Phosphoric acids and phosphates

Two or more orthophosphoric acid

For example, pyrophosphoric, triphosphoric and tetraphosphoric acids can be obtained by the reactions

$${\displaystyle {\begin{aligned}{\ce {2 H3PO4}}&\longrightarrow {\ce {H4P2O7 + H2O}}\\[2pt]{\ce {H4P2O7 + H3PO4}}&\longrightarrow {\ce {H5P3O10 + H2O}}\\[2pt]{\ce {H5P3O10 + H3PO4}}&\longrightarrow {\ce {H6P4O13 + H2O}}\end{aligned}}}$$

The "backbone" of a polyphosphoric acid molecule is a chain of alternating P and O atoms. Each extra orthophosphoric unit that is condensed adds 1 extra H (hydrogen) atom, 1 extra P (phosphorus) atom, and 3 extra O (oxygen) atoms. The general formula of a polyphosphoric acid is H_n+2P_nO_3n+1 or HO[−P(O)(OH)−O−]_nH.

Polyphosphoric acids are used in

Metaphosphoric acid (HPO₃) is a colorless, vitreous, deliquescent solid, density 2.2 to 2.5 g/cc, which sublimes upon heating. It is soluble in ethanol.[4]

Cyclic phosphoric acids

Phosphoric acid units can be bonded together in rings (cyclic structures). The simplest such compound is trimetaphosphoric acid or cyclo-triphosphoric acid having the formula H₃P₃O₉. Its structure is shown in the illustration. Since the ends are condensed, its formula has one less H₂O (water) than tripolyphosphoric acid.

The general formula of a phosphoric acid is H_n−2x+2P_nO_3n−x+1, where n is the number of phosphorus atoms and x is the number of fundamental cycles in the molecule's structure; that is, the minimum number of bonds that would have to be broken to eliminate all cycles.

• 
  Orthophosphoric acid
  H₃PO₄
• 
  Pyrophosphoric acid
  H₄P₂O₇
• 
  Tripolyphosphoric acid
  H₅P₃O₁₀
• 
  Tetrapolyphosphoric acid
  H₆P₄O₁₃
• 
  Trimetaphosphoric acid
  H₃P₃O₉
• 
  Phosphoric anhydride

  
  P₄O₁₀

The limiting case of internal condensation, where all oxygen atoms are shared and there are no hydrogen atoms (x = n+2/2) is an

Removal of the hydrogen atoms as protons H⁺ turns a phosphoric acid into a phosphate anion. Partial removal yields various hydrogen phosphate anions.

Orthophosphate

The anions of orthophosphoric acid H₃PO₄ are orthophosphate (commonly called simply "phosphate") PO3−4, monohydrogen phosphate HPO2−4, and dihydrogen phosphate H₂PO−4.

Linear oligophosphates and polyphosphates

Dissociation of pyrophosphoric acid H₄P₂O₇ generates four anions, [H_4−kP₂O₇]^k−, where the charge k ranges from 1 to 4. The last one is pyrophosphate [P₂O₇]⁴⁻. The pyrophosphates are mostly water-soluble.

Likewise, tripolyphosphoric acid H₅P₃O₁₀ yields at least five anions [H_5−kP₃O₁₀]^k−, where k ranges from 1 to 5, including

Branched polyphosphoric acids give similarly branched polyphosphate anions. The simplest example of this is triphosphono phosphate [OP(OPO₃)₃]⁹⁻ and its partially dissociated versions.

The general formula for such (non-cyclic) polyphosphate anions, linear or branched, is [H_n+2−kP_nO_3n+1]^k−, where the charge k may vary from 1 to n + 2. 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 [HPO₃]_x where x = number of phosphoric units in the molecule.

When metaphosphoric acids lose their hydrogens as H⁺, cyclic

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.^[5]

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 have been commonly used in

The −OH groups in phosphoric acids can also condense with the

References