Ferroin, also known as tris(''o''-phenanthroline)iron(II), is the

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

with the formula e(''o''-phen)₃O₄, where ''o''-phen is the abbreviation of ortho-phenanthroline for 1,10- phenanthroline, a bidentate

ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...

. The term "ferroin" is used loosely and includes salts of other

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

s such as chloride. Ferroin is one of many transition metal complexes of 1,10-phenanthroline.

Structure

Many salts of e(''o''-phen)₃sup>2+ have been characterized by

X-ray crystallography X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...

. The structures of e(''o''-phen)₃sup>2+ and e(''o''-phen)₃sup>3+ are almost identical, consistent with both being low-spin. These cations are octahedral with D₃

symmetry group In group theory, the symmetry group of a geometric object is the group of all transformations under which the object is invariant, endowed with the group operation of composition. Such a transformation is an invertible mapping of the amb ...

. The Fe-N distances are 197.3 pm.

Preparation and reactions

Ferroin sulfate can be prepared by combining phenanthroline to

ferrous sulfate Iron(II) sulfate or ferrous sulfate (British English: sulphate instead of sulfate) denotes a range of salts with the formula Fe SO4·''x''H2O. These compounds exist most commonly as the heptahydrate (''x'' = 7), but several values for ...

dissolved in water: : 3 phen + Fe²⁺ → e(phen)₃sup>2+ The

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

of this complex from Fe(II) to Fe(III), involving the fast and reversible transfer of only one electron, makes it a useful

redox indicator A redox indicator (also called an oxidation-reduction indicator) is an indicator which undergoes a definite color change at a specific electrode potential. The requirement for fast and reversible color change means that the oxidation-reduction Ch ...

aqueous solution An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, also known as sodium chloride (NaCl), in water ...

: : e(phen)₃sup>2+ → e(phen)₃sup>3+ + 1 e⁻ Addition of

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...

to an aqueous solution of e(phen)₃sup>2+ causes its

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 ...

and the formation of a neutral

ion pair In chemistry, ion association is a chemical reaction whereby ions of opposite electric charge come together in solution to form a distinct chemical entity. Ion associates are classified, according to the number of ions that associate with each ...

henHSO₄: : e(phen)₃sup>2+ + 3 H₂SO₄ + 6 H₂O → e(OH₂)₆sup>2+ + 3 henHsup>+HSO₄⁻ Addition of cyanide to an aqueous solution of precipitates .

Redox indicator

This

complex Complex commonly refers to: * Complexity, the behaviour of a system whose components interact in multiple ways so possible interactions are difficult to describe ** Complex system, a system composed of many components which may interact with each ...

is used as an indicator in

analytical chemistry Analytical skill, Analytical chemistry studies and uses instruments and methods to Separation process, separate, identify, and Quantification (science), quantify matter. In practice, separation, identification or quantification may constitute t ...

. The active ingredient is the e(''o''-phen)₃sup>2+ ion, which is a

chromophore A chromophore is the part of a molecule responsible for its color. The word is derived . The color that is seen by our eyes is that of the light not Absorption (electromagnetic radiation), absorbed by the reflecting object within a certain wavele ...

that can be oxidized to the ferric derivative e(''o''-phen)₃sup>3+. The potential for this redox change is +1.06 volts in 1 M H₂SO₄. It is a popular redox indicator for visualizing oscillatory

Belousov–Zhabotinsky reaction A Belousov–Zhabotinsky reaction, or BZ reaction, is one of a class of reactions that serve as a classical example of non-equilibrium thermodynamics, resulting in the establishment of a nonlinear chemical oscillator. The only common element in ...

s. Ferroin is suitable as a redox indicator, as the color change is reversible, very pronounced and rapid, and the ferroin solution is stable up to 60 °C. It is the main indicator used in

cerimetry Cerimetry or cerimetric titration, also known as cerate oximetry, is a method of volumetric chemical analysis developed by Ion Atanasiu. It is a redox titration in which an iron(II)– 1,10-phenanthroline complex (ferroin) color change indicates th ...

