Xu and Liu
9
−
1
added to prepare lactic acid solutions with concentrations
of about 50, 60, 70, 80, and 90 wt%. Their mass concentra-
tions were measured by back titration.
min . 1 L of solvent (500 mL H O + 500 mL CH OH) was
2
3
prepared as the mobile phase, and 1 mL of H PO was
3
4
added to adjust the pH value of the mobile phase to 2.5.
Each component of the sample has a particular peak
time, so it is feasible to determine what the specific compo-
nent is according to the peak time. The peak time of known
substances need be measured as the identification basis. 10
wt% of lactic acid aqueous solution and lactide standards
were dissolved in methanol and acetonitrile, respectively,
and then analyzed by HPLC to obtain the peak time of the
lactic acid monomer and cyclic dimer (Figures 1, 2, 5, and
2
5 g lactic acid solutions were placed into a two-
necked 100-mL round-bottom flask, and a high-tempera-
ture resistant rotor was added to enable stirring. The
reaction vessel was vacuum-pumped and decompressed
to 5~10 KPa. The reactants were heated from room tem-
perature to 210 °C in the oil bath. The heating process
lasted for about 1 h based on the heating rate of the oil
bath and heating was kept at 210 °C for 20 min. At the
end of the reaction, the products and distillates were col-
lected separately.
1
1). Their peak time is very slightly different in the two
solvents. (LA represents the lactic acid monomer; LT repre-
sents the lactic acid cyclic dimer).
Declaration of conflicting interests
1
1
The author(s) declared no potential conflicts of interest with
respect to the research, authorship, and/or publication of this
article.
H NMR analysis. H NMR spectroscopy was used to ana-
lyze tested samples. According to the obtained spectrum
Figure 12), the peak area of the hydrogen chemical shift of
the solvent CDCl (δ = 7.26) was taken as the benchmark
(
3
Funding
1
and that of –CH (δ = 5.0–5.1) and –CH (δ = 1.68–1.72)
3
The author(s) received no financial support for the research,
authorship, and/or publication of this article.
were recorded. Standard LT/CDCl solutions with concen-
trations of 5.0, 10.0, 15.0, 20.0, 25.0, and 30.0 g L were
prepared. The peak areas corresponding to the hydrogen
3
−1
ORCID iD
shifts of the –CH and –CH groups were linearly related to
3
Xiaolong Xu
the lactide concentration. After fitting the data, the lactide
concentration in the sample could be calculated according
to the formulae shown in Table 5.
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Titration of concentrations. Highly concentrated lactic acid
contains very little water. When standing, the upper layer is
a clear colorless liquid, while the lower layer is mostly
milky white crystals. Samples were taken from different
layers of HCLA, denoted as samples 1, 2, and 3, and their
appearance characteristics were as follows:
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Sample 1: taken from the surface layer of HCLA, col-
orless transparent liquid, no lactic acid crystals.
9
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Sample 2: taken from the middle layer of HCLA, with
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crystals.
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1
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sample was weighed and dissolved in 20 mL of CH OH; 10
3
1
1
2
mL of this solution was removed and 0.05 g of lactide stan-
dard was added for dissolution. Samples (0.5 mL) were taken
from the two solutions respectively and analyzed by HPLC.
2
2
Synthesis of lactide with lactic acid solution. The HCLA was
5
heated in a 60–70 °C water bath to melt all the crystals and 21. Van Wouwe P, Dusselier M, Vanleeuw E, et al. Chem Sus
shaken to mix the lactic acid evenly. Deionized water was
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