Sciencemadness Discussion Board

Available Bromine

Retard-3000 - 8-11-2011 at 06:43

Pool/Spa bromine granules consist of sodium bromide and sodium/potassium dichloroisocyanurate, my understanding is that when the granules come into contact with water the dichloroisocyanurate slowly decomposes releasing chlorine gas which displaces the bromide ions forming elemental bromine. My question is how much elemental bromine can be formed from these granules ?

Mixell - 8-11-2011 at 07:54

Stoichiometry is your friend.
If the limiting factor is the DCC salt, then for every mole of DCC a mole of Br2 forms.
If the sodium bromide is the limiting factor, then for every 2 moles of NaBr, 1 mole of Br2 forms.
But it does not sound like a viable way to produce bromine other than for its intended use- disinfection.
Better think about oxidation of bromide with H2O2 at acidic conditions or with HNO3. Or the reaction of bromate and bromide at acidic conditions.

Retard-3000 - 8-11-2011 at 08:41

I ask because I can't find any information on the amounts of DCC or NaBr that make up the bromine granules and thought someone on here may have bought them before. However I do agree that it probably is best left for it's intended use. I will probably go via the oxidation of bromide in acidic conditions, Do you think sodium hypochlorite will suffice as the oxidizer ?

Nicodem - 8-11-2011 at 10:30

Quote: Originally posted by Retard-3000  
Pool/Spa bromine granules consist of sodium bromide and sodium/potassium dichloroisocyanurate, my understanding is that when the granules come into contact with water the dichloroisocyanurate slowly decomposes releasing chlorine gas which displaces the bromide ions forming elemental bromine.

No, the bromide gets oxidized by the dichloroisocyanurate anion, which is a strong oxidant. It just is a simple redox reaction. Mind that a pH dependent equilibrium forms which can limit the amount of precipitable/extractable bromine. Remember that bromine is soluble in basic aq. solutions and that it can reversibly react with the cyanurate ions via N-bromination. You would have to reduce the pH with NaHSO4, H2SO4, H3PO4 or some other nonoxidizable acid in order to precipitate bromine, as the reaction mixture itself is basic. Cyanuric acid would therefore also precipitate.
Quote: Originally posted by Mixell  
Stoichiometry is your friend.
If the limiting factor is the DCC salt, then for every mole of DCC a mole of Br2 forms.

If the limiting reagent is the dichloroisocyanurate salt, then two equivalents of Br<sub>2</sub> form. Always count the electrons, when in doubts.

Mixell - 8-11-2011 at 11:35

Oh, I was building my answer on the assumption that the reaction proposed by retard is correct (The DCC releases a Cl2 molecules which in turn oxidize two bromide ions). Anyway, I'm not the best thinker on empty stomach :)