A Redox-Metric Study of the Formation
of Coloring Substances in the Synthesis of Pentaerythritol
O. A. Demchenko and D. I. Belkin
Institute of Chemical Technologies, Far East-Ukrainian National University, Rubezhnoe, Lugansk oblast, Ukraine
Received November 12, 2010
Abstract—The redox potentials of the reaction mixtures in the course of pentaerythritol synthesis and of
transformations of formaldehyde and glucose in a NaOH solution without catalyst and in the presence of
copper prepared by reduction of CuO were measured.
ORGANIC SYNTHESIS AND INDUSTRIAL
ISSN 1070-4272, Russian Journal of Applied Chemistry, 2011, Vol. 84, No. 8, pp. 1398–1403. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © O.A. Demchenko, D.I. Belkin, 2011, published in Zhurnal Prikladnoi Khimii, 2011, Vol. 84, No. 8, pp. 1333–1338.
When the synthesis of pentaerythritol is performed
in the presence of a relatively small excess of
formaldehyde, after the reaction completion it is
necessary to remove residual aldehydes from the
alkaline reaction mixture. To this end, the mixture is
heated to the boiling point.
In the course of such treatment, at a minimal
concentration of formaldehyde, a fast reaction yielding
colored substances starts. These impurities impart
color to pentaerythritol. The reaction can be stopped
by neutralizing the reaction mixture with an acid. It is
suggested to add an acid to the reaction mixture when
the weight fraction of formaldehyde in it decreases to
0.1–0.3%  or when aldehydes are already practically
absent and colored substances only start to form . It
is suggested to determine the moment of acid addition
by monitoring the aldehyde concentration , redox
potential , optical density , or electrical
To avoid formation of colored substances or
decrease their amount, it is suggested to use various
additives, e.g., copper, chromium, manganese, iron,
cobalt, or nickel oxalate , copper or silver hydroxide
or oxide, active copper, nickel, or platinum ,
aluminum, zinc, or tin compounds , hydrogen
peroxide , manganese compounds and atmospheric
oxygen , and boron compounds . According to
published data, the use of additives does not eliminate
the need for neutralization of the reaction mixture prior
to evaporation and hence the need for determining the
moment of acid addition.
In the above-cited papers, the nature of the pheno-
mena noted and the effect of additives on them were
not examined. In this paper we attempt to fill this gap.
We used Formalin containing 37 wt % formal-
dehyde and 10 wt % methanol, NaOH, acetaldehyde,
glucose, and copper prepared by reduction of CuO
with formaldehyde in the presence of NaOH. The
reactions were performed in a glass vessel submerged
in a bath and equipped with a magnetic stirrer, a
thermometer, and electrodes for measuring pH and
redox potential. The temperature was maintained with
an accuracy of ±0.5°C.
The redox potentials were measured with a
platinum electrode, and pH, with a glass electrode; as
reference electrode we used a silver chloride electrode.
When preparing mixtures and in the course of the
reactions, we took solution samples, transferred them
into flasks with a known excess amount of 0.1 N HCl,
and titrated excess HCl with a 0.1 N solution of NaOH
in the presence of phenolphthalein to determine the
NaOH concentration in the sample. Then a preset
amount of an aqueous solution of hydroxylamine
hydrochloride was added, and the concentration of
aldehydes was determined by titration with 0.1 N
alkali in the presence of Bromophenol Blue.
Three periods differing in the trend of variation of
the redox potential (Fig. 1) can be distinguished in the