Preparation of Bromine Water

Introduction

Prepare small quantities of bromine water from common laboratory chemicals.

Materials

Safety Precautions

Please read all safety precautions before proceeding with this activity.

• Only prepare as much bromine water as required for immediate use. Bromine water is difficult to store due to the reactive and corrosive properties of bromine. The following procedure to make bromine water is easy and takes only a few minutes. Please consider making bromine water fresh every time you use it.
• Prepare the bromine water in a working fume hood to avoid unnecessary exposure to bromine vapor.
• Keep a supply of sodium thiosulfate solution (hypo) on hand as a neutralizer whenever bromine water is prepared or used.
• Sodium bromide is mildly toxic by ingestion or inhalation; LD₅₀ 3,500 mg/kg.
• Sodium hypochlorite solution is a corrosive liquid, causes skin burns and reacts with acid to evolve chlorine gas; moderately toxic by ingestion and inhalation. In this lab sodium hypochlorite is reacted with hydrochloric acid to generate small amounts of very dilute halogen solutions for use by the students. This step should only be performed by the teacher and in the amounts indicated. Follow the directions carefully and work in an operating fume hood.
• Hydrochloric acid is toxic by ingestion or inhalation, corrosive to skin and eyes.
• Bromine water is toxic by inhalation and ingestion; severe skin and eye irritant, may cause skin burns or blindness: strong oxidizing agent.
• Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Bromine water may be reduced by sodium thiosulfate according to Flinn Suggested Disposal Method #12a.

Procedure

Preparing 100 mL

1. Dissolve 2.2 g of sodium bromide in 21.4 mL of 1 M hydrochloric acid.
2. Place solution in an amber glass bottle.
3. Add 15.2 mL of sodium hypochlorite solution to the bottle. Swirl gently to mix the reactants.
4. Dilute with 63 mL of deionized or distilled water to prepare 0.5 v/v% bromine water (0.5 mL Br₂ per 100 mL solution).
5. Place white polyethylene cap on bottle until use. Place bromine water label on bottle.

Preparation of Various Quantities of Bromine Water

Discussion

The preparation of bromine water is the result of several redox reactions. In the first step, chlorine, Cl₂, is formed from sodium hypochlorite and hydrochloric acid (Equation 1).(This is why it is important never to mix bleach with a cleaner containing acid.) In the second step (Equation 2) , the chlorine generated in the first step replaces the bromine in the sodium bromide producing elemental bromine in solution. The net balanced equation is shown in Equation 3.

The solubility of bromine in water is about 0.21 mol/L, which is equivalent to 3.36 g/100 mL or about 1.1 mL/100 mL. (Density of bromine is 2.928 g/mL.) The recipe described above produces 10.7 moles, or about 0.58 mL of bromine. Most laboratory activities work well with a 0.5 v/v% bromine water. If a more concentrated solution is required, add less water in the final step. At least 13 mL of water is required to make a saturated solution. More dilute solutions are safer to handle and store.

References

Special thanks to Walter Rohr, Eastchester High School, Eastchester, NY, for bringing this procedure to our attention.