The Analysis of Alkaline Reserve in Paper after Deacidification
by VLADIMIR BUKOVSKY
INTRODUCTION
The aim of deacidification is the elimination of acids occurring in paper and catalyzing hydrolytic
reactions leading to the cleavage of the cellulose fiber, the basic structural substance of paper. Acid
in groundwood paper is created by the hydrolysis of aluminum sulphate, one of the substances used
in the production of this kind of paper. Another source of acidity is formed by organic acids and
acid function groups being able to exchange H+ (or H3O+) ions. These acids and groups are formed
in handmade rag paper as well as in groundwood paper by natural ageing and are the components of
the cellulose (oxicellulose) molecules, lignin, sizing agents1, or they occur in paper as free lowmolecular, non-volatile2, 3 and low-molecular, slowly volatile degradation products4,5.
Another aim of deacidification is the creation in the paper of sufficient alkaline reserve (AR)6,7 to be
able to prevent the paper, during several centuries of storage, from becoming acidic, either by
natural oxidative degradation, further hydrolysis of aluminum sulphate, which is not removed from
paper during deacidification, or by the influence of environmental pollutants. Every deacidification
method, either mass or single sheet, results in the creation of such a reserve (usually formed by
magnesium carbonate or calcium carbonate).
As well as prolonging the lifetime of paper, resistance to outer detrimental factors such as, for
example, the dangerous influence of light and acidic air pollution, is closely related to the extent of
AR8.
The problem we tried to solve in this research was the difficulty of determining in advance, the
extent of alkaline reserve left in a paper after a deacidification treatment. This uncertainty is due to
the enormous differences to be found in the material generally called „paper", i.e. differences in
composition, quality, use, state of ageing, etc. The active element and the concentration of the
deacidification substances used to eliminate the acidity of paper, however, are known and can be
modified if necessary.
MATERIAL AND METHODS
Paper samples
We used the same paper samples as in our previous work6,8; their composition is not repeated here.
These are the samples used for this research:
• Nl: Newsprint made in Steti, 1996, Czech Republic;
• N2: Newsprint made in Vétrní, 1996, Czech Republic;
• C: Newsprint from Croatia, 1942;
• H: Newsprint from Hungary, 1881;
• Wl: Chromatographic paper Whatman 1 (pure cellulose);
Preparation of samples
Paper samples measuring 6 x l cm were conditioned for 2 days at 20±l°C and 45% RH and then
weighed. They were deacidified for 5 minutes using methano-lic solutions of methylmethoxymagnesium carbonate (MMMC) in different concentrations. The solutions were prepared
by diluting the 10% stock solution into weight concentrations of 1%, 2%, 4% and 6% active
substance. The 1% solution contained 0.41 mol/1 Mg or MMMC. After deacidification the papers
were reconditioned for 2 days at 45% RH, 20±l°C and then weighed again. The alkaline reserve
ARg , i.e. gravimetric, which should refer to magnesium carbonate, was calculated as a weight
increase. Parallel to this treatment, samples of the same paper were modified only by soaking in
pure methanol for 5 minutes.
Preparation of water extracts to determine pH and potentiometric titration
Water extracts were prepared by weighing the 1x6 cm paper strips cut into small pieces and
extracted in 20 ml distilled water (pH 7.0) for 10 minutes at 100°C. After cooling the pH of the
extract was measured and a potentiometric titration with 0.02 mol/1 HCl was done during which
time the pH was continually measured using a pH-meter M-120 (Mikrotechna, Czech Republic).
There were three parallel titrations.
Determination of alkaline reserve by potentiometric titration
Titrating this preparation and evaluating ∆pH at each 0.05ml addition of the titration solution, 6-7
peaks were found indicating 6-7 equivalent points (EP),
Table 1 : pH at the equivalent points found during titration of the acid-base systems provided by
deacidifying the samples with 4% and 6% MMMC solution.
which most probably refer to the same number of acid-base systems. This number of EP was found
in all analyzed samples, i.e. in samples that were deacidified and contained a measurable alkaline
reserve. Since there was one significant point in the titration record, the calculation of alkaline
reserve was made for the EP area 1-3 (AR1-3) and for the total EP area (AR1-7). The titration curves
are given in Fig. 1, pH of the peaks in Table 1. Despite their similarity there was a significant
difference in the pH of some EP of paper H.
RESULTS AND DISCUSSION
When evaluating the alkaline reserve in papers deacidified according to the classic non-aqueous
method using MMMC, we tried, using two types of modern and two selected historic newsprints, to
suggest procedures that would result in the required alkaline reserve. Additionally, chromatographic
pure cellulose paper was submitted to the same analysis.
Alkaline reserve: pure cellulose paper W1
Pure cellulose paper is neutral and its end-of-titration is reached at pH 5.2. After deacidification in
solutions of various MMMC concentrations the amount of created alkaline reserve was proportional
to the concentration of the solutions; the dependence was linear (Table 2, Fig. 2). Even a high
increase in pH of the paper extract was related to deacidification. If just 1% MMMC solution was
used, the pH reached pH 8.6, and with increasing MMMC concentrations it gradually rose to pH
9.8. The end-of-titration pH value in the range of AR1-3 was the same for various ARs in the paper
and the extracts made from them; it ranged from pH 5.7 up to pH 6.0, the average being
approximately pH 6.0.
