Direct Method of Measuring the Ph Value of Wood

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Article Direct Method of Measuring the pH Value of Wood

Anton Geﬀert *, Jarmila Geﬀertova and Michal Dudiak

Faculty of Wood Sciences and Technology, Technical University in Zvolen, T.G.Masaryka 24, 96001 Zvolen, Slovakia; geﬀ[email protected] (J.G.); [email protected] (M.D.) * Correspondence: geﬀ[email protected]; Tel.: +421-45-520-6378

Received: 16 August 2019; Accepted: 27 September 2019; Published: 30 September 2019

Abstract: A direct method of measuring the pH value of wood substance is proposed in the paper. The achieved results were completed by determining the pH value on the wood surface using the contact method. Moreover, the results were compared to the results achieved using the indirect methods to determine the pH value in cold water, as well as hot water, extract of wood. Using the direct method for measuring the pH value in drilled fresh sawdust, the pH value of beech was 5.11, of birch was 5.29, of alder was 4.88, and of maple was 4.65. Following the achieved results, the possibility to measure the pH value using a fast, accurate method useful in practice complying with the condition of the minimum free water in wood (moisture content of wood above the ﬁbre saturation point) was presented. The results of measurements of the pH value using the contact method on the wood surface showed that this method can be used in the case of coniferous as well as broadleaved trees with heartwood. The value of pH measured on the surface of pine was 4.50, of spruce was 4.79, of the heartwood of oak was 3.46, and of the sapwood of oak 5.04. The measurement of pH values of water extracts conﬁrmed great dependence of measured values on the conditions of wood extraction.

Keywords: pH value of wood; ﬂat electrode; piercing electrode; water extraction of wood; beech; spruce; pine; oak; birch; alder; maple

1. Introduction The pH value is a measure of the concentration of H+ ions in a solution and it is used to determine acidic, neutral, or base behaviour [1]. pH values are very important physiological parameter for plants, humans, and animals. The measurement of the pH value is used in almost every laboratory, manufacturing plant, or field-work [2]. All wood species have a pH and most species are naturally acidic, with the vast majority having a pH between 4.0 and 5.5. The values of pH of various tree species of temperate zone range from 3.3 to 6.4. Tropical tree species are of a weak acid or a weak base ranging from 3.7 to 8.2. Of course, there are examples of strongly acidic species like Douglas fir, with the pH value of 3.3, and alkaline species such as African tree species (Dalbergia melanoxylon Guill.; Terminalia superba Engl. & Diels), with the pH value of 8.0 and 8.2 [3–6]. The acidic reaction of most tree species is caused by free acids and acidic groups easy to be separated, that is, especially by acetic acid and acetyl groups. The acidity of wood is increased as a result of the wet environment and higher temperature [1]. Knowing the pH value of wood is very important in various areas of its use. Metals can corrode or the performance of adhesives, as well as the fixation mechanism of wood preservatives, can be affected when coming into contact with wood. The pH value of wood must be the centre of our attention, as well as in the case of producing the wood pulp, fibreboards, and particleboards or plasticising wood [3,6–10].

Forests 2019, 10, 852; doi:10.3390/f10100852 www.mdpi.com/journal/forests Forests 2019, 10, 852 2 of 9

The value of pH is a fast and easy to measure parameter; therefore, it is used to control and manage technology and processes. Several other methods of measuring the pH value of wood have been developed, as there are uncertainties facing the direct methods for measuring solid timber when the moisture content of wood is above the ﬁbre saturation point of wood [1]. The pH value of wood is measured using following methods, which requiring more time and handling:

The way of determining the pH value when the pH value of acidic or base solution is modiﬁed to • the pH value of disintegrated wood that is suspended [3]. The method of slope adjustment, graphical evaluation of the pH value of wood following the • measurement after it is immersed in diluted NaOH and the solution of HCl [4,11]. A more direct method of measuring the pH value is based on measuring the press-extracted wood • ﬂuids [12].

