Research Article Cite This: ACS Sustainable Chem. Eng. 2019, 7, 11521−11528
pubs.acs.org/journal/ascecg
Degradation of Deep-Eutectic Solvents Based on Choline Chloride and Carboxylic Acids Nerea Rodriguez Rodriguez,*,†,‡ Adriaan van den Bruinhorst,§ Laura J. B. M. Kollau,§ Maaike C. Kroon,∥ and Koen Binnemans† †
Department of Chemistry, KU Leuven, Celestijnenlaan 200F, P.O. Box 2404, B-3001 Leuven, Belgium SIM vzw, Technologiepark 935, B-9052 Zwijnaarde, Belgium § Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, Het Kranenveld 14, P.O. Box 513, 5600 MB Eindhoven, The Netherlands ∥ Khalifa University of Science and Technology, Department of Chemical Engineering, P.O. Box 2533, Abu Dhabi, United Arab Emirates
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‡
S Supporting Information *
ABSTRACT: Mixtures of carboxylic acids and choline chloride are one of the most commonly used families of deep-eutectic solvents. The thermal and long-term stability of carboxylic acid−choline chloride (ChCl) deep-eutectic solvents was investigated. This family of DESs was found to degrade due to an esterification reaction, mainly between the carboxylic acid and the alcohol moiety of ChCl. The esterification reaction occurs even at room temperature over extended periods of time and is promoted at elevated temperatures. The esterification reaction takes place independently of the preparation method used. Moreover, the deep-eutectic solvent malonic acid−ChCl (xChCl = 0.50) was found to decompose into acetic acid and carbon dioxide when prepared via the heating method, or when heated after preparation at room temperature. Therefore, the applicability of carboxylic acid−ChCl-based solvents is compromised. KEYWORDS: Deep-eutectic solvent, Decomposition, Carboxylic acid, Choline chloride, Malonic acid, Esterification
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avoid water uptake) until a clear liquid is formed.25 (3) The f reeze-drying method is where aqueous solutions of the initial components are mixed, frozen, and subsequently freeze-dried until a clear liquid is formed. The freeze-drying method is sometimes modified to evaporation in a rotary evaporator.26 (4) The twin screw extrusion method, where the solids are liquefied by high shear and compression forces induced by two co- or counter-rotating screws encased in a stainless steel barrel.27 In a very comprehensive work regarding the synthesis and properties of carboxylic acid-based DESs,25 it was stated that the preparation method (i.e., heating or grinding) had an effect on the purity of the DESs, and therefore on their physicochemical properties (mainly on density and viscosity). The authors observed that the DESs prepared via the heating method showed lower viscosities compared to those prepared via the grinding method. This behavior was attributed to an esterification reaction between the carboxylic acid and the OH group of the ChCl, which will take place when prepared via the heating method.25 A similar behavior has been observed for DESs formed by amino acids and hydroxy acids. In this case, the OH group of the hydroxy acid esterified with the carboxylic
INTRODUCTION Since deep-eutectic solvents (DESs) were first reported in 2003, their physicochemical properties quickly attracted the attention of the scientific community with more than 2000 publications in the past 15 years, as shown in the Supporting Information (SI), Figure S1.1 They have been used in a broad variety of applications, such as metal processing,2−5 synthesis,6,7 organometallics,8−11 biocatalysis,12,13 organocatalysis,12,13 solar technology,14 and separation processes such as CO2 capture,6,7 azeotrope breaking,15,16 or biomass processing.17,18 DESs are defined as a mixture of pure compounds for which the eutectic point temperature is below that of an ideal liquid mixture. For the mixtures used in this work, this behavior is attributed to the hydrogen-bonding between both components, which decreases the lattice energy of the system.1,19,20 Although several combinations of components have been reported to form DESs, the most often studied ones are based on choline chloride (ChCl) mixed with amides (e.g., urea),1 polyols (e.g., glycerol),21 or carboxylic acids (e.g., malic acid).22−24 Different methods have been reported for the preparation of DESs: (1) The heating method (by far the most commonly used in the literature) consists of stirring the two components at a certain temperature (normally 60−100 °C) until a clear liquid phase is formed.22 (2) The grinding method consists of grinding the two components in a mortar with a pestle at room temperature (occasionally in a glovebox to © 2019 American Chemical Society
