Gravimetric Analysis
Dr Ashwini Wadegaonkar
Unit 1: Gravimetric Analysis
Common ion effect and solubility product principles
Conditions for good precipitation
Factors affecting precipitation like acid, temperature, nature of solvent
Super saturation and precipitation formation
Precipitation from homogeneous solution and examples
Co-precipitation, post-precipitation and remedies for their minimization
Washing of precipitate and ignition of precipitate
Brief idea about method of filtration and drying of precipitate
Introduction to electrogravimetry: principle, applications, electrolytic
separations of Cu and Ni
Numerical problems only on gravimetric analysis
Introduction
Gravimetric analysis is a technique through which the amount of an
analyte (the ion being analyzed) can be determined through the
measurement of mass.
Gravimetric analyses depend on comparing the masses of two
compounds containing the analyte.
The principle behind gravimetric analysis is that the mass of an ion in a
pure compound can be determined and then used to find the mass
percent of the same ion in a known quantity of an impure compound.
In order for the analysis to be accurate, certain conditions must be met:
-The ion being analyzed must be completely precipitated.
- The precipitate must be a pure compound.
-The precipitate must be easily filtered.
Common ion effect and solubility product principles
Common Ion Effect
The role that the common ion effect plays in solutions is mostly visible in
the decrease of solubility of solids. Through the addition of common ions,
the solubility of a compound generally decreases due to a shift in
equilibrium.
The common ion effect also plays a role in the regulation of buffers.
Buffering solutions contain either an acid or base, accompanied by its
conjugate counterpart. Addition of more like conjugate ions will
ultimately shift the pH of the solution.
The common ion effect must be taken into consideration when
determining solution equilibrium upon addition of ions that are already
present in the solution.
To a weak acid, when another electrolyte containing a
common ion is added the degree of dissociation of the weak
electrolyte is suppressed.
Common ion effect gives valuable method to control the
concentration of the desired ion given by the weak
electrolyte.
Solubility
Solubility refers to the amount of material that is able to be
dissolved in a particular solvent.
For example, table salt (NaCl) placed in water eventually
dissolves.
However, if more table salt is continuously added, the solution
will reach a point at which no more can be dissolved; in other
words, the solution is saturated, and the table salt has
effectively reached its solubility limit.
https://opentextbc.ca/chemistry/chapter/11-3-solubility/
http://bioprofe.com/en/solubility-and-precipitation/
Chemical equilibrium is the chemical state where there are no net
physical or chemical changes between the reactant and the
products of a reaction.
This is because the rate of the forward (reactant to product) and
reverse (product to reactant) reactions are equal.
Solubility equilibrium refers to the state of chemical equilibrium
between a chemical compound in the solid state and a solution
composed of that dissolved compound.
This equilibrium is established when the rates of migration
between the solid and aqueous phases of the molecules (or ions)
are equal.
Common Ion Effect on Solubility
Adding a common ion decreases solubility, as the reaction
shifts toward the left to relieve the stress of the excess
product.
Adding a common ion to a dissociation reaction causes the
equilibrium to shift left, toward the reactants, causing
precipitation.
https://chem.libretexts.org/Bookshelves/Physical_and_Th
eoretical_Chemistry_Textbook_Maps/Supplemental_Modul
es_(Physical_and_Theoretical_Chemistry)/Equilibria/Solub
ilty/Common_Ion_Effect
Solubility product
In a saturated solution of a sparingly soluble electrolyte (salt), the
product of ionic concentrations when raised to proper powers is
constant at a given temperature, is called solubility product,
denoted by Ksp or S.
It is when compared to ionic product, the conclusions are as
follows –
1. When Ionic product = Ksp, the solution is saturated, no
precipitation
2. When Ionic product < Ksp, the solution is unsaturated, no
precipitation
3. When Ionic product > Ksp, the solution is supersaturated,
precipitation
Solubility product
The solubility products Ksp's are equilibrium constants in
hetergeneous equilibria (i.e. between two different phases).
If several salts are present in a system, they all ionize in the
solution.
If the salts contain a common cation or anion, these salts
contribute to the concentration of the common ion.