The absolute pH value of wood is not provided by the often used cold water and hot water extraction of wood by a certain amount of water [13–16]; thus, it can be used only to compare the determined values. The measurement of wood extractions is distorted as wood also contains acid insoluble groups in cell wall polymers [6]. The aim of the paper was to verify the possibility of determining the pH value using the contact method on the wood surface, to propose a fast and easy to be applied method in practice for measuring the pH value of wood substance at the diﬀerent moisture content. Moreover, the achieved results were compared to the pH values achieved using the indirect methods for determining the pH value in cold water, as well as hot water, extraction of wood.

2. Materials and Methods

2.1. Material Samples of air-dried and green wood of the widespread broadleaved and coniferous tree species, beech, oak, spruce, and pine, were used in the experimental measurements of the pH value of wood by the ﬂat electrode on the wood surface. Samples of the broadleaved tree species: green wood of beech (wa = 68.5%), birch (wa = 75.9%), alder (wa = 71.8%), and maple (wa = 60.8%) were used for direct measuring of the pH value of the wood substance.

2.2. Methods The samples of the wood trees were deﬁned in terms of chemical composition: Extractives ASTM D 1107-96. Standard Test Method for Ethanol-Toluene Solubilityof Wood [17]

Polysaccharide fraction Chlorite isolation method of Wise [14] Cellulose Kürschner-Hoﬀer method [14] Lignin ASTM D 1106-96. Standard Test Method for Acid Insoluble Lignin in Wood [18].

The fraction of sawdust from 0.5 mm to 1.0 mm from completely disintegrated boards of the wood trees (including surface and center part) was used to monitor the chemical characteristics. Contact measurement on the wood surface using the contact combination electrode with glass shaft SenTix Sur and inoLab pH metre was the ﬁrst mentioned method for determining the pH value of wood. The method was applied in accordance with the standard STN 50 0374 testing of pulp and paper [19]. Surface pH measurement of paper and pulp. Testing is based on the measurement of the pH value of the sample surface moistened with one drop of water using the planar combination electrode. In the developed method of direct determination of the pH value of wood, pH meter SI 600 with a piercing probe LanceFET+H from the company SENTRON (steel tipped probe for penetration Forests 2019, 10, 852 3 of 9

2270-010 and measurement of non-liquids) was used. The moisture content of samples of green wood of investigated tree species with the dimensions of 600 90 32 mm was above the ﬁbre saturation × × point. Three measurements of each sample were carried out in the centre, and 10 cm away from the edge.

SawdustForests 2019 was, 10, x FOR drilled PEER REVIEW using an electric drill machine, pressed back into the3 of opening,9 and subsequently, a piercing electrode was applied. After achieving the appropriate contact between sawdustinvestigated and the probe, tree species changes with in the the dimensions pH value of were 600 × recorded 90 × 32 mm every was 15above s for the a seven-minfibre saturation period. Thepoint. pH Three meter measurements Checker by of Hanna each sample and woodwere carried samples out in of the investigated centre, and 10 tree cm away species from in the the form edge. of dried sawdust of two qualities (ﬁne-grained fraction with 0.5–1.0 mm2 and fraction consisting of Sawdust was drilled using an electric drill machine, pressed back into the opening, and 2 sawdustsubsequently, with more thana piercing 1.0 mm electrode) were was used applied. in comparative After achieving indirect the measurementsappropriate contact of thebetween pH value of wood extracts.sawdust and the probe, changes in the pH value were recorded every 15 s for a seven-min period. When measuringThe pH meter the Checker pH value by Hanna in the and cold wood water samples extract, of investigated 2.0 g of sawdust tree species was in mixed the form with of 40 cm3 of distilleddried water sawdust in aof beaker. two qualities After 24-h(fine-grained contact fraction of sawdust with 0.5–1.0 with water mm2 and at the fraction laboratory consisting temperature, of 2 the pH valuesawdust of with cold more water than extract 1.0 mm was) were measured. used in comparative indirect measurements of the pH value of wood extracts. When measuring the pH value in the hot water extract, 2.0 g of sawdust with 40 cm3 of distilled When measuring the pH value in the cold water extract, 2.0 g of sawdust was mixed with 40 cm3 water wasof distilled used again. water in Sawdust a beaker. was After extracted 24-h contact in of the sawdust water with bath water at the attemperature the laboratory oftemperature, 95 ◦C for 15 min. After cooling,the pH value the pH of cold value water ofextract extract was was measured. measured. The measurementWhen measuring progress the pH of value the pHin the value hot wa ofter selected extract, tree2.0 g speciesof sawdust was with processed 40 cm3 of indistilled the program STATISTICAwater was 12. Theused one-wayagain. Sawdust analysis was ofextracted variance in the (ANOVA) water bath program at the temperature was used forof 95 the °C evaluationfor 15 of measuredmin. data. After cooling, the pH value of extract was measured. The measurement progress of the pH value of selected tree species was processed in the program STATISTICA 12. The one-way analysis of variance (ANOVA) program was used for the evaluation 3. Results and Discussion of measured data. Basic chemical analysis of wood of the examined wood trees: extractive composition in the 3. Results and Discussion ethanol-toluene solvent (TEE), holocellulose, cellulose, and lignin content was conducted in order to characteriseBasic the chemical sample. analysis Hemicellulose of wood of the content examined was wood calculated trees: extractive as the composition diﬀerence betweenin the the holocelluloseethanol-toluene and cellulose solvent content (TEE), holocellulose, (Table1). cellulose, and lignin content was conducted in order to characterise the sample. Hemicellulose content was calculated as the difference between the holocellulose and cellulose content (Table 1). Table 1. Chemical composition of wood of the examined wood trees.