Received: March 11, 2019 Revised: May 22, 2019 Published: May 28, 2019 11521
DOI: 10.1021/acssuschemeng.9b01378 ACS Sustainable Chem. Eng. 2019, 7, 11521−11528
Research Article
ACS Sustainable Chemistry & Engineering
heating block while stirring (500 rpm) using a magnetic stirrer (IKA RCT classic) with a temperature controller (VWR VT-5) until a clear liquid was formed. For the room-temperature stirring method, the same procedure as for the heating method was used, but no heat was applied. To study the effect of the temperature on the esterification reaction, the DESs were prepared following the same procedure as for the heating method, but the heat profile was changed. The mixtures were kept at 60 °C for 2 h (the DES malic acid−ChCl, xChCl = 0.5, was not formed in this period of time), heated at 80 °C for another 2 h, and finally were kept at 100 °C for another 2 h. Characterization Techniques. A nuclear magnetic resonance (NMR) spectrometer (Bruker Ascend 300) operating at 300 MHz was used to record the 1H NMR spectra of the DESs. All samples were dissolved in D2O. The chemical shift corresponding to the methyl group peak has been used as a reference for the 1H NMR spectra. The water content of the DESs was measured using a MettlerToledo V30S KF titrator where HYDRANAL−Composite 5 and HYDRANAL−Methanol dry (Honeywell, Fluka) were used as the titrant and medium, respectively. The decomposition temperature of the different DESs was measured using a thermogravimetric analyzer (TGA) Q500. Samples of 10−20 mg were heated from 30 to 350 °C at a heating rate of 5 °C min−1 in a 60 mL min−1 N2 flow. The decomposition temperatures were calculated as the extrapolated onset of the thermogravimetric curves. The uncertainty of the decomposition temperature determination was determined by performing a TGA analysis four times and calculating the standard deviations of the calculated decomposition temperature. The uncertainty of the experimentally determined decomposition temperature is 2 °C.
group of itself or the amino acid. For these DESs, it was reported that the esterification took place when prepared via the heating, freeze-drying, and the grinding method.28 The objective of this work is to show that the degradation of the carboxylic acid−ChCl-based DESs will take place regardless of the preparation method and that the high temperature used during the heating method accelerates the esterification reaction. Moreover, it will be demonstrated that, even if no esterification is noticed during the preparation, it will eventually occur if heat is applied or if enough time passes. The degree of esterification has been quantified under different conditions. In the past two years, more than 200 articles regarding the application of carboxylic acid-based DESs have been published, and in most of the cases, these DESs were being prepared, characterized, and used under conditions in which their stability could be questioned. The information on this work is important for fundamental studies, and for possible industrial applications in which DESs are used at high temperatures, where stability and reusability are required. The DESs selected for this work are lactic acid−ChCl (xChCl = 0.33), glycolic acid−ChCl (xChCl = 0.33), levulinic acid−ChCl (xChCl = 0.33), malic acid−ChCl (xChCl = 0.50), oxalic acid− ChCl (xChCl = 0.50), glutaric acid−ChCl (xChCl = 0.50), and malonic acid−ChCl (xChCl = 0.50). The molecular structure of the selected components is shown in Figure 1. This selection includes the most representative types of carboxylic acids used for the preparation of DESs, i.e., hydroxy acids, dicarboxylic acids, and keto acids.
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RESULTS AND DISCUSSION Esterification of Carboxylic Acid−ChCl DESs. The DESs listed in Table 1 were prepared via the commonly Table 1. DESs Studied in This Work Based on Carboxylic Acids and Choline Chloride (ChCl), Experimental Decomposition Temperature (TdExp) of the DESs Prepared via the Heating Method at 60 °C, and Decomposition Temperature Previously Reported in the Literature (TdLit)a
Figure 1. Structural formula of the components investigated in this work.
