What is the pH of alkaline paper?

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 spectrophotometric 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 CO2 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 CO2 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 CO2 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.