Zinc Phosphate (Zn3 (PO4) 2): Structure, Properties, Uses

The zinc phosphate is an inorganic compound with the chemical formula Zn 3 (PO 4 ) 2 , but it occurs in nature in its tetrahydrate form, Zn 3 (PO 4 ) 2 ·4H 2 O in mineral hopeite and parahopeíta . Likewise, a basic variety of it is found in the mineral tarbutite, Zn 2 (PO 4 ) (OH). These minerals are formed by the oxidation of sphalerite in phosphate-rich waters.

All the known uses for this compound are based on Zn 3 (PO 4 ) 2 · 4H 2 O, since its water molecules give it the property of being a good fixing agent. Therefore, its anhydrous form as such lacks uses of great economic demand.

Fragments of zinc phosphate. Source: Chemicalinterest [Public domain]

As can be seen in the image above, zinc phosphate is a white solid, present as a powder or caked in small fragments. Its white color has been used in the formulation of cosmetic products, as well as in the preparation of dental cements and phosphate pozzolanic cements.

Zinc phosphate is an anticorrosive agent that has been used in processes for the electrodeposition of zinc minerals (hopeite and phosphophyllite) on the surface of steels.

Article index

  • one

    Structure

  • two

    Zinc phosphate properties

    • 2.1

      Physical appearance

    • 2.2

      Molar mass

    • 23

      Melting point

    • 2.4

      Boiling point

    • 2.5

      Density

    • 2.6

      Refractive index

    • 2.7

      Water solubility

    • 2.8

      Flash point

  • 3

    Applications

    • 3.1

      Cosmetics

    • 3.2

      Antibacterial agent

    • 3.3

      Dental cement

    • 3.4

      Anticorrosive coating

  • 4

    References

Structure

The formula Zn 3 (PO 4 ) 2 indicates that the Zn 2+ and PO 4 3- ions make up the salt in a 3: 2 ratio, which means that for every three Zn 2+ cations there are two PO 4 3- anions . These ions interact with each other electrostatically, thus establishing a strong ionic bond due to the magnitude of their charges. Both ions are polyvalent.

Thus, the Zn 2+ and PO 4 3- end up orienting themselves in space until they define an ordered and repetitive structure: a crystal of zinc phosphate. This crystal adopts a monoclinic structure, α-Zn 3 (PO 4 ) 2 . It appears to be capable of undergoing phase transitions to other polymorphic forms: β-Zn 3 (PO 4 ) 2 and γ- Zn 3 (PO 4 ) 2 , all dependent on temperature.

The three polymorphs are isostructural, differing only in the spatial orientation of their ions; that is, they have different spatial groups.

On the other hand, zinc phosphate tends to appear mainly as a hydrate: Zn 3 (PO 4 ) 2 · 4H 2 O, whose crystalline structure is also monoclinic. This time the ions are accompanied by four water molecules, which interact with them through dipole-ion forces and hydrogen bonds.

Zinc phosphate properties

Zinc phosphate. Ondřej Mangl [Public domain]

Physical appearance

Dusty white solid.

Molar mass

454.11 g / mol

Melting point

900 ºC

Boiling point

No information. This could be due to possible thermal decomposition, or the unavailability of pressure conditions for the saline liquid to boil.

Density

3,998 g / cm 3

Refractive index

1,595.

Water solubility

Insoluble. This is due in part to the ionic bond between the Zn 2+ and PO 4 3- ions , which increases the crystalline lattice energy when the salt dissolves in water.

Flash point

Zinc phosphate is a non-flammable substance.

Applications

The uses of zinc phosphate correspond to those of its tetrahydrate, Zn 3 (PO 4 ) 2 · 4H 2 O, since it is its predominant form and is even found in the hopeite and parahopeite minerals. Therefore, it is unknown whether its anhydrous form, Zn 3 (PO 4 ) 2 , has a specific use.

Cosmetics

Zinc phosphate has been used as a white pigment, replacing zinc and titanium oxides in cosmetic and beauty products. The material, soft to the touch, of small and round particles to cover the surface of the skin without seeping through its pores, is synthesized from a mixture of phosphoric acid, H 3 PO 4 , and zinc nitrate, Zn (NO 3 ) 2 .

