The maximum ash content of these grades is intermediate between ashless and qualitative grades. They are particularly suitable for Büchner filtrations where it is desirable to recover the precipitate from the filter surface after filtration. Other characteristics include high wet strength and chemical resistance which are similar to the acid hardened ashless filter papers.
Hardened Low Ash Grades
Grade 50: 2.7 µm
Retention of very fine crystalline precipitates. The thinnest of all Whatman filter papers. Slow flow rate. Hardened and highly glazed surface. This finish also keeps the paper free from loose surface fibers. Highly suitable for qualitative or quantitative filtrations requiring vacuum assistance on Büchner or 3-piece filter funnels. Very strong when wet. Will withstand wet handling and precipitate removal by scraping. In the electronics industry, the virtual absence of fiber shedding is utilized in carriers for integrated circuits.
This grade is also available in Smear Tab format for wipe testing (e.g., testing of surfaces for radionuclide contamination).
Grade 52: 7 µm
The general purpose hardened filter paper with medium retention and flow rate. Very hard surface.
Grade 54: 22 µm
Very fast filtration for use with coarse and gelatinous precipitates. High wet strength makes this grade very suitable for vacuum assisted fast filtration of ‘difficult’ coarse or gelatinous precipitates.