Phosphonates are salts or esters of phosphonic acids, with a wide range of chemical structures and applications, including material design, industrial cleaning, and biologically active compounds. They are generally considered safe, but some in acid form may present risks.
Occasionally called a phosphite, a phosphonate is a salt or ester of one of a family of phosphonic acids. The simplest, unsubstituted phosphonic acid is H-(PO)-(OH)2, a white crystalline solid obtained by reacting phosphorus trioxide with water. Two of the hydrogen atoms are attached to the oxygen atoms and are more acidic than the lone hydrogen directly attached to the phosphorus atom. Sodium can form a mono- or di-basic salt of phosphonic acid, the disodium salt having the chemical structure, H-PO-(ONa)2H5O – the water molecules are called “water of crystallization”. Dimethylphosphonate is a liquid ester that boils at 2°F (340°C); its chemical structure is written H-PO-(OCH171)3.
The simplest phosphonic acid derivatives, such as the structures above, leave the hydrogen atom intact attached directly to the phosphorus. It can be replaced, however. A simple example of this is methylphosphonic acid, CH3PO-(OH)2, where the hydrogen is replaced by a methyl group, CH3. The disodium salt of methylphosphonic acid is written CH3PO-(ONa)2, while the diethyl ester structure — diethylmethylphosphonate — is written CH3PO-(OC2H5)2. Aryl derivatives can also be made, where the hydrogen is replaced by an aromatic organic structure, such as phenyl (-C6H5), naphthyl (-C10H7) or p-tolyl (-C6H4CH3).
Such a wide range of chemical structures allows for considerable reaction possibilities; textbooks have been written only on the topic of phosphonate chemistry. There is also a wide range of applications for phosphonates, such as in material design and industrial cleaning agents, 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 antibiotics and disease fungicides for uses ranging from applying golf course turf to protecting avocado roots. Phosphonates are even used as intermediates in the production of synthetic DNA.
Although many useful compounds are publicly cited for the health risks they may pose, there are few complaints about phosphonates. The HERA initiative, a European voluntary agency, indicates that three of the main phosphonates used in home applications – ATMP, HEDP and DTPMP – are safe and should not raise concerns regarding consumer use. Some of the phosphonates in acid form present risks, just like other strong acids do. Mono-salts or esters of phosphonic acids do not have both acidic hydrogen atoms substituted, and therefore still retain a measure of their acid reactivity. Of course, as with other chemicals, all material safety data sheets must be read carefully and the safety tips applied.
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