Phosphite anion
Properties
The phosphite ion is a polyatomic ion with phosphorus as its central atom. Its formula is PO33−. Its geometry is trigonal pryamidal; the three oxygen atoms are pushed out of plane by the lone pair on phosphorus. Additionally, phosphite is water soluble.
The acid of the phosphite ion is phosphorous acid (H3PO3). Other names for this acid are orthophosphorous acid and dihydroxyphosphine oxide. H3PO3 is also sometimes referred to as phosphorus trihydroxide and trihydroxyphosphine, though these names are misleading. Phosphorous acid is a diprotic acid, since the hydrogen bonded directly to the central phosphorus atom is not ionizable. That being said, a more logical chemical formula for phosphorous acid is HPO(OH)2, since three hydroxy groups are not actually present on the acid. This acid can be synthesized by reacting a carboxylic acid or an alcohol with phosphorus tribromide, or by simply reacting phosphorus tribromide with water in carbon tetrachloride.
PBr3 + 3 ROH → 3 RBr + HP(O)(OH)2
PBr3 + 3 RCOOH → 3 RCOBr + HP(O)(OH)2
PBr3 + 3 H2O → HP(O)(OH)2 + 3HCl
Synthesis of Phosphite Compounds
Organophosphorus compounds called phosphite esters (or sometimes just phosphites) have the formula (RO)3P. They are prepared from phosphorus trichloride (or phosphorus tribromide) and alcohols.
Uses
Historically, phosphate (PO43−) has been the active ingredient in fertilizers. Today, however, phosphite is beginning to be used. Ammonium phosphate is a very stable compound, and it is not very water soluble. Ammonium phosphite, on the other hand, has a relatively low stability allowing the compound to readily metabolize. Additionally, the compounds high solubility in water allows the fertilizer to be easily absorbed by plants. Thus, smaller quantities of phosphite fertilizers can be used to obtain the same effect as a large quantity of a phosphate fertilizer.
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