. Nitroferroin, the complex of iron(II) with 5-nitro-1,10-phenanthroline, has a transition potential of +1.25 volt. It is more stable than ferroin, but in

with Ce⁴⁺ ion, it requires a significant excess of titrant. It is, however, useful for titration in

perchloric acid Perchloric acid is a mineral acid with the formula H Cl O4. It is an oxoacid of chlorine. Usually found as an aqueous solution, this colorless compound is a stronger acid than sulfuric acid, nitric acid and hydrochloric acid. It is a powerful oxid ...

nitric acid Nitric acid is an inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but samples tend to acquire a yellow cast over time due to decomposition into nitrogen oxide, oxides of nitrogen. Most com ...

solution, where the cerium

redox potential Redox potential (also known as oxidation / reduction potential, ''ORP'', ''pe'', ''E_'', or E_) is a measure of the tendency of a chemical species to acquire electrons from or lose electrons to an electrode and thereby be reduced or oxidised respe ...

is higher. The redox potential of the iron-phenanthroline complex can be varied between +0.84 V and +1.10 V by adjusting the position and number of

methyl In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula (whereas normal methane has the formula ). In formulas, the group is often abbreviated as ...

groups on the phenanthroline core.

Fe²⁺ direct UV-visible spectrophotometric determination

, the

red Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength of approximately 625–750 nanometres. It is a primary color in the RGB color model and a seconda ...

color specific for the reduced form of ferroin was once used for the direct UV-visible

spectrophotometric Spectrophotometry is a branch of electromagnetic spectroscopy concerned with the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength. Spectrophotometry uses photometers, known as spe ...

determination of . The maximum absorbance of the Fe(II) ''o''-phenanthroline complex is at 511 nm. However, another related N-ligand called ferrozin (3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine-p,p′-disulfonic acid monosodium salt hydrate) is also used and must not be confused with ferroin. Ferrozin was specifically synthesised in the 1970s to obtain a less expensive reagent for automated

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. Separa ...

. Ferrozine reacts with to form a relatively stable

magenta Magenta () is a purple-red color. On color wheels of the RGB color model, RGB (additive) and subtractive color, CMY (subtractive) color models, it is located precisely midway between blue and red. It is one of the four colors of ink used in colo ...

-colored complex with a maximum

absorbance Absorbance is defined as "the logarithm of the ratio of incident to transmitted radiant power through a sample (excluding the effects on cell walls)". Alternatively, for samples which scatter light, absorbance may be defined as "the negative log ...

at 562 nm. The ferrozin method allows the determination of Fe(II)/Fe(III) speciation in natural

fresh Fresh may refer to: Arts, entertainment, and media Films and television * ''Fresh'' (1994 film), a crime film * ''Fresh'' (2009 film), a documentary film on sustainable agriculture * ''Fresh'' (2022 film), a thriller film *''Fresh with the Aust ...

or marine waters at the submicromolar level. In 2021, Smith ''et al.'' reexamined the formation kinetics and stability of ferroin and ferrozine Fe(II) complexes. They have found that while the kinetics of binding by ''o''-phenanthroline are very fast, the kinetics of complexation by ferrozine depend on ligand concentration. An excess ligand concentration provides a more stable absorbance, while the formation of Fe(II) complexes is pH-independent.{{cite journal , last1=Smith , first1=Gideon L. , last2=Reutovich , first2=Aliaksandra A. , last3=Srivastava , first3=Ayush K. , last4=Reichard , first4=Ruth E. , last5=Welsh , first5=Cass H. , last6=Melman , first6=Artem , last7=Bou-Abdallah , first7=Fadi , title=Complexation of ferrous ions by ferrozine, 2,2′-bipyridine and 1,10-phenanthroline: Implication for the quantification of iron in biological systems , journal=Journal of Inorganic Biochemistry , volume=220 , date=2021 , doi=10.1016/j.jinorgbio.2021.111460 , page=111460

Related complexes

* [Tris(bipyridine)ruthenium(II)

References

Iron complexes">Tris(bipyridine)ruthenium(II) chloride">[Tris(bipyridine)ruthenium(II)

References

Iron complexes Phenanthrolines

Structure

Preparation and reactions

Redox indicator

Fe2+ direct UV-visible spectrophotometric determination

Related complexes

References

References

Fe²⁺ direct UV-visible spectrophotometric determination