Technical Presentation Dt. 14/06/1998
SEQUESTERING AGENTS Sequestration may be defined as the ability of a compound to form a complex with a metal ion, keeping it in solution despite the presence of precipitating agent. Before dealing with the problem of sequestering agents, it is advisable to check through a few notions of general chemistry. Co-ordination compounds or complexes are those species where a central atom is surrounded by a group of atoms or small external molecules which go under the name of ligand. The formation of complex is due to an acid base reaction according to Lewis. The central atom (usually a metal ion) acts as an electron acceptor, the ligand acts as donors. A ligand may be uni-dentate or multi-dentate, the former case includes monoatomic ions (e.g. cl-) and all those molecules that have one single atom with an electronic doublet available for coordination (e.g. H2O: and :NH3) and the latter case includes those molecules which have several atoms with one free lone paid e.g. ethylenediamine (bidentate ligand) H2N - CH2 - CH2 - NH2 and the ethylenediamine tetra-acetate ion (hexa-dentate). -:OOCH2C
CH2COO:\ / N ---CH2 ----CH2---N / \ -:OOCH2C CH2COO:A multi dentate ligand is often structurally capable of enabling two or more of its donor atoms to form a bend simultaneously with the same metal atom, which is thus enclosed in a ring structure. Ligands of this type are called chelating agents and their complexes are called metal chelates. -OOC---CH2
CH2---COO/ N ---CH2 ----CH2---N / \ -OOC---CH2 CH2---COO\
CO / \ CH2 O| | | CH2--CO | / | \ N----------O------/ | CH2 | / Fe3+-----OH \
| N-------------O---|\ |/ | CH2-- CO | | CH2 O\ / \ / CO
EDTA Ferric Chelate Fig. 1
This definition draws its origin from Greek word, where the term chele, which is the scientific name for the claws of crabs and similar. The term chelate is expressive of the notion of enveloping the ion, consequently hiding its most evident characteristics. A complexing agent is thus a compound, which forms complexes of any type with metal ions. A sequestering agent is a compound which forms water soluble complexes with such ions. Generally speaking, the most stable complexes are the ones where the metal is enclosed is a 5 or 6 atoms ring structure. In conclusion, sequestering agents are always multidentate ligands capable of forming water soluble chelates with metal ions. When the complex is formed, the metal ion is practically removed from the solution and therefore ceases to exist as such. Obviously the reactions to which the pre-existing ions give rise can no longer take place. Among the best known and widely used sequestering agents one may count amino carboxylic acids and especially ethylene diamine tetra acetic acid. Owing to the great and not only historic importance of amino carboxylic acids, recently they have been replaced in many application by other types of sequestering agents, among them, to quote an instance, phosphonic sequestering agents. EDTA chelate compound with the Fe+3 ion is shown in Fig. 1.
3 Stabilizers for Hydrogen Peroxide : Hydrogen peroxide is commercially available as a 35% solution in water. These solutions are stable in the presence of sulphuric acid or phosphoric acid. Traces of heavy metals like gold, silver, platinum, iron, copper, manganese etc. Catalytically decompose hydrogen peroxide. 2H2O2 ---------> 2H2O + (O) (O) + (O) ---------> O2 ↑ -------------------------------------2H2O2 ----------> 2H2O + O2 ↑ An aqueous solution, hydrogen peroxide ionizes into hydrogen and perhydroxyl ions : (H2O) H2O2 -------------> H+ + HOOPerhydroxyl ions are supposed to be the active bleaching agent. In the presence of an alkali, like sodium hydroxide, the following equilibrium is set up : H2O2 + OH- ---------> HOO- + H2O <--------It is seen that an increase in the concentration of hydroxyl ions, i.e., increasing the pH, shifts the equilibrium to the right, thereby increasing the concentration of perhydroxyl ions. On the other hand, in the acidic medium, the backward reaction is favoured and the concentration of perhydroxyl ions decreases and the solution becomes stable. However, the decomposition of hydrogen peroxides is not a function of only the pH. Thus, sodium hydroxide or sodium carbonate decompose hydrogen peroxide faster than sodium silicate at the same pH. Whereas sodium silicate has a strong stabilizing effect on hydrogen peroxide, sodium carbonate has the opposite effect. Traditionally, sodium silicate used was a polysilicate (Na2O:SiO2, 1:3.3) referred to as sodium silicate (79°Tw or 42°Be) or water glass. However, its tendency to precipitate out of solution in hard water or upon acidification has resulted in its replacement by non-silicate stabilizers. Sodium silicate is available in various forms, such as the following : 1. 2. 3. 4. 5. 6.