Table 1. ChemicalTEE composition Holocellulose of wood of Cellulose the examined Hemicellulose wood trees. Lignin Sample of Wood Sample of TEE (%)Holocellulose (%) Cellulose (%) Hemicellulose (%) (%)Lignin WoodBeech (%) 2.3(%) 82.5 (%) 46.7 35.8 (%) 20.7 (%) Beech Birch2.3 2.9 82.5 84.2 46.7 45.4 38.835.8 17.7 20.7 Birch Alder2.9 6.6 84.2 77.2 45.4 44.1 33.138.8 22.0 17.7 AlderMaple 6.6 2.5 77.2 80.1 44.1 44.6 35.533.1 24.9 22.0 MapleSpruce 2.5 1.0 80.1 77.8 44.6 50.0 27.835.5 26.5 24.9 Spruce Pine1.0 5.2 77.8 73.1 50.0 47.3 25.827.8 25.6 26.5 Pine Oak5.2 9.4 73.1 69.4 47.3 39.1 30.325.8 22.8 25.6 Oak 9.4 TEE, 69.4 ethanol-toluene 39.1 solvent. 30.3 22.8 TEE, ethanol-toluene solvent. The measurement on the wood surface using the contact combination electrode SenTix Sur was the ﬁrst analysedThe measurement method for on determining the wood surface the using pH value the contact of wood combination (Figure electrode1). SenTix Sur was the first analysed method for determining the pH value of wood (Figure 1).

Figure 1. Measuring the pH value of wood using planar combination electrode. Figure 1. Measuring the pH value of wood using planar combination electrode.

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When measuring the air dried sample of beech wood (wa = 7.5%), the pH value could not be measured as the drop of water was absorbed by the wood structure very quickly and the value of pH was not stabilized. Fast penetration of liquids into beech wood relates to the chemical composition and capillary-porous structure, especially to high proportion of vessels and medullary rays [20], causing its good permeability. Reinprecht [21] mentioned that broadleaved tree species, beech, poplar, hornbeam, maple, and birch (but not the heartwood of oak and locust tree), are more permeable in comparison with coniferous trees. When measuring the pH value using the contact method on the surface of air-dried wood of pine and spruce, the value of pH stabilized very quickly. The average value of pH of pine wood was 4.50 ± 0.04 and in the case of spruce wood, the measured pH value was 4.79 0.11 (Figure2). ±

Figure 2. The pH value of the wood surface—spruce and pine.

