Removal of the hydrogen atoms as
protons turns a phosphoric acid into a phosphate anion. Partial removal yields various hydrogen phosphate anions.
Orthophosphate The anions of orthophosphoric acid are
orthophosphate (commonly called simply "phosphate") ,
monohydrogen phosphate , and
dihydrogen phosphate .
Linear oligophosphates and polyphosphates Dissociation of pyrophosphoric acid generates four anions, {{chem2|[H_{4-
k}P2O7]^{
k−}|}}, where the charge
k ranges from 1 to 4. The last one is
pyrophosphate . The pyrophosphates are mostly water-soluble. Likewise, tripolyphosphoric acid yields at least five anions {{chem2|[H_{5-
k}P3O10]^{
k−}|}}, where
k ranges from 1 to 5, including
tripolyphosphate . Tetrapolyphosphoric acid yields at least six anions, including tetrapolyphosphate , and so on. Note that each extra phosphoric unit adds one extra P atom, three extra O atoms, and either one extra hydrogen atom or an extra negative charge. Branched polyphosphoric acids give similarly branched polyphosphate anions. The simplest example of this is
triphosphono phosphate and its partially dissociated versions. The general formula for such (non-cyclic) polyphosphate anions, linear or branched, is {{chem2|[H_{
n+2−
k}P_{
n}O_{3
n+1}]^{
k−}|}}, where the charge
k may vary from 1 to . Generally in an aqueous solution, the degree or percentage of dissociation depends on the
pH of the solution.
Cyclic polyphosphates Salts or esters of cyclic polyphosphoric acids are often called "metaphosphates". What are commonly called
trimetaphosphates actually have a mixture of ring sizes. A general formula for such cyclic compounds is {{chem2|[HPO3]_{
x}|}} where
x = number of phosphoric units in the molecule. When metaphosphoric acids lose their hydrogens as , cyclic
anions called
metaphosphates are formed. An example of a compound with such an anion is
sodium hexametaphosphate (), used as a
sequestrant and a
food additive.
Chemical properties Solubility These phosphoric acids series are generally
water-
soluble considering the
polarity of the molecules.
Ammonium and
alkali phosphates are also quite soluble in water. The
alkaline earth salts start becoming less soluble and phosphate salts of various other metals are even less soluble.
Hydrolysis and condensation In aqueous solutions (solutions of water), water gradually (over the course of hours)
hydrolyzes polyphosphates into smaller phosphates and finally into ortho-phosphate, given enough water. Higher temperature or acidic conditions can speed up the hydrolysis
reactions considerably. Conversely, polyphosphoric acids or polyphosphates are often formed by dehydrating a phosphoric acid solution; in other words, removing water from it often by heating and evaporating the water off.
Uses Ortho-, pyro-, and tripolyphosphate compounds, such as
sodium tripolyphosphate, have been commonly used in
detergents (i. e. cleaners) formulations. Sometimes pyrophosphate, tripolyphosphate, tetrapolyphosphate, etc. are called
diphosphate,
triphosphate,
tetraphosphate, etc., especially when they are part of
phosphate esters in
biochemistry. They are also used for scale and
corrosion control by
potable water providers. As a corrosion inhibitor, polyphosphates work by forming a protective film on the interior surface of pipes. ==Phosphate esters==