Phosphine

Phosphine ($PH_3$) is the simplest hydride of phosphorus, and both its preparation and its chemistry closely parallel — while also contrasting with — ammonia.

Preparation

The most direct route is the reaction of calcium phosphide with water or with dilute hydrochloric acid:

In the laboratory, it is more commonly generated by heating white phosphorus with concentrated NaOH solution under an inert atmosphere of $CO_2$, a reaction that simultaneously yields sodium hypophosphite:

Pure phosphine is not itself inflammable, but gas prepared this way often ignites spontaneously in air because it carries traces of $P_2H_4$ or $P_4$ vapour as impurities. To obtain phosphine free of these, the gas is first absorbed in hydroiodic acid to form solid phosphonium iodide ($PH_4I$); treating this salt with potassium hydroxide then releases pure phosphine:

Properties

Phosphine is a colourless, highly poisonous gas with a distinctive rotten-fish smell. It is dangerously reactive toward oxidising agents — it explodes on contact with even traces of $HNO_3$, $Cl_2$ or $Br_2$ vapour.

It is only slightly soluble in water, and its aqueous solution slowly decomposes on exposure to light, depositing red phosphorus and releasing hydrogen gas. When passed into copper sulphate or mercuric chloride solution, it is absorbed to form the corresponding metal phosphides:

Like ammonia, phosphine is a weak base, and it reacts with acids to form phosphonium salts, e.g.:

Uses …