The results of measuring the pH value achieved by contact method on the surface of dry wood indicated that this method is suitable only in the case of tree species with a high content of resin and fatty acids such as coniferous trees. High resin acid content prevents the drop of distilled water from penetrating the structure. Assarssson and Akerlund [22] mention that the resin acid content in the petroleum ether extract of spruce is 27.37% and of fir is 29.03%. When measuring the pH value of the green beech wood sample using the flat electrode, the pH value stabilized for a long time. After repeated contact of the flat electrode with the wood sample taking 10 min, the measured values of pH ranged from 5.01 to 6.56. Contact method was also used to measure the pH values of heartwood and sapwood of the fresh sample of English oak with the moisture content above the fibre saturation point characterised by a higher content of resin and fatty acids [23–27]. Average values of pH determined after five-min contact of the flat electrode with the wood sample were 3.46 in the case of heartwood and 5.04 in the case of sapwood. According to the standard STN 50 0374 [19], the pH value is measured using the flat electrode only on the surface of wood moistened with a drop of water. The results of measurements indicated the fact that this method cannot be used to measure the pH values of broadleaved trees with low moisture content and low content of resin and fatty acids. When proposing the direct method of measuring the pH value of wood in the mass, the selection of the right electrode played an important role. Owing to its robust construction, piercing electrode LanceFET+H from the company SENTRON applied to partially disintegrated beech wood was the most suitable for the experiment. Measuring the pH value was carried out in the drilled sawdust pressed in the container of an appropriate size (Figure3).

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Figure 3. Measuring the pH value using the piercing electrode in sawdust. Figure 3. Measuring the pH value using the piercing electrode in sawdust.sawdust.

The moisture content of wood above the ﬁbrefibre saturation point is an essential condition of the The moisture content of wood above the fibre saturation point is an essential condition of the proposed direct method. Therefore, the method wa wass verified veriﬁed by measuring the pH value using the proposed direct method. Therefore, the method was verified by measuring the pH value using the sawdust moistened by distilled water in various proportions (Figure(Figure4 4).). sawdust moistened by distilled water in various proportions (Figure 4).

Figure 4. The pH values of wood with different moisture contents. Figure 4. The pH values of wood with different diﬀerent moisture contents.contents. Measured pH values were stabilized very quickly within a few tens of seconds. The pH value Measured pH values were stabilized very quickly within a few tens ofof seconds.seconds. The pH value was increasing, that is, the concentration of H3O+ ions (acidity) as well as the amount of substance of was increasing, thatthat is,is, thethe concentrationconcentration ofof HH33OO+ ionsions (acidity) (acidity) as as well well as as the the amount amount of substance of H3O++ was decreasing with the increased quantity of added water. Calculating the concentration of H3OO+ waswas decreasing decreasing with with the the increased increased quantity quantity of added water. Calculating the concentration of H3O++ ions and, subsequently, the amount of substance of H3O+ was based on the formula for the pH H3O+ ionsions and, and, subsequently, subsequently, the the amount of substance ofof HH33O+ waswas based based on on the the formula for the pH value for weak acids and the acidity was considered acetic acid with pKa = 4.756 (Table 2). value for weak acids and the acidityacidity waswas consideredconsidered aceticacetic acidacid withwith pKpKaa = 4.7564.756 (Table (Table2 2).).

Table 2. The pH value of sawdust moistened with different quantity of distilled water. Table 2. The pH value of sawdust moistened with different different quantity of distilled water.water. + 5 + 8 Ratio∇ + 5 H × 10 ∇ + 8 nH × 10 ∇ Ratio∇ Ratio pH H 10 H+ × 10n5 ∇H 10 nH+ × 108 ∇ Wood/Water pHpH × (mol/L) × (mol) Wood/Water Wood/Water (mol/L) (mol/L) (mol) (mol) 1:1 4.73 1.977 1.977 1:1 1:1 4.734.73 1.9771.977 1.977 1.977 1:2 4.91 0.893 1.726 1:2 1:2 4.914.91 0.8930.893 1.726 1.726 1:3 5.07 0.413 1.239 1:3 1:3 5.075.07 0.4130.413 1.239 1.239 1:4 5.30 0.143 0.573 1:4 1:4 5.305.30 0.1430.143 0.573 0.573 Changes in parameters in Table 2 indicate that final acidity is significantly affected not only by Changes in parameters in Table 2 indicate that final acidity is significantly affected not only by the released acetic acid, but also by the acid components in wood cell walls. the releasedChanges acetic in parameters acid, but inalso Table by 2the indicate acid components that final acidity in wood is significantly cell walls. a ffected not only by the The measurement results of the pH values of moistened sawdust indicated that the pH value of releasedThe aceticmeasurement acid, but results also by of the the acid pH componentsvalues of mo inistened wood sawdust cell walls. indicated that the pH value of wood with higher moisture content can be measured with accuracy and speed using this method. wood with higher moisture content can be measured with accuracy and speed using this method.