Sodium orthosilicate Sodium pyrosilicate Sodium metasilicate Sodium disilicate Sodium trisilicate Sodium tetrasilicate
(2Na2O . SiO2) (3Na2O . SiO2) (Na2O . SiO2) (Na2O . 2SiO2) (Na2O . 3SiO2) (Na2O . 4SiO2)
4 Mechanism of bleaching : Earlier, it was thought that during bleaching with hydrogen peroxide under alkaline conditions nascent oxygen is first produced, a part of which combines with itself produces molecular oxygen in the gaseous form and escapes into the atmosphere. This oxygen is not available for bleaching purposes. The other part attacks the coloured pigment present in cotton and bleaches (turning into white) the pigment. H2O2 ------> H2O + (O) ---------- (1) (O) + (O) -----> O2 ↑ ---------- (2) (O) + (Coloured pigment) -----> (White pigment) ---------- (3) Reaction (2) is the undesired, wasteful reaction, while reaction (3) is the desired one. The second mechanism suggested involves the formation of perhydroxyl ion. (OH-) H2O2 --------> HOO- + H2O HOO- -------> HO- + O | H-C ||
| H-C + O ----->
H-C H-C | | (Chromo(Oxirane) phore) | H-C | \ | O + H2O ------ > HO - C - H / | H-C H - C - OH | | (diol) Hydrogen peroxide is activated by alkali (OH-), which leads to the formation of the perhydroxy ion (HOO-). This decomposes into the more stable hydroxy (OH-) ion and singlet oxygen. This active form of oxygen reacts with the double bonds of the chromophore (e.g., carotenoid pigments) that impart the characteristic brown colour to raw cotton. A third mechanism --- with the formation of free radicals is also suggested. In this, hydrogen peroxide is cleaved to form two hydroxy free radicals. H - O - O - H -------> 2HO. Heavy metal compounds and other ill-defined impurities catalyze the decomposition of hydrogen peroxide, which then competes with the bleaching reaction. These metals can cause the formation of free radicals. The characteristic property of these metals is that they can exhibit in several valencies (Fe, Co, Mn, Cu, etc.). The free radicals can attack the pigments as well as cotton cellulose, leading to damage and can form “catalyst holes” in the cotton fabric.
5 Mechanism of peroxide stabilisation : When bleaching textile materials (cotton, wool, silk etc.) with hydrogen peroxide under alkaline conditions, bleach stabilizers must be used. These inhibit the decomposition of bleach-active perhydroxy anions (H-O-O-) and ensure a high oxidation potential over the whole bleaching time. The residual hydrogen peroxide content on the fabrics after bleaching in amounts of 15-40% of the original hydrogen peroxide content indicates that the bleaching process was satisfactory and that spontaneous catalytic decomposition has not occurred to a large extent.
Some stabilizers contain water soluble magnesium salts (producing Mg++ ions in aqueous solution and oxidation --- stable costabilisers. The magnesium cations stabilise the perhydroxy anions, while the anions of the costabilisers (e.g. phosphonate ions) form complexes with the heavy metal ions, thereby inactivating their catalytic effect. Thus, direct stabilisation is caused by the magnesium ions and indirect stabilization, by the costabilizer. As sodium silicate inactivates the heavy metal ions, its anion (silicate) also has an indirect stabilising action. Magnesium cation may act in the following way, magnesium perhydride being more stable that perhydroxy ion:
H-O-O-H -------> H+ + HOOOMg++ + 2HOO- ------> Mg (OOH)2
The stabilisers, available commercially, have various compositions. Apart from magnesium ions, responsible for direct stabilisation, there are complexing agents like EDTA, DTPA. Gluconic acid, phosphonic acid, poly (acrylic acid) derivatives. The organic stabilisers do not contain sodium silicate. The silicate-containing stabilisers include sodium metasilicate in aqueous solution at a concentration of 38째 - 40째 Be. Organic stabilisers containing surfactants are also marketed.
In pad-steam process with reaction items of upto 30 min. Silicate-free bleaching in the presence of organic stabilisers has been established. Stabilisation of bleach liquors with sodium silicate and magnesium ions has the disadvantages that silicate encrustations (scaling) form in the bleaching equipment. These scales are difficult to remove and damage the surface of the fabric. Silicate can also get deposited on the fabric and this spoils the hand of the fabric and reduces its absorbency.
1. 2. 3. 4. 5.
Good stabilizers should have the following properties Good stabilizing action Good resistance towards oxidants Prevention of silicate build-up on rollers in the steamer Inactivation of catalysts, like heavy metal ions, and Good metering and pumping properties.
6 Surfactant-containing stabilisers, used with sodium silicate in the bleach bath, must have silicate-dispersing properties. The surfactants used in bleaching have emulsifying, dispersing, and wetting properties, which promote the removal of hydrophobic impurities and soil, and assist in the transport of the reaction products formed by the bleaching process. The wetting properties are necessary to enhance the absorbency of the pre-treated goods and to make it uniform.
In order to meet these requirements, the surfactants used are usually mixtures of anionic surfactants, like alkyl sulphonates and alkyl aryl sulphonates, with nonionic surfactants, such as alkyl phenol ethoxylates, or the bio-degradable fatty alcohol ethoxylates. These surfactants must be stable in the bleach bath and must be suitable for metering equipment.
