Chemistry Project
2023 – 24
Double Salt Crystallisation
(Mohr’s Salt & Potash Alum)
Balakrishnan Meleparambottil
12 SCI J
Acknowledgement
I would like to begin by expressing my sincere
gratitude to all those who have helped me complete
this project.
Firstly, I want to specially thank my teacher and my
guide throughout the project, Mrs Bindu Rajendran
for her encouragement and invaluable guidance.
At any level, the project would’ve not been possible
without my parents’ support and the encouragement
of my friends.
I also want to acknowledge my sister who has been a
beacon of joy during the project.
Lastly, I thank God for giving me the strength to
overcome any hurdles and always showing me the
right path.
Index
SI.No
Name of the Topic
1.
Introduction
2.
Double Salts
3.
Potash Alum
4.
Mohr’s Salt
5.
Experiment
6.
Experiment
7.
Bibliography
Page No.
Introduction
What is a Salt?
In chemistry, a salt is a compound in which the positively charged
ions called cations and the negatively charged anions are held
together by ionic bonds.
They are typically formed from neutralization reactions between
an acid and a base. The hydrogen ion (H+) of the acid is replaced
by a cation and the hydroxide ion (OH-) is replaced by the anion.
The cations and anions attract each other in a salt and form a
crystal lattice structure which gives salts their characteristic solid,
crystalline appearance. They can either be soluble or insoluble in
water, depending upon their composition.
Some common examples of salts:
1. Sodium chloride (NaCl) – commonly called as the table salt
2. Calcium carbonate (CaCO3) – Chalk
3. Potassium Nitrate (KNO3) – Saltpetre
4. Magnesium Sulphate (MgSO4) – Epsom salt
Properties of Salts
1. Ionic nature - Salts are composed of ions, which are electrically
charged particles. They consist of positively charged cations and
negatively charged anions. The ionic nature of salts allows them
to conduct electricity when dissolved in water or melted.
2. Crystalline Structure – Salts generally have a crystalline
structure, forming regular, repeating patterns of ions in a threedimensional lattice. This arrangement gives salts their
characteristic shape and solid appearance.
3. High Melting & Boiling Points - Salts generally have high
melting and boiling points compared to molecular compounds.
The strong ionic bonds between the ions require a significant
amount of energy to break, resulting in high melting and boiling
points.
4. Solubility - The solubility of salts varies widely depending on
their composition. Some salts, like sodium chloride, are highly
soluble in water, while others, such as silver chloride, are only
sparingly soluble. The solubility of a salt is influenced by factors
such as temperature and the presence of other solutes.
5. Conductivity - When salts are dissolved in water or melted,
their ions become mobile and can carry electric charge.
Consequently, solutions of salts are good conductors of electricity.
However, solid salts do not conduct electricity as the ions are
fixed in the crystal lattice.
6. Color - Salts can exhibit a wide range of colors. Some salts are
colorless, while others display vivid colors due to the presence of
transition metal ions, which can absorb and reflect specific
wavelengths of light.
7. Hygroscopicity - Certain salts have the ability to absorb
moisture from the surrounding environment. These hygroscopic
salts can become damp or even dissolve in humid conditions.
8. Reaction with Acids and Bases - Salts, being the products of
neutralization reactions, are usually neutral compounds. They do
not exhibit acidic or basic properties. However, some salts may
undergo hydrolysis when dissolved in water, resulting in a
slightly acidic or basic solution.
Uses of Salts
Food Preservation and Flavouring: Sodium chloride (NaCl), is
widely used as a food preservative to inhibit the growth of
bacteria and other microorganisms. It is also a popular seasoning
that enhances the taste of food.
Water Softening: Certain salts, such as sodium chloride and
potassium chloride, are used in water softening systems to
remove hardness-causing minerals, such as calcium and
magnesium ions, from water. This helps prevent the buildup of
scale in pipes and appliances.
Chemical Manufacturing: Salts serve as essential raw materials in
the production of various chemicals. They are used in the
manufacturing of fertilizers, dyes, pigments, soaps, detergents,
and pharmaceuticals, among others.
Water Treatment: Salts like chlorine compounds (e.g., sodium
hypochlorite) are commonly used in water treatment processes
to disinfect and purify drinking water, swimming pools, and
wastewater.
