Zinc Bromide: Properties, Preparation & Lab Protocols

Zinc bromide is a versatile, highly soluble inorganic salt widely employed in research laboratories as a Lewis acid catalyst, electrolyte component, and synthetic reagent.

Chemical Formula

ZnBr₂

Molecular Weight

225.18 g/mol

Appearance

White to off-white hygroscopic crystalline powder

Solubility in Water

Highly soluble (>400 g/100 mL at 20 °C)

Melting Point

394 °C

CAS Number

7699-45-8

Storage Condition

Store in a tightly sealed container, cool and dry, away from moisture

Overview and Physicochemical Properties

Zinc bromide (ZnBr₂, CAS 7699-45-8) is an inorganic zinc salt of hydrobromic acid. It appears as a white to off-white crystalline powder with a molecular weight of 225.18 g/mol. One of its most notable characteristics is its extreme hygroscopicity: it absorbs atmospheric moisture rapidly and must be stored in airtight containers under anhydrous conditions.

ZnBr₂ has an exceptionally high water solubility (reported to exceed 400 g per 100 mL at 20 °C), making it suitable for preparing high-concentration aqueous electrolyte solutions. Its melting point is approximately 394 °C and it boils at around 697 °C. In solution, zinc bromide behaves as a Lewis acid, coordinating with electron-pair donors, which underpins many of its synthetic and catalytic applications.

The compound adopts a tetrahedral coordination geometry in the solid state and exhibits polymorphic forms depending on preparation conditions. Researchers should note that commercially supplied ZnBr₂ may contain variable levels of hydration; verification of actual purity and water content via thermogravimetric analysis (TGA) or Karl Fischer titration is recommended prior to moisture-sensitive applications.

Safety, Handling, and Storage

Personal Protective Equipment (PPE): Always wear chemical-resistant gloves (nitrile or neoprene), safety goggles, and a laboratory coat when handling zinc bromide. A dust mask or half-face respirator (P2/N95 rated) is recommended when weighing or transferring the powder to minimize inhalation of fine particulates.

Hazard Summary: ZnBr₂ is harmful if swallowed, inhaled, or absorbed through skin. It is classified as an irritant to skin, eyes, and the respiratory tract. Zinc compounds in general may pose aquatic toxicity risks; avoid releasing solutions to drains without appropriate neutralization and disposal according to local regulations.

First Aid: In case of skin or eye contact, flush immediately with copious amounts of water for at least 15 minutes and seek medical advice. If inhaled, move the affected individual to fresh air immediately.

Storage: Store in a tightly sealed, moisture-proof container in a cool (15–25 °C), dry, well-ventilated area. Keep away from strong oxidizing agents, strong acids, and strong bases. Segregate from incompatible materials. Check container integrity regularly, as ZnBr₂ can deliquesce and corrode metal fittings.

Note: Consult the current Safety Data Sheet (SDS) for complete hazard information before use.

Solution Preparation Protocol

The following protocol describes the preparation of a 1 M aqueous zinc bromide solution as a representative example. Adjust quantities proportionally for other concentrations.

1. Materials required: Zinc bromide (Aladdin, SKU Z1363855), ultrapure water (18.2 MΩ·cm), volumetric flask (appropriate volume), analytical balance (±0.1 mg), magnetic stir bar, and a sealed weighing vessel.
2. Calculation: To prepare 100 mL of 1 M ZnBr₂ solution: mass required = 1 mol/L × 0.100 L × 225.18 g/mol = 22.52 g. If working with a hydrated form, correct for water content using TGA or Karl Fischer data.
3. Procedure:
   a. Pre-dry the zinc bromide at 100–120 °C under vacuum for 2–4 hours if anhydrous stock is required; allow to cool in a desiccator before weighing.
   b. Tare the weighing vessel and accurately weigh 22.52 g of ZnBr₂ under ambient conditions, working quickly to minimize moisture uptake.
   c. Transfer the weighed salt to a 100 mL volumetric flask containing approximately 60 mL of ultrapure water.
   d. Stir gently until complete dissolution (dissolution is exothermic; allow the solution to cool to room temperature).
   e. Make up to the 100 mL mark with ultrapure water and mix thoroughly.
   f. Transfer to a sealed storage vessel and label with concentration, date, and preparer's name.
4. Verification: Confirm concentration by ICP-OES (for Zn²⁺) or gravimetric methods if high accuracy is required.

Important: ZnBr₂ solutions are mildly acidic due to hydrolysis of Zn²⁺. Adjust pH as required by your application.

