Whatman No. 41

Filter paper is very fast. ASTM initial filtration speed is 12 seconds. Paper is for filtration of coarse particles and gelatinous precipitates. Loading capacity is normal. Retention rating is 20-25 µm. Extensively used in metallurgical laboratories for filtering, e.g. iron or aluminum hydroxides. Exceptional for filtration of metallic aerosols in air pollution monitoring. 0.11 mg ash content per 12.5 cm circle. (For cost savings, see Ahlstrom equivalent, 445-200-02 Series)