Fig. 2: Left column: growth of alkaline reserve after deacidification; : ARg stated by weight; O:
AR1-3 stated by titration; •: AR1-7 stated by titration. - Right column: curves marked by • refer to the
pH of water extracts before titration. Lowest point • on the ordinate refers to the end-of-titration of
papers, which were not deacidified (0% MMMC). Empty columns represent the pH of end-oftitration in the range AR1-3 black columns represent the pH of end-of-titration in the range AR1-7,
when titration was completed.
Table 2: Alkaline reserve (ARg; weight %) created by deacidifying the papers with MMMC
solutions of several concentrations.
The end-of-titration pH values in the range of AR1-7 varied from pH 4.0 to pH 4.6. Comparing the
ARg amount stated by weighing (weight %) and by titration in the range of AR1-3 (pH EP
approximately pH 6.0) it seemed that this titration, despite lower values, approached ARg, i.e. those
found by weighing.
We can assume that the acid-base systems bound to cellulose account for the difference between the
amount of AR1-3 found by titration, the amount of ARg found by weighing (which are very similar)
and the amount of AR1-7. This probably even includes the value of pH 5.2 found at the end-oftitration of the pure cellulose paper Wl.
Alkaline reserve in newsprints: Paper number N1
The original paper was acid sized with an extract of pH 4.0; its titration in the range of AR1-7
ended at ca. pH 2.2. pH 7.0 of the extract was not achieved by deacidification with MMMC
concentrations lower than 1.5%, and pH 9.5 of the extract was not reached until higher
concentrations of MMMC (>4%) were used.
The end-of-titration of paper in the range of AR1-3, i.e. those papers the pH extract of which had
not reached pH 7.0, was completed at between pH 3.4 to pH 4.5. The end-of-titration pH of about
pH 6.0 was reached in those papers, the pH extracts of which were higher than pH 7.0, irrespective
of the amount of AR. Measured values in the extracts up to the pH 7.0 represent the neutralization
of individual acid functional groups in the original paper components. This applies even to very
similar ARg.
The titration in the range of AR1-7 of papers that were not deacidified because a MMMC solution
of lower EP concentration had been used, was very close to pH 2.5-3.1, and the end-of-titration at
approximately, pH 4.0-4.2 was reached in papers, the pH extract of which was pH 7.0 or higher,
comparable to Wl.
We assumed that, in relation to acidic papers, ARg and titration in the range of AR1-3 gave relatively
accurate information on the size of AR. AR found by AR1-7 titration was somewhat inaccurate.
The difference between ARg, AR1-3 and AR1-7 probably referred to the titration of the acid-based
systems of the cellulose and of lignin, or of some other non-cellulosic paper components.
Paper N2
The original paper was slightly acidic (pH 5.8) and its end-of-titration (EP) was at pH 4.O. pH 7.0
of the extract was achieved with 1% MMMC solution. The titration curve was similar to that of Nl.
When titrating the paper in its water extract in the range of AR1-3 , the EP was reached at pH 4.44.5. The end-of-titration at pH 5.6-6.2 was reached when the AR, created by deacidification using
higher concentrations of the MMMC solutions were larger. This is comparable to the situation
found for paper Nl.
Since there was a very low content of acid groups in this paper (the extract was pH 5.8), the course
of the curve of ARg, found by weighing and by titration in the range of AR1-3, was a little different
from that of Nl. We assumed that AR1-3 relatively precisely reflected the amount of AR in the
paper.
The course of titration in the range of AR1-7 was very similar to that of Nl. Again the difference
between the two titrations refers to different acid-base systems], i.e. of cellulose, lignin and other
paper components.
Table 3: Expected capacity of acid-base systems, expressed as percentage, derived from the titration
curves and from the consumption of the titration agent.
Paper C
The paper was in the acid region (pH 5.3) and its acidity was completely titrated off at pH 3.2. pH
7.0 of the extract was reached by deacidifying the paper with 1% MMMC solution. On the titration
curve we again found 6-7 EPs, but from Table 3 it can be seen that the majority of them, i.e. 70%
were in the acid area (EP 4-7). In Wl this amount was 43%, and in Nl 51%-57%.
Titrating the paper in the range of AR1-3 and when the pH of the extract was nearly pH 7.0, the endof-titration was reached at pH ca. 4.2. When the AR was higher than 2% and the pH of the extracts
higher than pH 7.0, the end-of-titration was at pH 6.1-6.2. The end-of-titration in the range of AR1-7
was completed at pH 3.5-3.7.
The determination of AR1-3 provided significantly lower values than that of ARg;; that of AR1-7,
however, provided similar values.
Alkaline reserve stated by weighing (ARg) referred to a high percentage of degradation products
having EP 4-7 in the significantly acid area. This referred to very similar values in the titration
curve of AR1-7.
In this case the ARg found by weighing was considerably higher than the true alkaline reserve, and
values found by weighing should be revised according to the values of AR1-3 found by titration.
