Filter paper

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Filter paper is a semi-permeable paper barrier placed perpendicular to a liquid flow. It is used to separate fine solids from liquids. In laboratories, filter paper is usually used with a filter funnel, Hirsch, or Buchner funnel.

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Filter paper comes in various porosities and grades. Different manufacturers have different standards, usually identified by parameters such as porosity, particle retention, and flow rate.

Choice of filter paper for a given task depends on four parameters: strength, compatibility, efficiency and capacity. A coffee filter is an adequate example to evaluate these parameters and how they affect performance and results.

If a coffee filter doesn't possess enough strength, it will tear or rupture, and many of the coffee grains will collect in the coffee pot. Compatibility describes a filter medium's resistance to degradation by heat and chemical attack; a filter that is not compatible with the liquid passing through it is likely to break down, losing strength (structural failure). (Filters made of certain plastics may not be compatible with organic solvents, for instance.)

Efficiency is the retention of particles in a target (size) category. The efficiency required is dictated by the particles or substances trying to be removed. A large-mesh filter may be very efficient at retaining large particles (e.g., leaves), but inefficient at retaining small particles (e.g., fine crystals, blood cells).

Capacity is the ability to "hold" previously removed particles while allowing further flow. A very efficient filter may show poor capacity, causing increased resistance to flow or other problems as it plugging up prematurely and increasing resistance or flow problems. A balance between particle capture and flow requirements must be met while ensuring integrity.

Filter paper is also sometimes called chromatography paper. Chromatography paper is pure white, so that colored compounds can be seen more easily. There are a number of different types of filter paper for specific applications: chemical analysis filter paper, qualitative filter paper, ashless filter paper, air filter paper, engine oil filter paper, fuel filter paper, etc.

Ashless filter paper is mainly used for gravimetric methods in quantitative chemical analysis. It has a base weight of 80 g/m²^{[citation needed]}.