Mohr's salt
Encyclopedia
Ammonium iron sulfate, or Mohr's Salt, is the inorganic compound
with the formula (NH4)2Fe(SO4)2·6H2O. Containing two different cations, Fe2+ and NH4+, it is classified as a double salt
of ferrous sulfate
and ammonium sulfate
. It is a common laboratory reagent. Like the other ferrous sulfate salts, ferrous ammonium sulfate dissolves in water to give the aquo complex
[Fe(H2O)6]2+, which has octahedral molecular geometry
.
, this salt is preferred over other salts of ferrous sulfate for titration
purposes as it is much less prone to oxidation by air to iron(III). The oxidation of solutions of iron(II) is very pH dependent, occurring much more readily at high pH. The ammonium ions make solutions of Mohr's salt slightly acidic, which slows this oxidation process.
Mohr's salt is named after the German chemist Karl Friedrich Mohr
, who made many important advances in the methodology of titration in the 19th century. Several similar salts are known, which were once called Tutton's salt
s.
in water containing a little sulfuric acid
, and then subjecting the resulting solution
to crystallization
. Ferrous ammonium sulfate forms light green crystals.
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...
with the formula (NH4)2Fe(SO4)2·6H2O. Containing two different cations, Fe2+ and NH4+, it is classified as a double salt
Double salt
Double salts are salts containing more than one cation or anion. They form when more than one salt is dissolved in a liquid and when together they crystallize in a regular pattern. A well-known double salt is alum containing two cations and a sulfate anion...
of ferrous sulfate
Iron(II) sulfate
Iron sulfate or ferrous sulfate is the chemical compound with the formula FeSO4. Known since ancient times as copperas and as green vitriol, the blue-green heptahydrate is the most common form of this material...
and ammonium sulfate
Ammonium sulfate
Ammonium sulfate , 2SO4, is an inorganic salt with a number of commercial uses. The most common use is as a soil fertilizer. It contains 21% nitrogen as ammonium cations, and 24% sulfur as sulfate anions...
. It is a common laboratory reagent. Like the other ferrous sulfate salts, ferrous ammonium sulfate dissolves in water to give the aquo complex
Metal aquo complex
Metal aquo complexes are coordination compounds containing metal ions with only water as a ligand. These complexes are the predominant species in aqueous solutions of many metal salts, such as metal nitrates, sulfates, and perchlorates. They have the general stoichiometry [Mn]z+. Their behavior...
[Fe(H2O)6]2+, which has octahedral molecular geometry
Octahedral molecular geometry
In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...
.
Applications
In analytical chemistryAnalytical chemistry
Analytical chemistry is the study of the separation, identification, and quantification of the chemical components of natural and artificial materials. Qualitative analysis gives an indication of the identity of the chemical species in the sample and quantitative analysis determines the amount of...
, this salt is preferred over other salts of ferrous sulfate for titration
Titration
Titration, also known as titrimetry, is a common laboratory method of quantitative chemical analysis that is used to determine the unknown concentration of an identified analyte. Because volume measurements play a key role in titration, it is also known as volumetric analysis. A reagent, called the...
purposes as it is much less prone to oxidation by air to iron(III). The oxidation of solutions of iron(II) is very pH dependent, occurring much more readily at high pH. The ammonium ions make solutions of Mohr's salt slightly acidic, which slows this oxidation process.
Mohr's salt is named after the German chemist Karl Friedrich Mohr
Karl Friedrich Mohr
Karl Friedrich Mohr was a German chemist famous for his early statement of the principle of the conservation of energy. Ammonium iron sulfate, 2Fe2.6H2O, is named Mohr's salt after him.-Life:...
, who made many important advances in the methodology of titration in the 19th century. Several similar salts are known, which were once called Tutton's salt
Tutton's salt
Tutton's salts are a family of salts with the formula M2M'26 or M2M'26 . These materials are "double salts," which means that they contain two different cations, M+ and M'2+.-Examples and related compounds:...
s.
Preparation of Mohr's salt
Mohr's salt is prepared by dissolving an equimolar mixture of hydrated ferrous sulfate and ammonium sulfateAmmonium sulfate
Ammonium sulfate , 2SO4, is an inorganic salt with a number of commercial uses. The most common use is as a soil fertilizer. It contains 21% nitrogen as ammonium cations, and 24% sulfur as sulfate anions...
in water containing a little sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
, and then subjecting the resulting solution
Solution
In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
to crystallization
Crystallization
Crystallization is the process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas. Crystallization is also a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid...
. Ferrous ammonium sulfate forms light green crystals.