**9.2.2. Properties of phosphoric acid**

316°C H P O 2 HPO H O 42 7 3 2

Hence, from the PO4 building block, a long series of two- and three-dimensional phosphates originates through P-O-P linkages. There is a continuous series of phosphates from ortho‐ phosphate (one P atom) to phosphorus pentaoxide (P2O5) followed by homologous series of straight-chained, branched and cyclic phosphates. The members of the series having one atom of phosphorus are called orthophosphates; the dimers (two P atoms) are pyrophosphates followed by the triphosphates (tripolyphosphates, three P atoms) and by the tetraphos‐ phates (four P atoms). The members of homologous series (H2PnO3n+1) with 5 – 15 P atoms are referred to as oligophosphates. In general, any phosphate having three or more P atoms is considered to be polyphosphate. Metaphosphates are cyclic with general formula (HPO3)n or

. The phosphates with three-dimensional structure are termed as ultraphosphates with

3 4 42 7 2 2 H PO H P O H O « + (16)

3− tetrahedra are given in polyhedral

In the crystalline state, phosphoric acid exists as prismatic crystal of H3PO4 and as hemihy‐ drate (H3PO4·1/2H2O, **Fig. 5**), but 10H3PO4·H2O hydrate was also reported. The crystals of H3PO4 have layered structure, where each molecule is connected to six others via hydrogen bonds. Heated to temperatures higher than the melting point (42.35°C), phosphoric acid slowly

**Fig. 5.** The H2O-H3PO4 phase diagram (a) and the structure of H3PO4·1/2H2O (b) viewed along the b-direction: large

and small empty cycles represent water and hydrogen atoms, respectively. PO4

® + (15)

³

424 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

the composition given by general formula: PnO3n+x, where 1 ≥ *x* ≤ *n*/2 [20].

(PnO3n) −

representation [5].

dehydrates according to the equation:

The solutions of phosphoric acid show unique phenomena of sonoluminescence (SL) in which the nonlinear oscillations of a single bubble under the influence of sufficiently strong sound field lead to a violent collapse of the bubble and to the production of very brief light pulse with the duration of several picoseconds at the end of the bubble collapse. The standard spectrum of SL radiation is a featureless continuum spectrum, ranging from 200 to 800 nm, which is well fitted by the black-body radiation with the temperature of 6000 – 20,000 K. The temperature inside the bubble has been predicted to be even much higher, as the spectrum of radiation from the regions close to the bubble center is fitted by thermal bremsstrahlung radiation from the plasma with temperature more than 106 K. The experiments show that the SL emission from sulfuric acid (**Fig. 7**) is about 2700 times greater than the brightest SL in water. Also, the SL radiation from phosphoric acid can be up to four orders of magnitude brighter than SL from water [22],[23],[24],[25].

Phosphoric acid is a key material for the manufacture of detergents, food products and alimentary supplies for cattle, toothpastes and fertilizers such as monoammonium phos‐ phate (MAP, (NH4)H2PO4, ammonium dihydrogen phosphate), diammonium phosphate (DAP, (NH4)2HPO4, diammonium hydrogen phosphate) and triple superphosphate (TSP) [26], [27], which are described in **Section 9.3**.

**Fig. 7.** Sonoluminescence occurs as the bubble collapse under some specific conditions including very low vapor pres‐ sure liquids [25].

Another applications of phosphoric acid include the treatment of surface of metals (**Sec‐ tion 10.8**), the utilization in dentistry [28],[29],[30],[31],[37],[38] (described in **Section 10.1.2**), phosphate binders [32], geopolymers [33], phosphoric acid fuel cells [34],[35],[36], gel-based electrolytes [39] and solid membranes [40],[41],[42] for fuel cells, the activation of carbon adsorbents [43],[44] and catalysts [45],[46], the modification of zeolites [47], catalytic decom‐ position of H2O2 [48] and organic syntheses, e.g. esterification [49],[50]. Direct applications of phosphoric acid are shown in **Fig. 8**.

**Fig. 8.** Direct applications of orthophosphoric acid [2].
