Occasionally called phosphites, a phosphonate is a salt or an ester of one of a family of phosphonic acids. The simplest — unsubstituted — phosphonic acid is H-(PO)-(OH)2, a crystalline white solid obtained by reacting phosphorous trioxide with water. Two of the hydrogen atoms are attached to oxygen atoms and are more acidic than the lone hydrogen attached directly to the phosphorous atom. Sodium can form either a mono- or a di-basic salt of phosphonic acid, the disodium salt having the chemical structure, H-PO-(ONa)2∙5H2O — the water molecules being called "water of crystallization." Dimethyl phosphonate is a liquid ester that boils at 340°F (171°C); its chemical structure is written H-PO-(OCH3)2.
Derivatives of the simplest phosphonic acid, such as the above structures, leave the hydrogen atom attached directly to the phosphorous untouched. It can be replaced, however. One simple example of this is methylphosphonic acid, CH3PO-(OH)2, in which the hydrogen is replaced by a methyl group, CH3. The disodium salt of methylphosphonic acid is written CH3PO-(ONa)2, while the structure of the diethyl ester — diethyl methylphosphonate — is written CH3PO-(OC2H5)2. Aryl derivatives can also be made, in which the hydrogen is replaced by an aromatic organic structure, such as phenyl (-C6H5), naphthyl (-C10H7) or p-tolyl (-C6H4CH3).
Such a broad range of chemical structures enables considerable reaction possibilities; textbooks have been written on just the subject of phosphonate chemistry. There is also a broad range of applications for phosphonates, such as in materials design and industrial cleaners, as well as in the formation of biologically active compounds. They are used to inhibit scale formation and to remove rust. Organic phosphonates are important in the development of disease antibiotics and fungicides for uses ranging from golf course turf application to avocado root protection. Phosphonates are even being used as intermediates in the production of synthetic DNA.
Although many useful compounds are cited publicly for health risks they may pose, there is little outcry over phosphonates. The HERA Initiative, a European volunteer agency, indicates three of the main phosphonates used in household applications — ATMP, HEDP and DTPMP — are safe and should cause no concern regarding consumer use. Some of the phosphonates in acid form do pose risks, even as other strong acids do. Mono-salts or esters of phosphonic acids do not have both acidic hydrogen atoms replaced, and so still maintain a measure of their acidic reactivity. Of course, as with other chemicals, all material safety data sheets should be carefully read and the safety suggestions applied.