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The measurement results of the pH values of moistened sawdust indicated that the pH value of woodForests 2019 with, 10 higher, x FOR PEER moisture REVIEW content can be measured with accuracy and speed using this method.6 of 9 In the following part, the results of direct measurement of the pH value of green wood of selected In the following part, the results of direct measurement of the pH value of green wood of selected broadleaved tree species (beech, birch, alder, and maple) are presented. Measuring the pH value was broadleaved tree species (beech, birch, alder, and maple) are presented. Measuring the pH value was carried out by piercing electrode in drilled sawdust pressed back into the drilled opening in three carried out by piercing electrode in drilled sawdust pressed back into the drilled opening in three various parts of the sample. In all cases of tree species, the measured pH values were stabilized after various parts of the sample. In all cases of tree species, the measured pH values were stabilized after 45 to 60 s of contact of the electrode with the sawdust. 45 to 60 s of contact of the electrode with the sawdust. The measurement progress of the pH value of beech, birch, alder, and maple is shown in Figure5. The measurement progress of the pH value of beech, birch, alder, and maple is shown in Figure The observed diﬀerences between the mean values of individual tree species were not assessed as 5. The observed differences between the mean values of individual tree species were not assessed as random errors, but as statistically signiﬁcant. random errors, but as statistically significant.

Figure 5. GraphGraph of of 95% 95% confidence conﬁdence intervals intervals of ofthe the pH pH value value of green of green wood—beech, wood—beech, birch, birch, alder, alder, and andmaple. maple.

Average pH values of green wood of selected broadleaved tree species at the deﬁneddefined moisture content wa areare illustrated in Table 3 3 in in descending descending order. order.

Table 3. Average values of the pH of green wood.wood.

BirchBirch BeechBeech AldeAlderr Maple Maple Tree SpeciesTree Species (wa =(w75.9%)a = 75.9%) (w(waa == 68.5%) (wa(w = a71.8%)= 71.8%) (wa = 60.8%)(wa = 60.8%) ± ± ± ± pHpH 5.295.290.12 0.12 5.11 5.11 0.130.13 4.88 4.88 0.190.19 4.65 0.25 4.65 0.25 ± ± ± ± When the sample of green beech wood was used for the time ranging from 60 to 420 s, the pH valueWhen increased the sample linearly of with green the beech narrow wood range was usedof 5.11 for ± the 0.13. time The ranging pH values from measured 60 to 420 s, in the three pH valuevarious increased parts of linearlythe sample with were the narrow almost rangeidentical of 5.11 as well.0.13. After The the pH stabilized values measured stage with in three the average various ± partspH value of the of sample green beech were almostwood of identical 5.29 ± 0.12, as well. a small After pH the range stabilized occurred. stage In with the thecase average of the sample pH value of ofalder green wood, beech the wood average of 5.29 pH value0.12, aof small 4.88 pH± 0.19 range was occurred. stabilized. In The the casepH values of the sampleof maple of alderwood wood, were ± thein the average observed pH valuerange of of 4.88 4.65 ±0.19 0.25. was In stabilized.the case of The the pHthird values measurement of maple wood of the were pH value in the observedof maple ± rangewood with of 4.65 the highest0.25. In values the case of pH, of the the third pH stabilit measurementy was the of lowest the pH owing value to of the maple change wood in moisture with the ± content resulting from the water evaporation and poor contact of sawdust with the electrode. Cold water and hot water extraction was performed using the sawdust of two different qualities (sawdust fraction of 0.5–1.0 mm2 and coarse fraction of sawdust) of selected tree species. The pH value of achieved extracts was measured using the pH meter Checker (Figure 6).

