Chloroform reacts photochemically with oxygen to form chlorine, phosgene and hydrogen chloride. To slow this process and reduce the acidity of the solvent, chloroform-d is stored in brown-tinted bottles, often over copper chips or silver foil as stabilizer. Instead of metals, a small amount of a neutralizing base like potassium carbonate may be added. It is less toxic to the liver and kidneys than  due to the presence of a  bond which is stronger than a  bond. The  bond is stronger than a  bond, making it somewhat less prone to form the destructive trichloromethyl radical ().