As a new approach, the acidity that wood exhibits under moderate conditions is assayed by stimulated dissociation of weak wood acids in lightly basic secondary phosphate solutions. To assure a sufficient dissociation of hardly soluble weak acids in the solution, the amount of wood suspended in
It is well known that, except for very few species, the woods of trees exhibit an acidic behavior of weak-to-moderate degree. The source of this property is found in both, main components and extractives as well. Acetyl groups and uronic acid residues linked to the polyoses and some organic acids occurring in free but also ester forms are particularly responsible for the wood acidity of many species. Additionally, the contribution of polyphenolic substances such as tannins is also of great importance in some hardwoods, like the heartwoods of oak and chestnut, for their acidic property. The nature and variety of wood acids are extensively reviewed by Choon and Roffael [
The role of acidic behavior of wood in its mechanical and chemical utilization is well recognized long ago and several methods for estimation of wood acidity, for example, its pH value have been developed [
Subramanian et al. [
The importance of wood acidity in its utilization demands the estimation of this property correctly and reliably. Used as massive wood, the natural wood acidity arising from free and soluble acids will be effective and the acidity of wood can easily be determined here by water extraction. However, during many technical and chemo-technical processes the heat and pressure and sometime the chemicals will cause splitting acetyl groups, esters or, sometimes, creating new acidic constituents as degradation products. As a new way to estimate the wood acidity which would make itself felt if the wood is exposed to not harsh but moderate conditions, we promoted the solubility and the dissociation of otherwise sparingly soluble and weak wood acids with Na2HPO4 solution at room temperature.
There are several acidic wood components many of which are bound as esters, some exist in form of their salts but few are found in the free form. The common nature of all wood acids is that they are weak; that is, their acid exponents are generally <5. Thus, depending also on the amount of acid content the most woods can exhibit pH values usually higher than 4, in average 4.5–5 [
Though numerous weak acids or acidic groups are present in wood, the most acidic ones will govern the wood acidity. Designating them by the common formula HA we will observe following equilibria for the slightly soluble weak acids when the wood is suspended in water:
To simplify the things let us consider one weak acid
The solubility and ionization of a hardly soluble wood weak acid can be increased when a weak base is introduced in the solution, where it reacts with hydronium ions released from wood acid and forms a conjugate weak acid (of the weak base). With disodium hydrogen phosphate as the weak base, the reactions below would take place:
For the extraction medium we found the disodium phosphate as an appropriate weak base. Its dissociation constant is 1.6
Chemicals of high purity (Merck) were used. 0.1 mol/L Na2HPO4 solutions were prepared by dissolving 71.628 g Na2HPO4
The wood specimens in form of discs were taken from the stems of black pine, alder, beech, spruce, and chestnut. Fresh wood was first chipped in a chopper machine and chips were air-dried in the shadow for about one month. After grinding and sieving, the 40 to 100 mesh fraction was used in the experiments.
The determination of wood acidity of each species was carried out in a series of eight to nine experiments where different weight of wood meal was suspended in 200.0 mL of 0.1 M disodium hydrogen phosphate for 24 hours. Based on the pH of the blank phosphate solution the lowest amount of wood was selected such that a pH drop of 0.5–0.6 was achieved, while this value for the greatest amount was kept between 1.0 and 1.1. The lowest and highest amounts are lying between one and ten g for most woods.
Titrations were performed in an automatic titrator (785 Titrino, Metrohm, with automatic temperature correction), the electrode of which is calibrated with the certificated buffers 7.00 and 9.00. Dynamic end-point titration method (DET, addition of variable volume increments to acquire about equal pH differences) was chosen with the equilibrium time of 30 seconds before the new reagent dispensing. The Metrohm software Tinet 2.4 enabled the evaluation of data. The desired portions of standard solutions up to 20.000 mL during some experiments were added with the dosimat (765 Dosimat, Metrohm). The phosphate solutions (100–200 mL) were measured with volumetric pipets.