Forests 2019, 10, 852 7 of 9 highest values of pH, the pH stability was the lowest owing to the change in moisture content resulting from the water evaporation and poor contact of sawdust with the electrode. Cold water and hot water extraction was performed using the sawdust of two diﬀerent qualities (sawdust fraction of 0.5–1.0 mm2 and coarse fraction of sawdust) of selected tree species. The pH value of achieved extracts was measured using the pH meter Checker (Figure6). Forests 2019, 10, x FOR PEER REVIEW 7 of 9 Forests 2019, 10, x FOR PEER REVIEW 7 of 9

Figure 6. Measurement of the pH value using Checker in water extracts from sawdust. Figure 6. Measurement of the pH valu valuee using Checker in water extracts from sawdust.

Measuring the the pH pH value value of of wood wood extracts extracts is isoften often used used as asthe the method method in practice. in practice. However, However, it is Measuring the pH value of wood extracts is often used as the method in practice. However, it is ittime is time consuming consuming and andthe information the information associated associated with withthe real the acidity real acidity of wood of wood is not is provided. not provided. The time consuming and the information associated with the real acidity of wood is not provided. The Themeasurement measurement is affected is affected but many but many factors, factors, especial especiallyly extraction extraction conditions—time, conditions—time, temperature, temperature, and measurement is affected but many factors, especially extraction conditions—time, temperature, and andhydro hydro module. module. hydro module. Therefore, the eeffectffect of selected factors and measured results were verifiedverified by comparativecomparative Therefore, the effect of selected factors and measured results were verified by comparative measurements and the measured datadata areare summarisedsummarised inin FigureFigure7 7.. measurements and the measured data are summarised in Figure 7.

Figure 7. The pH value of cold water and hot water extractions. Figure 7. The pH value of cold water and hot water extractions.extractions. When the effect of fraction size was assessed, the difference in the pH values of extracts after the When the effect of fraction size was assessed, the difference in the pH values of extracts after the cold Whenwater theextraction effect of taking fraction 24 sizeh between was assessed, both fractions the diff erencewas in inthe the range pH valuesfrom 0.04 of extracts for beech after wood the cold water extraction taking 24 h between both fractions was in the range from 0.04 for beech wood coldto 0.11 water for maple extraction wood. taking The 24finding h betweens of Sitholé both fractions [15], that was the inpH the va rangelues are from not 0.04 affected for beech significantly wood to to 0.11 for maple wood. The findings of Sitholé [15], that the pH values are not affected significantly 0.11by the for sawdust maple wood. fraction, The were findings confirmed. of Sithol é [15], that the pH values are not affected significantly by by the sawdust fraction, were confirmed. the sawdustAfter hot fraction, water extraction were confirmed. of sawdust in distilled water at the temperature of 95 °C for 15 min, After hot water extraction of sawdust in distilled water at the temperature of 95 °C for 15 min, the measured pH values ranged from 4.97 for birch wood to 5.90 for maple wood. Lower pH values the measured pH values ranged from 4.97 for birch wood to 5.90 for maple wood. Lower pH values in comparison with cold water extraction relate to the higher intensity of acetyl groups released as a in comparison with cold water extraction relate to the higher intensity of acetyl groups released as a result of higher temperature, despite the shorter time of extraction. result of higher temperature, despite the shorter time of extraction. In the process of cold water as well as hot water extraction, hydrolysis of wood polysaccharides In the process of cold water as well as hot water extraction, hydrolysis of wood polysaccharides is done in the form of the autocatalysis. Its speed increases owing to acetic acid released from the is done in the form of the autocatalysis. Its speed increases owing to acetic acid released from the acetyl groups of hemicelluloses. In later stages of hydrothermal extraction, the acidity of the acetyl groups of hemicelluloses. In later stages of hydrothermal extraction, the acidity of the environment is increased by formic acid and levulinic acid resulting from the decomposition of environment is increased by formic acid and levulinic acid resulting from the decomposition of

