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Sulfonic Acid Group Determination in Lignosulfonates by Headspace Gas Chromatography Philipp Korntner, Andreas Schedl, Ivan Sumerskii, Thomas Zweckmair, Arnulf Kai Mahler, Thomas Rosenau, and Antje Potthast ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00011 • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 16, 2018
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Sulfonic Acid Group Determination in
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Lignosulfonates by Headspace Gas
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Chromatography
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Philipp Korntner1, Andreas Schedl1, Ivan Sumerskii1, Thomas Zweckmair1, Arnulf Kai
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Mahler2, Thomas Rosenau1, Antje Potthast1*
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1
University of Natural Resources and Life Sciences, Department of Chemistry, Division of
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Chemistry of Renewable Resources, Konrad-Lorenz-Str. 24, A-3430 Tulln, Austria 2
SAPPI Europe, Gratkorn Mill, SAPPI Paper Holding, Brucker Str. 21, A-8101 Gratkorn,
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Austria Corresponding author:
[email protected] 12 13
Keywords
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Lignin, technical lignins, lignosulfonates, functional groups, sulfonate, quantification,
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titration, elemental analysis, gas chromatography
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Abstract
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A method for the determination of the sulfonic acid content in lignosulfonates via sulfur
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dioxide quantification upon treatment with H3PO4 by headspace gas chromatography is 1 ACS Paragon Plus Environment
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presented and compared to two already established alternatives, conductometric titration and
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elemental analysis. Several lignosulfonates, purified from various industrial sources, were
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examined by all three methods. Limitations and possible interference by other functional
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groups, such as carboxylic acids, are discussed and suitable solutions are presented. Results of
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the novel approach are comparable to those of the established techniques in terms of accuracy
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and precision. The LOD is 0.88 µmol SO3H and the corresponding LOQ is 3.78 µmol SO3H.
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At the same time, this method outperforms conductometric titration in terms of a higher
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sample throughput and a much smaller sample amount needed for analysis. Treatment of the
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sample with highly concentrated phosphoric acid and simple heating is straightforward
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enough to render the method a valuable tool in lignosulfonate analysis of industrial problem
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sets, such as screenings or optimization of process parameters, without compromising
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analytical accuracy.
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Introduction
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Lignin, as a unique, renewable resource for aromatics, has moved into the spotlight of
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industry and bioeconomy. Technical lignin, having a largely altered structure compared to the
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native parent polymer, is produced in vast amounts as a byproduct in the pulp and paper
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industry. Therefore, it possesses immense potential to be exploited as a feedstock for
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chemicals; while currently mostly being used to generate heat because of its high calorific
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value to ensure independence autarky of pulping mills.1,2,3
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The general term lignin, especially technical lignin, describes a family of polymers sharing
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similar structural features. So-called native lignin is mostly the product of a combinatorial-
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like radical coupling reaction of the three main phenylpropanoid units, the monolignols (p-
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coumaryl, coniferyl, and sinapyl alcohol). Unlike proteins or other biopolymers, this results in
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a purely chemically controlled synthesis with an astronomical number of possible lignin 2 ACS Paragon Plus Environment
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isomers. Additionally, the ratio of the starting monolignols is highly dependent on the type of
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biomass. In fact, lignin shows a high propensity for the incorporation of other structurally
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related monomers of phenylpropanoid type, sometimes called “metabolic plasticity” of
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lignin.4,5,6 The structure of the native lignins is significantly altered upon pulping processes,
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most of which aim at breaking down and/or dissolving the lignin. In terms of molecular
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structure, these processes leave behind a profoundly modified lignin with high polydispersity
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and a variety of functional groups, such as the hydroxyl, carboxyl, keto, thiol, and sulfonic
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acid groups.
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Functional groups containing sulfur are introduced by both of the two most commercially
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relevant pulping processes, the kraft and the sulfite processes. The sulfite process introduces
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sulfonic acid groups, mostly in the benzylic positions to the lignin, forming lignosulfonates
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(LS); this is one of the driving factors for the dissolution of the lignin in the pulping
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process.7,8 Today, the analysis of native or unaltered lignins as well as kraft lignins is rather
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well established, including structural information ranging from wet-chemical analysis5,9,10,11
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to advanced 2D and 3D nuclear magnetic resonance (NMR) protocols12,13,14,15 or various
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techniques for hydroxyl group determination. Examples are 1H NMR16 and
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derivatization,17
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of these methods can be used without modification for (LS)21,22,23 in the same way as they are
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used for most native and technical kraft lignins; others are limited by the low solubility of LS
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in nonpolar organic solvents.
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P NMR after
C NMR18 or gas chromatography/mass spectrometry (GC/MS).19,20 Many
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Sulfur contents of around 5–6% in LS are higher than those in Kraft lignins, and correspond
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to about 0.4–0.7 sulfonate groups per phenylpropanoid (C9) unit.24,25,26 Although sulfur may
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not be desirable for several applications,27 such as work with catalysts that might be poisoned,
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the sulfonic acid groups in LS provide unique physicochemical properties of a polyelectrolyte.