From (
To see the falsifying effect of the far going dilution in 0.1 M phosphate, the calculated mass of disodium phosphate dodecahydrate (7.1628 g) was transferred into 200.0 mL volumetric flasks to prepare 0.1 M phosphate solutions. A series of 0.1 M HCl standard solution (0.5 to 10.0 mL) was given to about 100 mL water portions which again were added to the disodium phosphate in volumetric flasks. The flasks were filled to specified volume with water and after inserting a stirring bar they were tightly stoppered, then, the solutions were stirred on a magnetic stirrer for 30 minutes.
Reacting strong hydrochloric acid with disodium phosphate should convert equivalent amount of weak base into its conjugate acid sodium dihydrogen phosphate. A 150.0 mL aliquot of each flask was titrated with standard alkali solution in titrator. A blank of 0.1 M phosphate solution was prepared and treated exactly in the same way, and its pH-value was measured under stirring. Since titrator acquires the pH value of solutions after 30-second equilibrium, the pH values of blank solution acquired for 1 minute after starting of stirrer were recorded. Stirring causes a pH fall in small extent (~0.05–0.09) occurring as a sharp drop at the very beginning (~0.09–0.11) followed by a slight rise. As the convention, the pH value of the blank was taken to mark the end point of titrations.
Though modern digital titrators with variable dynamic volume dispensing determine the inflection point(s) of a titration curve as end point(s), the Metrohm software Tinet 2.4 also enables to move along the curve and set these ones at desired pH values. In this way, the alkali consumption at a given pH value was read out for all titrations. Table
The effect of dilution on the titration of a strong acid in aqueous Na2HPO4 solution.
HCl | Alkali consumption mL | ||||||
added mL | End point set by device | % error | at pH 9.20 | % error | Corrected* | % error | Recovery**(%) |
0.50 | 1.3157 | 163.1 | 0.6333 | 26.7 | 0.4871 | −2.4 | 97.6 |
1.00 | 1.4693 | 46.9 | 1.1427 | 14.3 | 1.0044 | 0.5 | 100.5 |
1.50 | 1.7713 | 18.1 | 1.6347 | 9.0 | 1.5041 | 0.3 | 100.3 |
2.50 | 2.6199 | 5.0 | 2.6240 | 4.8 | 2.5089 | 0.2 | 100.2 |
4.00 | 4.1572 | 3.9 | 4.1053 | 2.6 | 4.0134 | 0.3 | 100.3 |
8.00 | 8.0636 | 0.8 | 8.0400 | 0.5 | 8.0095 | 0.1 | 100.1 |
10.00 | 9.9916 | −0.1 | 9.9947 | −0.1 | 9.9947 | −0.1 | 99,.9 |
*The data in the fourth column corrected according to (
**Based on corrected alkali consumption.
From the results in Table
The plots of alkali consumptions in second and fourth columns versus the HCl inputs (first column) result in the curves shown in Figure
Graphical evaluation of alkali consumption of HCl-titration in Na2HPO4 solution.
The plot of consumption at the given pH value (9.2) results in a linear correlation with an intercept on ordinate axis. Since addition of 10 mL HCl gives the concentration in which the acid is being titrated quantitatively, a correction of alkali consumption can be performed in the way that the intercept value is subtracted from the consumed amount by decreasing it gradually according to the formula
As the lowest allowable concentration has its limit so does the highest one too. Considering reversible reactions (
Stearic acid is a weak organic acid (MW = 284.49 g/mol) with very low solubility in water. It was used to test the ability of the weak bases sodium acetate and disodium phosphate to promote its dissociation. In order to estimate the acidity in the water or in a neutral electrolyte amounts between 0.2 to 1 g stearic acid were suspended in 200.0 mL of pure water or in 0.1 M NaCl solution in erlenmeyers with ground stopper. After 24 hours each suspension was filtered in a gooch crucible through fine porous ashless filter paper. Filtering is accomplished by applying light vacuum and 150.0 mL filtrate was taken for titration. The highest stearic acid recoveries in water and in NaCl solution were about 0.7 and 1%, respectively. A bit higher yield in NaCl solution is attributed to the well-known electrolyte effect in analytic chemistry [
A series of 7 weighing between 0.5 to 2.0 g of stearic acid was then suspended in 200 mL of 0.1 M sodium acetate solution in erlenmeyers which were treated in the same way. The results of titrations gave a nonlinear plot between mL consumption of alkali and the weight of acid, where the highest yield of ca. 5% is observed for the lowest weighing, while the yield decreased to about 1% in case of 2 g stearic acid. Since the highest weight (2 g), corresponding to about 7 mmol of stearic acid, was suspended in 20 mmol of acetate we thought in the beginning that this was the excess of sodium acetate enough during these experiments. However, because of the stearic acid is sparely soluble and since it is based on the whole stearic acid amount its insoluble part only lowered the yield. To determine the maximum acid recovery we decided to lower the amount of stearic acid down to one hundred milligrams in case of sodium acetate as extraction medium.