Thus, zinc phosphate white pigments are prepared by varying the Zn / P ratios. For this, when mixing the reagents, variable amounts of H 3 PO 4 and Zn (NO 3 ) 2 are added , until the product with the best cosmetic properties is obtained.

In a study carried out by the Kyoto Prefectural University, they found that pigments prepared with a Zn / P ratio equal to 2/1, 1/1 and 3/2 showed the best reflectances; therefore, they brightened the faces of those who applied the cosmetic compared to the brightness of other formulations.

Antibacterial agent

Zinc phosphate nanoparticles are in the arsenal destined to fight microbes and, thus, be an alternative to the use of antibiotics. In this way, the constant and progressive resistance that bacteria develop towards antibiotics is reduced, while at the same time it seeks to reduce costs in the treatment of infectious diseases.

These nanoparticles have shown great antibacterial activity against coliform bacteria, a study that was verified in rats without causing oxidative stress.

Dental cement

Zinc phosphate is used to prepare phosphate cement, which is used in restorations of many materials; Among them, our own teeth, behaving like a dental cement that has been quite popular in dentistry for a long time. This phosphate cement is used to fix and join several solids at the same time.

Zinc phosphate is used for the manufacture of dental cements.

It is prepared by dissolving zinc and magnesium oxides in phosphoric acid, which is why Zn 2+ and Mg 2+ ions are present , forming complex structures. This dental cement is essential for the final cementation of the teeth. However, due to its acidity, polycarboxylate cement is used instead for patients who are too sensitive to it.

Anticorrosive coating

Similar to cement, the surface of steels can also be phosphated.

To do this, the steel pieces are introduced into an alkalinized phosphoric acid bath, and after supplying an electric current, a protective film composed of hopeite (Zn 3 (PO 4 ) 2 · 4H 2 O) and phosphophyllite is formed on their surface. (Zn 2 Fe (PO 4 ) 2 · 4H 2 O), the latter compound being the most resistant against strongly alkaline media.

The chemical reactions involved are the following:

3Zn 2+ + 2H 2 PO 4 + 4H 2 O → Zn 3 (PO 4 ) 2 · 4H 2 O + 4H +

2Zn 2+ + Fe 2+ + 2H 2 PO 4 + 4H 2 O → Zn 2 Fe (PO 4 ) 2 · 4H 2 O + 4H +

The problem with these coatings lies in their degree of porosity, as it leaves exposed flanks where the steel can suffer corrosion.

On the other hand, pozzolanic cement containing zinc phosphate has been used for the development of concretes that are more resistant to corrosion.

In general, the anticorrosive property of zinc phosphate has been used to serve as coatings for the walls before applying the layers of paint, so that they last longer and show better colors.

References

  1. Shiver & Atkins. (2008). Inorganic Chemistry . (Fourth edition). Mc Graw Hill.
  2. Wikipedia. (2020). Zinc phosphate. Recovered from: en.wikipedia.org
  3. Elsevier BV (2020). Zinc Phosphate. ScienceDirect. Recovered from: sciencedirect.com
  4. National Center for Biotechnology Information. (2020). Zinc Phosphate. PubChem Database., CID = 24519. Recovered from: pubchem.ncbi.nlm.nih.gov
  5. Aref M. al-Swaidani. (2018). Inhibition Effect of Natural Pozzolan and Zinc Phosphate Baths on Reinforcing Steel Corrosion. doi.org/10.1155/2018/9078253
  6. Onoda, H., & Haruki, M. (2014). Mixing ratio of zinc nitrate and phosphoric acid for preparation of zinc phosphate white pigments. Ceramics, 60 (355), 392-396. dx.doi.org/10.1590/S0366-69132014000300010
  7. Horky, P., Skalickova, S., Urbankova, L. et al. (2019). Zinc phosphate-based nanoparticles as a novel antibacterial agent: in vivo study on rats after dietary exposure. J Animal Sci Biotechnol 10, 17. doi.org/10.1186/s40104-019-0319-8

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