Cerimetry
Encyclopedia
Cerimetry or cerimetric titration, also known as cerate oximetry, is a method of volumetric chemical analysis, a redox titration
in which a Fe
2+-1,10-phenanthroline
complex (ferroin
) color change indicates the end point. Ferroin can be reversibly discolored in its oxidized form upon titration
with a Ce
4+ solution. The use of cerium(IV) salts as reagents for volumetric analysis was first proposed in the middle of 19th century, but systematic studies did not start until about 70 years later. Standard solutions can be prepared from different Ce4+ salts, but often cerium sulfate
is chosen.
Since cerimetry is linked to the Fe3+/Fe2+ redox
pair, it can be used for analyses of nonstoichiometric levels that either oxidize Fe2+ or reduce Fe3+. For the case of oxidation, a precise excess of high-purity crystalline Mohr’s salt is added upon the oxide digestion in aqueous hydrogen chloride
(H
Cl
), while for the case of reduction
, an excess of 1 M iron trichloride
(FeCl3) is added. In the both cases, this is Fe2+ ions which will be titrated subsequently. Because the Ce4+ solution is prone to hydrolysis
, the titration is done in a strongly HCl-acidic solution into which some phosphoric acid
(H3P
O
4) is added to obtain a less colored phosphato complex of Fe3+.
According to tabulated values of standard potentials at pH = 0 for the first-row transition metals, any nonstoichiometry below the following oxidation states will reduce 1 M FeCl3 solution whereas any nonstoichiometry above them will oxidize the Mohr’s salt: Ti
4+, V
4+, Cr
3+, Mn
2+, Co
2+, and Ni
2+. In addition, any nonstoichiometry in the Fe(III)–Fe(II) range is titrated directly with no additives, any nonstoichiometry below Fe2+ will reduce 1 M FeCl3 whereas any nonstoichiometry above Fe3+ will oxidize Mohr’s salt. In the second- and third-row transition metals, only the early elements would be suitable for the titration, and the limiting oxidation states are Zr
4+, Nb
5+, Mo
4+, Hf
4+, Ta
5+, and W
6+. Standard potentials involving rhenium
ions are too close to E° for Fe3+/Fe2+ as well as to each other. Nonstoichiometry of oxides containing several elements in oxidation states suitable for cerimetry is determined in one titration
.
Redox titration
Redox titration is a type of titration based on a redox reaction between the analyte and titrant.Redox titration may involve the use of a redox indicator and/or a potentiometer.-Example:...
in which a Fe
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
2+-1,10-phenanthroline
Phenanthroline
Phenanthroline is a heterocyclic organic compound. As a bidentate ligand in coordination chemistry, it forms strong complexes with most metal ions...
complex (ferroin
Ferroin
Ferroin is the chemical compound with the formula [Fe3]SO4, where o-phen is an abbreviation for 1,10-phenanthroline, a bidentate ligand. The term "ferroin" is used loosely and includes salts of other anions such as chloride.-Redox indicator:...
) color change indicates the end point. Ferroin can be reversibly discolored in its oxidized form upon 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...
with a Ce
Cerium
Cerium is a chemical element with the symbol Ce and atomic number 58. It is a soft, silvery, ductile metal which easily oxidizes in air. Cerium was named after the dwarf planet . Cerium is the most abundant of the rare earth elements, making up about 0.0046% of the Earth's crust by weight...
4+ solution. The use of cerium(IV) salts as reagents for volumetric analysis was first proposed in the middle of 19th century, but systematic studies did not start until about 70 years later. Standard solutions can be prepared from different Ce4+ salts, but often cerium sulfate
Cerium(IV) sulfate
Cerium sulfate, also called ceric sulfate, is a yellow to yellow/orange chemical compound. It exists as the anhydrous salt Ce2; a few hydrated forms are also known: Ce2 ⋅ xH2O, with x equal to 4, 8, or 12. Ceric sulfate is available commercially.-Properties:Ceric sulfate is moderately soluble in...
is chosen.