Key Laboratory Applications

Zinc bromide is a multifunctional reagent with a broad range of research applications:

• Lewis Acid Catalysis: ZnBr₂ acts as a mild Lewis acid catalyst in organic synthesis reactions including Friedel–Crafts alkylations, Diels–Alder cycloadditions, and the activation of epoxides. It is often preferred over stronger Lewis acids when milder conditions are needed to improve selectivity.
• Zinc–Bromine Flow Batteries: Concentrated ZnBr₂ solutions (typically 2–4 M) serve as the primary electrolyte in zinc–bromine redox flow battery research, a subject of active investigation for grid-scale energy storage applications.
• Protecting Group Chemistry: ZnBr₂ in combination with appropriate nucleophiles is used for selective removal of protecting groups (e.g., t-butyl ethers and certain carbamates) under mild conditions in multi-step synthesis.
• Coordination Chemistry and Materials Science: ZnBr₂ is employed as a precursor or co-ligand in the synthesis of zinc coordination polymers, metal–organic frameworks (MOFs), and luminescent materials.
• Analytical Chemistry: Dense ZnBr₂ solutions (density up to ~2.0 g/mL at high concentration) have historically been used as heavy liquids for density separations of minerals and particulates.

Troubleshooting and Quality Notes

Researchers frequently encounter the following issues when working with zinc bromide:

• Caking and Deliquescence: If the solid has caked or appears wet, it has absorbed significant moisture. Re-dry under vacuum at 100–120 °C before use, or use freshly opened stock. Store remaining material immediately with a desiccant.
• Discoloration of Solutions: Pale yellow or brown discoloration of ZnBr₂ solutions may indicate trace bromine (Br₂) liberation due to oxidation or contamination. Use freshly prepared solutions and store protected from light. Discolored solutions should be discarded.
• Incomplete Dissolution: Although ZnBr₂ is highly water-soluble, caked or partially hydrated material may dissolve slowly. Gentle heating (40–60 °C) with stirring typically resolves this; avoid prolonged boiling which may cause hydrolysis.
• pH Considerations: Freshly prepared 1 M ZnBr₂ solution typically exhibits a mildly acidic pH (approximately 4–6). Applications requiring neutral or buffered conditions should include a pH adjustment step.
• Corrosivity: Bromide-containing solutions are corrosive to many metals. Use glass, PTFE, or polypropylene labware and avoid stainless steel fittings where prolonged contact is expected.

Frequently asked questions

How should zinc bromide be stored to prevent moisture absorption?

Zinc bromide is extremely hygroscopic and must be stored in a tightly sealed, airtight container with a desiccant (e.g., silica gel or molecular sieves) in a cool, dry environment. Opened containers should be resealed immediately after use. For long-term storage, consider packaging under an inert atmosphere (nitrogen or argon).

Is zinc bromide compatible with stainless steel equipment?

Zinc bromide solutions are generally not recommended for prolonged contact with stainless steel, as bromide ions are highly corrosive to most grades of stainless steel and can cause pitting corrosion. Glass, PTFE, polypropylene, or high-grade Hastelloy C are preferred materials for labware and fittings when working with ZnBr₂ solutions.

Can zinc bromide be used as a catalyst in organic synthesis?

Yes, ZnBr₂ is a well-established mild Lewis acid catalyst used in a variety of organic transformations, including Friedel–Crafts reactions, epoxide ring-opening, and cycloadditions. Its activity is generally milder than AlCl₃ or BF₃, making it suitable for reactions where harsh conditions would cause undesired side reactions or substrate decomposition.

What concentration of zinc bromide is typically used in flow battery electrolytes?

In zinc–bromine redox flow battery research, ZnBr₂ electrolyte concentrations typically range from 2 to 4 M. The exact concentration is optimized based on factors including ionic conductivity, viscosity, and the specific cell design used in a given study.

How do I remove residual moisture from zinc bromide before a moisture-sensitive reaction?

Pre-dry ZnBr₂ under vacuum at 100–120 °C for 2–4 hours. Allow the dried material to cool in a sealed desiccator before transferring to a dry, inert-atmosphere glove box or Schlenk line for weighing and use. Confirm dryness by TGA or Karl Fischer titration if quantitative anhydrous purity is required.

References

1. Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd ed.; Butterworth-Heinemann: Oxford, 1997; pp 1220–1222.
2. Lide, D. R. (Ed.) CRC Handbook of Chemistry and Physics, 84th ed.; CRC Press: Boca Raton, FL, 2003.
3. Lex, A. et al. Zinc–bromine flow batteries: electrolyte formulation and performance. Journal of Power Sources (generic reference — specific article not identified).

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