Neutralization of Raw Furfural

When the raw furfural produced by the decanter of the azeotropic column turns out to be acid, due to an insufficient number of trays between the feed inlet and the side stream withdrawal, it is customary to submit this raw furfural (the heavy phase of the decanter) to a neutralization with sodium carbonate. This, however, is problematic, the principal cause being the fact that the dielectric constant of furfural amounts to a mere 48 percent of the value for water (38.0 versus 78.5 at 25 °C). Thus, raw furfural, exhibiting a typical furfural content of 92 % by weight, will have a dielectric constant only half as great as that of water. This has far-reaching consequences. 

From this, it can be estimated that the dissociation constant of acetic acid in raw furfural is in the order of 10 mole/liter, i.e. roughly 1/20000 of the value in water. In other words, while the dissociation of acetic acid in water is already poor, it is almost nonexistent in raw furfural. This changes the course of neutralization. 

Dissolved in water, sodium carbonate hydrolyzes according to Na2C03 + H2O 2 Na + HC03 + OH (1) 

for various ratios (3) and with the known dissociation constant, the hydrogen ion concentration can be calculated, which means that the degree of neutralization, expressed by (3), can be related to the pH value observed. For instance, when (3) is 90/10, corresponding to a 10 % neutralization, equation (4) yields [HVio = Kx9 

In raw furfural, where a dissociation constant K = 10 mole/liter applies, this amounts to [H ]90/io = 9 X 10 mole/liter 

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As the neutralization of raw furfural is commonly carried out to a pH of 9, it can be concluded from Figure 115 that under such circumstances only 50 percent of the acid is neutralized whereas in water a pH of 9 would effect 100 % neutralization. 

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