The short answer is that ~1/3 weight bicarb will liberate the FB from ~ 1 weight of cocaine.
Using molecular weights to demonstrate;
1 mole of Coke HC
l = 339.82g
1 mole of Sodium Carbonate = 105.99
1 mole of Sodium Bicarbonate = 84.01g (1/3 weight of 1 mole of coke = ~113g = 1.34 moles bicarb)
The main reaction is: 1 mole of sodium
carbonate will liberate 2 moles of freebase from 2 moles of Coke-HC
l salt
2COKE-HC
l + Na2CO3 --> 2NaC
l + H20 + CO2 + 2COKE-FB
But, it takes 2 moles of Bicarb to produce 1 mole of sodium carbonate (naturally formed in a warm solution of bicarb in water)
2NaHCO3 --> CO2 + Na2CO3 + H20
So if a 1/3 weight bicarb is used (and lets say for simplicity we're working with a 1 molar amount of coke) then from the ~ 113g of bicarb (~1.34 moles), 0.67 moles or ~ 71.2g of carbonate is produced.
Repeating the above reaction;
2COKE-HC
l + Na2CO3 --> 2NaC
l + H20 + CO2 + 2COKE-FB
As 2 moles of coke-HC
l react with 1 mole of carbonate we multiply the 2 moles of cocaine (2 x 339.82) by the
molar amount of carbonate we obtained from the 1/3 weight bicarb This is done because, being less than 1 mole in weight, the carbonate is the *limiting reactant*
(2 moles coke) 679.64 x 0.67 moles carbonate = 455g of coke that can be freebased from the 1/3 bicarb used above
We can now see (hopefully) that it is effectively more than enough to react with the 339g or 1 mole of coke-HC
l. Of course it's not expected that anyone would be freebasing this sort of quantity, but when using moles, it's easier to explain in whole molar amounts. To use the same math with smaller quantities, the amount of coke first needs to converted to moles e.g. 1 gram = 1/ 339.82 or 2.94E-03 moles.
Some notes on the procedure:
The solution of bicarb and coke must be warmed first to allow the formed sodium carbonate to properly dissolve and the reaction go to completion. When fizzing is complete, the solution is cooled and the FB should crystallise out of solution.
Going over board with bicarb is usually no concern, but if the added amount of water is minimal, it may cause some excess carbonate to crystallise out with the freebase as the solution cools down.
Remember that FB weighs less than the HCl salt and must be accounted for if you're attempting to gauge purity. **
1 mole of COKE FB = 303.35g so a theoretical 100% coke would give 339.82/ 303.35 = ~89% of starting weight.
**Also important to note is that this method will not remove amphetamine, as it will freebase with the coke. If it's present in large amounts it will likely make the resulting FB hard to crystallise.
See Le Junk's posts for Removing amphetamine from cocaine
As a last couple of notes: If you are new to FB coke, then before deciding this is the way to go, please understand the dangers of using FB. It is far more addictive IMO. Also be aware that if you're caught performing the above procedures, you could face more serious charges than that of possession.