This method measures the hydrogen ion concentration (pH) of a cold aqueous extract (unfiltered) of paper. It is suitable for writing, printing, and sized industrial papers, but is not for unbuffered types such as insulating and consdenser papers. The determined values may not be exact and should not be interpreted in terms of solution theory. pH values are emperically correlated with end use requirements and paper qualities. This method avoids change of acidity or alkalinity resulting from heat-induced hydrolysis. It consists of a cold water (25 degrees Celcius plus or minus 5 degrees) extraction of the specimen for one hour followed by measurement with a commercial pH meter. The pH determination measures the extent to which the paper alters the hydrogen - hydroxyl equilibrium of pure water. The pH (acidity) is important because of its effect on the permanence of the paper. Although the acidity may be determined as the amount of water soluble acidity by titration with alkali, the pH is more indicative of the stability of paper than is the total acidity.