De-icing and Anti-Icing: Salts, particularly calcium chloride and
sodium chloride, are used as de-icing agents to melt ice and snow
on roads, sidewalks, and airport runways. These salts lower the
freezing point of water, preventing ice formation or aiding in its
removal.
Agriculture and Fertilizers: Salts containing essential nutrients
like nitrogen, phosphorus, and potassium (NPK) are used as
fertilizers to enhance crop growth and increase yields. Examples
include ammonium nitrate, potassium nitrate, and triple
superphosphate.
Medicinal and Health Applications: Certain salts are used in
medicinal preparations and healthcare. For instance, Epsom salt
(magnesium sulphate) is used in baths for its soothing and
muscle-relaxing properties. Salts like magnesium citrate and
sodium bicarbonate can be used as antacids or laxatives.
Photography: Silver salts, such as silver chloride and silver
bromide, are utilized in traditional black-and-white photography.
These salts are light-sensitive and undergo chemical changes
when exposed to light, forming the basis of capturing images on
photographic film.
Industrial Processes: Salts play a role in various industrial
processes. For example, sodium hydroxide (caustic soda) is used
in chemical manufacturing and as a pH regulator. Molten salts,
such as sodium chloride or potassium nitrate, can be used as heat
transfer fluids in high-temperature applications.
Batteries: Some types of batteries, such as lithium-ion batteries,
utilize salts as electrolytes to facilitate the flow of ions between
the electrodes, enabling the storage and release of electrical
energy.
Types of Salts
There are mainly 4 types of salts: 1. Simple salts- Simple salts are composed of only one type of
cation and one type of anion. They are the most common type of
salts and are formed through neutralization reactions between an
acid and a base.
Examples: Sodium chloride (NaCl)
Potassium sulphate (K2SO4)
Calcium carbonate (CaCO3)
There are 3 sub-types under simple salts: i. Acidic Salts - Acid salts, also known as hydrogen salts, are
formed when a portion of the hydrogen ions in an acid are
replaced by cations. They are acidic in nature (pH < 7) and can
still release hydrogen ions in solution.
They are formed in a reaction between a strong acid and a weak
base.
Examples: -
Ammonium Chloride (NH4Cl)
HCl + NH4OH  NH4Cl + H2O
Copper Sulphate (CuSO4)
H2SO4 + Cu(OH)2  CuSO4 + H2O
ii. Basic Salts - Basic salts are formed when only a portion of the
hydroxide ions in a base are replaced by anions. They are basic in
nature which means their pH value is more than 7.
It is formed when a weak acid is reacted with a strong base.
Examples: Sodium Sulphide (Na2S)
H2S + NaOH  Na2S + H2O
Sodium Acetate (CH3COONa)
CH3COOH + NaOH  CH3COONa + H2O
iii. Neutral Salts – A neutral salt is formed when all the hydrogen
ions and the hydroxide ions in the acid and base are replaced by a
cation and an anion respectively. They are neutral, which means
their pH is equal to 7.
They are formed during a neutralization reaction between an acid
and a base of equal strength.
Examples: Sodium Chloride i.e., Common salt (NaCl)
HCl + NaOH  NaCl + H2O
Potassium Nitrate (KNO3)
HNO3 + KOH  KNO3 + H2O
2. Complex Salts
Complex salts, also known as coordination compounds, are salts
that contain a central metal ion coordinated with surrounding
molecules or ions called ligands.
Examples: 1. Tetra amine copper (II) sulphate [Cu(NH3)4]SO4
CuSO4 + 4NH3 + H2O → [Cu(NH3)4]SO4 + H2SO4
2. Ferricyanide [Fe(CN)6]3FeCl3 + 6K3[Fe(CN)6] → [Fe(CN)6]3- + 3FeCl2 + 18KCl
3. Mixed Salts
Mixed Salts are those salts in which the anions share a common
cation, or the cations share a common anion.
For example,
Calcium Oxychloride (CaOCl2)
Ca(OH)2 + Cl2  CaOCl2 + H2O
In this salt Oxygen ion and Chlorine ion which are anions share a
common cation Calcium ion.
4. Double Salts
The salts that contain more than one cation or anion are known
as double salt.
