The phosphoric acid It is an oxoacid of phosphorus that has the chemical formula H3PO4. It consists of a mineral acid in which three acidic protons are bound to the phosphate anion (PO43-). Although it is not considered a strong acid, its improper use can present a health risk.
It can be found in two states: as a solid in the form of thick orthorhombic crystals, or a crystalline liquid with a syrupy appearance. Its most common commercial presentation has a concentration of 85% w / w and a density of 1.685 g / cm3. This density comes from the hand of concentration.
The three OH groups are responsible for donating acidic hydrogens. Due to the presence of them in its structure, it can react with different hydroxides originating various salts.
In the case of sodium hydroxide, it can form three: monobasic sodium phosphate (NaHtwoPO4), dibasic sodium phosphate (NatwoHPO4) and tribasic sodium phosphate (Na3PO4).
However, depending on which base is used for its neutralization, or which cations are very close to it, it can form other phosphate salts. Among them are: calcium phosphate (Ca3(PO4)two), lithium phosphate (Li3PO4), ferric phosphate (FePO4), and others. Each one with its different degrees of protonation of the phosphate anion.
On the other hand, phosphoric acid can "sequester" divalent cations such as Fetwo+, Cutwo+, ACtwo+ and Mgtwo+. At elevated temperatures it can react with itself with the loss of an H moleculetwoOr, forming dimers, trimers and polymers of phosphoric acids.
This type of reaction makes this compound capable of establishing a large number of structures with phosphorus and oxygen skeletons, from which a wide range of salts known as polyphosphates can also be obtained..
Regarding its discovery, it was synthesized in 1694 by Robert Boyle, dissolving PtwoOR5 (phosphorous pentoxide) in water. It is one of the most useful mineral acids, its function as a fertilizer being the most important. Phosphorus, along with potassium and nitrogen, constitute the three main plant nutrients.
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Phosphoric acid consists of a P = O bond and three P-OH, where the latter are the carriers of the acidic hydrogens released in a dissolution medium. With the phosphorus atom in the center, the oxygens draw a kind of molecular tetrahedron.
In this way, phosphoric acid can be visualized as a tetrahedron. From this perspective, said tetrahedra (by units of H3PO4) interact with each other through hydrogen bonds; that is, its vertices closely approximate.
These intermolecular interactions allow phosphoric acid to crystallize into two solids: the anhydrous and the hemihydrate (H3PO41 / 2HtwoO), both with monoclinic crystal systems. Its anhydrous form can also be described by the formula: 3HtwoO · PtwoOR5, which is equal to a tri-hydrated phosphorous pentoxide.
Tetrahedra can even become covalently linked, but for this one of their units must eliminate a water molecule through dehydration. This occurs when the H3PO4 is subjected to heating, and consequently generates the formation of polyphosphoric acids (PA).
The simplest of all PAs is diphosphoric acid (H4PtwoOR7), also known as pyrophosphoric acid. The chemical equation of its formation is as follows:
2H3PO4 <=> H4PtwoOR7 + HtwoOR
The balance depends on the amount of water and the temperature. What is its structure? In the image in the section, the structures of orthophosphoric acid and pyrophosphoric acid are illustrated in the upper left corner..
Two units are covalently joined when a water molecule is eliminated, forming a P-O-P oxygen bridge between them. Now they are not three acidic hydrogens, but four (four -OH groups). Because of this, the H4PtwoOR7 has four ionization constants kto.
Dehydration can proceed with pyrophosphoric acid if heating continues. Why? Because at each end of its molecule there is an OH group that can be removed as a water molecule, thus promoting the subsequent growth of the P-O-P-O-P skeleton ...
Examples of these acids are tripolyphosphoric and tetrapolyphosphoric acids (both illustrated in the image). It can be seen how the P-O-P skeleton lengthens in a kind of chain made up of tetrahedra.
These compounds can be represented by the formula HO (POtwoOH)xH, where HO is the extreme left that can be dehydrated. POtwoOH is the phosphorus backbone with the P = O and OH bonds; and x are the units or molecules of phosphoric acids necessary to obtain said chain.
When these compounds are completely neutralized with a base, so-called polyphosphates are created. Depending on which cations surround them, they form a wide variety of polyphosphate salts.
On the other hand, if they react with ROH alcohols, their backbone hydrogens are replaced by the R- alkyl substituents. Thus, phosphate esters (or polyphosphates) arise: RO (POtwoOR)xR. It is enough to replace the H by R in all the structures of the image of the section to obtain them.
The P-O-P chains can even close on a phosphoric ring or cycle. The simplest of this type of compound is trimetaphosphoric acid (upper right corner of image). Thus, APs can be linear, cyclical; or if their structures exhibit both types, branched.
The nomenclature of phosphoric acid is dictated by the IUPAC and how the ternary salts of oxo acids are named.
Because in the H3PO4 the P atom has valence +5, the highest value, its acid is assigned the suffix -ico to the prefix phosphor-.
However, phosphoric acid is also commonly called orthophosphoric acid. Why? Because the word 'ortho' is Greek and means 'true'; which would translate into the "true form" or "more hydrated" of the same.
When phosphoric anhydrous is hydrated with an excess of water (P4OR10, the phosphor “cap” in the picture above) is produced H3PO4 (3HtwoO · PtwoOR5). Thus, the prefix ortho is assigned to those acids formed with abundant water.
The prefix pyro refers to any compound originated after the application of heat, since diphosphoric acid arises from the thermal dehydration of phosphoric acid. It is therefore called pyrophosphoric acid (2HtwoO · PtwoOR5).
