3. Results and discussion
3.1. Comparison of the titration method developed in this study with
the ASTM D664 method
Considering the significant differences between bio-oils and
petroleum oils, the standard method used in petroleum products
cannot always be directly applied to bio-oil [23]. Compared with
petroleum products, bio-oil contains more phenolic compounds
with pKa around 10 (in water). However, as the ASTM D664 states,
it was only applicable for the determination of the acids with a pKa
lower than 9. So the ASTM D664 cannot be used to measure bio-oil
having large amounts of phenolic compounds. The ASTM D664
method and our method were used to measure typical wood biooils.
As shown in Fig. 1a, only one inflection point of carboxylic
acids was obtained when following ASTM D664, while Fig. 1b indicated
that three inflection points can be obtained with the method
herein proposed. In this method, the mixture of tert-butanol and
acetone was used to dissolve bio-oil, and the potassium hydroxide
recommended by ASTM D664 was replaced with an organic base of
tetramethylammonium hydroxide, which are the keys for the sensitivity
of this method.
Choosing solvent in non-aqueous titration was vital to avoid
levelling effect and obtain distinguishable endpoint. The use of
the mixture of tert-butanol and acetone as solvent was based on
several considerations. Firstly, the mixture of tert-butanol and acetone
can dissolve bio-oils very well and provide a non-aqueous
system for titration.
Secondly, tert-butanol has a fairly high dielectric constant
(n = 12) and good solvating ability for bio-oil. A high dielectric constant
ensures the steady potential readings. Good solvating ability
of tert-butanol prevents the occurrence of homoconjugation
during the titration of acids. In solvents with good solvating
properties, solutes interact with the solvent instead of undergoing
self-condensation, and homoconjugation does not occur.