Examples: Potash Alum - KAl(SO₄)₂·12H₂O
Mohr’s Salt (Ammonium Iron (II) Sulphate) - NH₄)₂Fe(SO₄)₂·6H₂O
We shall investigate double salts further.
Double Salts
A double salt is a salt that contains two or more different cations
or anions. It is made up of 2 simple salts with a crystalline
structure that does not resemble either of them.
Double salts are found exclusively in the solid state and when
dissolved in water they behave as a mixture of 2 different salts, in
other words, it completely dissociates into its constituent ions.
Properties of double salts: 




Exist only in solid state.
They fully ionize in the aqueous state.
Dissolve in water
No isomerism qualities
One or more crystal water molecules are present in each
double salt molecule.
Examples: Potash Alum (Potassium Aluminium Sulphate) - KAl(SO₄)₂·12H₂O
Mohr’s Salt (Ammonium Iron(II) Sulphate)- (NH₄)₂Fe(SO₄)₂·6H₂O
Iron Alum (Ammonium Iron(III) Sulphate)– (NH4)Fe(SO4)2.6H2O
Ferric Ammonium Oxalate – (NH4)3Fe(C2O4)3
Phosgenite (Lead(II) carbonate chloride) – Pb2(CO3)Cl2
Ammonium Cobalt (II) Phosphate – NH4CoPO4
We shall discuss about potash alum and Mohr’s salt in further
detail.
Potash Alum
Potassium alum, potash alum, or potassium aluminium
sulphate is a chemical compound: the double salt of potassium
sulphate and aluminium, with chemical formula KAl(SO4)2.
It is commonly encountered as the dodecahydrate
KAl(SO4)2·12H2O.
The compound is the most important member of the generic class
of compounds called alums and is often simply called alum.
History
Potassium alum was also known to the Ancient Egyptians, who
obtained it from evaporites in the Western desert and reportedly
used it as early as 1500 BCE to reduce the visible cloudiness
(turbidity) in the water.
Potassium alum is also mentioned in some Ayurvedic texts and is
used in traditional Chinese medicine.
Potassium alum was imported into England mainly from
the Middle East, and, from the late 15th century onwards,
the Papal States for hundreds of years where it was used as a dyefixer for wool.
The textile dyeing industry in Bruges, and many locations in Italy,
and later in England, required alum to stabilize the dyes onto the
fabric and also to brighten the colors.
Identification of Formula
In early 1700s George Stahl made the first attempt at identifying
the formula of potash alum. He claimed that reacting Sulphuric
acid with limestone produced an alum.
He was corrected by Johann Pott and Andreas Marggraf, who
demonstrated that the precipitate obtained when an alkali is
poured into a solution of alum, namely alumina is different from
lime and chalk. They also said that it is one of the ingredients in
common clay.
Marggraf also showed that crystals with properties of alum can be
obtained by dissolving alumina in sulphuric acid and adding
potash or ammonia.
In 1767, Torbern Bergman observed the need for potassium or
ammonium sulphates to convert aluminium sulphate into alum,
while sodium or calcium wouldn’t work.
In 1797, contrary to popular belief that potash was found
exclusively on plants, Martin Klaproth discovered the presence of
potassium in the mineral’s leucite and lepidolite.
Louis Vauquelin speculated that potassium was an ingredient in
many other minerals. He suspected that the alkali used in
Marggraf’s experiments was an essential ingredient of natural
alum.
In 1797, Louis published a thesis demonstrating that potash alum
is a double salt which is composed of sulphuric acid, alumina and
potash.
Characteristics
Potash alum crystallises in a regular octahedra with flattened
corners.
It is very soluble in water.
Solution is slightly acidic and bitter to taste.
Neutralizing a potash alum solution with Potassium hydroxide
(KOH) will cause the separation of alumina [Al (OH)3].
When heated to red hot it will give a porous friable mass known
as burnt alum. It fuses at 92°C (198°F) in its own water of
crystallisation.
Occurrence
Potash alum occurs in nature as a sulphate material called alum(K), on rocks in areas weathering.
Potash alum historically has been obtained from alunite
KAl(SO4)2.11H2O.
Methods of Production
Historically, the main method of preparation was from alunite.
Alunite is first calcined (heating a material to high temperature
without fusing) after which it is exposed to air for a long time.
