Phosphorus(III) iodide, also known as phosphorus triiodide, is a chemical compound. It is made of phosphorus and iodide ions. It contains phosphorus in its +3 oxidation state. Its chemical formula is PI3.
It is a red solid. It is very unstable and a powerful reducing agent. It reacts violently with water. The reaction with water makes phosphorous acid and hydrogen iodide. It is used in making chemicals. Some think that it is too unstable to be stored, but it can be stored and can be bought.
Phosphorus(III) iodide is made by reacting iodine with white phosphorus dissolved in carbon disulfide. It is dissolved in carbon disulfide because it does not dissolve in water. It can also be made by reacting phosphorus(III) chloride with hydrogen iodide or some other iodide.
It reacts with alcohols such as methanol to make "alkyl iodides" such as methyl iodide. Many times phosphorus(III) iodide is made in the reaction. The alkyl iodides are used in many chemical reactions.