They are probably closer to the true AR.
Paper H
The paper was very acidic (pH 3.7) and the end-of-titration of its extract was completed at about pH
2.0. pH 7.0 in the paper extract was reached after deacidification with 4% MMMC solution, and a
6% solution modified the pH of the extract to numbers higher than pH 8.0. This situation was the
result of the paper being highly sized, which meant that it was only able to absorb relatively lower
amounts of the deacidification solutions9.
The titration curve was similar to other curves, but the share of acid-based systems in the acid area
was similar to that of the historic newsprint paper C. That is on the AR4-7 range paper H has a result
of 66.3% and paper C a result of 70.9% (Table 3).
The end-of-titration of extracts of pH higher than pH 6.0 in the range of AR1-3 was completed at pH
4.7-5.0, that of AR1-7 at ca. pH 3.0.
The titration in the range of AR1-3 recorded a low content of AR created by the deacidification of
this paper. It reached 1% when a 6% deacidification solution was used and 2% AR with the
approximately 10% MMMC solution (range of AR1-3).
Similarly as in paper C, the values of ARg were very close to the values of AR1-7. Compared to the
titration in the range of AR1-3, ARg referred to a large number of acid-based systems., which could
be titrated in acid area.
CONCLUSIONS
Pure cellulose paper Wl
• It is relatively easy to determine precisely the preparation of AR in such a paper and assign it to a
certain MMMC concentration. For potentiometric determination of AR it is necessary to titrate to
pH 6.0 (range of AR1-3). In these conditions there is a good congruence between AR determination
by weighing and by titration.
• It is possible to prepare a large alkaline reserve.
• The amount of titratable substances below pH 6 is small. Newsprints Nl and N2
• In modern newsprints it is possible to calculate relatively precisely the size of AR and prepare it
by means of various MMMC concentrations if the pH in extracts is higher than pH 7.0. The analysis
indicated a good congruence between ARg determined by weighing and AR1-3 determined by
titration to pH 6.0.
• At lower extract pH the end-of-titration in the range of AR1-3 is reached at lower pH and does not
refer to real AR.
• The amount of titratable substances below pH 6.0 is relatively large.
• The method of determining ARg, i.e. by weighing, provides relatively accurate results.
Historic newsprints C and H
• It is relatively difficult to reach a desired amount of AR in these papers using preparations of the
deacidification solution.
• There is a big difference between AR1-3 stated by titration and ARg stated by weighing.
• Even using high MMMC concentrations, the AR reached in these papers is only 1-2%.
• The amount of substances able to be titrated below pH 6.0 or pH 5.0 is large.
• Determination of AR1-3 by titration represents only 29%-34% of the capacity of acid-based
systems in the papers referring to the true AR; it is just about 50% of the ARg.
SUMMARIES
The analysis of alkaline reserve in paper after deacidification
The creation of sufficiently large alkaline reserves is, besides elimination of acidity, another important aim of any deacidification of groundwood papers. It is problematic reaching a required
alkaline reserve of 2% because the amount of alkaline reserve retained in the paper is determined by
the type of paper and its initial acidity. It is also difficult because the amount and concentration of
the chosen deacidification agent necessary for both aims must be adjusted to take both into account.
With a mass method this adjusting is almost impossible. The research reported aimed at finding the
relationship between the alkaline reserve measured as change of paper weight before and after
deacidification on one side, and by titration on the other
Analyse de la réserve alcaline du papier après la désacidification
Parallèlement à l'élimination de l'acidité un autre but important recherché par la désacidification de
papiers est de créer une réserve alcaline suffisante. Ce n'est pas facile d'obtenir la quantité souhaitée
de 2% de réserve alcaline car il faut ajuster la quantité et la concentration de la solution de
neutralisation à chaque type de papier et à son pH d'origine. Ceci est presque impossible lors d'une
neutralisation de masse. L'objectif poursuivi par les recherches en question était d'établir une
relation entre les quantités de réserve alcaline obtenues en mesurant le poids du papier avant et
après la désacidification et les résultats obtenus par titration.
Messen der alkalischen Reserve in Papier nach einer Neutralisierung
Das Ziel der konservatorisch-restauratorischen Neutralisierung von sauer gefertigtem holzhaltigem
Papier ist, neben dem Neutralisieren von dort vorhandener Säure, die Schaffung einer
ausreichenden alkalischen Reserve. Es ist schwierig eine angestrebte Menge von ca. 2% zu erreichen, weil Menge und Konzentration der Neutralisierungslösung entsprechend dem Papiertyp und
seinem Ausgangs-pH eingestellt werden müssen. Bei einer Massenneutralisierung ist dies geradezu
unmöglich. Ziel der Untersuchungen, über die berichtet wird, war es, eine Relation zwischen den
durch Wiegen und den durch Titration gemessenen Mengen an alkalischer Reserve zu finden.
REFERENCES
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Vladimir Bukovsky
Slovak National Library
Preservation Service Branch
Nám.J.C. Hronského I, SK-
036 01 Martín
Slovak Republic
E-mail: bukovsky@snk.sk