Standard: TAPPI
Method: T 435
Title: Hydrogen ion concentration (pH) of paper extracts (hot extraction method)
Overview
This method measures the hydrogen ion concentration, expressed in terms of pH, of an aqueous extract of paper obtained by hot extraction (unfiltered and extracted by boiling water for one hour). It may be applied to writing, printing and sized industrial paper, but it is not intended for unbuffered types such as electrical insulating and condenser papers. Values determined by this method will reflect changes resulting from heat-induced hydrolysis. Additives, such as those used in filled and coated papers can have an effect on the extract pH. The cold extraction method is described in TAPPI T 509, “Hydrogen Ion Concentration (pH) of Paper Extracts (Cold Extraction Method).” Surface pH measurement of paper is described in TAPPI T 529, “Surface pH Measurement of Paper.” pH determinations by the different methods are not comparable.
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 hydrogen ion concentration (pH) is more indicative of the stability of paper than is the total acidity.
Samples
This test is performed in duplicate. At least 5 sheets of paper should be submitted for each sample. Gloves should be worn when handling the papers. Do not include any papers touched with the fingers.
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