Electrolytic Bromine

Preparation of one of the most interesting elements - bromine by its precipitation in yellow water solution prepared with electrolysis

Scientific name: Electrolytic oxidation of NaBr to NaBrO3 with following con-proportionation of bromine

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Electrolytic Bromine, Part 1

by mrhomescientist

Electrolytic Bromine, Part 2

by mrhomescientist

Safety

Wear eye protection goggles. Potassium dichromate is carcinogenic. Use only glass glassware. Beginning with step 12 carry out the experiment only in a fume hood or outside. Bromine forms very toxic fumes!

Always follow general safety recommendations. Please note that conducting chemistry experiments you must comply with the relevant legal procedures in your country.

Reaction formula

2 H2O + 2 e- → 2 OH- + H2

2 Br- - 2 e- → Br2

3 Br2 + 6 OH- → 5 Br- + BrO3- + 3 H2O

5 KBr + KBrO3 + 6 NaHSO4 → 3 Br2 + 3 K2SO4 + 3 Na2SO4 + 3 H2O

Step-by-step instruction

WARNING! This experiment is dangerous! You should NOT perform this at home. ONLY carry out this experiment, if you are a trained chemist, and you understand local safety and legal requirements, which are required to perform such experiments

  1. Dissolve 100 g of sodium bromide in 300 ml of water using a large beaker.
  2. Dissolve 0.5 g of potassium dichromate in 5 ml of water using a small beaker. The solution should be orange.
  3. Pour the solution of potassium dichromate in the beaker with a solution of sodium bromide. It is necessary to completely transfer the potassium dichromate, so rinse the glass with it with small amounts of distilled water. The resulting solution must be yellow.
  4. Assemble the electrolytic cell. For this purpose, it is necessary to pour the solution into a convenient container and plunge 2 graphite electrodes connected to a DC power supply into it.
  5. Apply voltage to the circuit. Set the voltage at which the current is equal to 1.5 amperes (about 4-5 volts, depending on the distance between the electrodes).
  6. Observe the release of gas bubbles on one of the electrodes. Probably, this gas is hydrogen and it is released at the anode.
  7. Meanwhile you can observe the formation of a small amount of bromine and its accumulation at the bottom layer in the form of bromine water.
  8. After 3 hours the voltage should be turned off and electrodes should be taken out of the solution. Mix the layers until these layers are completely mixed. You need to observe no evidence of reaction between alkali and bromine.
  9. Start the process of electrolysis again (see paras. 4-5).
  10. After 23 hours turn off the voltage and dismantle the electrolytic cell. If necessary (significant accumulation of bromine water) sometimes you need to stir the solution (see paras. 8-9).
  11. Filter the resulting solution to a stoppered bottle to dispose of particles of the graphite electrode using a glass filter paper and a funnel.
  12. Dissolve 150 g of sodium bisulfate in 300 mL of distilled water using a round bottom flask.
  13. Slowly add a solution of sodium bisulfite to the resulting solution of potassium bromate. After addition of each portion, the reaction mixture should be stirred thoroughly. Avoid the excessive heating of the reaction mixture and close the bottle with a stopper every time.
  14. Observe the darkening of the reaction mixture and the formation of bromine vapor over the solution.
  15. Leave the closed bottle with the solution of bromine in cold water overnight.
  16. Observe the segregation of the reaction mixture. A small amount of bromine forms at the bottom.
  17. Drain most of the top layer. Transfer the remaining part of the mixture to a separate funnel.
  18. Separate the lower layer into a small round-bottomed flask.
  19. Cool the flask with ice. Transfer bromine into an ampoule and seal it.
  20. All of the used dishes should be rinsed with sodium thiosulfate solution to remove residual bromine and its vapors.

Scientific background

For the production of bromine from bromide an oxidant is required. For the oxidation process, electrolysis can be used. Thus, bromate anion is formed. Potassium dichromate was added to the solution for the reason that the formed bromate anion on one of the electrodes can be restored back to bromide on the other electrode. If potassium dichromate isn't added, electrolysis efficiency is practically zero. During the electrolysis process, a small amount of bromine is also released. Its accumulation occurs due to the fact that the heavy bromine water is in the bottom layer and the lighter alkaline solution is at the top. Consequently, the reaction does not proceed. That is why, it is necessary to mix the solution.

Only 1/6 of the bromide passes into bromate. However, it is the ratio of the reactants required for the next step.

The total time of the electrolysis is calculated according to the formula. T= 1.5FM/I = 26 h

The use of hydrochloric acid instead of sodium bisulfite is undesirable due to the fact that during the last stage interhalides (Br-Cl) can be formed. The acidic solution of bromide and bromate cause con-proportionation of bromine.

Danger:
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Published on 30 June 2015

  • Fire
  • Heating with fire
  • Explosion
  • Poisoned gas
  • Organic
  • Electricity
  • Solution
  • Oxidation reduction
  • Color change
  • Precipitate
  • Gassing
  • Catalyst