Chloroform reacts with oxygen in the air to form phosgene (COCl2), a poisonous gas, according to the equation:
2CHCl3 + O2 ---> 2COCl2 + 2HCl
In order to prevent this oxidation, ethanol or methanol should be added if you were planning to store it for an extended period of time. You should ensure that the chloroform contains 1-2% ethanol. Ethyl carbonate is produced and is harmless.
COCl2 + 2CH3CH2OH ---> (C2H5O)2CO + 2HCl
Phosgene is the chemical compound with the formula COCl2. This colorless gas gained infamy as a chemical weapon during World War I.
It is also a valued industrial reagent and building block in synthesis of pharmaceuticals and other organic compounds. In low concentrations, its odor resembles freshly cut hay or grass. In addition to its industrial production, small amounts occur naturally from the breakdown and the combustion of organochlorine compounds, such as those used in refrigeration systems.

In this video Mr. Lalit Sardana (IIT-JEE AIR 243) is explaining students about phosgene gas, its characteristics, negative catalyst. Mr. Lalit Sardana knows the supreme art to awaken joy in creative expression and knowledge. So experience the same through his online teaching.
The oxidation of chloroform is a very important topic which is asked in 11th CBSE, 12th CBSE, JEE Mains, JEEAdvanced, VITEEE, MPPET, AIPMT, WBJEE, KCET engineering, AFMC, etc.
