Filter Papers and Membrane filters.

Laboratory filter paper is a semi-permeable barrier placed perpendicular to a liquid or air flow. It is used to separate fine solids from liquids or air.

Filter paper comes in various porosities and grades depending on the applications it is meant for. The important parameters are wet strength, porosity, particle retention, flow rate, compatibility, efficiency and capacity.

There are two mechanisms of filtration with paper; volume and surface. By volume filtration the particles are caught in the bulk of the filter paper. By surface filtration the particles are caught on the paper surface. Filter paper is mostly used because even a small piece of filter paper will absorb a significant volume of liquid.

The raw materials are different paper pulps. The pulp may be from softwood, hardwood, fibre crops, mineral fibres.

For laboratory use filter papers are made in a variety of ways since specific applications require specific types of papers. The raw materials might be acid washed wooden fibres, carbon or quartz fibres.

In laboratories, filter paper is usually used with a filter funnel, Hirsch, or Buchner funnel.

Ashless filter paper is mainly used for gravimetric methods in quantitative chemical analysis.

 

The link here is useful as it contains a basic guide to choosing the right filter paper grade.


Glass fibre filters are commonly used in laboratories.  The manufacturer Whatman for example offers two types of glass microfiber filters manufactured from 100% borosilicate glass: binder free glass microfiber that is chemically inert and binder glass microfiber.

These depth filters combine fast flow rates with high loading capacity and the retention of very fine particle, extending into the sub-micron range. Glass microfiber filters can be used at temperatures up to 500°C and are ideal for use in applications involving air filtration and for gravimetric analysis of volatile materials where ignition is involved.

 

There are a number of manufacturers and include Whatman, Millipore, Sartorius, Munktell, Pall, Nalgene and more.

 

A Membrane Filter typically traps contaminants larger than the pore size on the addressed surface of the membrane. Contaminants smaller than the rated pore size may pass through the membrane or may be captured within the membrane by other mechanisms. Membrane filters are typically used for critical applications such as sterilising and final filtration.

A useful weblink can be found here.

 

When choosing a membrane filter a number of factors have to be considered, for example:-

• Depth vs Membrane filtration
• Chemical compatability
• Hydrophilic vs Hydrophobic
• Pore size
• Thermal stability

Filtration is a huge area to cover with a short blog article but hopefully the links will prove useful for anyone looking to broaden their knowledge.