Reacts slowly with oxygen or oxidising agents, when exposed to air and light, giving, mainly, phosgene, Cl₂ and HCl. Commercial (CAS NO. 67-46-3) is usually stabilized by addn of up to 1% EtOH or of dimethylaminoazobenzne. Simplest purifications involve washing with water to remove the EtOH, drying with K₂CO₃ or CaCl₂, refluxing with P₂O₅, CaCl₂, CaSO₄ or Na₂SO₄, and distilling. It must not be dried with sodium. The distd CHCl₃ should be stored in the dark to avoid photochemical formation of phosgene. As an alternative purification, CHCl₃ can be shaken with several small portions of conc H₂SO₄, washed thoroughly with water, and dried with CaCl₂ or K₂CO₃ before filtering and distilling. EtOH can be removed from CHCl₃ by passage through a column of activated alumina, or through a column of silica gel 4-ft long by 1.75-in diameter at a flow rate of 3mL/min. (The column, which can hold about 8% of its weight of EtOH, is regenerated by air drying and then heating at 600 °C for 6h. It is pre-purified by washing with CHCl₃, then EtOH, leaving in conc H₂SO₄ for about 8hr, washing with water until the washings are neutral, then air drying, followed by activation at 600 °C for 6h. Just before use it is reheated for 2h to 154 °C.)

yl-containing impurities can be removed from CHCl₃ by percolation through a Celite column impregnated with 2,4-dinitrophenylhydrazine, phosphoric acid and water. (Prepared by dissolving 0.5g DNPH in 6mL of 85% H₃PO₄ by grinding together, then mixing with 4mL of distilled water and 10g of Celite). Chloroform can be dried by distn from powdered type 4A Linde molecular sieves. For use as a solvent in IR spectroscopy, Chloroform is washed with water (to remove EtOH), then dried for several hours over anhydrous CaCl₂ and fractionally distd. This treatment removes material absorbing near 1600 cm^-1. (Percolation through activated alumina increases this absorbing impurity).

Rapid purification: Pass through a column of basic alumina (Grade I, 10g/mL of CHCl₃), and either dry by standing over 4A molecular sieves, or alternatively, distil from P₂O₅ (3% w/v). Use immediately.