Cerimetry or cerimetric titration, also known as cerate oximetry, is a method of volumetric

chemical analysis Analytical chemistry studies and uses instruments and methods to separate, identify, and quantify matter. In practice, separation, identification or quantification may constitute the entire analysis or be combined with another method. Separati ...

developed by

Ion Atanasiu Professor Ion A. Atanasiu (25 September 1894 – 19 December 1978) was the founder of the Romanians, Romanian School of Electrochemistry and the first to teach this subject in Romania. He is known as the originator of cerimetry, an analytical ...

. It is a

redox titration A redox titration is a type of titration based on a redox reaction between the analyte and titrant. It may involve the use of a redox indicator and/or a potentiometer. A common example of a redox titration is treating a solution of iodine with a ...

in which an

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...

(II)–

1,10-phenanthroline 1,10-Phenanthroline (phen) is a heterocyclic organic compound. It is a white solid that is soluble in organic solvents. The 1,10 refer to the location of the nitrogen atoms that replace CH's in the hydrocarbon called phenanthrene. Abbreviated ...

complex (

ferroin Ferroin is the chemical compound with the formula e(''o''-phen)3O4, 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 ...

) color change indicates the end point. Ferroin can be reversibly discolored in its oxidized form upon

titration Titration (also known as titrimetry and volumetric analysis) is a common laboratory method of quantitative chemical analysis to determine the concentration of an identified analyte (a substance to be analyzed). A reagent, termed the ''titrant'' ...

with a Ce⁴⁺ 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, also called ceric sulfate, is an inorganic compound. It exists as the anhydrous salt Ce( SO4)2 as well as a few hydrated forms: Ce(SO4)2(H2O)x, with x equal to 4, 8, or 12. These salts are yellow to yellow/orange solids th ...

is chosen. Since cerimetry is linked to the Fe³⁺/Fe²⁺

redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate (chemistry), substrate change. Oxidation is the loss of Electron, electrons or an increase in the oxidation state, while reduction ...

pair, it can be used for analyses of

nonstoichiometric In chemistry, non-stoichiometric compounds are chemical compounds, almost always solid inorganic compounds, having elemental composition whose proportions cannot be represented by a ratio of small natural numbers (i.e. an empirical formula); mos ...

levels that either oxidize Fe²⁺ or reduce Fe³⁺. For the case of

oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

, a precise excess of high-purity crystalline

Mohr's salt Ammonium iron(II) sulfate, or Mohr's salt, is the inorganic compound with the formula (NH4)2Fe(SO4)2(H2O)6. Containing two different cations, Fe2+ and NH4+, it is classified as a double salt of Iron(II) sulfate, ferrous sulfate and ammonium sulfa ...

is added upon the oxide digestion in aqueous

hydrogen chloride The compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colourless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hydrogen chloride ga ...

( H Cl), while for the case of reduction, an excess of 1 M

iron trichloride Iron(III) chloride is the inorganic compound with the formula . Also called ferric chloride, it is a common compound of iron in the +3 oxidation state. The anhydrous compound is a crystalline solid with a melting point of 307.6 °C. The colo ...

(FeCl₃) is added. In both cases, Fe²⁺ ions will be titrated subsequently. Because the Ce⁴⁺ solution is prone to

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

, the titration is done in a strongly HCl-acidic solution into which some

phosphoric acid Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solution, w ...

(H₃ P O₄) is added to obtain a less colored phosphato complex of Fe³⁺.{{cite journal, last=Karen, first=Pavel, title=Nonstoichiometry in oxides and its control, journal=Journal of Solid State Chemistry, year=2006, volume=179, issue=10, pages=3167–3183, doi = 10.1016/j.jssc.2006.06.012, bibcode = 2006JSSCh.179.3167K According to tabulated values of standard potentials at pH 0 for the first-row

transition metals In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...

, any

nonstoichiometry In chemistry, non-stoichiometric compounds are chemical compounds, almost always solid inorganic compounds, having elemental composition whose proportions cannot be represented by a ratio of small natural numbers (i.e. an empirical formula); mos ...

below the following

oxidation states In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. Co ...

will reduce 1 M FeCl₃ solution whereas any nonstoichiometry above them will oxidize the Mohr's salt: Ti⁴⁺, V⁴⁺, Cr³⁺, Mn²⁺, Co²⁺, and Ni²⁺. 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 Zr⁴⁺, Nb⁵⁺, Mo⁴⁺, Hf⁴⁺, Ta⁵⁺, and W⁶⁺. Standard potentials involving

rhenium Rhenium is a chemical element with the symbol Re and atomic number 75. It is a silvery-gray, heavy, third-row transition metal in group 7 of the periodic table. With an estimated average concentration of 1 part per billion (ppb), rhenium is one ...

ions are too close to E⁰ for Fe³⁺/Fe²⁺ as well as to each other.

Nonstoichiometry In chemistry, non-stoichiometric compounds are chemical compounds, almost always solid inorganic compounds, having elemental composition whose proportions cannot be represented by a ratio of small natural numbers (i.e. an empirical formula); mos ...

of oxides containing several elements in oxidation states suitable for cerimetry is determined in one

References

Romanian inventions Analytical chemistry Cerium Titration

See also

References