Contributions from all salts must be included in the calculation
of concentration of the common ion.
Consideration of charge balance or mass balance or both leads to
the same conclusion.
When NaCl and KCl are dissolved in the same solution, the Cl− ions
are common to both salts.
In a system containing NaCl and KCl, the Cl− ions are common ions.
NaCl ⇌ Na+ + Cl−
KCl ⇌ K++ Cl−
CaCl2 ⇌ Ca2++ 2Cl−
AlCl3 ⇌ Al3++ 3Cl−
AgCl ⇌ Ag++ Cl−
Precipitation
•To precipitate is to form an insoluble
compound, either by decreasing the
solubility of a compound or by reacting two
salt solutions.
• The solid that forms via a precipitation
reaction is called the precipitate.
• Precipitation reactions serve important
functions. They are used for purification,
removing or recovering salts, for making
pigments, and to identify substances in
qualitative analysis.
https://www.chemicool.com/definition/precipitate.html
https://media.iupac.org/publications/analytical_compendiu
m/Cha09sec50.pdf
Conditions for good precipitation
What is precipitation gravimetry?
Precipitation gravimetry is an analytical technique that uses a
precipitation reaction to separate ions from a solution.
The chemical that is added to cause the precipitation is called
the precipitant or precipitating agent.
The solid precipitate can be separated from the liquid
components using filtration, and the mass of the solid can be
used along with the balanced chemical equation to calculate
the amount or concentration of ionic compounds in solution.
Ideally, an analytical precipitate for gravimetric analysis
should consist of perfect crystals large enough to be easily
washed and filtered.
The perfect crystal would be free from impurities and be
large enough so that it presented a minimum surface area
onto which foreign ions could be adsorbed.
The precipitate should also be "insoluble”
It has been shown (Von Weimarn) that the particle size of
precipitates is inversely proportional to the relative
supersaturation of the solution during precipitation
The following methods are used to approach these criteria -
Precipitation from hot solution. The solubility S of precipitates
increases with temperature and so an increase in S decreases the
supersaturation.
Precipitation from dilute solution. This keeps Q low. Slow addition of
precipitating reagent with effective stirring. This also keeps Q low,
stirring prevents local high concentrations of the precipitating agent.
Precipitation at a pH near the acidic end of the pH range in which
the precipitate is quantitative. Many precipitates are more soluble at
the lower (more acidic) pH values and so the rate of precipitation is
slower.
Digestion of the precipitate. Also the digestion period results in some
improvement in the internal perfection of the crystal structure
[sometimes called ripening], here some internal foreign atoms may be
expelled.
Coprecipitation
This is anything unwanted which precipitates with the thing you
do want. Coprecipitation occurs to some degree in every
gravimetric analysis (especially barium sulfate and those
involving hydrous oxides). It can be minimized by careful
precipitation and thorough washing.
Surface adsorption
Here unwanted material is adsorbed onto the surface of the
precipitate. Digestion of a precipitate reduces the amount of
surface area and hence the area available for surface adsorption.
Washing can also remove surface material.
Occlusion
This is a type of coprecipitation in which impurities are
trapped within the growing crystal.
Postprecipitation
Sometimes a precipitate standing in contact with the mother
liquor becomes contaminated by the precipitation of an
impurity on top of the desired precipitate.
Factors affecting precipitation
1. Effect of Acids on the Solubility of a Precipitate
The solubility of a sparingly soluble salt of strong acid
increases upon addition of strong acid. Addition of strong
acid increases the ionic strength of the solution and hence
decreases the activity coefficients of both ions of sparingly
soluble salt.
2. Effect of Temperature on the Solubility of a
Precipitate
The solubility of a precipitate increases with rise in
temperature. The dissolution of a solute is mostly an
endothermic process, so the solubility product constant
increases as the temperature is increased.
3. Effect of the nature of Solvent on the Solubility of a
Precipitate
A solvent has a large effect upon solubility of a solute. Generally polar
solutes are more soluble in polar solvents and non polar solutes are
soluble in non polar solvents. Solubility of inorganic compounds is
reduced when an organic solvents is added to water.
