What is magnesium phosphate?

The magnesium phosphate is a term used to refer to a family of inorganic compounds consisting of magnesium and alkaline earth metal phosphate oxyanion. The simplest magnesium phosphate has the chemical formula Mg 3 (PO 4 ) 2 . The formula indicates that for every two PO 3– anions  there are three Mg 2+ cations interacting with them.

Likewise, these compounds can be described as magnesium salts derived from orthophosphoric acid (H 3 PO 4 ). In other words, the magnesium “piggybacks” between the phosphate anions, regardless of their inorganic or organic presentation (MgO, Mg (NO 3 ) 2 , MgCl 2 , Mg (OH) 2 , etc.).

Due to these reasons magnesium phosphates can be found as various minerals. Some of these are: catteite —Mg 3 (PO 4 ) 2 · 22H 2 O—, struvite – (NH 4 ) MgPO 4 · 6H 2 O, whose microcrystals are represented in the upper image—, holtedalite —Mg 2 (PO 4 ) (OH) – and bobierrite —Mg 3 (PO 4 ) 2 · 8H 2 O—.

In the case of bobierrite, its crystalline structure is monoclinic, with crystalline aggregates in the shape of fans and massive rosettes. However, magnesium phosphates are characterized by exhibiting rich structural chemistry, meaning that their ions adopt many crystalline arrangements.

Magnesium phosphates are derived from the substitution of H 3 PO 4 protons . When orthophosphoric acid loses a proton, it remains as the dihydrogen phosphate ion, H 2 PO  .

How to neutralize the negative charge to create a magnesium salt? If Mg 2+ counts for two positive charges, then you need two H 2 PO  . Thus, magnesium diacid phosphate, Mg (H 2 PO 4 ) 2, is obtained .

Next, when the acid loses two protons, the hydrogen phosphate ion, HPO 2– remains . Now how do you neutralize these two negative charges? Since Mg 2+ only needs two negative charges to neutralize, it interacts with a single HPO 2– ion . In this way, magnesium acid phosphate is obtained: MgHPO 4 .

Finally, when all the protons are lost, the phosphate anion PO 3– remains . This requires three Mg 2+ cations and another phosphate to assemble into a crystalline solid. The mathematical equation 2 (-3) + 3 (+2) = 0 helps to understand these stoichiometric ratios for magnesium and phosphate.

As a result of these interactions, tribasic magnesium phosphate: Mg 3 (PO 4 ) 2 is produced . Why is it tribasic? Because it is capable of accepting three equivalents of H + to form the H 3 PO 4 again :

PO 3– (aq) + 3H + (aq) <=> H 3 PO 4 (aq)

Magnesium phosphates with other cations

The compensation of negative charges can also be achieved with the participation of other positive species.

For example, to neutralize PO 3– , ions K + , Na + , Rb + , NH + , etc., can also intercede, forming compound (X) MgPO 4 . If X equals NH + , the mineral anhydrous struvite, (NH 4 ) MgPO 4, is formed .

Given the situation where another phosphate intervenes and negative charges increase, other additional cations can join the interactions to neutralize them. Thanks to this, numerous crystals of magnesium phosphate can be synthesized (Na 3 RbMg 7 (PO 4 ) 6 , for example).


Magnesium phosphate

The upper image illustrates the interactions between the Mg 2+ and PO 3– ions that define the crystal structure. However, it is only an image that rather demonstrates the tetrahedral geometry of phosphates. So the crystal structure involves phosphate tetrahedra and magnesium spheres.

In the case of anhydrous Mg 3 (PO 4 ) 2 , the ions adopt a rhombohedral structure, in which Mg 2+ is coordinated with six O atoms.

The above is illustrated in the image below, with the notation that the blue spheres are made of cobalt, it is enough to change them for the green spheres of magnesium:

Magnesium phosphate

Right in the center of the structure can be located the octahedron formed by the six red spheres around the bluish sphere.

Likewise, these crystalline structures are capable of accepting water molecules, forming magnesium phosphate hydrates.

This is because they form hydrogen bonds with phosphate ions (HOH — O-PO 3– ). Furthermore, each phosphate ion is capable of accepting up to four hydrogen bonds; that is, four molecules of water.

Since Mg 3 (PO 4 ) 2 has two phosphates, it can accept eight molecules of water (which is the case with the mineral bobierrite). In turn, these water molecules can form hydrogen bonds with each other or interact with the positive centers of Mg 2+ .


It is a white solid, forming crystalline rhombic plates. It is also odorless and tasteless.

It is very insoluble in water, even when it is hot, due to its high energy of the crystalline lattice; This is the product of strong electrostatic interactions between the polyvalent ions Mg 2+ and PO 3– .

That is, when the ions are polyvalent and their ionic radii do not vary much in size, the solid shows resistance to dissolution.

It melts at 1184 ºC, which is also indicative of strong electrostatic interactions. These properties vary depending on how many water molecules it absorbs, and if the phosphate is in some of its protonated forms (HPO 2– or H 2 PO  ).

Uses / applications


It has been used as a laxative for states of constipation and heartburn. However, its harmful side effects – manifested by the generation of diarrhea and vomiting – have limited its uses. In addition, it is likely to cause damage to the gastrointestinal tract.

Bone repair

The use of magnesium phosphate in bone tissue repair is currently being explored, investigating the application of Mg (H 2 PO 4 ) 2 as a cement.

This form of magnesium phosphate meets the requirements for this: it is biodegradable and histocompatible. In addition, its use in the regeneration of bone tissue is recommended for its resistance and fast setting.

Orthopedic material

The use of amorphous magnesium phosphate (AMP) as a biodegradable and non-exothermic orthopedic cement is being evaluated. To generate this cement, AMP powder is mixed with polyvinyl alcohol, to form a putty.

In living beings

The main function of magnesium phosphate is to serve as a contribution of Mg to living beings. This element intervenes in numerous enzymatic reactions as a catalyst or intermediate, being essential for life.

A deficiency of Mg in humans is associated with the following effects: decreased Ca levels, heart failure, Na retention, decreased K levels, arrhythmias, sustained muscle contractions, vomiting, nausea, low circulating levels of parathyroid hormone and stomach and menstrual cramps, among others.

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