Gravity filtration is a laboratory technique used to separate a solid substance (residue) from a liquid mixture (filtrate) using only the force of gravity. The process involves pouring the mixture through a porous barrier, typically filter paper, which acts as a sieve to trap the solid particles. This simple, cost-effective method requires minimal specialized equipment, making it a common procedure for purifying solutions or isolating materials in educational and research settings.
Fundamental Principles and Purpose
Gravity filtration operates on the principle of particle size exclusion, driven by gravity. The suspension is introduced into a funnel containing a filter medium with microscopic pores. These pores allow liquid molecules and dissolved solutes to pass through freely. Separation occurs because the suspended solid particles are physically larger than the pores, retaining the solid material on the paper’s surface. The primary purpose is often to remove solid impurities from a desired liquid product (clarification), or to isolate a solid product from a reaction mixture.
The speed of filtration is proportional to the gravitational force acting on the liquid column. A tighter filter paper captures finer particles but slows the process, while a looser paper allows for faster flow but only traps larger solids. This technique is suited for separating relatively coarse solids or precipitates from a solution.
Essential Equipment and Preparation
The basic setup for gravity filtration involves three main components: a filter funnel, filter paper, and a receiving vessel. A standard long-stem glass funnel is supported over the collection flask, often an Erlenmeyer flask. The flask is preferred over a beaker because its narrow neck minimizes solvent evaporation and prevents splashing of the collected filtrate.
The filter paper must be sized so its top edge sits below the rim of the funnel when folded. While a simple cone fold is used, fluting is a more efficient method involving alternating folds that create multiple pleats. This fluting significantly increases the effective surface area, improving the flow rate by allowing air to escape. Before pouring, the folded filter paper is moistened with a small amount of the solvent being filtered. This step seals the paper against the glass, ensuring a tight fit and preventing the mixture from flowing around the paper instead of through it.
Executing the Filtration Process
Filtration begins by carefully pouring the suspension into the filter funnel. A glass stirring rod is used to guide the flow of the liquid from the source container, controlling the stream and avoiding splashing. Initially, only the liquid portion, known as the supernatant, should be poured, allowing the solid to settle to the bottom of the original container.
The liquid level in the funnel must be maintained below the top edge of the filter paper at all times to prevent the unfiltered mixture from spilling over and contaminating the filtrate. This decanting process helps prevent the filter paper from becoming prematurely clogged. Once most of the liquid has passed through, the remaining solid residue is carefully transferred to the filter paper. A wash bottle containing the pure solvent is used to rinse the solid from the walls of the original container, ensuring a quantitative transfer. Finally, the solid residue is washed one or more times with fresh solvent to remove any remaining soluble impurities, with each wash allowed to fully drain before the next is applied.
When to Use Gravity Filtration
Gravity filtration is the preferred technique when the goal is to collect the liquid, or filtrate, free of solid contaminants. It is commonly used for removing insoluble solid impurities, such as drying agents or undesired side products, from a solution. This method is effective for mixtures containing relatively large, coarse solid particles that do not easily clog the filter medium.
A special case is “hot filtration,” typically performed during a recrystallization procedure. The solution is kept hot to ensure the desired compound remains dissolved while insoluble impurities are removed. Using vacuum filtration in this case would cause rapid cooling and premature crystallization of the desired product, blocking the process.
Because the process relies only on gravity, it is inherently slow, making it inefficient for filtering large volumes or for mixtures where the solid consists of extremely fine particles. Fine particles quickly block the pores of the filter paper, severely reducing the flow rate and requiring an alternative method like vacuum filtration.