Thorin (chemistry)

Thorin is an analytical reagent used as a metallochromic indicator in complexometric titrations, particularly for the determination of thorium and other tetravalent metals. It is an azo dye, typically used in the form of its disodium salt.

Thorin's chemical structure features an azo group (-N=N-) connecting two aromatic rings, one of which contains sulfonic acid groups (-SO3H) and the other an arsenic acid group (-AsO3H2). The presence of these acidic groups makes Thorin water-soluble and imparts its ability to form colored complexes with certain metal ions.

The indicator works by undergoing a color change when it binds to the metal ion being titrated. Before the endpoint, the solution will contain free metal ions. As the titrant is added, it reacts with these metal ions. At the endpoint, when essentially all the metal ions have reacted with the titrant, the added titrant will start to react with the Thorin, causing a distinct color change that signals the end of the titration. The color change is due to the formation of a metal-Thorin complex.

Thorin is also used in spectrophotometric methods for determining trace amounts of various metals. The intensity of the color developed after the metal ion reacts with Thorin is measured using a spectrophotometer, and this absorbance is proportional to the concentration of the metal ion.