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EXPERIMENTAL PROCEDURE a
Chemicals. All the chemicals were purchased from trustful sources and used as received unless otherwise stated. Choline chloride (99%), malonic acid (99%), glutaric acid (99%), and glycolic acid aqueous solution (70%) were purchased from Acros Organics. Levulinic acid (99%), oxalic acid (99%), and aqueous solution of L-(+) lactic acid (90%) were purchased from J&K Scientific BVBA. DL-Malic acid (≥98%) was purchased from Sigma-Aldrich. Before DES preparation, the aqueous solutions of glycolic acid and lactic acid were dried in a rotary evaporator (Büchi Rotavapor R-300) at 50 °C for 2 h while the pressure was gradually decreased from 70 to 2 mbar. Thereafter, they were kept under a vacuum in a Schlenk line at room temperature and 1 mbar for 1 week. After that period, both chemicals were kept in a desiccator with silica gel. The glycolic acid solidified during the drying process (and was removed from the Schlenk line upon solidification), while the lactic acid remained liquid. The water content of both chemicals was measured before use by volumetric Karl Fischer titration and was found to be 2.40 wt % for glycolic acid and 1.38 wt % for lactic acid. Preparation of DESs. The DESs were prepared using different methods. For the heating method, the carboxylic acid and ChCl were weighed, placed in closed vials, and heated (60 °C) in a metallic
carboxylic acid
ChCl mole fraction
TdExp (°C)
TdLit (°C)
lactic acid glycolic acid levulinic acid malic acid oxalic acid glutaric acid malonic acid
0.33 0.33 0.33 0.50 0.50 0.50 0.50
173 218 159 211 161 235 125
16620 22725 17725 160,2516232 23925 125,2512832
u(Td) = 2 °C.
used heating method at 60 °C. The selected mole fractions correspond to the most widely studied compositions and do not necessarily reflect their eutectic composition.29,30 For oxalic acid−ChCl and malonic acid−ChCl mixtures, the ideal eutectic phase behavior was calculated and compared to previously reported S−L phase behavior, and the obtained results are shown in the SI, Figure S2.22 The oxalic acid−ChCl and malonic acid−ChCl systems do show significant negative deviations from ideality and can therefore be considered deep eutectics. To the best of our knowledge, the binary phase diagrams of malic, levulinic, glutaric, lactic, and glycolic acid with ChCl were not reported in the literature. Hence, it could not be verified whether these mixtures comply with the definition. Yet, they were designated DESs in order to increase the visibility of these decomposition studies to those applying these binary eutectic mixtures as “stable solvents.” 11522
DOI: 10.1021/acssuschemeng.9b01378 ACS Sustainable Chem. Eng. 2019, 7, 11521−11528
Research Article
ACS Sustainable Chemistry & Engineering The decomposition temperature was measured via dynamic TGA. The thermogravimetric curves are shown in the SI (Figure S3). The obtained decomposition temperatures, calculated as the extrapolated onset31 of the thermogravimetric curve, are also included in Table 1. The decomposition temperatures of most of the selected DESs have been previously reported in the literature (Table 1) and are in the same range as our experimental results. The observed variations can be attributed to the differences in the experimental conditions used (i.e., heating rate), the different sources of the chemicals, and the different water content in the DESs. The decomposition temperature of all the selected DESs is between 125 and 235 °C. On the basis of the dynamic TGA results, the carboxylic acid−ChCl DESs were generally referred to as thermally stable solvents with a broad operational window.33 However, the esterification reaction between the carboxylic acid and the OH group of the ChCl (Figure 2) will
Figure 3. 1H NMR spectra of oxalic acid−ChCl (xChCl = 0.50) prepared via the heating method for 2 h at 60 °C and heating steps of 2 h at 80 °C and 100 °C. The chemical shift corresponding to the methyl group peak has been used as a reference.
Figure 2. Esterification reaction taking place in the investigated DESs.
