Chemistry:Zinc phosphate

Zinc phosphate

Names
IUPAC name Zinc phosphate
Identifiers
CAS Number	7779-90-0 Y
3D model (JSmol)	Interactive image
ChemSpider	22927 Y
PubChem CID	24519
RTECS number	TD0590000
UNII	1E2MCT2M62 Y
InChI InChI=1S/2H3O4P.3Zn/c2*1-5(2,3)4;;;/h2*(H3,1,2,3,4);;;/q;;3*+2/p-6 Y Key: LRXTYHSAJDENHV-UHFFFAOYSA-H Y InChI=1/2H3O4P.3Zn/c2*1-5(2,3)4;;;/h2*(H3,1,2,3,4);;;/q;;3*+2/p-6 Key: LRXTYHSAJDENHV-CYFPFDDLAR
SMILES [Zn+2].[Zn+2].[Zn+2].[O-]P([O-])(=O)[O-].[O-]P([O-])([O-])=O
Properties
Chemical formula	H4O12P2Zn3
Molar mass	454.11 g·mol−1
Appearance	white solid
Density	3.998 g/cm3
Melting point	900 °C (1,650 °F; 1,170 K)
Boiling point	158 °C (316 °F; 431 K)
Solubility in water	insoluble
Magnetic susceptibility (χ)	−141.0·10−6 cm3/mol
Refractive index (nD)	1.595
Structure
Crystal structure	monoclinic
Thermochemistry
Std enthalpy of formation (ΔfH⦵298)	− 2891.2 ± 3.3
Hazards
NFPA 704 (fire diamond)	0 2 0
Flash point	Non-flammable
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Zinc phosphate is an inorganic compound with the formula Zn₃(PO₄)₂. This white powder is widely used as a corrosion resistant coating on metal surfaces either as part of an electroplating process or applied as a primer pigment (see also red lead). It has largely displaced toxic materials based on lead or chromium, and by 2006 it had become the most commonly used corrosion inhibitor. Zinc phosphate coats better on a crystalline structure than bare metal, so a seeding agent is often used as a pre-treatment. One common agent is sodium pyrophosphate.

Minerals

Natural forms of zinc phosphate include minerals hopeite and parahopeite. A somewhat similar mineral is natural hydrous zinc phosphate called tarbuttite, Zn₂(PO₄)(OH). Both are known from oxidation zones of Zn ore beds and were formed through oxidation of sphalerite by the presence of phosphate-rich solutions. The anhydrous form has not yet been found naturally.

Use

Dentistry

Zinc phosphate cement is the classic dental cement par excellence. It is commonly used for luting permanent metal and zirconium dioxide restorations and as a base for dental restorations. Zinc phosphate cement is used for cementation of inlays, crowns, bridges, and orthodontic appliances and occasionally as a temporary restoration.

It is prepared by mixing zinc oxide (ZnO) and magnesium oxide (MgO) powders with a liquid consisting principally of phosphoric acid, water, and buffers. It is the standard cement to measure against. It has the longest track record of use in dentistry.

In recent years, newer adhesive cements on a different chemical basis have been added (e.g. glass ionomer cement), but they have not displaced the classic phosphate cement, which continues to hold its own in the dental market with its simple and safe processing and good price-performance ratio. Zinc phosphate cement has only a low flexural strength and it does not stick to the dentin (it is a cement and not an adhesive).

Zinc phosphate cement has high compressive strength, low film thickness, minimal setting shrinkage and thermal expansion and is biocompatible. Compared to other luting materials such as glass ionomer cement or composites, zinc phosphate cement is less sensitive to moisture. The excess produced during the cementation of dental restorations can be easily removed.

Zinc phosphate cement has a high adhesive capacity to the tooth, metal, or even zirconium oxide.

Despite its strong acidity, zinc phosphate cement does not damage the pulp (or the tooth nerve) during the setting phase. It is therefore used as liner to protect the pulp under composite fillings.

Well-known dental brands in Germany and the world for zinc phosphate cement are Harvard cement and Hoffmann's cement. Otto Hoffmann invented this cement in 1892 and had it patented. Until the beginning of the First World War, he had a worldwide monopoly position with his cement.