por What is magnesium phosphate?
He magnesium phosphate is a term used to refer to a family of inorganic compounds made up of magnesium, the alkaline earth metal, and the oxoanion phosphate. The simplest magnesium phosphate has the chemical formula Mg3(PO4)2. The formula indicates that for every two PO43– anions there are three Mg2+ cations interacting with them.
Likewise, these compounds can be described as magnesium salts derived from orthophosphoric acid (H3PO4). In other words, magnesium «tails» between phosphate anions, regardless of their inorganic or organic presentation (MgO, Mg(NO3)2, MgCl2, Mg(OH)2, etc.).
Due to these reasons magnesium phosphates can be found as various minerals. Some of these are: catteite —Mg3(PO4)2 22H2O—, struvite —(NH4)MgPO4 6H2O, whose microcrystals are represented in the image above—, holtedalite —Mg2(PO4)(OH)— and bobierrite —Mg3( PO4)2 8H2O—.
In the case of bobierrite, its crystalline structure is monoclinic, with fan-shaped crystalline aggregates and massive rosettes. However, magnesium phosphates are characterized by exhibiting rich structural chemistry, meaning that their ions adopt many crystalline arrangements.
Forms of magnesium phosphate and the neutrality of their charges
Magnesium phosphates are derived from the substitution of the protons of H3PO4. When orthophosphoric acid loses a proton, it remains as the dihydrogen phosphate ion, H2PO4–.
How to neutralize the negative charge to create a magnesium salt? If Mg2+ counts for two positive charges, then you need two H2PO4–. Thus, magnesium dihydrogen phosphate, Mg(H2PO4)2, is obtained.
Next, when the acid loses two protons, the hydrogen phosphate ion, HPO42–, remains. Now, how to neutralize these two negative charges? Since Mg2+ only needs two negative charges to be neutralized, it interacts with a single HPO42– ion. This is how magnesium acid phosphate is obtained: MgHPO4.
Finally, when all the protons are lost, the phosphate anion PO43– remains. It requires three Mg2+ 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 is produced: Mg3(PO4)2. Why is it tribasic? Because it is capable of accepting three equivalents of H+ to form H3PO4 again:
PO43–(aq) + 3H+(aq) <=> H3PO4(aq)
Magnesium phosphates with other cations
The compensation of the negative charges can also be achieved with the participation of other positive species.
For example, to neutralize PO43–, the ions K+, Na+, Rb+, NH4+, etc., can also intercede, forming the compound (X)MgPO4. If X equals NH4+, the anhydrous struvite mineral, (NH4)MgPO4, is formed.
Given the situation that another phosphate intervenes and the negative charges increase, other additional cations can be added to the interactions to neutralize them. Thanks to this, numerous magnesium phosphate crystals can be synthesized (Na3RbMg7(PO4)6, for example).
Structure
The top image illustrates the interactions between Mg2+ and PO43– 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 tetrahedrons and magnesium spheres.
In the case of anhydrous Mg3(PO4)2, the ions adopt a rhombohedral structure, in which Mg2+ is coordinated with six O atoms.
This 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 magnesium spheres:
Right in the center of the structure can be located the octahedron formed by the six red spheres around the blue 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-PO33–). Furthermore, each phosphate ion is capable of accepting up to four hydrogen bonds; that is, four water molecules.
Since Mg3(PO4)2 has two phosphates, it can accept eight molecules of water (which occurs with the bobierrite mineral). In turn, these water molecules can form hydrogen bonds with others or interact with the positive centers of Mg2+.
Properties
It is a white solid, forming crystalline rhombic plates. Also, it has no smell and no taste.
It is highly insoluble in water, even when hot, due to its high lattice energy; this is a product of the strong electrostatic interactions between the polyvalent ions Mg2+ and PO43–.
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 the 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 (HPO42– or H2PO4–).
Uses/applications
Laxative
It has been used as a laxative for states of constipation and gastric acidity. 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, and the application of Mg(H2PO4)2 as cement is being investigated.
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 rapid setting.
material for orthopedics
Amorphous magnesium phosphate (AMP) is being evaluated for use as a non-exothermic, biodegradable orthopedic cement. 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 provide Mg to living beings. This element is involved in numerous enzymatic reactions as a catalyst or intermediary, being essential for life.
A Mg deficiency 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.
References
Mineral Data Publishing. (2001-2005). bobierrite. Retrieved from handbookofmineralogy.org
Sahar Mousa. (2010). Study on synthesis of magnesium phosphate materials. Phosphorus research bulletin Vol. 24, pp 16-21.
Wikipedia. (2018). Magnesium phosphate tribasic. Retrieved from en.wikipedia.org
Pubchem. (2018). Magnesium Phosphate Anhydrous. Retrieved from pubchem.ncbi.nlm.nih.gov
Barbie, E., Lin,B., Goel,VK and Bhaduri, S. (2016) Evaluation of amorphous magnesium phosphate (AMP) based non-exothermic orthopedic cement. Biomedical Mat. Volume 11(5): 055010.
Yu, Y., Yu, CH. and Dai, H. (2016). Preparation of a degradable magnesium bone cement. Regenerative Biomaterials. Volume 4(1): 231