Taking secondary phosphate solution too, another series experiments was conducted in the same way described above. The related data of both experiments were summarized and visualized in Table
Determination of stearic acid dissociated in CH3COONa and Na2HPO4 solution.
Sodium acetate | Disodium hydrogen phosphate | ||||
Alkali consumption at pH 7.8 (mL) | Stearic acid weight (g) | Yield* (%) | Alkali consumption at pH 9.2 (mL) | Stearic acid weight (g) | Yield* (%) |
0.672 | 0.1040 | 6.78 | 2.344 | 0.1430 | 28.85 |
0.831 | 0.1526 | 7.58 | 3.967 | 0.2904 | 30.26 |
0.936 | 0.2044 | 7.13 | 5.536 | 0.4298 | 30.94 |
1.052 | 0.2505 | 7.13 | 6.673 | 0.5580 | 29.69 |
1.228 | 0.3008 | 7.60 | 8.376 | 0.7173 | 29.92 |
1.335 | 0.3513 | 7.37 | 9.764 | 0.8572 | 29.69 |
1.451 | 0.4045 | 7.22 | 11.467 | 1.0011 | 30.31 |
1.573 | 0.4565 | 7.16 | 12.801 | 1.1401 | 29.97 |
Avg. Yield | 7.25 | Avg. Yield | 29.95 |
*Based on standard alkali consumption corrected for intercept.
Titration of different amount of stearic acid in Na2HPO4 and CH3COONa solutions.
The third and last columns (Table
The stearic acid recovery in secondary phosphate solution is about 4 times bigger than that in acetate. In woods too, we obtained 3-4 times higher acidities with phosphate in comparison to the acetate (unpublished results). The stronger are the wood acids the higher will be the yield by the extraction with a weak base. The stearic acid should be considered as an example of a very weak acid with sparing solubility and its contribution to the wood acidity is then negligible. Even so, around one-third of stearic acid can be caught by the secondary phosphate as demonstrated above.
The advanced ionization of stearic acid in secondary phosphate allowed for the development of a new method by using Na2HPO4 solutions to estimate the wood acidity when the wood is exposed to the moderate conditions. In this connection, the appropriate concentration of secondary phosphate solution and the time for reaching the equilibrium were searched and optimized first.
At high ionic strengths, the interpretation of the behavior of solutions is difficult and therefore 0.1 M solution was selected as the highest concentration. Together with the 0.05 and 0.025 M Na2HPO4 solutions, three different concentrations were tested for the periods up to 24 hours (1, 2, 4, 8, 16, and 24 h). On absolute dry wood basis,
Effect of the time and concentration of Na2HPO4 solution on the extractability of wood acids.
As expected from the reversible equations given in Section
Series of experiments including 8 to 9 determinations for each wood species were carried out to assay the related acidity. The results of three examples from soft- and hardwoods including one with extremely acidic wood (black pine, alder and chestnut heartwood) are given in Table
Estimation of wood acidity by means of titration of wood-phosphate solutions.
