- Structure
- Nomenclature
- Properties
- Physical state
- Molecular weight
- Melting point
- Density
- Solubility
- Other properties
- Obtaining
- Use in ceramics
- Aluminum Phosphate Ceramic Manufacturing
- Other uses
- In obtaining concrete
- In dental cements
- In vaccines
- As a flame retardant in polymers
- References
The aluminum phosphate is an inorganic solid formed by an aluminum ion to the 3+ and a phosphate ion PO 4 3-. Its chemical formula is AlPO 4. It is a white solid whose crystalline structure is similar to that of silica SiO 2. It is insoluble in water.
It can be obtained from alumina (Al 2 O 3) and phosphoric acid (H 3 PO 4). It can also be obtained starting from aqueous solutions of aluminum chloride (AlCl 3) and sodium phosphate (Na 3 PO 4).
Aluminum phosphate AlPO 4. Ondřej Mangl. Source: Wikimedia Commons.
Aluminum phosphate has a very high melting point, which is why it is widely used as a component of refractory ceramics, that is, ceramics that withstand very high temperatures.
It is also used as an antacid for the stomach, in mixtures for the repair of teeth and as an adjuvant to vaccines, that is, to stimulate the body's immune response.
Some refractory concretes have AlPO 4 in their composition, which increases the mechanical and high-temperature support properties of this type of cement.
It has been used as a protective shield to prevent combustible materials such as certain polymers from burning.
Structure
AlPO 4 is made up of an Al 3+ aluminum cation and a PO 4 3- phosphate anion.
Ionic structure of aluminum phosphate. Author: Marilú Stea.
Crystalline aluminum phosphate is also called berlinite or alpha phase (α-AlPO 4) and its crystals are similar to quartz.
Synthetic berlinite crystals (α-AlPO 4). DMGualtieri. Source: Wikimedia Commons.
The alpha phase of aluminum phosphate is a solid formed by a covalent network of tetrahedra of PO 4 and AlPO 4 that alternate and are linked by oxygen atoms.
This structure is isomorphic with silica, that is, it has the same shape as that of silica SiO 2.
Nomenclature
- Aluminum phosphate
- Aluminum monophosphate
- Aluminum salt of phosphoric acid.
Properties
Physical state
Crystalline white solid.
Molecular weight
121.93 g / mol
Melting point
1800 ºC
Density
2.56 g / cm 3
Solubility
Insoluble in water
Other properties
The structure of AlPO 4 is very similar to that of silica SiO 2, so it shares many physical and chemical properties with it.
Aluminum phosphate is a highly refractory material, that is, it resists very high temperatures without changing its physical state or structure and without decomposing.
The crystalline AlPO 4 or berlinite when heated converts to a tridymite-type structure and then a cristobalite-type structure, other forms of this compound that resemble silica SiO 2.
Aluminum phosphate. Chemicalinterest. Source: Wikimedia Commons.
Obtaining
Aluminum phosphate AlPO 4 can be obtained by the reaction between phosphoric acid H 3 PO 4 and alumina Al 2 O 3. Temperature application is required, for example between 100 and 150 ° C.
Al 2 O 3 + 2 H 3 PO 4 = 2 AlPO 4 + 3 H 2 O
It can also be obtained by joining an aqueous solution of aluminum chloride AlCl 3 with an aqueous solution of sodium phosphate Na 3 PO 4:
AlCl 3 + Na 3 PO 4 = AlPO 4 + 3 NaCl
Use in ceramics
Aluminum phosphate AlPO 4 is often found in the constitution of alumina ceramics.
Ceramic with a high content of alumina is one of the materials that, due to its hardness, is used in applications where it is required to resist high loads and severe conditions.
This type of ceramic is resistant to corrosion, to high-temperature environments, to the presence of hot steam or to reducing atmospheres such as carbon monoxide (CO).
Alumina ceramic also has low electrical and thermal conductivity, which is why it is used to make refractory bricks and electrically insulating components.
Refractory brick linings which may contain AlPO 4 aluminum phosphate. These bricks protect from high temperatures. Alexknight12. Source: Wikimedia Commons.
Because aluminum phosphate forms at a much lower temperature than silica SiO 2, it is cheaper to produce, which is an advantage in the manufacture of ceramics suitable for demanding services.
Aluminum Phosphate Ceramic Manufacturing
Alumina Al 2 O 3 and phosphoric acid H 3 PO 4 are used in aqueous medium.
The preferred formation pH is 2-8, as there is an abundance of dissolved phosphoric acid species, such as H 2 PO 4 - and HPO 4 2-. At acid pH the concentration of Al 3+ ions is high, coming from the dissolution of Al 2 O 3 alumina.