The prefix meta, which is also a Greek word, means 'after'. It is added to those substances whose formula has eliminated a molecule, in this case, that of water:
H3PO4 => HPO3 + HtwoOR
Note that this time the addition of two phosphoric units does not occur to form diphosphoric acid, but instead metaphosphoric acid is obtained (of which there is no evidence of its existence).
It is also important to note that this acid can be described as HtwoO · PtwoOR5 (similar to hemidrate, multiplying HPO3 For 2). The meta prefix comes perfectly in line with cyclic PA, since if the triphosphoric acid dehydrates, but does not add another H unit3PO4 to become tetraphosphoric acid, then it must form a ring.
And this is the same with other polymetaphosphoric acids, although the IUPAC recommends calling them cyclic compounds of the corresponding PA.
H3PO4
97.994 g / mol
In its solid form it presents orthorhombic, hygroscopic and transparent crystals. In liquid form it is crystalline with the appearance of a viscous syrup..
It is commercially available in aqueous solution with a concentration of 85% w / w. In all these presentations it lacks odor.
158º C (316º F at 760 mmHg).
108º F (42.2º C).
548 g / 100 g HtwoOr at 20 ° C; 369.4 g / 100 ml at 0.5 ° C; 446 g / 100m at 14.95º C.
1,892 g / cm3 (solid); 1.841 g / cm3 (100% solution); 1.685 g / cm3 (85% solution); 1,334 g / cm3 50% solution) at 25 ° C.
Relative to air 3,4 (air = 1).
It is not flammable.
3.86 mPoise (40% solution at 20 ° C).
pH: 1.5 (0.1 N solution in water)
pKa: pKa1 = 2.148; pKa2 = 7.198 and pKa3 = 12.319. Therefore, your most acidic hydrogen is the first.
When heated, it releases phosphorous oxides. If the temperature rises to 213º C or more, it becomes pyrophosphoric acid (H4PtwoOR7).
Corrosive to ferrous metals and aluminum. Reacting with these metals gives rise to the fuel gas hydrogen.
Polymerizes violently with azo compounds, epoxides and polymerizable compounds.
-Phosphoric acid serves as the base for making phosphates, which are used as fertilizers because phosphorus is a main nutrient in plants..
-It has been used in the treatment of lead poisoning and other conditions in which significant amounts of phosphate are required and the production of mild acidosis.
-It is used to control the pH of the urinary tract of mink and expenses in order to avoid the formation of kidney stones.
-Phosphoric acid gives rise to Na saltstwoHPO4 and NaHtwoPO4 which constitute a pH buffer system with a pKa of 6.8. This pH regulating system is present in man, having importance in the regulation of intracellular pH, as well as in the management of the hydrogen concentration in the distal and collecting tubules of the nephrons..
-It is used in the removal of the moldy layer of iron oxide that accumulates on this metal. Phosphoric acid forms iron phosphate that can be easily removed from the metal surface. It is also used in the electrical polishing of aluminum and is a binding agent for refractory products such as alumina and magnesia.
-Phosphoric acid is used as a catalytic agent in the manufacture of Nylon and gasoline. It is used as a dehydrating agent in lithographic engraving, in the production of dyes for use in the textile industry, in the latex coagulation process in the rubber industry and in the purification of hydrogen peroxide.
-Acid is used as an additive in carbonated beverages, thus contributing to its flavor. Diluted is applied in the sugar refining process. It also acts as a buffer system in the preparation of ham, gelatin and antibiotics..
-It is involved in the production of detergents, in the acid catalysis of acetylene production.
-It is used as an acidulant in balanced food for the livestock industry and pets. The pharmaceutical industry uses it in the manufacture of antiemetic drugs. It is also used in a mixture to make asphalt to pave the ground and repair cracks..
-Phosphoric acid acts as a catalyst in the hydration reaction of alkenes to produce alcohol, mainly ethanol. In addition, it is used in the determination of organic carbon in soils.
It is used by dentists to clean and condition the tooth surface prior to placement of dental braces. It also finds use in teeth whitening and the removal of dental plaques. In addition, it is used in the manufacture of adhesives for dental prostheses.
Phosphoric acid is used to adjust pH in cosmetic and skin care products. It is used as a chemical oxidizing agent for the production of activated carbon..
-Phosphoric acid is prepared from phosphate rocks of the apatite type, by digestion with concentrated sulfuric acid:
AC3(PO4)two + 3 HtwoSW4 + 6 htwoO => 2 H3PO4 + 3 (CaSO4.2HtwoOR)
The phosphoric acid obtained in this reaction is of low purity, so it undergoes a purification process that includes precipitation, solvent extraction, crystallization and ion exchange techniques..
-Phosphoric acid can be produced by dissolving phosphorus pentoxide in boiling water.
-It can also be obtained by heating phosphorus with a mixture of air and water vapor:
P4 (l) + 5 Otwo (g) => P4OR10 (s)
P4OR10 (s) + HtwoO (g) => 4H3PO4 (l)
-Because its vapor pressure is low at room temperature, its vapors are unlikely to be inhaled unless acid is sprayed. If so, the symptoms of your inhalation include: cough, sore throat, shortness of breath and labored breathing..
-The literature cites the case of a sailor who was exposed to phosphoric acid fumes for a long period of time. He suffered general weakness, a dry cough, chest pain, and breathing problems. Within one year of exposure, reactive airway dysfunction was observed.
-Skin contact with phosphoric acid can cause redness, pain, blisters and burns of the skin..
-The contact of the acid with the eyes, depending on its concentration and the duration of contact, can cause corrosive tissue injuries or severe burns with permanent eye damage..
-Ingestion of the acid causes burning of the mouth and throat, burning sensation beyond the breastbone, abdominal pain, vomiting, shock and collapse.
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