When recovering an acid from its salt dissolved in water, we typically add HCl until the pH of the solution is strongly acidic (pH ~ 2). Why don’t we stop when the pH is neutral? When recovering basic compounds from their aqueous salts we again skip neutral and adjust the pH until the solution is strongly basic. Why? (hint: Henderson-Hasselbalch) Solution The extraction is done with bicarbonate first and then with NaOH. But if you are doing it in reverse order, then extraction will not be proper. The three compounds mentions in the text are 2-methoxy naphthalene, 2-naphthol and naphthoic acid. Naphthol is a weak acid, naphthoic acid is strong acidic and naphthalene is neutral. NaOH is a strong base and bicarbonate is a weak base. Naphthol being a weak acid will only solubilise in strong base. Naphthoic acid is a strong acid, so it will be soluble in both strong and weak base, i.e. NaOH and NaHCO3. If you add aqueous NaOH solution to the mixture, then both naphthoic acid and naphthol will go in water layer and the naphthalene will go in organic layer, which is ether here. If you separate these layers and add NaHCO3 solution to the ether layer, you will not be left with anything which will be soluble in NaHCO3 solution. So, extraction will be difficult. On the other side, If you add NaHCO3 initially to the mixture of compounds, then naphthoic acid will go in aqueous layer and naphthalene will go in ether layer. Then when NaOH will be added, naphthol will go in this layer. Thus, all the compounds are in different layers, and easy to recover. If concentrated HCl is used instead of dilute HCl, even if the pH is kept same to recover the compounds, then there are more chances of side reactions during recovery, which is undesirable. And by adding concentrated HCl, there is increased evolution of heat. When NaHCO3 is neutralized with HCl, CO2 is evolved. If HCl is concentrated, then there will be lot of CO2 evolved and we have to handle the system with a lot of care during the recovery. These problems are not there while using dilute HCl. .

1. When recovering an acid from its salt dissolved in water, we typically add HCl until the pH of
the solution is strongly acidic (pH ~ 2). Why don’t we stop when the pH is neutral? When
recovering basic compounds from their aqueous salts we again skip neutral and adjust the pH
until the solution is strongly basic. Why? (hint: Henderson-Hasselbalch)
Solution
The extraction is done with bicarbonate first and then with NaOH. But if you are doing it in
reverse order, then extraction will not be proper. The three compounds mentions in the text are 2-
methoxy naphthalene, 2-naphthol and naphthoic acid. Naphthol is a weak acid, naphthoic acid is
strong acidic and naphthalene is neutral. NaOH is a strong base and bicarbonate is a weak base.
Naphthol being a weak acid will only solubilise in strong base. Naphthoic acid is a strong acid,
so it will be soluble in both strong and weak base, i.e. NaOH and NaHCO3. If you add aqueous
NaOH solution to the mixture, then both naphthoic acid and naphthol will go in water layer and
the naphthalene will go in organic layer, which is ether here. If you separate these layers and add
NaHCO3 solution to the ether layer, you will not be left with anything which will be soluble in
NaHCO3 solution. So, extraction will be difficult. On the other side, If you add NaHCO3
initially to the mixture of compounds, then naphthoic acid will go in aqueous layer and
naphthalene will go in ether layer. Then when NaOH will be added, naphthol will go in this
layer. Thus, all the compounds are in different layers, and easy to recover.
If concentrated HCl is used instead of dilute HCl, even if the pH is kept same to recover the
compounds, then there are more chances of side reactions during recovery, which is undesirable.
And by adding concentrated HCl, there is increased evolution of heat. When NaHCO3 is
neutralized with HCl, CO2 is evolved. If HCl is concentrated, then there will be lot of CO2
evolved and we have to handle the system with a lot of care during the recovery. These problems
are not there while using dilute HCl.