Decantation is a laboratory technique used to separate components of a mixture without filtration equipment. This method relies solely on the physical principle of gravity acting upon substances with different densities. It is employed in chemistry and various industries. Learning to perform this technique correctly is important for many chemical procedures, especially when preparing a sample for further analysis or purification. This guide provides practical instruction on executing a precise chemical decantation.
Understanding Decantation and Its Uses
The mechanism of decantation relies on the difference in density between the components of a mixture. When a mixture of an insoluble solid and a liquid is left undisturbed, the denser solid particles will settle to the bottom under gravity, a process known as sedimentation. The lighter liquid phase remaining above the settled solid is referred to as the supernatant. Decantation is the careful pouring off of this supernatant liquid, leaving the sediment behind.
Decantation is commonly used in the laboratory to separate a liquid phase from a solid precipitate formed during a chemical reaction. This technique is also effective for separating two immiscible liquids, such as oil and water, where the less dense liquid floats on top of the denser one. For the process to be successful, the difference in density must be significant, and the solid must be allowed sufficient time to fully settle and compact at the base of the container.
Necessary Tools and Preparation
Decantation requires a few pieces of laboratory glassware and a specialized tool to guide the flow. The mixture is typically held in a beaker or an Erlenmeyer flask, which provides a stable base and a spout for pouring. A clean receiving vessel, such as another beaker or flask, must be positioned to collect the separated liquid.
The primary tool is the glass stirring rod, which is used as a guide during the pour. Using the rod ensures a controlled flow rate and prevents the liquid from dribbling down the outside of the container due to surface tension. Before starting, the container must be placed on a level surface and the mixture must be allowed to rest until the interface between the solid and liquid layers is completely distinct.
Step-by-Step Guide to Decanting
The first step is ensuring complete sedimentation, which may take minutes or hours depending on the particle size and density of the solid. Once the solid has formed a compact layer at the bottom, the receiving vessel is placed next to the original container. The glass stirring rod is then held vertically across the lip or spout of the container holding the mixture.
The original container is tilted slowly and deliberately, allowing the supernatant liquid to flow down the length of the glass rod. The rod guides the liquid stream into the center of the receiving vessel, maintaining a slow and steady pour. This controlled movement minimizes turbulence inside the container, which prevents the re-suspension of settled solid particles.
It is important to hold the glass rod firmly against the pouring lip to maintain continuous contact and control. The tilting should stop well before the liquid reaches the level of the settled sediment. Leaving a small residual amount of the supernatant liquid behind is necessary to ensure that no solid particles are accidentally carried over into the receiving vessel.
Achieving Maximum Purity
While the initial decantation separates the bulk of the liquid, achieving maximum purity requires additional steps. The solid sediment often retains a small amount of the original liquid, which contains impurities. To remove these residual contaminants, a process called “washing” is performed.
Washing involves adding a small volume of fresh solvent to the sediment, gently stirring the mixture to suspend the solid, and then allowing it to settle again. The newly added solvent dissolves the trapped impurities, and the subsequent second decantation removes the wash liquid. This washing and decanting process can be repeated multiple times to reach a high level of purity.
For mixtures containing very fine solid particles, decantation alone may not be sufficient, as the smallest particles may remain suspended in the supernatant. In such cases, decantation serves as a preliminary step to remove the majority of the liquid before the remaining mixture is transferred for a more rigorous separation technique, such as filtration. Filtration, using specialized filter paper, is generally more effective at capturing the smallest particles.