A representation of these ions is shown in the image below. (NH 4 ) 3 PO 4 is also known as tribasic ammonium phosphate or triammonium phosphate. These names are due to the fact that, as observed, PO 4 3- has available three negative oxygens capable of accepting protons, H + , from a surrounding acid.
And this is precisely what happens with the NH 4 + of its own structure. Therefore, ammonium phosphate is unstable, decomposing into NH 4 H 2 PO 4 and NH 3 . That is why this salt gives off a strong ammonia smell.
The other ammonium phosphate salts are more stable and useful. For example, they are used in agriculture as fertilizer materials, and the crystals of some of them even have optical and physical applications.
This family of salts is simple but diverse when the acidity of the medium where it is found is changed.
Ammonium phosphate is composed of NH 4 + and PO 4 3- ions , both of tetrahedral geometries. The electrons of PO 4 3- are distributed among the three oxygen atoms of the PO – bonds , whose negative charge strongly attracts NH 4 + . Therefore, it could be thought that the crystal structure of (NH 4 ) 3 PO 4 is strong.
In it, the NH 4 + and PO 4 3- ions are found in a 3: 1 ratio, which means that in its crystals there are three times as many NH 4 + ions as there are PO 4 3- ions . Thus, their electrostatic interactions end up defining a tetragonal crystal.
The same occurs in part with the other salts of this phosphate: (NH 4 ) 2 HPO 4 and NH 4 H 2 PO 4 , in which PO 4 3- has gained one or two H + , respectively. In its crystals, mainly tetragonal, the electrostatic interactions are weaker due to their lower ionic charges.
However, they are more energetically stable, especially NH 4 H 2 PO 4 , as it has the highest melting point.
On the other hand, ammonium phosphate salts can also form hydrates, incorporating water molecules into their crystalline structures.
Properties of ammonium phosphate
(NH 4 ) 3 PO 4
This formula, strictly following the inorganic nomenclature, corresponds to the ammonium phosphate salt. Other accepted names are:
-Tibasic ammonium phosphate
-Triammonium or triammonium phosphate
(NH 4 ) 2 HPO 4
This salt has several names according to the nomenclature of preference. The most common, apart from diammonium phosphate, are:
-Dibasic ammonium phosphate
Unlike (NH 4 ) 3 PO 4 , it is composed of NH 4 + and HPO 4 2- ions in a 2: 1 ratio.
NH 4 H 2 PO 4
This salt corresponds to the “most acidic version” of ammonium phosphate. In it, PO 4 3- has gained two H + , thus reducing its ionic charge to H 2 PO 4 – . Therefore, their most common names are:
-Monobasic ammonium phosphate (because it barely has an oxygen capable of accepting protons)
-Diacid ammonium phosphate
Some of the above names can also be changed to:
-Ammonium dihydrogen phosphate
As can be seen, there are many possible names. The important thing is to know how to relate them directly to the formulas of any of the three salts corresponding to the family of ammonium phosphates.
(NH 4 ) 3 PO 4 : 149.09 g / mol
(NH 4 ) 2 HPO 4 : 132.06 g / mol
NH 4 H 2 PO 4 : 115.025 g / mol
The ammonium phosphate salts share in common that they are all white. With the exception of triammonium or tribasic ammonium phosphate, the other salts do not give off an ammonia odor under normal conditions.
(NH 4 ) 3 PO 4 decomposes before it can melt. Meanwhile, (NH 4 ) 2 HPO 4 melts at 155 ºC, and NH 4 H 2 PO 4 at 190 ºC. The higher melting point of NH 4 H 2 PO 4 indicates greater stability and stronger ionic bonding .
Immeasurable, since they decompose when melted.
Ammonium phosphate salts are very soluble in water, with (NH 4 ) 3 PO 4 being a hygroscopic solid. However, they are insoluble in ethanol, acetone, and liquid ammonia.
All ammonium phosphate salts have the potential to release ammonia upon decomposition. Also, depending on the intensity of the heat, they decompose into phosphoric acid to finally release harmful gases of nitrogen oxides and phosphorus.
Ammonium phosphate salts are obtained through the neutralization reaction that occurs between phosphoric acid and ammonia:
H 3 PO 4 + 3 NH 3 → (NH 4 ) 3 PO 4
As can be seen, if it is desired to obtain “less basic” ammonium phosphates, which is the same as saying “more acidic”, it is necessary to vary the proportions of the reagents that are added to the reactor. Generally, phosphoric oxide, P 2 O 5 , is used as a source of phosphorous for its respective acid.
The more H 3 PO 4 there is, the greater the production of NH 4 H 2 PO 4 or (NH 4 ) 2 HPO 4 (more acidic) than of (NH 4 ) 3 PO 4 (more basic).
In fact, they are the NH 4 H 2 PO 4 (MAP its acronym: monoammonium phosphate ) and (NH 4 ) 2 HPO 4 (DAP) salts ammonium phosphate having greater commercial value. Meanwhile, (NH 4 ) 3 PO 4 is the least commercialized, precisely because it is the most unstable and possibly the most expensive to prepare.
One of the immediate consequences of the previous paragraph is that most of the uses of ammonium phosphate salts are due to NH 4 H 2 PO 4 and (NH 4 ) 2 HPO 4 . (NH 4 ) 3 PO 4 , on the other hand, has almost no use, as it decomposes quickly.
Salts of ammonium phosphate fertilizers are good because they provide two essential nutrients for the development of plants : nitrogen and phosphorus. In addition, they are soluble in water, so they quickly become available in the soil for proper absorption through the roots.
They are stored without any problem, and unless there is a fire, they do not give off ammonia, a gas that can damage plant tissues.
The decomposing ammonium phosphate salts release various gases that, although harmful, displace oxygen from the air in full fires. That is why they are used as fire retardants. In this way, the advance of the flames is slowed or slowed down.
Optics and physics
NH 4 H 2 PO 4 is used in optical applications because its crystals show birefrigeration, that is, they have double refraction. They also have piezoelectric properties, which is why they are used in transducers.