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Sequestering Agent

Reviewed by Raghvendra GopalCheckmark
Last updated: March 15, 2021

What Does Sequestering Agent Mean?

In chemistry, a sequestering agent is an organic compound capable of linking metal ions or molecules together to form complex ring-like structures known as chelates. Although similar, a sequestering agent is different than a chelating agent.

Sequestering agents are used to link undesirable metal ions together to form a stable structure that does not readily decompose. This limits the metal ions’ ability to react with other ions, clays or polymers.

Sequestering agents are commonly used for water treatment purposes to reduce water hardness. They combine with calcium, magnesium and other heavy metal ions in hard water to form molecules in which the ions are held so securely (sequestered) that they can no longer react. Chelation is used to treat metal poisoning and for the industrial extraction of metals.


Corrosionpedia Explains Sequestering Agent

A sequestering agent surrounds another molecule or atom and holds it "in seclusion." In this process, the sequestering agent hides the molecule or atom and prevents it from entering into chemical reactions.

Chelation enables plants to take up metal ions such as iron that may not be readily available in its raw state.

Chelation agents are commonly used to bind metal ions into a ring structure. They serve as a stabilizing agent and prevent the precipitation of damaging compounds. As acids react and the pH increases, reaction products may be produced in the form of a gelatinous and insoluble mass. Chelating agents prevent precipitation, thus keeping ions in a soluble form until the treatment fluid can be put back into the formation during cleanup.

Sequestering is not the same as precipitation, because sequestering does not form a solid.

The main types of commercial sequestering agents include:

  • Aminocarboxylic acid base products
  • Phosphates and phosphonates
  • Hydroxy carboxylates
  • Polyacrylates
  • Sugar acrylates

Using Sequestering Agents for Cleaning and Water Treatment

Unfortunately, calcium and other hard water ions readily combine with surfactants in detergents, thus reducing their cleaning effectiveness. By inactivating the ions with a sequestering agent, detergents are better able to clean clothes.

Metal sequestering agents can be either liquid or granular, and are used to treat water discoloration, stains and scale.

Polyphosphate is used to sequester soluble iron atoms that remain in settled water before it is chlorinated or that leach off of iron pipes in water distribution systems. By surrounding and sequestering these soluble iron atoms, the typical reddish colors associated with iron oxides and iron hydroxides are prevented. Scale can be controlled through the use of sequestering agents and chelates.

The Difference Between a Chelating Agent and a Sequestering Agent

A chelating agent is a chemical compound that can bind with a metal ion and prevent that metal from undergoing other chemical reactions, while a sequestering agent is a chemical compound that is capable of forming a complex with metal ions and helps to remove these ions from a solution. Chelating agents have one active site per molecule, while sequestering agents have several active sites per molecule. Chelating agents are less reactive compared to sequestering agents. Sequestering agents are more reactive due to the presence of several active sites.

A sequestering agent does the same job that a chelating agent does to cover a metal ion and form a stable soluble complex and thus prevent metal ions from taking part in chemical reactions. A sequestering agent can envelop multiple ions at a time because it has multiple active sites that can bind with more than one metal ion. Thus the only difference between a chelating agent and a sequestering agent is that chelating agent can bind with a single metal ion at a time while sequestering agent can envelop multiple ions at a time. Sodium tripoly phosphate is a typical sequestering agent.


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