Eriochrome Black T
Overview
Complexometric indicator
A complexometric indicator is an ionochromic dye that undergoes a definite color change in presence of specific metal ions. It forms a weak complex with the ions present in the solution, which has a significantly different color from the form existing outside the complex.Complexometric indicators...
that is part of the complexometric titration
Complexometric titration
Complexometric titration is a form of volumetric analysis in which the formation of a colored complex is used to indicate the end point of a titration. Complexometric titrations are particularly useful for the determination of a mixture of different metal ions in solution...
s, e.g. in the water hardness determination process. It is an azo dye.It is also known as ET-00.
In its protonated form, Eriochrome Black T is blue. It turns red when it forms a complex
Complex (chemistry)
In chemistry, a coordination complex or metal complex, is an atom or ion , bonded to a surrounding array of molecules or anions, that are in turn known as ligands or complexing agents...
with calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
, magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
, or other metal ions. Its chemical formula can be written as HOC10H6N=NC10H4(OH)(NO2)SO3Na.
When used as an indicator in an EDTA
EDTA
Ethylenediaminetetraacetic acid, widely abbreviated as EDTA , is a polyamino carboxylic acid and a colourless, water-soluble solid. Its conjugate base is named ethylenediaminetetraacetate. It is widely used to dissolve limescale. Its usefulness arises because of its role as a hexadentate ligand...
titration, the characteristic blue end-point is reached when sufficient EDTA is added and metal ions form complexes with the EDTA instead of the Eriochrome.
Eriochrome Black T has also been used to detect the presence of rare earth metals.
