January 16, 2005

[Basically a direct copy from my Sciencemadness post]

Premise:
Upon mixing HBr(aq) with KBrO3 a top water layer and a bottom bromine layer was observed, the bromine appeared to be clean, no precipitate, and appeared in large quantity indicating a good yield. When the reaction is done with NaBr and H2SO4 with KBrO3 dirty bromine (almost a suspension of bromine with precipitate) appears in the bottom. My attempt today was to make a solution of HBr in situ with NaBr and H2SO4, cool to allow the sulfate to precipitate, then add the bromate in the cold weather.

Equation:
10NaBr + 2KBrO3 +6H2SO4 ---> 6Br2 + 5Na2SO4 + K2SO4 + 6H2O

Procedure:
To 200 ml of room temperature distilled water in a 1 L Erlenmeyer was added 120 g (~1 mol) NaBr*xH2O, with slight agitation the sodium bromide was dissolved in 7 minutes. 92 ml (a slight excess) of concentrated H2SO4 was measured out in a graduated cylinder and was added in several portions with significant swirling to the sodium bromide solution. A slight red color from bromine evolution was observed and the reaction mixture heated to an estimated 40 C once all the H2SO4 had been added. The mixture was allowed to cool for two hours outside at -10C. Upon inspection of the mixture it was noted that it was now a slush, the expected precipitate of Na2SO4 had either been over shot or occluded by the mixture freezing entirely, it was wrong to assume that the Na2SO4 would precipitate out nicely and the rest of the solution would remain liquid due to the freezing point depressive abilities of the HBr contained therein.

The slush and chunks were broken up to a homogenous consistency and 33.5 g of KBrO3 was weighed out, roughly .2 mol and half of this was added to the slush mixture. The addition produced a hissing sound and the mixture quickly started to turn red from bromine evolution. After the fist half had been added a watch glass was placed on top, lifting the mixture drops of bromine were evident on the bottom, at these temperatures the bromine was significantly less volatile and only small amounts wafted off the top.

The mixture was allowed to cool for twenty minutes further outside. The addition of the second half of the bromate was smoother then the first addition, no sound or other indicative factors of a fast reaction. The color slightly deepened and bromine was more pronounced at the opening of the container and little droplets of it clung to the sides at the water line. The top was covered yet again and the mixture was allowed to react to completion over the course of another 20 minutes.

The bromine at the bottom was heavily contaminated with precipitate, separation may have been accomplished by straining through glass wool however due to the temperature outside I was ready to call it quits so I had to find something to store it in. The entire mixture was poured into a glass reagent bottle which was accompanied by significant fuming. After the bottom precipitate layer was poured in there was sediment left in the container, shaking the flask the sediment sounded distinctly like stones in the flask, removing one and prodding it with a glass stirring rod they were found to be very hard, their composition is unknown.

In the smaller reagent bottle the bromine at the bottom seems to be divided into layers, a top green/red layer, a bottom bromine/sediment layer, if this separation persists through the winter actions will be taken to ascertain the identity of the layer constituents. Theoretically roughly 80 grams of bromine (26 ml), although not measured directly the container appears to contain at least 20 ml of bromine, which would be quite good considering the bromine lost to evaporation and solvated in the water above.

Conclusion:
HBr should be previously purified to give a somewhat pure bromine product in one shot. Filtration through glass frit/wool should be considered if taking this on following this procedure to remove suspended particulate, distillation is always an option. Unlike a previous attempt, bromine hydrate was not a problem. Overall this reaction quickly gave quantities of bromine in a high yield without the need for distillation, if a pipette were to have glass wool inserted into the intake then the filtration may be accomplished by simply withdrawing the bromine from the container. 

The picture above was taken the next day after allowing it to cool all night in the cold, here are my observations.  After sitting for 15 hours or so the bromine has separated into a distinct phase, the particulate has frozen into a layer above it and does not travel with the bromine. It is now believed the solid rock like pieces left in the reaction flask are pieces of bromate that went into the mixture too quickly and formed an inert crust. I plan on just suctioning off the bromine only a thin sheet of ice stands in my way, the mixture is for the most part slushy. The above container has a volume of 450 ml.

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