Test for Bromide Radical

Theoretical Background / Context

• Bromide is the anion of bromine anion that possesses -1 charge. 
• Bromide forms the acid radical (negative portion) of some inorganic salts such as sodium bromide, potassium bromide, calcium bromide, etc.

 Abundance of bromide in nature: 

• Bromide is present in nature in seawater with a concentration of 65 mg/L. 
• This is considered to be around 0.2% of all dissolved salts in seawater. 
• High levels of bromide salts are found in seafoods and deep sea plants. 
• Crystalline silver bromide, commonly known as bromargyrite, is one of the most commonly known bromide salts although it is very rare. 
• Another commonly known bromide salt is sodium bromide.

 Preparation of Sodium Bromide: 

• Sodium bromide (NaBr) can be prepared through the reaction of sodium hydroxide and hydrogen bromide as the following reaction:

NaOH + HBr → NaBr + H2O

 Chemical Properties of Sodium Bromide: 

• Sodium bromide is a white crystalline inorganic salt that dissolves in water producing colorless aqueous solution. It is both chemically and thermally stable. 
• Sodium bromide is used as a bromine source and a brominating agent in organic synthesis.

 Uses of Sodium Bromide: 

• Sodium bromide is used as a disinfectant for swimming pools and as a hypnotic in sedatives preparation.
• It is also used in the preparation of dense fluids in oil wells. In addition, it has been used in photography, films, dyes, and other industries.
•  Bromide ion is classified as a member of acidic radicals of the second group in which concentrated sulphuric acid is used as the group reagent. 
• Sulphuric acid displaces bromide ions in its salts leading to the liberation of reddish-brown HBr gas. 
• In addition, potassium permanganate is used as a reagent for detecting the presence of the bromide ion. 
• Furthermore, bromide could be detected through some confirmatory tests using silver nitrate solution or lead acetate solution since they react together forming silver chloride or lead chloride as pale yellow or sugary white precipitates, respectively. 
• This returns back to the low solubility product of silver and lead chloride salts so they precipitate very easily at very low concentrations.
• Finally, the cyclohexane test for bromide ions is carried out as a specific test for detection of bromide ion, where the result gives an orange color in the cyclohexane layer.

Principle of Work

• In this experiment, bromide ion in sodium bromide is detected through some identification and confirmatory tests. 
• In addition, these tests can be used to differentiate between halide class and other acid radical classes.
• Moreover, confirmatory test for bromide ion and specific tests are used to differentiate between the presence of different halides.

First: Dry Heat Test: 

• In this test, a sample of the bromide salt is heated to be tested for its ability to decompose into bromine gas.

Second: Concentrated Sulphuric Acid Test: 

• It depends on the fact that conc. sulphuric acid can displace bromide ions in its sodium salt-forming sodium bisulfate salt and reddish-brown hydrogen bromide gas.

The reaction of Sodium bromide with Sulphuric Acid:

NaBr + H2SO4 → NaHSO4 + HBr↑

Third: Potassium Permanganate Test: 

• It depends on the fact that permanganate oxidizes bromide in an acidified medium into bromine gas, while permanganate gets reduced and discolored.

Fourth: Silver Nitrate Test for bromide:

• Silver nitrate solution is added to a solution of sodium bromide leading to the precipitation of silver bromide salt as a yellow precipitate due to its low solubility product.
• The reaction of the test is:

NaBr + AgNO3 → NaNO3 + AgBr↓

Fifth: Lead Acetate Test: 

• Lead acetate solution is added to sodium bromide solution resulting in precipitation of lead bromide as a sugary white precipitate due to low solubility product as shown in the following chemical reaction that shows lead bromide formula :

2 NaBr + (CH3COO)2Pb → 2 CH3COONa + PbBr2↓

Sixth: Cyclohexane Test: 

1. Cyclohexane test is considered as a specific bromide test for detection of the presence of bromide ion as the acid radical of a salt. 
2. The test depends on adding cyclohexane to a sodium bromide solution which results in the formation of a separate cyclohexane layer above the salt aqueous solution. 
3. Afterward, adding drops of chlorine water will displace bromide in its salt leading to the liberation of bromine gas that imparts an orange color to the cyclohexane layer through inorganic salt analysis for bromide radical.

NaBr + Cl2 → NaCl + Br2 ↑ Reddish-brown vapors