Sequestering agent, also known as complexing agents, are organic or inorganic compounds that chelate with heavy metal ions to form stable water-soluble complexes and passivate heavy metal ions. The molecule of the compound contains electron donors that can coordinate and bind with heavy metal ions, so it has a series of special effects, such as softening, descaling, rust protection, stability, and synergism. The common sequestering agent used in printing and dyeing is as follows.

(1)Phosphates: mainly including sodium tripolyphosphate, sodium polyphosphate, sodium hexametaphosphate, sodium pyrophosphate and so on.
Because of its ion exchangeability, this reagent is the first softener used in printing and dyeing industry. Sodium pyrophosphate can form complex ions with iron ions, so it can be used as hydrogen peroxide stabilizer. However, inorganic phosphate is prohibited in some areas.

（2）Aminocarboxylic acid: mainly contains ethylenediaminetetraacetic acid (ED-TA), namely softener B; nitrogen triacetic acid (NTA), namely softener A. In addition, there are diethylenetriamine pentaacetic acid (DTPA), N-hydroxyethyl ethylamine triacetic acid (HEDIA), ethylene glycol monobis (B-aminoethyl ether) - N, N-tetraacetic acid (EGTA).The ligands of aminocarboxylic acid chelating agents are nitrogen atoms and negatively charged carboxylic acid radical ions (COO-). The more ligands, the stronger the complexation with metal ions. Among them, DTPA cooperates most strongly with most metal ions, followed by EDIA and HEDIA, and NTA is the worst. Among them, DTPA is the best hydrogen peroxide stabilizer. However, NTA, EDTA, DTPA, and others have very poor biodegradability due to metal chelation and have been banned in some European countries in recent years.