not be detected in a dynamic measurement at fast heating rates, because the initial water content will evaporate simultaneously with the water produced due to the esterification. Recent studies have investigated the thermal stability of carboxylic acid-based DESs with static TGA measurements.32,34,35 In all cases, mass losses ranging from 10 to 40 wt % were found at temperatures lower than the decomposition temperature calculated from the dynamic measurements. However, no explanation was given for this behavior, besides the fact that fast heating rates tend to overestimate the decomposition temperature. Nonetheless, mass losses of up to 40 wt % at temperatures 40 °C lower than the measured decomposition temperature should not be caused by an overestimation of the experimental technique. We attribute this behavior to the esterification reaction shown in Figure 2, the mechanism of which can be found elsewhere.36 In the 1H NMR spectra, the esterification reaction can be noticed from the partial shift of several signals. The signals corresponding to the hydrogen atoms of the ethylene groups of the ChCl (numbered as 1 and 2 in Figures 2, 3, and 4) are partially shifted when an ester is formed (numbered as 4 and 5 in Figures 2, 3, and 4). Throughout this work, the integration of these peaks was used for the quantification of the esterification yield (E%). A detailed explanation of the procedure used for the calculation can be found in the SI. More visual evidence of the esterification reaction is the displacement of the methyl group peak of the ChCl (numbered as 3 in Figures 2, 3, and 4), which is split in two peaks when the ester is present (numbered as 6 in Figures 2 and 3) or significantly broadened before it splits into two differentiated peaks, as shown in Figure 4. The 1H NMR spectra of the DESs prepared via the heating method at 60 °C were recorded after the preparation. It was found that all the investigated DESs had esterified to a certain extent. In order to investigate the effect of the temperature on the esterification of the DESs, the mixtures were stirred at 60 °C for 2 h then heated at 80 °C for 2 h and finally at 100 °C for another 2 h. A sample was taken and analyzed after each heating step. The 1H NMR spectra of all the studied DESs as a
Figure 4. 1H NMR spectra of levulinic acid−ChCl (xChCl = 0.33) prepared via the heating method for 2 h at 60 °C and heating steps of 2 h at 80 °C and 100 °C. The chemical shift corresponding to the methyl group peak has been used as a reference.
function of temperature can be found in the SI (Figures S7− S13). The 1H NMR spectra of oxalic acid−ChCl and levulinic acid−ChCl are shown as an example in Figures 3 and 4. Those DESs were selected because they are the most and least affected by the esterification reaction, respectively. From Figure 3, it can be observed that the peak of the methyl group peak (2.98 ppm, number 3) gradually splits when the DES is heated (3.03 ppm, number 6). Simultaneously, the ethylene peaks (3.83 ppm, number 1; and 3.30 ppm, number 2) are also split, and two new peaks can be found (4.50 ppm, number 4; and 3.61 ppm, number 5). The integration of the 1 H NMR spectra shows that the esterification accounts for 10 mol % of ChCl already when prepared at 60 °C, and it reaches 34 mol % of ChCl at 100 °C. Figure 4 shows the magnified 1H NMR spectra of levulinic acid−ChCl. The integration of the peaks shows that the degree of esterification increases from 2 mol % of ChCl after 2 h at 60 °C to 3 mol % after 2 h at 80 °C to 6 mol % after 2 h at 100 °C. The aforementioned changes in the 1H NMR spectra are not as evident as in Figure 3, because the degree of esterification is the lowest of the studied DESs. However, they can be noticed at high temperatures. The degree of esterification as a function of temperature has been calculated for all the investigated DESs, and the obtained 11523
DOI: 10.1021/acssuschemeng.9b01378 ACS Sustainable Chem. Eng. 2019, 7, 11521−11528
Research Article
ACS Sustainable Chemistry & Engineering
the acid−ChCl esterification increased. Glycolic acid was found to be more sensitive to both types of esterification, and increasing the temperature increased both the yield of acid− acid and acid−ChCl esterification. The glycolic acid selfesterification decreased upon DES formation, but it increased to values higher than those of the initial component when the temperature was increased. On the basis of the release of water during the esterification reaction (Figure 2), the water content is expected to increase with the esterification reaction. The water content of the DESs was measured at each of the temperature steps (60 °C, 80 °C, and 100 °C) using volumetric Karl Fischer titration. The obtained results are depicted in Figure 5 and confirm that the
results are shown in Table 2. The yield of esterification increases significantly with an increasing temperature for all the Table 2. Fraction of Esterified Choline Chloride (ChCl) in mol % for the Different DESs after Being Heated for 2 h at Different Temperaturesa ChCl esterification (mol %) DES
xChCl
60 °C
80 °C
100 °C
lactic acid−ChCl glycolic acid−ChCl levulinic acid−ChCl malic acid−ChCl oxalic acid−ChCl glutaric acid−ChCl malonic acid−ChCl
0.33 0.33 0.33 0.50 0.50 0.50 0.50
2 2 2
4 10 3 6 29 3 8
7 17 6 17 34 7 17
10 1 3
a
u(E%) = 1.