Black pine | Chestnut | Alder | ||||||
mmol/100 g* | mmol/100 g* | mmol/100 g* | ||||||
1.959 | 1.0059 | 15.60† | 2.191 | 0.1498 | 65.67† | 2.053 | 1.5297 | 9.76 |
2.823 | 1.5016 | 16.21 | 3.121 | 0.2489 | 76.91 | 2.616 | 2.0236 | 10.16 |
3.716 | 2.0215 | 16.46 | 4.365 | 0.3994 | 79.08 | 3.161 | 2.5155 | 10.34 |
4.485 | 2.4981 | 16.39 | 5.531 | 0.5483 | 78.86 | 3.736 | 3.1660 | 10.03 |
5.383 | 3.1080 | 16.07 | 6.497 | 0.6974 | 75.86 | 4.176 | 3.5751 | 10.11 |
6.148 | 3.5011 | 16.45 | 7.693 | 0.8465 | 76.63 | 4.712 | 4.0781 | 10.18 |
6.871 | 4.0128 | 16.15 | 8.800 | 0.9912 | 76.60 | 5.707 | 5.0634 | 10.16 |
7.657 | 4.4847 | 16.20 | 9.897 | 1.1486 | 75.66 | 7.629 | 7.0145 | 10.08 |
9.296 | 5.5042 | 16.18 | ||||||
Avg: | 16.26 | Avg: | 77.1 | Avg: | 10.10 |
*Based on alkali consumption corrected for intercept, †not included to the average.
Graphical evaluation of consumed alkali of titration in dependence on sample weight.
The highest acidity found in the heartwood of chestnut originates particularly from tannins, while the resin acids in black pine are apparently responsible for the somewhat greater value. The acidities of the beech and spruce woods (not shown in the table) amounted to 11.5 and 12.0 mmol/100 g, respectively. Because of the very low resin acid content, the spruce exhibits a decrease in the wood acidity comparing to another softwood black pine.
On the other side, the acetyl groups and 4-O-Me-glucuronic acid residues attached to the polyoses (xylans in hardwoods, mannans in softwoods) can be considered as the main source of wood acidity [
During the treatment with secondary phosphate, many weak acids in wood participate in reactions and at the end they are practically converted to one weak acid (NaH2PO4, conjugate acid of secondary phosphate). This is reflected by the titration curve where a modern and sensitive titrator itself can detect only one inflection point as the end point (Figure
The typical titration curve of the Na2HPO4/NaH2PO4 solution obtained as filtrate from the wood suspension.
On the other side, the titration curves of the cold and hot water extracts as well as the extract from sodium acetate gave in average 3-4 end points indicating the presence of the acids of different strength (Figure
The titration curve of a hot water wood extract.
From the results evaluated above and from our experiences with the phosphate solutions the following methods can be suggested for the determination of wood acidity that can be effective on the wood under moderate conditions in practice like production of plywood, particle- and fiberboard.
(1) Method with a series of determinations: as explained and applied in this paper, 7 to 9 determinations should be preferred to assay the wood acidity. Therefore, the preparation of 2.0 liters 0.1 M Na2HPO4 solution is recommended as stock. (Use always distilled and briefly boiled water observing precautions that the water should not get excessive contact with air during cooling down and during experimenting otherwise.) For example, 9 determinations will use 1.8 liters of stock solution and about 100 mL blank are also kept in an erlenmeyer flask under same conditions. 300 mL erlenmeyer flasks with ground stopper should be used. First the wood is weighed in the flasks and they should be tightly closed soon after the 200.0 mL of phosphate solution is added. Two or three trials can be made before to estimate the pH drop caused by a given amount of wood. A series of weighing can then be made between the smallest and largest one, corresponding to a pH fall between ca. 0.6 and 1.1 from the pH of blank solution. Preparing vacuum flask, Gooch filter, and Filtering takes about 15 minutes, so it is advisable to put 20-minute intervals between determinations. The blank solution should be treated in the same way as the others and its pH value should be measured first under the same conditions the titrations made. This value serves the reference pH for end point of titrations. We observed a very small difference (0.01–0.02 decrease) in pH between untreated and filtered blank solutions.