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After hot water extraction of sawdust in distilled water at the temperature of 95 ◦C for 15 min, the measured pH values ranged from 4.97 for birch wood to 5.90 for maple wood. Lower pH values in comparison with cold water extraction relate to the higher intensity of acetyl groups released as a result of higher temperature, despite the shorter time of extraction. In the process of cold water as well as hot water extraction, hydrolysis of wood polysaccharides is done in the form of the autocatalysis. Its speed increases owing to acetic acid released from the acetyl groups of hemicelluloses. In later stages of hydrothermal extraction, the acidity of the environment is increased by formic acid and levulinic acid resulting from the decomposition of monosaccharides. At the same time, hemicellulose degradation as well as amorphous proportion in cellulosic sample appear [1]. Four diﬀerent methods of measuring the pH value were focused especially on the beech owing to its dominant position in wood-processing industry in the Slovak Republic. The achieved results are summarised in the Table4.

Table 4. Measured values of pH of beech wood.

on the Surface in the Mass Cold Water Extraction Hot Water Extraction Method Flat Electrode Piercing Electrode Glass Electrode pH 5.01–6.56 5.11 0.13 5.87 5.66 ±

Measuring the pH value on the surface of wood using the contact method did not work because of the long time necessary for stabilization of the pH value and large range of measured values (5.01–6.56). During the indirect measurement of pH value using the cold water and hot water extraction, + the pH value of beech wood was the highest (5.87 and 5.66). It is caused by the dilution of H3O ions during the wood extraction. The fastest and the most accurate information about the acidity of measured green wood is provided by the pH value measured in the wood substance using the piercing electrode (5.11).

4. Conclusions The results showed that the piercing electrode can be used for measuring pH value from sawdust with the higher moisture content—at the moisture content above the fibre saturation point. The method is usable in the laboratory as well as in operational practice for its speed and sufficient accuracy. Measuring the pH value using the contact method on the surface of the wood sample indicated that this method is influenced not only by moisture, but also by other factors (structure and chemical composition of wood). The measurement of pH values of water extracts confirmed great dependence of measured values on the conditions of wood extraction.

Author Contributions: A.G. and J.G. designed the whole study; J.G. and M.D. conducted data collection, modeling and results analysis; J.G. wrote the original draft paper; A.G. revised and edited the paper. Funding: This research was funded by project APVV-17-0456 “Thermal modification of wood with water vapor for purposeful and stable change of wood colour”. Acknowledgments: This experimental research was carried out under the grant project APVV-17-0456 “Thermal modification of wood with water vapor for purposeful and stable change of wood colour”. The contribution was also prepared within the project agency IPA TUZVO, which contributed significantly to the creation of this contribution through the project: IPA TUZVO 15/2019. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Fengel, D.; Wegener, G. Wood: Chemistry, Ultrastructure, Reactions, 2nd ed.; Walter de Gruyter: Berlin, Germany, 1989; p. 613. Forests 2019, 10, 852 9 of 9

