DETERMINATION OF pH OF PAPER
Drago Kočár*1, Matija Strlič1, Jana Kolar2, Boris Pihlar1
1 University of Ljubljana, Faculty of Chemistry and Chemical Tehnology, Aškerčeva 5, SI-1000
Ljubljana, Slovenia 2 National and University Library, Turjaška 1, SI-1000 Ljubljana, Slovenia *
corresponding author: drago.kocar@uni-lj.si
1. Introduction
Of all paper properties, acidity/alkalinity is probably the crucial parameter for stability of its most
important structural element, cellulose1. Considering the role of this parameter also in permanent
paper standards, its proper determination is of primary importance. Several procedures for
determination of paper pH have been evaluated, ranging from standard extraction procedures,
surface pH, "micro'" pH determination, and use of coloured acid/base indicators, on 55 different
samples. This work is published as a full paper elsewhere.2
2. Materials and methods
Paper samples
Papers and pulps of various origins were used and the type of fibres was determined according to
SCAN-G3:90 and SCAN-G4:9() standards. From books, only paper parts without print were used.
Deacidified samples were prepared by immersion in solutions of 0.04 mol L-1 Mg(HCO3), or 0.01
mol L-1 Ca(HCO3)2. All samples were equilibrated at room conditions prior to use.
Determination of pH
The following procedures were used:
- Tappi 509 om-02 (cold extraction, 1 g of sample per 70 mL of water, 1 h);
- Tappi 529 om-88 (pH of paper surface using a flat combined glass electrode);
- Determination of pH using colour acid/base indicators-containing pen (pHydrion, Talas. New
York, USA);
- Determination of equilibrium pH: to 1.00 ±0.01 g sample, 70 mL MilliQ water was added. The
sample was stirred at 250 r.p.m. and pH was followed continuously until an equilibrium (up to three
days) was achieved.
- New method: to 0.070 + 0.005 g sample in a reaction tube, 5 mL MilliQ water is added,
previously aerated for 30 min in order to obtain a CO2-saturatcd solution (for samples with pH > 7).
Samples are defibrillated in a reaction tube using a precision hand drill with a stand at 25,000 r.p.m.
for 15 min.
- "Micro" determination of pH: samples are cut out with a sharpened surgical needle of inner
diameter 0.8 mm. The average weight of such samples depended on the grammage of a particular
paper, but was approximately 10 μg. Aerated water was added (2 μL) and the pH was determined
once the reading
stabilised (after max. 5 min) using the MI-415-2cm micro combined glass electrode
(Microcleclrodes Inc., Bedford, USA).
3. Results and discussion
Water in paper, if present in sufficient quantities, may contain dissolved matter, some of which in
ionic state. While, at present, the concentration of H3O+ ions in this matrix cannot be measured, the
compounds can be extracted and the pH of extract determined. It is evident, though, that pH in
paper will be affected by its water content.
Determination of pH is relatively straightforward if paper samples arc acidic. For such samples, all
extraction procedures will give data in good agreement with the standard cold extraction procedure.
Irrespective of the fact that the true pH in cellulosic fibres is difficult to determine, it seems that
CO2 should be taken into account when determining pH of alkaline paper, since the material is
usually in equilibrium with the atmosphere in which it is stored and this regularly contains CO2.
None of the present standard procedures take this into account.
A comparison between the equilibrium values and the values obtained with the standard extraction
method (Fig. 1) shows that the standard method gives values up to 1 pH unit too low for gelatine
surface sized samples and values up to 2 pH units too high for samples containing MgCO3. The
agreement between the two methods is excellent only for acidic samples.
Figure I: A comparison of the determinations of equilibrium pH values with those obtained with the
standard procedure. Only acidic samples are included in the correlation.
Figure 2: A comparison of pH values oľ paper extracts obtained by the new method (N = 5) and the
determinations of equilibrium pH. All samples included in the correlation.
With the new procedure, a remarkable correlation with the determinations using the method of
determination of equilibrium pH is evident (Fig. 2), in which dispersions of paper samples are given
ample time to equilibrate with the atmosphere. In the correlation, all samples arc included, even the
surface-sized, although these samples are not homogeneous.
If samples are alkaline, the following can be concluded:
- If the duration of extraction is short or if the mixing is not adequate, the slow dissolution of
alkali-earth metal carbonates and the slow absorption of atmospheric CO2 will not allow for proper
equilibrium to be achieved. Determinations of pH of such alkaline extracts using the standard
extraction methods will lead to pH values which may be up to 1.5 pH units too high. The
repeatability is nevertheless below 0.15 pH units, yet sample consumption is high: 1 g is needed.
- With the surface method, the repeatability is lower, up to 0.30 pH units, yet this may also relied
sample inhomogeneity. The time of dissolution and the amount of water arc sufficient for rapid
dissolution of an equilibrium amount ol CaCO3 meaning that the pH of such samples determined by
the surface method will not correlate with the measurements obtained by the standard cold
extraction procedure. Determination of surface pH cannot be considered as a non-destructive
method, due to marks left at the edge of the wetted part of paper, along which paper degradation
proceeds faster. Gelatine used as surface sizing leads to lower surface pH of papers than the bulk
pH by up to a few units. This may be problematic if the decision on whether to deacidify or
not is based on such information.
- If using the standard cold extraction procedure, the time to achieve equilibrium is excessively
long especially in the case of MgCO3-containing papers yet it can be shortened to 15 min if
intensive mixing is used. Having optimised the type of mixing and the sample to water ratio, we
propose a new procedure. Using this new procedure, pH of CaCO3 containing pulps was determined
to be approx. 8.5, and the pH of MgCO3-containing pulps approx. 9.5. In paper, the type and
amount of additives may influence these values considerably. Repeatability of determinations using
the proposed procedure is lower than 0.12 pH units, the amount of sample needed is 0.07 g. A rapid
equilibration (15 min) with atmospheric CO, can be achieved, which makes this method suitable for
routine determination of paper pH.
- Miniaturisation of the procedure using micro-combined pH electrodes leads to a satisfactory
correlation with the proposed method, yet with lower repeatability: values of 1.0 pH unit are
acceptable. The amount of sample needed for such determination is low (circular cut of diameter <
1 mm), yet several samples arc needed to obtain a satisfactory average value.
- Determination of pH using pH pens may provide-rough estimations, with errors up to several pH
units. Caution should be taken especially if degraded, intensively yellow papers are under
observation, as colour evaluation is then difficult. This may be problematic if the decision on
whether to deacidify or not is based on such information.
- Due to the slow dissolution of alkali-earth metal carbonates, changes in relative atmospheric
humidity and the resulting changes ol water content in paper lead to fluctuations in local pH in
paper. This should be taken into account in paper stability studies, if experiments with dynamic
humidity (humidity cycling accelerated ageing) are performed.
4. Acknowledgements
Ministry of Education, Science and Sports of the Republic of Slovenia is gratefully acknowledged
for supporting this research (Programme PI-0153, project LI-5237).
5. Reference
1. L. F. McBurney, Degradation of Cellulose. 1. Kinetics of degradation reactions, in: E. Ott, H.
M. Spurlin. M. W. Giaťllin (Eds.), Cellulose and cellulose derivatives. Vol. I, Interscience, New
York. 1954.
2. M. Strlič. J. Kolár, D. Kočar, T. Drnovšek, V. S. Šelih, R. Sušič. B. Pihlar, What is the pH of
alkaline paper?. e-PS. 2004, I. 35-47. Available on-line: www.e-preservationscience.org.