**9.2.1. Orthophosphoric acid**

being transported countercurrently from below, whereupon phosphorus condenses as a liquid. Solid phosphorus is formed in the second condensation tower, which uses water with the temperature of 10 to 25°C. CO gas is recovered for the use as a fuel in the sintering operation. The by-product calcium silicate is drawn off from the bottom of the furnace as molten liquid. Iron phosphide, "ferrophos" or ferrophosphorus (**Section 9.2.7**) formed from the iron

The mechanism of phosphate reduction is complex and there is no complete agreement among the exact path of each step in the reaction sequence. The overall reaction can be presented by

Ca PO 3 SiO 5 C 3 CaSiO 5 CO P 34 2 ( )<sup>2</sup> + +® + + 3 2 (4)

3 4 ( )<sup>2</sup> 3 2 5 Ca PO 40 C 5 Ca P 40 CO +® + (5)

3 4 ( )<sup>2</sup> 3 2 <sup>2</sup> 3 Ca PO 5 Ca P 24 CaO 8 P +®+ (6)

3 4 ( )<sup>2</sup> <sup>2</sup> 3 4 10 2 Ca PO 6 SiO 6 CaSiO P O +® + (7)

Ca PO 5 CO 3 CaO 5 CO P 3 4 ( )<sup>2</sup> +® + +2 2 (9)

The phosphide theory is generally considered as unlikely due to the thermodynamic reasons,

but the acid displacement mechanism has considerable experimental support [6].

P O 10 C 2 P 10 CO 4 10 + ®+<sup>2</sup> (8)

<sup>2</sup> 5 CO 5 C 10 CO + ® (10)

impurities present in the phosphate ore is also drawn off as a melt [6],[12].

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

the following equation [6]:

Proposed mechanisms for the reactions are:

**1.** Phosphide mechanism [14]:

**2.** Acid displacement mechanism:

**3.** CO reduction mechanism:

Orthophosphoric acid6 [15] is a colorless polyprotic weak acid with stepwise dissociation that involves three equilibrium reactions described by the following equations [2],[16], [17],[18]:

$$\begin{aligned} \text{H}\_3\text{PO}\_4 &\leftrightarrow \text{H}^+ + \text{H}\_2\text{PO}\_4^- & \text{K}\_1 = 7.52 \cdot 10^{-3} \\ 2.15 \le \text{pH} & \le 7.2 \end{aligned} \tag{11}$$

$$\begin{aligned} \text{pH}\_2\text{PO}\_4^- &\leftrightarrow \text{H}^+ + \text{HPO}\_4^{2-} & \text{K}\_2 = 6.23 \cdot 10^{-8} \\ \text{7.2} \leq \text{pH} & \le 12.37 \end{aligned} \tag{12}$$

$$\begin{aligned} \mathrm{HPO\_4^{2-}} & \leftrightarrow \mathrm{H^+} + \mathrm{PO\_4^{3-}} & \quad \mathrm{K\_3} = 2.2 \cdot 10^{-13} \\ \mathrm{pH} & \ge 12.3 \, 7 \end{aligned} \tag{13}$$

However, due to very low values of the equilibrium constants associated with reactions (**Eqs. 12** and **13**), orthophosphoric acid has only one strongly ionizing hydrogen atom and only the first acidic dissociation (**Eq. 11**) has significant effect on the system composition. HPO4 2− and PO4 3− ionic species are present at significant concentrations in very highly diluted solutions only [2].

Orthophosphoric acid gives three series of salts (**Eqs. 11** – **13**):


When heated, orthophosphoric acid condenses to pyrophosphoric acid (250°C) and then to metaphosphoric acid (316°C) [19]:

$$2\ \text{H}\_3\text{PO}\_4 \xrightarrow{\text{\tiny \cdot 230^\circ \text{C}}} \text{H}\_4\text{P}\_2\text{O}\_7 + \text{H}\_2\text{O} \tag{14}$$

<sup>6</sup> Orthophosphoric acid (H3PO4, phosphoric(V) acid) is often termed simply as phosphoric acid, but other phosphoric acids exist: metaphosphoric acid (HPO3) and pyrophosphoric acid (H4P2O7) [15].

$$\text{H}\_4\text{P}\_2\text{O}\_7 \xrightarrow{\text{x316°C}} \text{2HPO}\_3 + \text{H}\_2\text{O} \tag{15}$$

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 (PnO3n) − . The phosphates with three-dimensional structure are termed as ultraphosphates with the composition given by general formula: PnO3n+x, where 1 ≥ *x* ≤ *n*/2 [20].

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 dehydrates according to the equation:

$$2\ \text{H}\_3\text{PO}\_4 \leftrightarrow \text{H}\_4\text{P}\_2\text{O}\_7 + \text{H}\_2\text{O} \tag{16}$$

**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 3− tetrahedra are given in polyhedral representation [5].

The crystal structure of H3PO4 (**Fig. 6**) is monoclinic with the space group P21/C and the cell parameters *a* = 5.80 Å, *b* = 4.85 Å, *c* = 11.62 Å, *β* = 95.20° and *Z* = 4. H3PO4·1/2H2O is monoclin‐ ic (space group P21/A) with the cell parameters *a* = 7.92 Å, *b* = 12.99 Å, *c* = 7.47 Å, *β* =109.9° and *Z* = 8 [5],[21].

**Fig. 6.** The projection of the structure of H3PO4 along the b-axis (a) and c-axis (b). H-bonds are figured by dotted lines. This view induces the feeling that some H-bonds connect two O atoms of the same phosphoric tetrahedron, but they are, in fact, established between two superimposed tetrahedra [5].

Furthermore, polyphosphoric acids and deutronic phosphoric acid are known. Deutrophos‐ phoric acid (D3PO4) can be prepared by dissolving phosphorus pentaoxide in D2O or by the hydrolysis of POCl3 with D2O. This acid has slightly higher melting point, density and viscosity but lower electrical conductivity than the hydrogen analogue [2],[5].