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At present, applications of LS include oil well drilling additives, several types of dispersants,
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emulsion stabilizers, and plasticizers in concrete,28 which all fall into the lower-value 3 ACS Paragon Plus Environment
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segment. Most of the applications exploit the surface activity of the polymer, which in turn is
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connected to the molecular weight and the content of functional groups, sulfonic acid groups
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in particular. Traditionally, LS was used in the form it was extracted from the spent pulping
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liquor. However, for replacing higher-performance petro-based chemicals, the tailoring of its
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physicochemical properties, and thereby the degree of sulfonation, becomes important.3,29,30 It
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is obvious that appropriate analytical techniques to quantify those active functional groups are
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needed, especially when considering the rising number of potential LS applications.
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Available techniques to address sulfonic acid groups in LS can be separated into two
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groups: indirect methods measuring total sulfur contents, assuming that all sulfur is present in
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the form of sulfonic acid groups, and those measuring sulfonic acid groups directly.31
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Examples of indirect determination are X-ray fluorescence spectroscopy,32 combustion/ion
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chromatography,33,34 and elemental analysis. With regard to direct sulfonic acid
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determination, well-known examples are the quantitation of retained benzidinium ions by
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UV-spectroscopy,35 or the direct conductometric titration of the acid form by a strong base.36
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While most methods have certain advantages for different problem sets, many of them share
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the disadvantage of tedious sample preparation or the necessity of relatively high sample
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amounts, limiting their value to industrial sample sets.
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This study presents a comparison of two established methods for sulfonic acid
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determination, namely elemental analysis41 and conductometric titration,31,9 with a novel
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headspace GC/MS method, by means of various LS samples. The two conventional methods
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provided reference data against which the new method for the determination of sulfonic acid
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was evaluated. The method aims at higher throughput using smaller sample volumes and
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easier sample handling.
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Material and Methods
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Chemicals. All LSs were provided by project partners in the FLIPPR (Future Lignin and
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Pulp Processing Research) project. All solutions of hydrochloric acid (HCl) and sodium
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hydroxide (NaOH) for titration were prepared from 0.1M FIXANAL® (Sigma-Aldrich)
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standard solutions. Solutions of lithium hydroxide (LiOH) were prepared from LiOH
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monohydrate (p.a.). Ortho-phosphoric acid was of p.a. quality, and 3,4-dihydroxybenzoic acid
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(protocatechuic acid, DHBA) had a purity of >97% (purum). All other solvents and chemicals
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were of p.a. quality or higher. All chemicals were purchased from Sigma-Aldrich.
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Preparation of lignin samples. The LS was prepared by Amberlite® XAD7 resin
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purification according to Sumerskii et al.25 in order to remove all processing chemicals and
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low molar mass carbohydrates. In short, a 5 ml polypropylene syringe was filled with 0.5 g of
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Dowex 50WX8 (H-form) and 1 g of XAD-7 resin. Glass wool was plugged in the tip to
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prevent the resins from leaking. The final concentration of LS solution to be purified was
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chosen to be 100–150 mg LS/g resin. The syringe filled with the LS solution was closed and
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placed on a shaker for 5–6 hours. After shaking to ensure complete adsorption of the LS to the
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resin, the supernatant was released from the syringe. The LS was desorbed and re-dissolved
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by repeated washing of the resin with ethanol to a volume of about 15 ml (ensuring complete
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desorption of the LS). As a last step, the ethanol was evaporated on a rotary evaporator after
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diluting the solution with water by a factor of approximately two. The volumes will vary to a
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certain extend for different LS samples. The resulting aqueous solution of the LS was freeze-
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dried and carefully homogenized with a glass rod.
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Alternatively, the LS samples were also purified by ultrafiltration of black liquor through a
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1 kDa membrane and ion exchange into the acid form by Dowex 50WX8 resin. Subsequently,
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the resulting aqueous solutions were freeze-dried and carefully homogenized with a glass rod.
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Solid-state nuclear magnetic resonance (NMR) spectroscopy. Solid-state nuclear
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magnetic resonance (NMR) experiments were performed on a Bruker Avance III HD 400
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spectrometer (resonance frequency of 1H of 400.13 MHz, and
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C of 100.61 MHz, 5
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respectively), equipped with a 4 mm dual broadband cross-polarization magic-angle spinning
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(CP-MAS) probe.
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(TOSS) sequence at ambient temperature with a spinning rate of 5 kHz and a CP contact time
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of 2 ms. Quantitative 13C NMR spectra were recorded according to the multiple CP approach
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described by Johnson and Schmidt-Rohr38 at a rotational frequency of 12 kHz. A total of ten
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CP blocks were implemented, each with a contact time of 1 ms and a ramped CP contact 1H
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(70–100%) followed by a 1 s repolarization delay. For both experiments, a recycle delay of 2
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s, SPINAL-64 1H decoupling, and an acquisition time of 49 ms were used, with the spectral
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width set to 250 ppm. Chemical shifts were referenced externally against the carbonyl signal
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of glycine with δ = 176.03 ppm. The integration limits for carboxylic acids were δ = 160 ppm
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to δ = 210 ppm. Apodization with an exponential function prior to Fourier transformation
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(FT) was applied.
