Zinc chloride hydroxide monohydrate or more accurately pentazinc dichloride octahydroxide monohydrate is a zinc hydroxy compound with chemical formula Zn5(OH)8Cl2·H2O. It is often referred to as tetrabasic zinc chloride (TBZC), basic zinc chloride, zinc hydroxychloride, or zinc oxychloride. It is a colorless crystalline solid insoluble in water. Its naturally occurring form, simonkolleite, has been shown to be a desirable nutritional supplement for animals.
Natural occurrence The naturally occurring mineral form, simonkolleite, was described as a new mineral in 1985 for samples collected at Richelsdorf, Germany. It is a rare secondary mineral formed by weathering of zinc-bearing slag, and is associated with native zinc, hydrocerussite, diaboleite, zincite and hydrozincite. It is named after Werner Simon and Kurt Kolle, Mineral collectors of Cornberg, near Michelsdorf who submitted the samples for investigation. Simonkolleite is frequently found as a corrosion product of Zinc bearing metals.
Structure Simonkolleite is rhombohedral, space group R3m. There are two crystallographically distinct zinc sites in Simonkolleite, both of which are fully occupied by zinc. The Zn(1) site is coordinated by six hydroxyl (OH) groups in an octahedral geometry [Zn(OH)6]. The Zn(2) site is coordinated by three OH groups, and one Cl atom in a tetrahedral geometry [Zn(OH)3Cl]. The [Zn(OH)6] octahedra form an edge-sharing dioctahedral sheet similar to that observed in dioctahedral micas. On each site of the vacant octahedron, a [Zn(OH)3Cl] tetrahedron is attached to three anions of the sheet and points away from the sheet. Intercalated between adjacent sheets are interstitial water (H2O) groups. The sheets are held together by hydrogen bonding from OH groups of one sheet to Cl anions of adjacent sheets, and to interstitial H2O groups. The [Zn(OH)6] octahedra have four long equatorial bonds (at 2.157 Å) and two short apical bonds (at 2.066 Å). This apical shortening is a result of the bond-valence requirements of the coordinating OH groups and the connectivity of polyhedra in the structure. The equatorial OH groups [O(1)H] are coordinated by two Zn(1) cations and one Zn(2) cation, whereas the apical OH groups [O(2)H] are coordinated by three Zn(1) cations. As Zn(1) is six-coordinated and Zn(2) is four-coordinated, the local bond-valence requirements require the Zn(1)-O(1) bonds to be considerably longer than the Zn(1)-O(2) bonds. The [Zn(OH)3Cl] tetrahedron has three short Zn(2)-O(1) bonds (at 1.950 Å) and one long Zn(2)-Cl bond (2.312 Å) (Figure 1).
Properties Simonkolleite is colorless, forms tabular hexagonal crystal up to 1 mm in diameter, and has perfect cleavage parallel to (001). Thermal stability studies have shown that simonkolleite decomposes to ZnO at several stages upon heating (eq. 1-3). The decomposition starts with loss of a single mole of the lattice water. Further dehydration at 165−210 °C produces a mixture of ZnO and an intermediate Zn(OH)Cl. At 210−300 °C, the intermediate Zn(OH)Cl decomposes to ZnO and ZnCl2. At higher temperature, volatilization of zinc chloride occurs, leaving a final residue of zinc oxide.
The dehydrated mixture (Zn(OH)Cl and ZnO) is easily rehydrated and converted back to simonkolleite upon exposure to cool moist air (eq. 4).
Simonkolleite is virtually insoluble in water and organic solvents, soluble in mineral acids yielding the corresponding zinc salts (eq. 5), soluble in ammonia, amine and EDTA solutions under complex formation. It can easily be converted to zinc hydroxide by reacting with sodium hydroxide (eq. 6). Its pH in water is 6.9 measured by EPA method SW846-9045.
Preparation
From hydrolysis of ZnCl2 Basic zinc chloride can be prepared by hydrolysis of a ZnCl2 solution in the presence of a base such as sodium hydroxide or ammonia (eq. 7-8).
Simonkolleite nanodisks with a width of 40 nm have been successfully synthesized via a hydrothermal method using zinc chloride and ammonia as the starting materials.
From reaction of ZnCl2 with ZnO Basic zinc chloride can be synthesized from the reaction of a ZnCl2 solution with ZnO (eq. 9).
It can be synthesized from nano-sized ZnO particles aged in aqueous ZnCl2 solution at 6–140 °C for 48 h. Elevating the aging temperature increases the crystallinity of basic zinc chloride.
Applications
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