What is the pH of Alkaline Paper?
| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English Slovak |
| ISSN: | ISSN 1581-9280 |
| Online Access: | http://www.viks.sk/chk/preservscience1.doc |
| Abstract: | CONCLUSION. Determination of operational pH is relatively straightforward if paper samples are 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 of paper is difficult to determine, we believe that CO2 should be taken into account when determining pH of alkaline paper. As with the present standard methods, this is not the case, a new method is proposed. If samples are alkaline, the following can be concluded: - If the duration of extraction is too 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 according to the standard extraction methods will lead to pH values which may be up to 1.5 pH units too high. The repeatability is still 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 reflect sample inhomogeneity. The time of dissolution and the amount of water are sufficient for rapid dissolution of the equilibrium amount of CaCO3, the pH of such samples determined by the surface method will therefore not correlate with the measurements obtained by the 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 low values of surface pH of otherwise neutral to moderately alkaline papers. - 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. - 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 are 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 of the indicator is then difficult. - If a solution of appropriate coloured acid/base pH indicators is added to paper, dried, and the reflectance spectrum of the spot is determined, the resulting spectra correlate with sample pH. The correlation depends on the choice of calibration standards. Furthermore, washing out the coloured degradation products may lead to erroneous results. SUMMARIES. In a comparative evaluation of seven procedures for determination of pH of paper, 55 different paper and pulp samples were used. Six procedures were based on cold extraction and subsequent determination of extract pH, either using a variety of combined pH electrodes or using a mixture of coloured acid/base indicators. One procedure involved impregnation of the sample with a solution of indicators and subsequent spectrophotomet-ric determination of pH of air-dry samples. The correlation of determinations showed that most extraction methods give comparable data for acidic samples, with the exception of gelatine surface-sized samples. For samples with alkaline aqueous extracts, the effect of atmospheric CO and slow dissolution of earth-alkali metal carbonates should be taken into account, which is not the case with any of the standardised methods for determination of paper pH. Since CO enters into equilibria as a weak acid, it will decrease the equilibrium pH of solutions of CaCO3 and MgCO3, the difference amounting to more than 1.5 pH units. A new procedure is therefore proposed for determination of pH of alkaline papers basing on rapid equilibration with CO and dissolution of carbonates. The repeatability of determinations is satisfactory, providing data with standard deviation less than 0.15 pH units. A discussion of the concept of paper pH follows, in view of the role of water content in air-dry paper and implications on stability of paper. The different procedures are compared in view of sample consumption, measurement repeatability and systematic error. |
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| ISSN: | ISSN 1581-9280 |
