| Abstract: | SUMMARY. 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. 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. |
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