Electrodes
consists of a platinized
platinum electrode in HCl solution with hydrogen at
atmospheric pressure bubbled over the platinum surface.
Normal Hydrogen Electrode
the electrode which responds to change in the
potential of analyte solution.
Indicator electrode
It is convenient to classify
metallic indicator electrodes as electrodes of the first kind,
electrodes of the second kind, and inert redox electrodes.
Metallic Indicator Electrodes
is a pure metal electrode that is in direct
equilibrium with its cation in the solution. A single
reaction is involved.
Electrodes of the First Kind
Metals not only
serve as indicator electrodes for their own cations but
also respond to the activities of anions that form
sparingly soluble precipitates or stable complexes
with such cations.
Electrodes of the second Kind
Several relatively inert conductors respond to redox
systems. Such materials as platinum, gold, palladium,
and carbon can be used to monitor redox systems.
Inert Metallic Electrodes for Redox Systems
the most convenient
method for determining pH has involved measurement of the
potential that appears across a thin glass membrane that
separates two solutions with different hydrogen ion
concentrations.
Membrane Indicator Electrodes
a type of
ion-selective electrode made of a doped glass membrane that
is sensitive to a specific ion.
The Glass Electrode for measuring pH
Also known as the liquid junction, used to prevent the mixing
or interference of the analyte solution with that of the reference solution.
Salt bridge
are potentiometric sensors that include a selective membrane to
minimize matrix interferences.
Ion – selective Electrode (ISE) or specific ion electrode (SIE)
Most common ISE:
pH electrode, which contains a thin glass
membrane that responds to the H+ concentration in a solution.
TYPES OF ION SELECTIVE ELECTRODE (ISE):
Any electrode that preferentially responds to one ion species.
- Glass Membrane Electrode: H+
- Solid State Electrode
- Liquid Membrane Electrode
- Gas Sensing Electrode
- Enzyme electrodes
