A Comparison of Three Methods for Determining the pH of Wood and Paper

The procedure generally used for determining the pH of wood and paper consists of immersing the material in distilled water for various lengths of time, with or without heating or stirring, and then determining the pH of the water extract. A number of variables in this procedure can affect the pH determination, including degree of extraction of the components of the test material. The question is also raised as to whether this method gives the pH of air-dry material as well as the pH of the equilibrium-solid water system. A second method consists of suspending the wood in dilute unbuffered solutions of acids and bases which have different pH values. The pH of the solution which is not changed by adding the wood, or the intersection point at which no change in pH occurs from plots of the pH change against the original pH, represent the true pH of air-dry wood. A recently developed glass electrode for use on semi-plastic surfaces was used to determine the pH of various wood species. Thin cross sections of four different softwoods, four hardwoods, and multiple thicknesses of four. different papers, each weighing approximately 4 grams, were soaked in 25 milliliters of distilled water and in 25 milliliters of unbuffered dilute solutions of sodium hydroxide and of hydrochloric acid having pH values ranging from 2 to 12. The pH of solutions was determined after various periods of contact with the samples using the new pressure-type glass electrode. Surface contact pH values were measured on specimens after removal from the distilled water. Comparisons were made of the determinations made in distilled water, on surfaces of specimens removed from distilled water, and from the intercept of the plot of the equilibrium pH of the various unbuffered solutions against their original pH with the line representing equal original and equilibrium pH values. The values determined in these three methods showed good agreement when specimens were allowed to soak for at least 1/2 hour before making measurements. The plots show that both wood and paper exhibit significant buffering action upon the solutions that increases in degree with time and varies considerably between different samples.

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