I see many people asking questions about how much HBr and H2O2 to add, why the solution turns red even though not all of the H2O2 has been added, etc. After reading several publications and conducting several tests, I will try to provide a clear answer to these questions.
Troubleshooting :
- How much HBr should be added?
Between 1eq and 1.2eq is fine. Anything above that is pointless. Personally, I find 1eq to be perfect.
- How much H2O2 should be added?
This was the most debated part. According to publications and reaction equations, it seems that at least 1eq is needed, sometimes 3, etc. In reality, 0.5eq is sufficient in theory. This is why your solution sometimes turns red even though you haven't added everything yet, because here we use 0.76eq of H2O2, which is a small excess. Moreover, after numerous tests, I determined that this was indeed a well-chosen excess, and that adding more would not change anything, and would only risk contaminating your product with Br2 and forming the dibrominated product if the temperature exceeded 65°C.
I therefore advise you to stick with this amount of H2O2 to add (0.76eq). In addition, 12% H2O2 works very well for this method, but can sometimes cause the product to crystallize more quickly, leading to the stirrer becoming blocked.
- What temperature is required for the reaction?
The ideal temperature is between 30°C and 40°C. Do not go higher than 65°C! I strongly advise you to use a water bath when adding H2O2, as the reaction is very exothermic, and if not controlled, it will produce Br2 vapor everywhere and result in an impure product (usually orange and green), which still works for the animation, but gives a lower yield.
- Why is my RM orange/red and won't change color?
When enough H2O2 has been added, the RM will no longer change color because it will be saturated with Br2. Heating and adding more HBr will therefore have no effect. However, this does not necessarily mean that the reaction is complete, as sometimes the Br2 just takes longer to react towards the end of the reaction. In this case, I recommend stopping the addition of H2O2 and leaving the RM for an hour with stirring. Keep the setup in a cold water bath because sometimes the reaction can suddenly restart! After waiting an hour, I recommend adding the remaining H2O2 if the RM is no longer red, otherwise the reaction is complete.
- What should I do if I have added all the H2O2 and my RM is colorless?
This means that you haven't added enough H2O2, so keep adding it. Perhaps your H2O2 is less concentrated than before because it degrades over time.
- The mixture hasn't solidified:
It happens, you can add a 2b4m crystal seed if you have one, otherwise you can just wait or use a technique that works every time: put everything in the freezer. Once everything has solidified, leave it aside until the water has turned back into liquid and your product has crystallized.
- The product has crystallized, but I haven't finished adding H2O2:
The problem with this is that when it crystallizes, the product often blocks stirring. If this happens, stop adding H2O2, filter the product, recover the liquid, and continue adding H2O2. In general, I consider the reaction to be complete after adding all the H2O2 and waiting a few hours for everything to react. You don't necessarily have to wait 12 hours.
- My product is slightly yellow/orange:
This happens with traces of Br2. Basically, I recommend that you always wash your product with a NaHCO3 solution, then with plenty of water. Even if your product is orange/yellow, you can proceed with the animation. Personally, I like to add a few crystals of sodium thiosulfate to my washing solutions to make my product white by neutralizing the remaining Br2. However, this will produce a little sulfur, so it's up to you to decide.