4. Supersaturation and precipitate formation
Smaller the particle size more is the solubility of a substance.
A supersaturated solution the one that contains a greater
concentrations of solute than expected equillibrium conditions at a
given temperature.
Supersaturation plays important role in determining particle size of
the precipitate.
particle size of the precipitate decreases with increasing concentration
of the reactants.
Super saturation and precipitation formation
A supersaturated solution is an unstable solution that
contains more solutes than a saturated solution, with time,
supersaturation is relieved by precipitation of the excess
solute.
To increase the particle size of a precipitate, minimize the
relative supersaturation during the precipitate formation.
Relative supersaturation = (Q-S)/S
where Q is the concentration at any instant and S is the
equilibrium solubility.
5. The Purity of the Precipitate
Co-precipitation – the contamination of the precipitate by substances which
are normally soluble in the mother liquor is termed as co-precipitation.
Two types of co-precipitation –
a. Due to surface adsorption
b. Due to formation of solid solution – Occlusion
Co-precipitation can be minimized by –
Taking dilute and pure solution
Use double precipitation
Operations like digestion or aging
Use of organic solvent
Slow addition of reagent
Stirring
Proper temperature
Post precipitation
The process in which precipitation of some component
(impurity) occurs on the surface after its formation is called
precipitation.
It occurs with sparingly soluble substances
The longer thee time for precipitate to stand, the greater is the
error due to due to this cause
Post-precipitation can be minimized by –
Complex formation
Change in oxidation state
Rapid filtration of precipitate
Concentration of impurities
Distinction between Co-Precipitation and Post-Precipitation
https://image.slidesharecdn.com/gravimetry-
160407142410/95/gravimetry-21-638.jpg?cb=1460039131
Introduction to Electrogravimetry
Electrogravimetry is a method in which the element / metal to be
determined is deposited electrolytically upon a suitable electrode.
From the gain in the weight of electrode the amount of the metal
in the sample is calculated.
The method of electrogravimetry is rapid and accurate.
Electrogravimetry is a method for the separation of the metal
ions by using the electrodes.
The deposition takes place on the one electrode. The weight of this
electrode is determined before and after deposition.
This gives the amount of the metal present in the given sample
solution.
Principle of Electrogravimetry
The main principle
involved in this method is
the deposition of the
solid on an electrode
from the analyte solution.
Electrogravimeter
The material is deposited by means of potential application. The
electrons are transported to electrode by the following
mechanisms:
Diffusion
Migration
Convection
THEORY
A metal is electrolytically deposited on the electrode by increasing the mass
of the electrode.
M+2 + 2e− M(S)
Therefore,
Eelectrolysis = Ecathode − Eanode
The electrons deposition is governed by Ohm's and Faraday's laws of
electrolysis which states that the amount of the electrons deposited on the
electrode is directly proportional to the amount of the current passed
through the solution and the amount of different substances deposited is
directly proportional to the molar masses divided by the number of electrons
involved in the electrolysis process.
That is the current (I) is directly proportional to the electromotive force (E)
and is indirectly proportional to the resistance (R).
E = IR
From the above equation, we get the following:
Eelectrolysis = Ecell − IR
Ecell = Ecathode − Eanode
where
Therefore,
Eapplied = Ecathode − Eanode − IR
I = (−Eapplied/R) +1/R(Ecathode − Eanode)
I = (Ecell − Eapplied/R)
I = (−Eapplied/R) + K
where K is the constant.
A plot of the current of the applied potential in an electrolytic cell should be
straight line with a slope equal to negative reciprocal of the resistance.
Electrogravimetric plot
http://rxpharmaworld.blogspot.com/2016/12/electrogravi
metry.html
Electrolytic separation of Copper and Nickel
Constant current electrolysis is used in the separation and
determination of metals in alloys.
Some alloys, mainly contain copper and nickel as - Monel
metal, coinage alloys, cupro-nickel alloys etc. they contain
impurity like iron.
The impurity is removed and the two metals are electro-
deposited at different applied potentials.
