Filter paper is a specialized porous material used across scientific fields to separate solid particles from a liquid or gaseous suspension. This technique, known as filtration, relies on the paper acting as a barrier, allowing the fluid to pass through while trapping the solid material. Understanding how to select the correct paper and properly execute the filtration process ensures the efficient removal of solids from the liquid phase.
Understanding Filter Paper Types
The selection of filter paper depends on the purpose of the filtration, dictating the required purity and particle retention. Filter papers are broadly classified into qualitative and quantitative grades, distinguished by their ash content after ignition. Qualitative paper is suitable for general separations where the solid is not being measured, typically containing a higher ash content (generally not exceeding 0.13%).
Quantitative filter paper, often called ashless paper, is designed for precise analytical techniques like gravimetric analysis, where the solid material is dried and weighed. These papers are acid-treated to achieve an extremely low ash content, often less than 0.0009%, ensuring the paper’s residue does not interfere with the measurement. Papers are also specified by their pore size, which determines the smallest particle they can reliably retain and affects filtration speed. Coarse papers have larger pores for fast flow and retention of large particles, while fine papers retain minute precipitates but result in a slower flow rate.
Performing Standard Gravity Filtration
Gravity filtration is the most common technique and relies solely on the weight of the liquid to pull it through the filter medium. The process begins with preparing the filter paper, which should be folded to fit within a glass funnel, with the paper’s edge sitting slightly below the funnel rim. A simple cone fold involves folding the paper twice to form a quarter circle, creating a cone shape for the funnel. For faster filtration, the paper can be “fluted” or pleated, creating multiple folds that maximize the effective surface area and allow air to escape easily.
The apparatus is assembled by supporting the funnel over a receiving flask, ensuring the funnel stem touches the inside wall of the flask to prevent splashing. Once the paper is placed in the funnel, it should be moistened with the solvent to secure it against the glass wall and create a seal. When pouring the liquid mixture, a stirring rod should be used to guide the flow of the suspension onto the filter paper, preventing the liquid from running down the outside of the beaker.
It is important to keep the liquid level below the top edge of the filter paper at all times, as overfilling would allow the solid particles to bypass the paper and flow into the receiving flask. Initially, the supernatant liquid—the liquid above the settled solid—is poured through the filter first to prevent the paper from becoming prematurely clogged. After most of the liquid has passed through, the remaining solid residue is carefully transferred to the filter using a wash bottle to rinse the particles from the original container. The solid is then rinsed with a small amount of clean solvent to remove any residual impurities, with the washings also passing through the paper.
to Vacuum Filtration
Vacuum filtration is an alternative method used when the gravity technique is too slow or when the solid precipitate is the desired product that needs to be collected quickly. This method is significantly faster because it uses a pressure differential to actively pull the liquid through the filter medium. A vacuum pump or aspirator is used to reduce the air pressure inside a specialized thick-walled filter flask, which creates a suction effect.
The specialized equipment includes a Büchner funnel, which has a flat, perforated base to hold the filter paper, and the side-arm filter flask. The filter paper must be selected to fit flat and completely cover the holes on the funnel’s base. This technique is particularly effective for separating fine particles or filtering viscous solutions that would otherwise take a long time to pass through a gravity filter.