studied DESs. From the 1H NMR spectra, no evidence of other decomposition mechanisms besides the esterification reaction were found, with the exception of malonic acid−ChCl, which is discussed further in the text. In the 1H NMR spectra of lactic acid−ChCl and glycolic acid−ChCl, additional peaks can be distinguished (SI Figures S14 and S16). These peaks correspond to oligomer formation of the hydroxy acids, which contain both the OH and carboxyl functionality, and were observed for pure lactic and glycolic acid as well (Figures S15 and S17). The self-esterification of the initial components and of the lactic acid−ChCl and glycolic acid−ChCl mixtures has been quantified from the 1H NMR spectra. The procedure used for the calculation is included in the SI, and the obtained results are shown in Table 3. Interestingly, the self-esterification initially decreases upon
Figure 5. Increase of the water content of the DESs after stepwise heating for 2 h at 60, 80, and 100 °C. Data on malic acid−ChCl (xChCl = 0.50) at 60 °C are missing because a clear liquid was not formed after 2 h.
Table 3. Esterification of Hydroxy-Acid in mol % per Type of Ester, Acid−Acid and Acid−Choline Chloride (ChCl), before Mixing with Choline Chloride (Pure), after Preparation via the Heating Method for 2 h at 60 °C, Heating Steps of 2 h at 80 °C, and Heating Steps of 2 h at 100 °Ca
water content of all the DESs increases when heated. The yield of esterification obtained from the integration of the NMR spectra can also be used for the calculation of the water content increase (the procedure is reported in the SI). The water content values obtained from the Karl Fischer and the NMR measurements are compared in the SI (Figure S6), and it was found that both values are in reasonable agreement. In the case of glycolic and lactic acid, it was necessary to take into account the self-esterification of the acid (see Table 3). It is worth noting that the vials were kept closed to avoid water uptake. The water formation during esterification explains why the TGA static measurements reveal significant mass losses at temperatures much lower than the decomposition temperature determined by dynamic measurements.32,34,35 Notice that when the water is evaporated by the elevated temperature and constant nitrogen flow in the TGA, the equilibrium is shifted; thus, much higher weight losses will be induced than when the DES would be kept in a closed environment. Moreover, it explains the differences in density and viscosity observed when different preparation methods were used.25,33 The experimental data show evidence of ester formation when heat is applied. Nevertheless, since the esterification is an equilibrium reaction, it should also be observed at room temperature after a certain time (regardless of the preparation method used) as shown for proline-based DESs earlier.28 In order to extend these findings to ChCl-based DESs and to investigate the equilibrium condition, the DESs were prepared via the room-temperature stirring method, a modification of the
acid esterification (mol %) DES
xChCl
ester
pure
glycolic acid−ChCl
0.33
lactic acid−ChCl
0.33
acid−acid acid−ChCl total acid−acid acid−ChCl total
18 0 18 43 0 43
60 °C 80 °C 100 °C 13 1 14 31 1 32
17 5 22 31 2 32
23 9 32 29 3 32
a
u(E%) = 1.
mixing with ChCl for both glycolic and lactic acid, emphasizing that esterification is governed by an equilibrium reaction. This behavior could be attributed to the presence of the ChCl water, which displaces self-esterification equilibrium to the nonesterified acid side. For the lactic acid−ChCl mixture, the temperature has little effect on the total lactic acid of esterification, i.e., acid−acid and acid−ChCl, but it has an effect on how the lactic acid esters are distributed. The acid− acid esterification upon DES formation was lower than of pure lactic acid independently of the temperature. The acid−acid esterification decreased with an increasing temperature, while 11524
DOI: 10.1021/acssuschemeng.9b01378 ACS Sustainable Chem. Eng. 2019, 7, 11521−11528
Research Article
ACS Sustainable Chemistry & Engineering heating method in which the two components are also mixed under constant stirring until a clear liquid is formed; in contrast to the heating method, no heat is applied. All the investigated mixtures became a transparent clear liquid using this preparation method. The 1H NMR of the DESs prepared via this method was recorded immediately after preparation, and the obtained spectra are shown in the SI (Figures S14− S22). Similarly to the grinding method,25 signs of esterification were hard to recognize at first glance from the 1H NMR spectra recorded after preparation. However, integration of the zoomed spectra showed thatalthough often less than 2 mol % of ChCl esterifiedthe esterification was unavoidable during the preparation of all the DESs, even at room temperature. For