First, a hydrated aluminum diphosphate trihydrogen AlH 3 (PO 4) 2.H 2 O gel is formed:
Al 3+ + H 2 PO 4 - + HPO 4 2- + H 2 O ⇔ AlH 3 (PO 4) 3.H 2 O
However, there comes a time when the pH of the solution drops and becomes neutral, where the alumina Al 2 O 3 has low solubility. At this time the insoluble alumina forms a layer on the surface of the particles preventing the reaction from continuing.
Therefore it is necessary to increase the solubility of the alumina and this is achieved by gently heating. On heating to 150 ° C the gel continues the reaction with alumina Al 2 O 3 releasing water and crystalline berlinite (alpha-AlPO 4) is formed.
Al 2 O 3 + 2 AlH 3 (PO 4) 3.H 2 O → AlPO 4 + 4 H 2 O
The berlinite binds the individual particles and forms the ceramic.
Other uses
AlPO 4 is used as an antacid, as an adsorbent, as a molecular sieve, as a catalyst support and as a coating to improve resistance to hot corrosion. Here are other applications.
In obtaining concrete
Aluminum phosphate is an ingredient in refractory or heat resistant concretes.
It provides excellent mechanical and refractive properties to these concretes, such as resistance to heat. In the temperature range between 1400-1600 ° C, cellular concrete based on aluminum phosphate is one of the most efficient materials as a thermal insulator.
It does not require drying, its hardening is achieved through a self-propagating exothermic reaction. It is possible to prepare bricks of this material of any shape and size.
In dental cements
Aluminum phosphate is part of dental cements or materials used to heal decayed teeth.
In dental cements, alumina is used as a moderator of acid-base reactions, where the moderating effect is due to the formation of aluminum phosphate on the particles of other materials.
These cements present a very high resistance to compression and tension, which is due to the presence of aluminum phosphate.
Dental cements used to heal cavities may contain aluminum phosphate. Author: Reto Gerber. Source: Pixabay.
In vaccines
AlPO 4 has been used for many years in various human vaccines to enhance the body's immune response. AlPO 4 is said to be an "adjuvant" to vaccines. The mechanism is not yet well understood.
It is known that the immunostimulatory effect of AlPO 4 depends on the process of adsorption of the antigen to the adjuvant, that is, on the way in which it adheres to it. An antigen is a compound that upon entering the body generates the formation of antibodies to fight a specific disease.
Antigens can be adsorbed to AlPO 4 by electrostatic interactions or by binding with ligands. They are adsorbed on the surface of the adjuvant.
It is further believed that the size of the AlPO 4 particles also has an influence. The smaller the particle size the antibody response is greater and longer lasting.
Vaccines may contain aluminum phosphate AlPO 4 to increase their effectiveness. Author: Tumisu. Source: Pixabay.
As a flame retardant in polymers
AlPO 4 has been used as a fire retardant and to prevent the combustion or burning of certain polymers.
The addition of AlPO 4 to a polypropylene polymer that already has a flame retardant causes a synergistic effect between both retarders, which means that the effect is much greater than that of both fire retardants separately.
When the polymer is subjected to combustion or burned in the presence of AlPO 4, an aluminum metaphosphate is formed which penetrates the charred surface and fills the pores and cracks in the surface.
This leads to the formation of a highly efficient protective shield to prevent burning or combustion of the polymer. In other words, AlPO 4 seals the charred surface and prevents the polymer from burning.
With AlPO 4 the combustion of certain polymers can be retarded. Author: Hans Braxmeier. Source: Pixabay.
References
- Abyzov, VA (2016). Lightweight Refractory Concrete Based on Aluminum-Magnesium-Phosphate Binder. Procedia Engineering 150 (2016) 1440-1445. Recovered from sciencedirect.com.
- Wagh, AS (2016). Aluminum Phosphate Ceramics. In Chemically Bonded Phosphate Ceramics (Second Edition). Chapter 11. Recovered from sciencedirect.com.
- Mei, C. et al. (2019). Aluminum Phosphate Vaccine Adjuvant: Analysis of Composition and Size Using Off-Line and In-Line Tools. Comput Struct Biotechnol J. 2019; 17: 1184-1194. Recovered from ncbi.nlm.nih.gov.
- Qin, Z. et al. (2019). Synergistic barrier effect of aluminum phosphate on flame retardant polypropylene based on ammonium polyphosphate / dipentaerythritol system. Materials and Design 181 (2019) 107913. Recovered from sciencedirect.com.
- Vrieling, H. et al. (2019). Stabilized aluminum phosphate nanoparticles used as vaccine adjuvant. Colloids and Surfaces B: Biointerfaces 181 (2019) 648-656. Recovered from sciencedirect.com.
- Schaefer, C. (2007). Gastrointestinal drugs. Antacids. In Drugs During Pregnancy and Lactation (Second Edition). Recovered from sciencedirect.com.
- Rouquerol, F. et al. (1999). Properties of Some Novel Adsorbents. In Adsorption by Powders and Porous Solids. Recovered from sciencedirect.com.