ISSN 1070-4272, Russian Journal of Applied Chemistry, 2008, Vol. 81, No. 7, pp. 1278!1282. + Pleiades Publishing, Ltd., 2008.
Original Russian Text + N.V. Shul
ga, L.A. Gomolko, N.P. Krut
ko, 2008, published in Zhurnal Prikladnoi Khimii, 2008, Vol. 81, No. 7, pp. 1198!
AND POLYMERIC MATERIALS
Potentiometric Titration of Lignosulfonic Acids
N. V. Shul
ga, L. A. Gomolko, and N. P. Krut
Institute of General and Inorganic Chemistry, National Academy of Sciences of Belarus, Minsk, Belarus
Received December 11, 2007
Abstract-The conditions of potentiometric titration of lignosulfonic acids allowing quantitative determina-
tion of strongly and weakly acidic functional groups in lignosulfonates were determined.
Lignosulfonic acids (LSAs) are complex and
poorly studied compounds. The analytical proce-
dures existing today allow fairly accurate determina-
tion of the majority of functional groups in lignins,
precursors of lignosulfonates . Differentiation of
acid groups in lignin and its derivatives is compli-
cated because of wide range of their ionization con-
stants and of certain specific features related to
macromolecular structure of lignin, such as accessi-
bility of acid functional groups [2, 3].
In this study we made an attempt to find poten-
tiometric titration conditions that would furnish
evidence for the existence of acid groups of different
strenghths in lignosulfonates and allow their quanti-
As starting material for preparing LSAs we used
technical-grade lignosulfonates (LSTs) from the
Sokol Pulp-and-Paper Combine. Their characteris-
tics and the conditions for their purification and
synthesis of LSAs are described in detail in .
In potentiometric titration of LSAs, we used as
solvents distilled water, mixed aqueous3organic solv-
ent (water, ethanol, and acetone taken in 35 : 21: 4
volume ratio), and dimethyl sulfoxide (DMSO) of
chemically pure grade. As base titrant we used an
aqueous or aqueous3alcoholic (1: 19) NaOH solu-
tion (0.0630.75 N).
When choosing the solvent, we took into account
the fact that the apparent strength of acids and
bases depends on the extent of their interaction with
the solvent. In aqueous medium, the leveling effect
of the solvent smears the inflection points in poten-
tiometric titration curves because of close ionization
constants of acids of different strengths in water and
hydrolysis of the forming salt of a weak acid. How-
ever, this factor does not prevent determination of
the total amount of acidic protons.
In nonaqueous titrations (in DMSO), we did not
make special efforts to remove residual water (about
1wt %) from the mixture being titrated. It is known
that glass electrodes in many organic solvents give
unstable data, and small amounts of H
the pH-meter readings . Furthermore, an experi-
ment showed that dry LSA is practically insoluble
in water and DMSO. Therefore, in some cases in
titration of more saturated (~2%) solutions of ligno-
sulfonic acid in DMSO, to decrease the water con-
tent of the mixture being titrated, a sample of LSA
aqueous solution containing 20323 wt % dry matter
was transferred into a titration beaker and carefully
evaporated at 35340oC to a resinous state, after
which the organic solvent (60380 ml) was added and
the titration was started.
Titration was performed at room temperature
wich continuous stirring in a flow of an inert gas
(He or N
) after its passing through a column
packed with KOH. As indicator electrode we used
an ESL 43-07 electrode. The reference electrode was
an EVL-1-M3 silver chloride electrode filled with
a saturated aqueous KCl solution. According to the
existing experience , the glass electrode is prefer-
able in titration of model lignin compounds in
DMSO and DMSO3water mixtures.
The EV-74 pH meter was calibrated using stand-
ard buffer mixtures. The correctness of readings was
checked every time before the measurements.
The base titrant was added to an LSA solution in