It is continually moistened with water, so that it falls to a very fine
powder. The powder is then lixiviated with hot water.
The liquor is decanted, and the alum is allowed to crystallise.
Potash alum is now produced industrially by adding potassium
sulphate to a concentrated solution of aluminium sulphate.
The aluminium sulphate is usually obtained by treating minerals
like alum schist, bauxite and cryolite with sulfuric acid. If much
iron should be present in the sulphate, then it is preferable to
use potassium chloride in place of potassium sulphate.
Uses
1. Medicine
Potash alum is used as an astringent and anti-septic.
Also used for cauterization purposes as it has a significant drying
effect.
They are active ingredients in some antiperspirants and
deodorants.
Potassium alum was the major immunologic adjuvant used to
increase the efficacy of vaccines and has been used since the
1920s. But it has been almost completely replaced by aluminium
hydroxide and aluminium phosphate in commercial vaccines.
In the 1950s, men sporting crewcut or flattop hairstyles
sometimes applied alum to their hair, as an alternative to pomade,
to keep the hair standing up.
2. Culinary
The chief culinary use is in pickling recipes, to preserve and add
crispness to fruit and vegetables.
3. Flame retardant
Potassium alum is used as a fire retardant to render cloth, wood,
and paper materials less flammable.
4.Tanning
Potassium alum is used in leather tanning, to remove moisture
from the hide and prevent rotting. Unlike tannic acid, alum
doesn't bind to the hide and can be washed out of it.
5.Dyeing
Alum has been used since antiquity as mordant to form a
permanent bond between dye and natural textile fibers like wool.
It is also used for this purpose in paper marbling.
6.Chemical flocculant
Potassium alum has been used since antiquity for purification of
turbid liquids. It is still widely used in the purification of water for
drinking and industrial processes water, treatment
of effluents and post-storm treatment of lakes to precipitate
contaminants.
Mohr’s Salt
Ammonium Iron (III) Sulphate also known as Mohr’s Salt is an
inorganic compound with the formula (NH₄)₂Fe(SO₄)₂·6H₂O.
It contains 2 different cations Fe2+ and NH4+.
It is classified as a double salt of ferrous sulphate and ammonium
sulphate. It is a common laboratory reagent because it is readily
crystallised, and the crystals resist oxidation by air.
The mineral form is Mohrite.
It dissolves in water to give aquo complex [Fe(H2O)6]2+.
Structure
Mohr’s salt belongs a family of double sulphates collectively called
Tutton’s salts (Schonites). All members of this family form salts
with a monoclinic geometry.
The bonding patterns include octahedral centres made of
[Fe(H2O)6]2+ centres.
They also form hydrogen bonds with ammonium and sulphate
ions.
Mohr’s Salt is named after the German chemist Karl Friedrich
Mohr who made many significant advances in titration in the 19th
century.
Properties
The molar mass of Mohr’s salt (in its anhydrous state) is equal to
284.05 grams per mole. The hexahydrate of Mohr’s salt is known
to have a molar mass of 392.13 grams per mole.
Under standard conditions for temperature and pressure (often
abbreviated to STP), the appearance of Mohr’s salt is bluish-green
in colour. It exists as a crystalline solid under these conditions.
The density of Mohr’s salt under standard conditions is equal to
1.86 grams per cubic centimetre.
The solubility of the hexahydrate of Mohr’s salt in water is
roughly equal to 269 grams per litre at STP.
Uses
In the field of analytical chemistry, Mohr’s salt is one of the most
preferred compounds that act as a source of Fe2+ ions. The
advantage of using this solid as a source of ferrous ions is that it
has a relatively long shelf life and is also resistant to oxidation due
to atmospheric exposure. The oxidation of this compound is
known to occur relatively quicker when the pH of the
environment is high (basic medium).
Another important application of Mohr’s salt is in Fricke’s
dosemeter, where it helps measure high doses of gamma
radiation.
Experiment: Double Salt Crystallisation
AIM:
To prepare double salts: potash alum and ammonium iron (II)
sulphate.
THEORY:
When a mixture containing equimolar proportions of potassium
sulphate and aluminium sulphate or ferrous sulphate and
ammonium sulphate is crystallised from its solution, a double salt
is formed.