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C NMR spectra were acquired with the total sideband suppression
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Quantitative 13C NMR (solution state). LS (200 mg) was dissolved in 600 µl of a 0.01 M
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chromium(III) acetylacetonate (Cr(acac)3) solution in DMSO-d6. The spectra were acquired
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on a Bruker Avance II 400 instrument (resonance frequency 400.13 MHz for 1H and 100.61
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MHz for
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acquisition of 64 k data points and 30.000 scans. The spectral width was set to 240 ppm. An
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acquisition time of 1.4 s and a relaxation delay of 2 s were used. Prior to Fourier
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transformation (FT), apodization with an exponential window function (lb = 70 Hz) was done.
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The integration limits for carboxylic acids were δ = 160 ppm to δ = 210 ppm18. Standard
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Bruker NMR experiments were used for acquisition. Data processing was performed with
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Topspin 2.1 (Bruker, Rheinstetten, DE).
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C), equipped with a 5 mm broadband probe head (BBFO) with a z-gradient,
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Conductometric titration. Conductometric titration was performed on a Metrohm Titrando
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device (Metrohm, Herisau, CH), equipped with an 856 conductivity module and an electrode
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with a 5-ring conductivity cell combined with a PT1000 thermo sensor. 40–80 mg of sample
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(in protonated form) were dissolved in 40 ml of HQ water and titrated in 0.05 ml steps against 6 ACS Paragon Plus Environment
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0.1 M LiOH solution (titer determined by 0.1 M HCl standard solution) and subsequently
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back-titrated with 0.1 M HCl solution. The titration was stopped after 6 ml of base or acid
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were added to the LS solution. Metrohm tiamo™ software was used for data acquisition.
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The sulfonic acid groups were determined by straight-line fitting of the linear regions of the
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titration curves and determination of the points where the lines intersect. The first intersection
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point of the base titration provided the strong acid content, which was equal to the sulfonic
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acid content (since no other strong acids were present in the sample). The second intersection
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point accounted for the weak acids, mainly phenolic hydroxyl groups and, to a lesser extent,
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carboxylic acids31,9. The curve of the back titration was evaluated in the same way, providing
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the data in the reversed order for confirmation.9 Fig. 1 provides a graphic illustration of the
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data evaluation procedure.
Conductivity (mS/cm (20°C))
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1.2
0.8 c
0 0.4
b
2
a
4
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Volume HCL 0.1M, ml a
0.0 0
b
c
2
4
6
Volume LiOH 0.1M, ml
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Figure 1: Conductometric titration of a LS sample with 0.1 M LiOH and the back titration
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with 0.1 M HCl. Strong acids (sulfonic acids) were calculated from volume a, weak acids
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(such as carboxylic acids and phenolic hydroxyls) from volume b, and the excess amount of
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LiOH is shown as volume c.
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Elemental analysis. All samples (3 mg per analysis) were thoroughly dried prior to
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elemental analysis and stored under an inert atmosphere. Elemental analysis was performed as
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C/H/N/S (oxygen was determined indirectly) analyses on an EA 1108 CHNS-O (Carlo Erba
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Instruments, CE Elantech, Inc.) elemental analyzer.41
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Preparation of lignin samples for headspace GC/MS. For the determination of sulfonic
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acid groups in LS, 5–15 mg of LS was weighed into 10 ml headspace vials. As an internal
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standard, approximately 5 mg DHBA was added to the sample. Then 3 ml of 85% ortho-
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phosphoric acid was added. The vials were tightly sealed with crimp caps, shaken, and heated
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to 110 °C for three hours. In particular the heating of the reaction mixture should be done with
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suitable safety protection during operation since boiling of the concentrated phosphoric acid
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(b.p=158°C) can lead to exploding vials and severe spills of hot, concentrated acid.
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Calibration curve. Aliquots from 5–50 mg of a selected LS were prepared as described
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above, again with the addition of around 5 mg of DHBA as an internal standard. The selected
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LS was well characterized beforehand, in terms of sulfonic acid content, by conductometric
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titration and elemental analysis, ash content, residual carbohydrates, and methoxyl groups. A
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minimum of 14 calibration points have been measured evenly distributed over the calibration
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range.
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Determination of the analytical figures of merit. Estimation of cLOD (Limit of detection)
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was done according to DIN 32645:2008-1140 by measurement of blank samples (n=8) within
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a confidence level of 99%. A 3-point calibration with a minimum concentration of 5*LOD
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was used for calculation. The Limit of Quantification, cLOQ was calculated according to
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cLOQ=3.3*cLOD
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Relative standard deviation (RSD) was determined by independent analysis of sample material (the values of n are provided) ). Accuracy was evaluated by comparison to reference methodologies, such as elemental analysis or conductometric titration. 8 ACS Paragon Plus Environment
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Table 1: Lignin standard used for calibration. The number of repeated determinations and the
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RSDs (%) of the corresponding analysis are shown in brackets if applicable.
Pulping process
Magnefite
Residual non-cellulosic polysaccharide, wt% < 1 (n=2) Ash content, wt% (n=2)