oxalic acid−ChCl and malonic acid−ChCl, significant esterification was observed upon initial preparation, around 11 mol % and 8 mol % of ChCl had esterified, respectively. No signs of other types of decomposition besides the degradation were observed. In previous studies,25 malonic acid−ChCl showed both esterification and decomposition peaks in the 1H NMR spectra when prepared via the grinding method, although due to the wrong assignment of the peaks, none of these behaviors were noticed. The unavoidable ester formation when carboxylic acid−ChCl DESs are prepared implies that they cannot exist as a binary eutectic mixture, as usually reported. Instead, they should be considered as a quaternary eutectic mixture, where all four components play a role in the solid−liquid equilibrium. After 11 months, the 1H NMR of the DESs prepared via the room-temperature stirring method were recorded and compared to those of the freshly prepared DESs. The 1H NMR spectra of levulinic acid−ChCl are shown in Figure 6 as an example. The integrated 1H NMR spectra of the DESs 11 months after the preparation via the room-temperature stirring method are shown in the SI (Figures S23−S29). The degrees of esterification immediately after preparation and after 11 months are compared in Figure 7. It can be observed that for most investigated DESs, more than 20 mol % of ChCl was esterified after a long period of time at room temperature. The least esterified DES was found to be levulinic acid−ChCl (xChCl = 0.50) with 10 mol % of ChCl, which is in agreement with the results shown in Table 2. The degree of esterification after 11 months at room temperature is higher than that after preparation at 100 °C for most DESs. It is expected that the long-term degree of esterification of carboxylic acid−ChClbased DESs at elevated temperatures is significantly higher than those presented in Figure 7, because an elevated temperature shifts the esterification equilibrium (Figure 2) further to the right. Besides an increase in ChCl esterification, a strong increase in oligomer formation was observed for the glycolic and lactic acid based DESs (Table 4). As previously shown, glycolic acid is very sensitive to ester formation with almost half of the carboxyl moieties converted to esters after 11 months of equilibration at room temperature. For this mixture, higher esterification yields are obtained after 11 months at room temperature than after heating for short times (as shown in Table 3), and both the acid−acid and the acid−ChCl esterification increased with the time. For lactic acid, the total acid esterification remains nearly constant with time, although the self-esterification decreased while the acid−ChCl esterification increased. This behavior was also observed when the lactic acid−ChCl was heated (Table 3), and it could be
Figure 6. 1H NMR of levulinic acid−ChCl (xChCl = 0.33) newly prepared via the room-temperature stirring method (top) and after 11 months (bottom). The chemical shift corresponding to the methyl group peak has been used as a reference.
Figure 7. Fraction of esterified ChCl, in mol %, after preparation via the room-temperature stirring method and 11 months after preparation.
attributed to the higher self-esterification yield of the pure lactic acid compared to glycolic acid (probably due to the longer drying time). During drying at the Schlenk line, the selfesterification equilibrium was shifted due to the fast water removal, and the water created from the acid−ChCl esterification might be used to restore the acid−acid esterification equilibrium. For malic acid, no self-esterification could be observed besides the malic acid−ChCl esters at room 11525
DOI: 10.1021/acssuschemeng.9b01378 ACS Sustainable Chem. Eng. 2019, 7, 11521−11528
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ACS Sustainable Chemistry & Engineering
properties of the pure DES and obtained reproducible results. This protocol is not suitable for carboxylic acid−ChCl DESs; the water content should be adequately quantified and reported instead. Decomposition of Malonic Acid−ChCl (xChCl = 0.50). Besides the esterification reaction, the DES malonic acid− ChCl shows an additional type of thermal decomposition. This decomposition can be observed in Figure 8, by the appearance
Table 4. Esterification of Hydroxy Acid in mol % per Type of Ester, Acid−Acid, and Acid−Choline Chloride (ChCl), before Mixing with Choline Chloride (Pure), Directly after DES Preparation with the Room-Temperature Stirring Method (Initial), and after Equilibration (11 Months) acid esterification (mol %) DES
xChCl
ester
pure
initial
11 months
glycolic acid−ChCl
0.33
lactic acid−ChCl
0.33
acid−acid acid−ChCl total acid−acid acid−ChCl total
18 0 18 43 0 43
15 1 16 30 0a 30
33 14 47 18 11 29
a
rounded down from 0.2−0.4 mol %. u(E%) = 1.
temperature. Only after heating to 100 °C could a very minor (
