2020-07-19

Bromine isolation


Today we will proceed with isolation of bromine. We will need it for further syntheses.

Reagents:
    52.4g  KBr (0.44 mole)
    43.2g H2SO4 98%  (0.44 mole, 24ml)
    50.5g H2O2 30% (0.44 mole, 45.5ml)
    80ml of distilled water
    H2SO4 98% for dehydration
    NaOH solution (trap)
    Na2 S2O3 solution

Procedure:
52.4 grams of KBr was dissolved in 80ml of distilled water and poured to 250ml 3-neck round bottom flask.  Simple distillation setup was mantled with a trap containing NaOH solution connected to the vacuum adapter. Then 43.2 grams of concentrated sulphuric acid were added slowly.

Addition funnel with pressure equalizer was mounted on one of the remaining necks and was charged with 50.5 grams of 30% hydrogen peroxide.The neck which was unused was closed by the stopcock.

Receiver flask was submerged in ice-cold water, ice-cold water was also running through the condenser.
Hydrogen peroxide was added drop-wise. During addition, the temperature in the flask raised so much, that external heating was not needed. This reaction is highly exothermic and pouring it fast is a recipe for huge problems. After the addition has completed heating mantle was enabled and distillation was continued until the content of reaction flask changed the colour to yellow.




During the distillation, another portion of concentrated sulphuric acid was prepared and cooled down in ice bath. After the distillation has completed, the acid was poured in separatory funnel, then the distillate was poured also. The content was shaken and after 5 minutes the bottom layer containing elementary, dehydrated bromine was collected to the beaker. Then it was pipetted to glass ampoule and sealed. The ampoule was placed in container secured with sodium thiosulphate in case of any leaks.



Stoichiometry of this reaction: 2KBr+2H2SO4+H2O2→Br2+2KHSO4 +2H2O

Theoretical yield – 0.22 mole of Br_2 (35.156 grams) – based on starting KBr
Practical yield – 0.115 mole (18.4 grams)

Notes: yield could be improved by providing better cooling. The amount of ice which were used for cooling the reaction flask and condenser water was too small and huge amount of bromine evaporated and was trapped in NaOH solution.

Safety:
    bromine is corrosive and toxic
    sulphuric acid, sodium hydroxide and hydrogen peroxide are corrosive
Inhalation of bromine vapours can be fatal – the procedure must be performed in fume hood or well-ventilated area with respirator. Storage of bromine is problematic. The only one reliable solution is sealed glass ampoule.
In case of any bromine spills it is necessary to have sodium thiosulphate solution prepared. It will neutralize elementary bromine to bromine anions which are non-toxic.
Dismantling the apparatus is easy. Bromine has strong vapour pressure and it need just few minutes to evaporate. After that the apparatus should be rinsed with sodium thiosulphate solution.


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