Sequestering / Chelating agents In wet processing of textile materials, the quality of water used is of utmost importance. The presence of alkaline earth (calcium and magnesium) and / or heavy metal (iron, copper, manganese, etc.) salts create problems. Thus copper, iron and manganese lasts, even in very small quantities, catalytically decompose hydrogen peroxide used in the bleaching of cotton materials and cause local damage to these materials.
Formation of sparingly soluble salt-like compounds with anionic dyes (direct, acid, reactive, mordant and metal complex dyes) by these metal salts, lead to filtering out problems in package dyeing, levelling problems and impairement rubbing and washing fastness. Certain dye molecules (capable of chelating metal ions) can form stable complexes with metal ions. Causing changes in shade / tone, accompanied by loss of brilliance. In the case of dyeing of cotton with vat dyes, especially blue vat dyes, the presence of calcium salts like calcium chloride in the water (hardness) produces insoluble calcium carbonate by reaction with sodium carbonate (formed by contact with stock solution of sodium hydroxide with carbon dioxide of the atmosphere) and gets deposited in the cotton material. After the oxidation of the leuco vat dyes, the brightness of the final dyeing is impaired by the presence of calcium carbonate in the fabric. A treatment with dilute hydrochloric acid solution at the room temperature for a few minutes, followed by thorough washing (calcium chloride and hydrochloric acid) brings back the brilliance of the vat dyeing. These and other problems can be overcome by adding sequestering/chelating agents to the dyebath to form water-soluble complexes with the metal ions, which then lose their metallic nature and hence will not interfere with the process being carried out. Sequestering agents differ with respect to the stability of the metal complex they form and the specific effect on metal cations. Further, the stability of the complex depends on the pH of the treatment bath.
7 Ethylene diamine tetra-acetic acid (as various sodium salts) (EDTA), diethylene triamine penta acid (as various sodium salts) (DTPA), nitrilo triacetic acid (as sodium salts) (NTA), phosphonic acid-based salts are some of the sequestering agents that are very effective on a wide range of cations, including heavy metal ions (iron, copper, manganese etc.) and those from hard water (calcium and magnesium). Specific compounds to combat the effects of hard water salts include mild complexing agents, such as polyphosphates and various polycarboxylic acids. These also have a dispersing action on the precipitates from water hardness, which the strong complexing agents do not have. Specific mild complexing agents for heavy metal ions such as copper, iron and manganese include various polyhydroxy compounds such as sorbitol, gluconic acid, gluco heptaonic acid and alkanolamines.
Chelate compounds : Compounds in which a metal ion is joined to two or more donor groups of a single ion are called chelate compounds. The donor molecule or ligand is known as unidentate, bidentate, tridentate, etc. according to whether it forms one, two, three, etc. covalent linkages with the metal atom. For example, glycine (amino acetic acid) is a bidentate agent, which forms two covalent bonds with a cupric ion, giving five membered, ring structure A. In this the actual ligand is the glycinate anion, two of which neutralize the positive charges on the original cupric ion, resulting in an uncharged chelate. 2H2N-CH2-COOH + Cu++ â†“ O=C-O H2N-CH2 \ | Cu | \ CH2-NH2 O- C=O (A)
Sodium hexametaphosphate sequesters calcium and magnesium ions from hard water and these metal ions are held in the anion of the complex, thereby losing their metallic properties
8 (NaPO3)6 --------->Na2(Na4P6O18) <--------Na2(Na4P6O18) + 2CaCl2 -----> Na2(Ca2P6O18) + 4NaCl Na2(Na4P6O18) + 2MgCl2 -----> Na2(Mg2P6O18) + 4NaCl EDTA (tetra sodium salt) holds calcium ions by sequestering : NaOOC-CH2
CH2COONa \ / N-CH2-CH2-N / \ NaOOC-CH2 CH2COONa
↓ (-4 NaCl) COO OOC / \ / \ CH2 Ca CH2 \ / N-CH2-CH2-N / \ CH2 Ca CH2 \ / \ / COO COO The structures of some conventional sequestering agents are given below : Hydroxycarboxylates
H H OH H | | | | HOCH2 C---C---C---C---CO2H | | | | OH OH H OH Gluconic Acid
H H OH H H | | | | | HOCH2 C --- C--- C--- C--- C---CO2H | | | | | OH OH H OH OH Glucoheptonic Acid
9 Amino Carboxylates CH2CO2H N
Nitrilotriacetic Acid (NTA) HO2CCH2
Ethylene diamine tetra Acetic Acid (EDTA) HO2CCH2
CH2CO2H NCH2CH2NCH2CH2N | CH2CO2H CH2CO2H
Diethylene triamine pentaacetic Acid (DTPA)
10 Organophosphonates H2O3PCH2
CH2PO3H2 N | CH2PO3H2
Aminotri (methylene phosphonic Acid) (ATMP)
OH | CH3---C---PO3H2 | PO3H2 1- Hydroxethylidene-1, 1-diphosphonic Acid (HEDP)
Ethylenediaminetetra (methylenephosphonic Acid) (EDTMP)
NCH2CH2NCH2CH2N | CH2PO3H2 CH2PO3H2
Diethylenetriaminepenta (methylenephosphonic Acid) (DTPMP)