Since cerimetry is linked to the Fe3+/Fe2+ redox
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
pair, it can be used for analyses of nonstoichiometric levels that either oxidize Fe2+ or reduce Fe3+. For the case of oxidation, a precise excess of high-purity crystalline Mohr’s salt is added upon the oxide digestion in aqueous hydrogen chloride
Hydrogen chloride
The compound hydrogen chloride has the formula HCl. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric humidity. Hydrogen chloride gas and hydrochloric acid are important in technology and industry...
(H
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
Cl
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
), while for the case of reduction
Reduction
Reduction, reduced, or reduce may refer to:- Chemistry :* Reduction, part of a reduction-oxidation reaction where oxygen is being removed from a compound.** Reduced gas, a gas with a low oxidation number...
, an excess of 1 M iron trichloride
Iron(III) chloride
Iron chloride, also called ferric chloride, is an industrial scale commodity chemical compound, with the formula FeCl3. The colour of iron chloride crystals depends on the viewing angle: by reflected light the crystals appear dark green, but by transmitted light they appear purple-red...
(FeCl3) is added. In the both cases, this is Fe2+ ions which will be titrated subsequently. Because the Ce4+ solution is prone to hydrolysis
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
, the titration is done in a strongly HCl-acidic solution into which some phosphoric acid
Phosphoric acid
Phosphoric acid, also known as orthophosphoric acid or phosphoric acid, is a mineral acid having the chemical formula H3PO4. Orthophosphoric acid molecules can combine with themselves to form a variety of compounds which are also referred to as phosphoric acids, but in a more general way...
(H3P
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
O
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
4) is added to obtain a less colored phosphato complex of Fe3+.
According to tabulated values of standard potentials at pH = 0 for the first-row transition metals, any nonstoichiometry below the following oxidation states will reduce 1 M FeCl3 solution whereas any nonstoichiometry above them will oxidize the Mohr’s salt: Ti
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
4+, V
Vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...
4+, Cr
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
3+, Mn
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
2+, Co
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....
2+, and Ni
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
2+. In addition, any nonstoichiometry in the Fe(III)–Fe(II) range is titrated directly with no additives, any nonstoichiometry below Fe2+ will reduce 1 M FeCl3 whereas any nonstoichiometry above Fe3+ will oxidize Mohr’s salt. In the second- and third-row transition metals, only the early elements would be suitable for the titration, and the limiting oxidation states are Zr
Zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...
4+, Nb
Niobium
Niobium or columbium , is a chemical element with the symbol Nb and atomic number 41. It's a soft, grey, ductile transition metal, which is often found in the pyrochlore mineral, the main commercial source for niobium, and columbite...
5+, Mo
Molybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
4+, Hf
Hafnium
Hafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in zirconium minerals. Its existence was predicted by Dmitri Mendeleev in 1869. Hafnium was the penultimate stable...
4+, Ta
Tantalum
Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as tantalium, the name comes from Tantalus, a character in Greek mythology. Tantalum is a rare, hard, blue-gray, lustrous transition metal that is highly corrosion resistant. It is part of the refractory...
5+, and W
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
6+. Standard potentials involving rhenium
Rhenium
Rhenium is a chemical element with the symbol Re and atomic number 75. It is a silvery-white, heavy, third-row transition metal in group 7 of the periodic table. With an average concentration of 1 part per billion , rhenium is one of the rarest elements in the Earth's crust. The free element has...
ions are too close to E° for Fe3+/Fe2+ as well as to each other. Nonstoichiometry of oxides containing several elements in oxidation states suitable for cerimetry is determined in one 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...
.
See also
- IodometryIodometryIodometry, also known as iodometric titration, is a method of volumetric chemical analysis, a redox titration where the appearance or disappearance of elementary iodine indicates the end point....
- Redox titrationRedox titrationRedox titration is a type of titration based on a redox reaction between the analyte and titrant.Redox titration may involve the use of a redox indicator and/or a potentiometer.-Example:...
- Nonstoichiometry
- Redox indicatorRedox indicatorA redox indicator is an indicator that undergoes a definite color change at a specific electrode potential....
- Oxidizing agentOxidizing agentAn oxidizing agent can be defined as a substance that removes electrons from another reactant in a redox chemical reaction...
- Reducing agentReducing agentA reducing agent is the element or compound in a reduction-oxidation reaction that donates an electron to another species; however, since the reducer loses an electron we say it is "oxidized"...