2. VWR International. Hamilton pH Basics. Available online: https://sk.vwr.com/cms/hamilton_ electrochemistry_phbasics/ (accessed on 23 August 2019). 3. Sandermann, W.; Rothkamm, M. The determination of pH values of woods and their practical importance. Holz Roh- Werkstoff 1959, 17, 433–441. [CrossRef] 4. McNamara, W.S.; Sullivan, C.E.; Higgins, J.C. pH measurements on northeastern woods. Wood Sci. 1970, 3, 48–51. 5. Liptáková, E.; Sedliaˇcik,M. Chémia a aplikácia pomocných látok v drevárskom priemysle [Chemistry and application of auxiliaries in the wood industry], 1st ed.; Alfa: Bratislava, Slovakia, 1989; p. 520. 6. Irle, M. pH and Why You Need to Know it. Available online: http://www.wbpionline.com/features/ph-and- why-you-need-to-know-it/ (accessed on 20 August 2019). 7. Gray, V.R. The acidity of wood. J. Inst. Wood Sci. 1958, 1, 58–64. 8. Kehr, E.; Schilling, W. Untersuchungen über die Eignung verschiedener Holzarten und -sortimente zur Herstellung von Spanplatten. 7. Mitteilung: Eiche, Aspe, Pappel, Hainbuche, Ulme, Lärche sowie als Vergleichsholzarten Fichte und Kiefer. Holztechnology 1965, 6, 225–232. 9. Lábsky, O. Zmeny pH dreva po modifikácii amoniakom [Changes in wood pH after ammonia modification]. Wood Res. 1974, 19, 125–133. 10. Johns, W.E.; Niazi, K.A. Effect of pH and buffering capacity on the gelation time of urea formaldehyde resin. Wood Fiber Sci. 1980, 12, 255–263. 11. Stamm, A.J. A comparison of three methods for determining the pH of wood and paper. For. Prod. J. 1961, 11, 310–312. 12. Kubinsky, E.; Ifju, G. A simple and fast method of pH measurement for wood. For. Prod. J. 1973, 23, 54–56. 13. Campbell, W.G.; Bryant, S.A. Determination of pH in Wood. Nature 1941, 147, 357. [CrossRef] 14. Kaˇcík, F.; Solár, R. Analytická chémia dreva [Analytical Chemistry of Wood], 1st ed.; Technical University in Zvolen: Zvolen, Slovakia, 2000; p. 369. 15. Sitholé, B. New method of measuring the pH of wood chips. Pulp Pap. Ca. 2005, 106, 42–45. 16. Wang, W.; Cao, J.; Cui, F.; Wang, X. Effect of pH on Chemical Components and Mechanical Properties of Thermally Modified Wood. Wood Fiber Sci. 2012, 44, 46–53. 17. ASTM D1107-96(2013), Standard Test Method for Ethanol-Toluene Solubility of Wood; ASTM International: West Conshohocken, PA, USA, 2013. Available online: www.astm.org (accessed on 23 August 2019). 18. ASTM D1106-96(2013), Standard Test Method for Acid-Insoluble Lignin in Wood; ASTM International: West Conshohocken, PA, USA, 2013. Available online: www.astm.org (accessed on 23 August 2019). 19. STN 50 0374; Skúšanie buniˇcinya papiera. In UrˇceniePovrchového pH Papiera a Buniˇciny[Testing of Pulp and Paper. Surface pH Measurement of Paper and Pulp]; ÚNMS SR: Bratislava, Slovakia, 1996. 20. Kúdela, J.; Cunderlˇ ík, I. Bukové Drevo –Štruktúra, Vlastnosti, PoužItie [Beech Wood - Structure, Properties, Use], 1st ed.; Vydavatel’stvo TU vo Zvolene: Zvolen, Slovakia, 2012; p. 152. 21. Reinprecht, L. Procesy Degradácie Dreva [Wood Degradation Processes], 1st ed.; Vydavatel’stvo TU vo Zvolene: Zvolen, Slovakia, 1998; p. 150. 22. Assarsson, A.; Akerlund, G. Studies on wood resin, especially the change in chemical composition during seasoning of the wood. Sven. Papperstidn 1966, 69, 517–525. 23. Výbohová, E. Lipophilic Wood Extractives – Possibilities of Analysis and Degradation. Sel. Process. Wood Process. 2004, 5, 250–258. 24. Výbohová, E. Degradation of lipophilic extractives from oak wood by storage. Acta Fac. Xylologiae Zvolen 2007, 49, 47–52. 25. Geffertová, J.; Geffert, A.; Furták, V. Influence of different characteristics of oak heartwood and sapwood on selected characteristics of sulphate pulp. Acta Fac. Xylologiae Zvolen 2006, 48, 23–32. 26. Geffertová, J.; Hanzel, P. Oak sessile and oak moscupped in the process of sulphate pulp. Sel. Process. Wood Process. 2007, 7, 57–66. 27. Laurová, M.; Výbohová, E.; Mamoˇnová, M. Heartwood and sapwood lipophilic extractives of oak. Acta Fac. Xylologiae Zvolen 2007, 49, 17–23.

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