The ion PH₄⁺, or the corresponding organic derivatives of the type R₃PH⁺, RPH₃⁺. The phosphonium ion PH₄⁺ is formally analogous to the ammonium ion NH₄⁺ but PH₃ has a much lower proton affinity than NH₃ and reaction of PH₃ with acids is necessary for the production of phosphonium salts.