The formation of double salt may be shown as follows:
K2SO4 + Al2 (SO4 )3 + 24H2O → K2 SO4 . Al2(SO4 )3 . 24H2O
FeSO4 + (NH4 )2 SO4 + 6H2O → FeSO4 . (NH4 )2 SO4 . 6H2O
Fe2+ and Al3+ ions undergo hydrolysis, therefore, while preparing
aqueous solutions of ferrous sulphate and aluminium sulphate in
water, 2-3 mL dilute sulphuric acid is added to prevent the
hydrolysis of these salts.
APPARATUS REQUIRED:
- Beaker
- Conical flask
- Trough
- Glass rod
- Tripod stand
- Funnel
- Wire gauze
CHEMICALS REQUIRED:
- Potassium sulphate
- Aluminium sulphate
- Ferrous sulphate
- Ammonium sulphate
- Dilute sulphuric acid
PROCEDURE:
a)
PREPARATION OF POTASH ALUM
STEP 1: Take 10 mL of distilled water in a 50 mL beaker and heat
it to about 40°C. Dissolve 6.6 g of aluminium sulphate in it and
add about 0.4 mL of dilute sulphuric acid.
STEP 2: Weigh 2.4 g of powdered potassium sulphate and transfer
it to the above solution.
STEP 3: Heat the solution with constant stirring till potassium
sulphate dissolves completely.
STEP 4: Allow the solution to cool to room temperature slowly.
STEP 5: On cooling, white crystals of potash alum separate out.
STEP 6: Dry these between the folds of a filter paper and note the
yield.
b) PREPARATION OF MOHR’S SALT
STEP 1: Dissolve 3.5 g of ferrous sulphate and 1.7 g of ammonium
sulphate (weighed separately), in 5 mL of distilled water
contained in a 50 mL conical flask by heating. Add about 0.5 mL of
dilute sulphuric acid to the flask and concentrate the solution by
heating till the crystallization point is reached.
STEP 2: Allow the mixture to cool to room temperature slowly.
STEP 3: On cooling, light green crystals of ferrous ammonium
sulphate separate out.
STEP 4: Dry between the folds of a filter paper and record the
yield.
PRECAUTIONS:
(a)
Cool the solution slowly to get good crystals. Avoid rapid
cooling.
(b) Do not disturb the solution while cooling.
(c)
Avoid prolonged heating while preparing crystals of
ferrous ammonium sulphate, as it may oxidise ferrous
ions to ferric ions and change the stoichiometry of the
crystals.
NOTES
- The formation of double salt typically involves the exchange
of ions between the reactant salts. If their mixture is not
equimolar, there will be an excess of one of the ions. This
excess ion will not have a corresponding ion to react with in
the other salt, which prevents the formation of double salt.
- In an equimolar mixture, the ratio of ions from both salts is
balanced, allowing the exchange and combination of ions to
form a double salt.
- In the preparation of double salt, dilute sulphuric acid is
used to prevent hydrolysis of the salt. Dilute sulphuric acid
is used as it provides the necessary acidity without
providing excessive heat. The usage of concentrate sulphuric
acid would lead to the generation of excessive heat upon
reaction with water, leading to violent, uncontrollable
reactions.
Bibliography
- https://en.wikipedia.org/wiki/Double_salt
- https://www.allthescience.org/what-is-a-doublesalt.htm#:~:text=Used%20in%20commercial%20food%20prep
aration,buffering%20in%20the%20cosmetics%20industry.
- https://en.wikipedia.org/wiki/Ammonium_iron(II)_sulfate
- https://en.wikipedia.org/wiki/Potassium_alum
- https://byjus.com/chemistry/mohrssalt/#:~:text=What%20are%20the%20applications%20of,oxida
tion%20due%20to%20atmospheric%20exposure
- https://byjus.com/chemistry/preparation-of-ferrousammonium-sulphate/
- https://unacademy.com/content/nda/studymaterial/chemistry/salts-different-types-ofsalts/#:~:text=There%20are%20different%20types%20of,neutr
al%20salt%2C%20and%20double%20salt.&text=A%20salt%20
that%20is%20formed,called%20an%20%27Acidic%20Salt%27
- https://dewwool.com/uses-of-salt/
- https://www.medicalnewstoday.com/articles/146677
- https://en.wikipedia.org/wiki/Salt