Phosphoric acid
Phosphoric acid (also known as orthophosphoric acid or phosphoric [V] acid)
Molecular formula : H3PO4
(O = 65,3 %, P = 31,64%, H = 3,06 %)

Molar mass = 97,9952 ± 0,0014 g·mol-1

Phosphoric acid is a mineral (inorganic) acid . Orthophosphoric acid molecules can combine with themselves to form a variety of compounds which are also referred to as phosphoric acids, but in a more general way. The term phosphoric acid can also refer to a chemical or reagent consisting of phosphoric acids, such as pyrophosphoric acid or triphosphoric acid, but usually orthophosphoric acid.
The conjugate base of phosphoric acid is the dihydrogen phosphate ion, H2PO4-, which in turn has a conjugate base of hydrogen phosphate, HPO42-, which has a conjugate base of phosphate, PO43-.
The most common source of phosphoric acid is an 85% aqueous solution; such solutions are colourless, odourless, and non-volatile. Rather viscous, syrupy liquids, but still pourable. Because it is a concentrated acid, an 85% solution can be corrosive, although nontoxic when diluted. Because of the high percentage of phosphoric acid in this reagent, at least some of the orthophosphoric acid is condensed into polyphosphoric acids. For the sake of labeling and simplicity, the 85% represents H3PO4 as if it were all orthophosphoric acid. Dilute aqueous solutions of phosphoric acid exist in the ortho- form.

Phosphoric acid can be prepared by three routes :

The more expensive thermal process produces a purer product that is used for applications in the food industry. The wet process dominates in the commercial sector.

Wet process phosphoric acid is prepared by adding sulfuric acid H2SO4 to tricalcium phosphate rock Ca5(PO4)3F, typically found in nature as apatite.
The reaction is:

Ca5(PO4)3X + 5 H2SO4 + 10 H2O >>> 3 H3PO4 + 5 CaSO4·2 H2O + HX

where X may include OH, F, Cl, and Br.

Digestion of the phosphate ore using sulfuric acid yields the insoluble calcium sulfate (gypsum, CaSO4.2H2O), which is filtered and removed as phosphogypsum or hemihydrate CaSO4 .1/2 H2O. Wet-process acid can be further purified by removing fluorine to produce animal-grade phosphoric acid, or by solvent extraction and arsenic removal to produce food-grade phosphoric acid.
The nitrophosphate process is similar to the wet process except that it uses nitric acid in place of sulfuric acid. The advantage to this route is that the coproduct, calcium nitrate is also a plant fertilizer. This method is rarely employed.
Thermal process.
Very pure phosphoric acid is obtained by burning elemental phosphorus to produce phosphorus pentoxide P4O10 (empirical formula, P2O5), which is subsequently dissolved in dilute phosphoric acid. This route produces a very pure phosphoric acid, since most impurities present in the rock have been removed when extracting phosphorus from the rock in a furnace. The end result is food-grade, thermal phosphoric acid; however, for critical applications, additional processing to remove arsenic compounds may be needed.

Elemental phosphorus is produced by an electric furnace. At a high temperature, a mixture of phosphate ore, silica and carbonaceous material (coke, coal etc...) produces calcium silicate, phosphorus gas P and carbon monoxide CO. The P and CO off-gases from this reaction are cooled under water to isolate solid phosphorus. Alternatively, the P and CO off-gases can be burned with air to produce phosphorus pentoxide and carbon dioxide CO2.

Main properties.
At room temperature, phosphoric acid is a crystalline solid density 1.834, melting point at 42.35 °C > colorless viscous liquid.
Pur is a hygroscopic solid (deliquescent). The pure acid is not commercially available.

1.885 g/mL (liquid)
1.685 g/mL (85% solution)
Melting point
42.35 °C (108.23 °F) (anhydrous) 29.32 °C (84.78 °F) (hemihydrate)
Boiling point (decomposition)
158 °C (316 °F; 431 K)
392.2 g/100 mL (-16.3 °C)
369.5 g/100 mL (0.5 °C)
5.48 g/mL (20 °C)
miscible (42.3 °C)
2.4–9.4 cP (85% aq. soln.)
147 cP (100%)

The oxidation state of phosphorus (P) in ortho- and other phosphoric acids is +5; the oxidation state of all the oxygen atoms (O) is -2 and all the hydrogen atoms (H) is +1. Triprotic means that an orthophosphoric acid molecule can dissociate up to three times, giving up an H+ each time, which typically combines with a water molecule, H2O, as shown in these reactions:

For each of the dissociation reactions shown above, there is a separate acid dissociation constant, called Ka1, Ka2, and Ka3 given at 25 °C.

Aqueous solution.
For a given total acid concentration [A] = [H3PO4] + [H2PO4-] + [HPO42-] + [PO43-];
([A] is the total number of moles of pure H3PO4 which have been used to prepare 1 liter of solution), the composition of an aqueous solution of phosphoric acid can be calculated using the equilibrium equations associated with the three reactions described above together with the [H+] [OH-] = 10-14 relation and the electrical neutrality equation. Possible concentrations of polyphosphoric molecules and ions is neglected. The system may be reduced to a fifth degree equation for [H+] which can be solved numerically, yielding:

For strong acid concentrations, the solution is mainly composed of H3PO4. For [A] = 10-2, the pH is close to pKa1, giving an equimolar mixture of H3PO4 and H2PO4-.
For [A] below 10-3, the solution is mainly composed of H2PO4- with [HPO42-] becoming non negligible for very dilute solutions. [PO43-] is always negligible.
Since this analysis does not take into account ion activity coefficients, the pH and molarity of a real phosphoric acid solution may deviate substantially from the above values.

In water treatment, it is used by injection in the form of water diluted reagent (solutions).

A formulation for the water treatment.
Reaction mechanism of calcium bicarbonates (simplified equations):

..................... ....................2H3PO4 +.......................3Ca[HCO3]2 . ...... >>>.... .........6 CO2 ..........+ 6H2O.... +Ca3[PO4]2
(98x2=196)...... (162x3 or 30°F or 300 mg/L as CaCO3) ..................(6x44=264) .......

With 1 mg as H3PO4 added, there has (30/98x2) = 0.15°F for lower TAC (Alkalinity), and 1.347 mg as free CO2 and 0.97 mg/l as PO4 formed.

A entreposer dans un récipient tenu fermé, portant une identification claire de son contenu, placé dans un endroit frais, sec et bien ventilé, à l'abri des bases, des matières combustibles.

(Phosphoric acid and its derivatives are pervasive and find many niche applications)

It can cause severe burns.
In soft drinks : Phosphoric acid, used in many soft drinks (primarily cola), has been linked in epidemiological studies to chronic kidney disease and lower bone density.

Toxicological profile (French) - Acide phosphorique (FT 37) par l'Institut national de recherche et de sécurité (INRS).
Also, see NIOSH Pocket guide to chemical hazards (The National Institute for Occupational Safety and Health - US).

Sources : personal and Wikipedia, the free encyclopedia.

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