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 Concen-tration (pH) of Paper Extracts (Cold Extraction Method). Surface pH measurement of paper is described in TAPPI T 529, Surface pH Measurement