Eder reaction

What is Eder reaction?

The Eder reaction was first reported by Eder in 1880 and involves the photochemical reduction of mercuric chloride HgCl2 to calomel (Hg2Cl2) by oxalates C2O42-.

Eder reaction - general reaction scheme
Eder reaction

This reaction has been found to be an energetically efficient chain reaction with a chain length of nearly 1000000 and is commonly utilized in actinometry, including x-ray actinometry. The reaction can be induced through illumination, oxidizing reagents (such as KMnO4 and MnO2), and reducing reagents, but it can also be inhibited by a few substances such as oxygen, phenols, and other inorganic chlorides. Additionally, some photochemical sensitizers such as fluorescent dyes, potassium trioxalatocobaltiate, quinine, and FeCl4 can also function in this reaction.


Eder, J.M. (1880), Ein neues chemisches Photometer mittelst Quecksilberoxalat zur Bestimmung der Intensität der ultravioletten Strahlen des Tageslichtes und Beiträge zur Photochemie des Quecksilberchlorides. [A new chemical photometer using mercuric oxalate to determine the intensity of ultraviolet rays of daylight and contributions to the photochemistry of mercuric chloride.] Ber. Dtsch. Chem. Ges., 13: 166-168. https://doi.org/10.1002/cber.18800130150