Phosphoric acids and phosphates

This article compares various kinds of phosphoric acids and phosphates.

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Orthophosphoric acid

The simplest compound of a series of phosphoric acids is sometimes called by its common name, orthophosphoric acid, but more often called by its IUPAC name, simply phosphoric acid, by both non-technical people and even many chemists. The chemical formula of orthophosphoric acid is H₃PO₄ and its chemical structure is label in the illustration. There is a separate article on this most important compound in the series at: Phosphoric acid. However, two or more orthophosphoric acid molecules can be joined by condensation into larger molecules by elimination of water. This way, a series of polyphosphoric acids can be obtained.

Orthophosphate

Orthophosphoric acid has three hydrogen atoms bonded to oxygen atoms in its structure. All three hydrogens are acidic to varying degrees and can be lost from the molecule as H⁺ ions (alternatively referred to as protons). When all three H⁺ ions are lost from orthophosphoric acid, an orthophosphate ion (PO₄^-3) is formed. Orthophosphate is the simplest in a series of phosphates, and is usually just called phosphate by both non-technical people and many chemists alike; see a separate article on phosphate for details.

Because orthophosphoric acid can undergo as many as three dissociations or ionizations (losses of H⁺ ions), it has three acid dissociation constants called K_a1, K_a2, and K_a3. Another way to provide acid dissociation constant data is to list pK_a1, pK_a2, and pK_a3 instead. Orthophosphate is in a sense the triple conjugate base of phosphoric acid and has three related basicity constants, K_b1, K_b2, and K_b3, which likewise have corresponding pK_b1, pK_b2, and pK_b3 values.

Polyphosphoric acids

When two orthophosphoric acid molecules are condensed into one molecule, pyrophosphoric acid (H₄P₂O₇) is obtained as follows:

2 H₃PO₄ → H₄P₂O₇ + H₂O

The chemical structure of pyrophosphoric acid is also shown in the illustration. There is also a separate article on Pyrophosphoric acid. Three orthophosphoric acid molecules can condense in a row to obtain tripolyphosphoric acid (H₅P₃O₁₀), which is also shown in the illustration. This condensation process can continue with additional orthophosphoric acid units to obtain tetrapolyphosphoric acid (H₆P₄O₁₃, pictured) and so on. Note that each extra phosphoric unit adds 1 extra H (hydrogen) atom, 1 extra P (phosphorus) atom, and 3 extra O (oxygen) atoms. The "backbone" chain of these types of molecules consists of alternating P and O atoms covalently bonded together. Polyphosphoric acid molecules can have dozens of such phosphoric units bonded in a row. A general formula for such poly-acid compounds is HO(PO₂OH)_xH, where x = number of phosphoric units in the molecule. The four oxygen atoms bonded to each phosphorus atom are in a tetrahedral configuration with the phosphorus in the center of the tetrahedron and the oxygens in each of the four corners.

Polyphosphates

In a pyrophosphoric acid molecule, there are four hydrogens bonded to oxygens, and one, two, three, or all four can be lost as H⁺ ions. When all four are lost from pyrophosphoric acid, a pyrophosphate ion is formed. Because pyrophosphoric acids can undergo four dissociations, there are four K_a values for it, as well as four corresponding pK_a values. Similarly, pyrophosphate is a base with four K_b and, of course, four pK_b values for regaining the H⁺ ions in reverse order.

The situation with higher order polyphosphoric acids and polyphosphates continues in a similar way. Tripolyphosphoric acid can lose up to five H⁺ ions to form a tripolyphosphate ion, tetrapolyphosphoric acid can lose up to six H⁺ ions to form tetrapolyphosphate, etc. As more dissociations per molecule are possible, the intervals between individual pK_a and pK_b values now start becoming smaller on the pH scale.

As the polyphosphoric molecules grow increasingly larger and more complex, practically any number of the somewhat acidic -OH groups in them can dissociate to become negatively charged oxygens, forming numerous combinations of multiple-charged polyphosphoric/polyphosphate anions. Generally in an aqueous solution, the degree or percentage of dissociation depends on the pH of the solution.

Ortho-, pyro-, and tripolyphosphate compounds have been commonly used in detergents (i. e. cleaners) formulations. For example, see Sodium tripolyphosphate. Sometimes pyrophosphate, tripolyphosphate, tetrapolyphosphate, etc. are called diphosphate, triphosphate, tetraphosphate, etc. , especially when they are part of phosphate esters in biochemistry.

Cyclo- or metaphosphoric acids and metaphosphates

The phosphoric acid units can be bonded together in rings (cyclic structures) forming metaphosphoric acid molecules. 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. A general formula for such cyclic compounds is (HPO₃)_x where x = number of phosphoric units in the molecule.

When these metaphosphoric acids lose their hydrogens as H⁺, cyclic anions called metaphosphates are formed. An example of a compound with such an anion is sodium hexametaphosphate (Na₆P₆O₁₈) used as a sequestrant and a food additive.

Branched polyphosphates

The third -OH group on an orthophosphoric acid unit can also be used for condensation with other phosphoric groups to form branches in the polyphosphoric/polyphosphate chains. The ultimate example of cyclic and branching condensation would be the cyclic four-phosphate unit double-branched to form the phosphoric anhydride P₄O₁₀; see illustration.

Hydrolysis of polyphosphoric/polyphosphates

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. 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.

Phosphate and phosphite esters

The -OH groups in phosphoric acids can also condense with the hydroxyl groups of alcohols to form phosphate esters. Since orthophosphoric acid has three -OH groups, it can esterify with one, two, or three alcohol molecules to form a mono-, di-, or triester. See the general structure image of an ortho- (or mono-) phosphate ester below on the left, where any of the R groups can be a hydrogen or an organic radical. Pyro- (or di-) phosphate esters and tripoly- (or tri-) phosphate esters, etc. are also possible. Any -OH groups on the phosphates in these ester molecules may lose H⁺ ions to form anions, again depending on the pH in a solution. In the biochemistry of living organisms, there are many kinds of (mono)phosphate, diphosphate, and triphosphate compounds (essentially esters), many of which play a significant role in metabolism such as adenosine diphosphate (ADP) and triphosphate (ATP).  

Similarly, phosphorous acid can bond with alcohol molecules to form a phosphite ester. See the general structure image below on the right. The two dots on the P represent the lone electron pair of the phosphorus atom.

References

• Determination of Polyphosphates Using Ion Chromatography with Suppressed Conductivity Detection, Application Note 71 by Dionex