The flasks are then kept at 25 ± 2°C for 24 hours. The higher and lower temperatures affect the equilibrium resulting in some increase and decrease in the acidity. It is then recommended to keep the flasks in a water bath with a thermostat when the room temperature exceeds the limits.
After the titrations with standard 0.1 M NaOH solution, the weights of wood are plotted against mL consumption of alkali (
(2) A simpler way to determine the acidity is to prepare such 2-3 wood suspensions, each in 200 mL phosphate, where after 24 hours a decrease in pH about 1.2 from the pH-value of blank can be achieved. The mL alkali consumptions can then be taken and averaged for the calculation of wood acidity in good approximation to the first method.
In the presence of Na2HPO4, the reactions of wood acids generate the conjugate acid NaH2PO4, thus a buffer system occurs in the solution. On the other side the conjugate bases of wood acids are also present in the medium. The excess of phosphate buffer will mainly determine and stabilize the pH value of solution. Indeed, the phosphate solutions after the suspended wood was filtered off displayed a stable pH which can be measured as exactly as ±0.001 in the still state. This observation brought us to the idea, to determine the acidity by using the decrease in pH which would then correspond to a known amount of standard alkali solution that is equal to the promoted wood acidity.
Graphical evaluation of the titration data of NaH2PO4/Na2HPO4-system.
The plot of pH versus calculated amount of mL (7.500-alkali consumption at the titration) results in a curve which is expressed by a polynomial equitation of the power of four. Achieving equitation with very high
Care should be exercised at the measurements of pH-value of wood filtrates. Here again the same conditions were applied, which were effective during the titration of Na2HPO4/NaH2PO4 solution with the device. Table
Estimation of wood acidity as a function of pH-value of wood-phosphate solutions.
Black pine | Chestnut | Alder | |||||||||
pH | mmol/100 g | pH | mmol/100 g | pH | mmol/100 g | ||||||
8.78 | 1.9820 | 1.0059 | 15.49† | 8.715 | 2.3614 | 0.1498 | 61.18† | 8.763 | 2.0772 | 1.5297 | 9.68† |
8.647 | 2.8075 | 1.5016 | 15.87 | 8.576 | 3.3368 | 0.2489 | 76.01 | 8.673 | 2.6304 | 2.0236 | 10.05 |
8.529 | 3.7283 | 2.0215 | 16.35 | 8.437 | 4.6052 | 0.3994 | 79.13 | 8.598 | 3.1690 | 2.5155 | 10.15 |
8.449 | 4.4818 | 2.4981 | 16.24 | 8.334 | 5.7905 | 0.5483 | 79.26 | 8.525 | 3.7677 | 3.1660 | 9.95 |
8.365 | 5.4087 | 3.1080 | 16.04 | 8.264 | 6.7404 | 0.6974 | 75.93 | 8.481 | 4.1711 | 3.5751 | 9.93 |
8.309 | 6.1153 | 3.5011 | 16.26 | 8.192 | 7.8575 | 0.8465 | 75.75 | 8.424 | 4.7421 | 4.0781 | 10.03 |
8.254 | 6.8867 | 4.0128 | 16.10 | 8.127 | 9.0021 | 0.9912 | 76.24 | 8.340 | 5.7148 | 5.0634 | 9.98 |
8.206 | 7.6283 | 4.4847 | 16.06 | 8.075 | 10.020 | 1.1486 | 74.66 | 8.204 | 7.6607 | 7.0145 | 9.96 |
8.115 | 9.2286 | 5.5042 | 16.00 | ||||||||
Avg: | 16.11 | Avg: | 76.7 | Avg: | 10.1 |
†Not included into the average calculation.
Graphical evaluation of calculated alkali consumption in dependence on sample weight.
In comparison to the titrations, smaller yields were obtained from the calculations of wood acidity as a function of pH. Somewhat bigger intercept values notable here are responsible for the slight decrease in yields but after all a good agreement between the predicted (calculations, Table