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Acid-soluble iron is considered that portion of the iron present which is potentially chemically reactive, as distinguished from the insoluble or "fixed" iron which might occur as a silicate or other complex compound in clay filler. This procedure for acid-soluble iron is preferable to that of extracting the ash from the paper with acid, because it requires less time and avoids change in the nature of iron compound which may result from ashing. The method is applicable to paper or paperboard containing approximately 10-200 ppm acid-soluble iron.