What Is Metal Chelation?

Metal chelating agent (metal chelating agent) can bind metal ions to the inside of the chelating agent through the strong binding effect of chelator molecules and metal ions to become stable, larger molecular weight compounds, thereby preventing metal ions from functioning. It is used for detoxification, printing and dyeing, and scale inhibition.

Metal chelator

Right!

Metal chelating agent (metal chelating agent) can bind metal ions to the inside of the chelating agent through the strong binding effect of chelator molecules and metal ions to become stable, larger molecular weight compounds, thereby preventing metal ions from functioning. It is used for detoxification, printing and dyeing, and scale inhibition.

Chinese name

Metal chelator

Foreign name

metal chelating agent

Category

Reagent

Use

It is used for detoxification, printing and dyeing, and scale inhibition.

By a simple

Certain dyes (such as CI Disperse Red 60, CI Disperse Blue 79, etc.) bring in iron, copper, magnesium, calcium and other ions during the chemical synthesis process, causing adverse effects such as darkening of the color when the dye is colored. To avoid the intrusion of these ions, metal chelants are often used to chelate these ions so that they do not affect the printing effect of the dye. The metal chelating agents used are citric acid, ethylenediaminetetraacetic acid (edetic acid), etc., and the amount is usually a few thousandths of the amount of the dye (if the metal content is too high, it may be more appropriate). In addition to the above-mentioned disperse dyes, it can also be used for other dyes sensitive to these metal ions.

Inorganic metal ion chelator

The disadvantage of polyphosphate chelating agents is that they will hydrolyze and decompose at high temperature, which weakens or loses the chelating ability. Moreover, its chelating ability is greatly affected by pH, and it is generally only suitable as a chelating agent under alkaline conditions. In general, these inorganic chelants have a poor chelating ability for heavy metal ions, especially iron ions. Due to the above disadvantages, the use of inorganic chelating agents is limited. Usually, it is only used to chelate calcium and magnesium ions, so it is often used as a hard water softener.

Organic metal ion chelator

There are many organic compounds that can chelate with heavy metal ions, such as carboxylic acid type and organic polyphosphonic acid.

Carboxylic acid

The carboxylic acid type chelating agent used in the cleaning agent mainly includes aminocarboxylic acids and hydroxycarboxylic acids.

(1) Aminocarboxylic acidsAminocarboxylic acids used as chelating agents are ethylenediaminetetraacetic acid (EDTA), aminotriacetic acid (also known as NTA), diethylenetriaminepentaacetic acid and its salts Wait. They have strong chelation effect on calcium and magnesium ions. From the number of calcium ions chelated by the three acids per unit mass, NTA chelated the most, EDTA was the second, and DTPA was again. From a practical point of view, the chelate formed by NTA and calcium ions is quite stable and cheap, so it is often used as a substitute for sodium tripolyphosphate in detergents.

Ethylenediamine tetraacetic acid disodium salt molecule contains two nitrogen atoms and four oxygen atoms, which can provide an electron pair forming a coordination bond. A K-membered ring composed of six coordination bonds with calcium ions is as follows: Very stable in water Not easy to dissociate. However, due to the high cost of ethylenediaminetetraacetic acid, it is currently only used in special scale cleaning such as non-stop cleaning and is commonly used in disodium or tetrasodium salts with greater solubility in water. The pH of the disodium salt solution was 4.4 and the pH of the tetrasodium salt solution was 10.8. It should be noted that under basic conditions, some metal ions will form hydroxide precipitates without being sequestered. For example, Fe3 + will form Fe (OH) 3 precipitates in aqueous solutions of pH> 8, and EDTA cannot be used to chelate Fe3 +. Therefore, if metal ions generate hydroxides under high pH conditions, they cannot be chelated. Pay attention in practice.

(2) Hydroxycarboxylic acids. Such carboxylic acids used as chelating agents are mainly citric acid (CA), tartaric acid (TA) and gluconic acid (GA). Since these chelating agents have weaker chelating properties under acidic conditions, their salts are often used as builders in detergent formulations. Among them, sodium gluconate is a good all-around chelating agent, which has a good chelating ability for a variety of metal ions, and sodium tartrate and sodium citrate can also chelate most divalent and trivalent metal ions.

Citric acid and its salts have strong chelating ability in the acidic range, and the suitable range of use is pH = 4-8. The chelate formed by citric acid and iron ions has low solubility and will form a precipitate in water. In order to increase its solubility, the appropriate amount of ammonium salt is added to generate monoammonium citrate and chelate with Fe3 + and Fe2 + ions to form ferrous citrate with higher solubility, respectively. Ammonium and ferric ammonium citrate so that no precipitation occurs when removing rust.

When ammonium citrate is used in combination with glycolic acid, its chelating ability is increased. The order of dissolving ability of several carboxylic acids to rust is: ammonium citrate-glycolic acid> glycolic acid> ammonium citrate.

Because the citric acid chelating agent has a wider applicable pH range than other hydroxycarboxylic acids, it has become the most widely used hydroxycarboxylic acid chelating agent in industrial production.

(3) Typical examples of hydroxyaminocarboxylic acids used as chelating agents are hydroxyethylethylenediaminetriacetic acid (HEDTA) and dihydroxyethylglycine (DEG).

They can chelate iron ions under weakly alkaline conditions at pH = 9, but have poor chelating ability to other ions. For example, dihydroxyethylglycine cannot chelate calcium and magnesium ions, so they can generally be used to remove iron scale They are not used to remove scales such as calcium carbonate. Most carboxylic acid chelating agents have the advantages of being easily biodegradable, not polluting the environment, and non-toxic. Other carboxylic acid chelating agents include 1,2,3, tetracarboxylic cyclopentane, 2-oxoacetic acid malonate and 2-oxoacetate butyric acid.

Organic polyphosphonic acid

Organic polyphosphonic acid has good chemical stability compared with inorganic polyphosphate, is not easy to hydrolyze, and can withstand higher temperatures. It has excellent chelating ability for many metal ions such as calcium, magnesium, copper and zinc. It is often used as a scale inhibitor in industrial cleaning to prevent the formation of scale. Has been widely used.

Polycarboxylic acid

There are polyacrylic acid, polymethacrylic acid, hydrolyzed polymaleic anhydride, fumaric acid (fumaric acid) -propylene sulfonic acid copolymer. They contain polymeric anions that are excellent chelating agents for metal ions and are therefore also used as scale inhibitors.

Among them, polyacrylic acid and its sodium salt are currently the most widely used polycarboxylic acid scale inhibitors.