Hügelwilli wrote: ↑Fri Sep 11, 2020 5:21 am
atarijedi wrote: ↑Tue Sep 08, 2020 6:09 pm
I'd just like to note, because it doesn't seem to have been clearly stated, that neither sodium bicarbonate, nor sodium carbonate, will break down esters. They are not nucleophiles in this situation, like sodium hydroxide is, and a nucleophile is absolutely required for hydrolysis of an ester using a base.
So say I want to make a buffer solution for fermenting fruits at low pH with a low pH tolerating yeast.
If the buffer solution is made from citric acid and sodium hydroxide, does it destroy esters compared to a buffer solution made with citric acid and sodium carbonate with same pH? (Of course it needs more of the carbonate than of the hydroxide to target the same pH. This is not my question.)
Buffers need to be made of a weak acid and it's conjugate base, or a weak base and it's conjugate acid. Citric acid is a weak acid, but it's conjugate base is a citrate salt, so sodium citrate, calcium citrate, etc... Adding sodium hydroxide, or sodium carbonate, won't make a buffer with citric acid, they will attempt to neutralize each other, and if there is any hydroxide left over, it will react with any organic compounds there, like esters.
Just to make it more clear, you cannot make a buffer with strong acids or strong bases, hydrochloric acid, nitric acid, sulfuric acid, are examples of strong acids, because they completely disassociate when put into water. All the hydroxides are strong bases, so sodium hydroxide, calcium hydroxide, potassium hydroxide, etc... because they also completely disassociate when put into water. Buffers need partial disassociation, not full disassociation.
Disassociation is when the anion and cation separate, so for hydrochloric acid, you have HCl -> H+ and Cl-, becomign a hydrogen ion and a chloride ion. With sodium hydroxide you have NaOH ---> Na+ and OH-, becoming a sodium ion and a hydroxide ion. So all the HCl turns into H+ and Cl-, and all the NaOH turns into Na+ and OH-. Doesn't work as a buffer because of that.
Acetic acid on the other hand CH3COOH, a weak acid, only some of it disassociates into acetate and a free hydrogen ion, CH3COOH -> CH3COO- and H+.
The purpose of a buffer is that only some of the weak/conjugate acid, and only some of the conjugate/weak base, has disassociated at any one time. Once that disassociated portion of the weak/conjugate acid/base has reacted to neutralize the offending base/acid, than some more of the original weak/conjugate acid/base will then disassociate, and it will continue doing that, trying to keep the pH at/around a specific value.
All that said, a buffer solution won't destroy esters. A citric acid/sodium citrate buffer, isn't strong(nucleophilic) enough. They won't be strong enough. But they can stop new esters from being created, by reacting with some of the acids (like fatty acids) that would normally react with alcohols to become esters (like fatty acid esters), preventing that from happening.