Aluminium phosphate is a chemical inorganic compound made up of aluminium and phosphate ions. Any chemical compound is known as a phosphate when it consists of a phosphate anion which is a polyanion made up of phosphorus and oxygen. The phosphate polyanion has the molecular formula as PO43- with a -3 electrical charge. Hence, for a phosphate compound or salt, there should be a cation with a positive charge and the same value which is +3. In this case, aluminium is the cation as it has the property to lose three of its electrons in the outermost shell to get an octet configuration, which is Al3+. Hence, when the two combine to form a neutral salt they form aluminium phosphate. The chemical formula of aluminium phosphate is therefore given as, AlPO4. Thus, the neutral chemical formula of aluminium phosphate is made up of the aluminium cation and phosphate anion.
Properties of Aluminium Phosphate
In nature, aluminium phosphate occurs as a mineral known as berlinite. It is usually found as a white crystalline solid. The structural formula of the compound representing the arrangement of the ions shown in the molecular formula of aluminium phosphate is shown as below:
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It also has many different synthetic forms with a network of different structure types like the zeolites. Berlinite has a quartz like structure and it also looks like it which cannot be known from the formula for aluminium phosphate. The difference between the two is that in quartz silicon is replaced by Al and P. The structure of AlO4 and PO4 are tetrahydrate alternates of each other. Just like quartz the structure of aluminium phosphate shows chirality. The crystalline aluminium phosphate when heated it usually changes into tridymite and the cristobalite forms.
Some of the other physical properties that are commonly used to define the aluminium phosphate compound are as follows:
The best identifying method for any chemical compound is the molecular weight. The molecular weight of this compound as determined by the aluminium phosphate formula is 121.952 g/mol.
The density of the compound is 2.56 g.cm3.
The solubility of the compound is also varying. The solubility of the compound having the aluminium phosphate formula is insoluble in water and is only slightly soluble in HCl and HNO3.
There is no boiling point of the compound as it decomposes on heating.
It is isoelectronic with silicon dioxide and is also known to have piezoelectric properties.
Aluminium Phosphate Utility
Most of the use of aluminium phosphate involves its use in aluminium phosphate molecular sieves which are generally known as “AlPO’s”. These structures share similarity with the structures of aluminium phosphate with many of them having microporous cavities. The structures of these molecular sieves are composed of alternating aluminium phosphate and phosphate tetrahedra.
Aluminium phosphate in combination with aluminium hydroxide is used mostly as immunological adjuvants or efficiency enhancers in vaccinations. These immunological adjuvants are used commonly because of their cheap price, long time use, and the most safety and efficiency with most of the antigens. Although so commonly used these salts function as adjuvants.
Another use of the aluminium phosphate compound is its use as an antacid. This function is because of the neutralization of the stomach acid by aluminium phosphate which then forms aluminium chloride. The use of aluminium phosphate is considered to be safe and is also normal for use during pregnancy and breastfeeding. Yet there are some reports that suggest that a maximum of 20% of the aluminium can be absorbed from the given amount taken as an antacid from the gastrointestinal tract. Still, there are no short or long term effects that have been reported to cause any damage to the health of a person in its use as an antacid.
Some other additional uses for aluminium phosphate where it can be used solely or in combination with other compounds include white colourants for pigments, corrosion inhibitors, cements and the dental cements. One such example is the use of Al(H2PO4)3 which are used in dental cements, metal coatings, glaze compositions and refractory binders.