720 KNOWLEDGE FILTRATION Hint on the ideal filter diameter Diameter of the filter hopper d1 in mm versus recommended diameter d2, filter circles folded double (or diameter of folded filters) The major filter types: Ashless filter papers for quantity analysis are acid-washed and water-neutralized. Free of minerals and metal ions, they are ideal for the detection of trace elements (mean value 0.004% ash), gravimetric routine tests and for sample preparation for instrumental analysis. For their high purity (mean ash content 0.08%), highly pure filter papers for quality analysis are recommended for precise identification of materials and for sample preparation prior to sensitive detection methods. These reliable papers are perfectly qualified for food control, beverage analysis and environmental monitoring. Glass fibre filters with and without binder are particularly recommended for monitoring air and water pollution. Compared to paper filters, they have a high flow rate and air permeability, extremely low metal content, and good stability at high temperatures. Cellulose or glass fibre extraction thimbles allow for reliable, fast analyses in food control and environmental monitoring. Suitable for Soxhlet type, Tecator type or similar devices, to extract certain components out of solid material with an appropriate solvent. The high, even porosity of the sleeves ensures a fast flow. The selection of the appropriate filter type for analytic separation depends on many different factors. The chemical and physical properties of the sample you want to filter must be considered, as well as those of the subsequent analysis or further processing of the precipitation or filtrate. The following questions help you to reliably find the filter which best meets your requirements: Important questions for selecting the appropriate filter type: • Are you looking for a filter for filtration or absorption (impregnation, sampling), blotting or chromatography? Select the appropriate product group with the respective properties! • Clarification (liquid is used further) or separation filtration (analysis of the separated particles)? In clarification, all particles should be removed. In separation filtration, only the particles of a specified size are to be captured. Choose the appropriate retention rate! • What is the size of the particles you wish to capture? Choose a paper type with a matching retention rate! • What does a retention rate of e.g. 12–15 µm signify? This paper retains only some of the particles of size 12 µm but all particles of size 15 µm and up. If you need to capture all 12 µm particles, use a retention rate of e.g. 8-12 µm. • Do you wish to filter air? Often, glass fibre papers are better suited for filtering air because they have a higher permeability but still capture particles down to 1 µm. • How many particles do you wish to capture? Thick paper can carry more particles than thin paper! • Which is the sample volume? Because creped papers have a larger filter surface, they are better suited for large volumes in the laboratory funnel. This applies independent of the filtration time and is due to the particle retention in µm. • What is the pH and the pressure? With high pressure or vacuum and/or extreme pH values, hard filtration papers are more appropriate than the regular wet-strength types. • What is the temperature and viscosity? Highly viscous samples should be warmed to reduce viscosity! • Which is the optimum filter type? The type that ensures complete separation of the target particles at an acceptable filtration rate. d1 mm 35 45 55 70 80 100 120 150 180 200 250 d2 mm 45–55 55–70 70–90 110–125 125–150 150–185 185–240 240–270 270–320 320–385 385–400
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