Carbonyl sulfide

Carbonyl sulfide is the chemical compound with the formula OCS. Commonly written as COS, it is a colourless gas with an unpleasant odor. It is a linear molecule consisting with a carbonyl group double bonded to a sulfur atom. Carbonyl sulfide can be considered to be a compound of carbon dioxide and carbon disulfide.

This compound is found to catalyze the formation of peptides from amino acids. This finding is an extension of the Miller-Urey experiment and it is suggested that carbonyl sulfide played a significant role in the origin of life.

Occurrence and applications
Carbonyl sulfide is the major sulfur compound naturally present in the atmosphere at 0.5 (± 0.05) ppb because it is emitted from volcanos and deep sea vents. As such it participates in the global sulfur cycle. It is also present in foodstuffs such as cheese and prepared vegetables of the cabbage family. Traces of COS is naturally present in grains and seeds in the range of 0.05-0.1 mg kg−1. It is a significant sulfur-containing impurity in synthesis gas.

Measurements on the Antarctica ice cores provide a detailed picture of OCS concentrations from 1640 to the present day separating anthropogenic and non-anthropogenic sulfur sources. Carbonyl sulfide is transported into the stratospheric sulfate layer where it is oxidized to sulfuric acid.

Carbonyl sulfide is a potential fumigant and a replacement for methyl bromide and phosphine. Carbonyl sulfide has been observed in interstellar medium.

Synthesis
It was first described in 1841, but was apparently mischaracterized as a mixture of carbon dioxide and hydrogensulfide. Carl von Than first characterized the subtance first in 1867. It forms when carbon monoxide reacts with elemental sulfur. This reaction reverses above 1200 K. A laboratory synthesis entails the reaction potassium thiocyanate and sulfuric acid. The resulting gas contains significant amounts of byproducts and requires purification.
 * KNCS +  2 H2SO4 + H2O  →   KHSO4  +  NH4HSO4  +  COS