LNC Technical Presentation Dt. 14/06/1998 SEQUESTERING AGENTSSequestration may be defined as the ability of a compound to form a complex with ametal ion, keeping it in solution despite the presence of precipitating agent.Before dealing with the problem of sequestering agents, it is advisable to check througha few notions of general chemistry.Co-ordination compounds or complexes are those species where a central atom issurrounded by a group of atoms or small external molecules which go under the nameof 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 asdonors. A ligand may be uni-dentate or multi-dentate, the former case includesmonoatomic ions (e.g. cl-) and all those molecules that have one single atom with anelectronic doublet available for coordination (e.g. H2O: and :NH3) and the latter caseincludes those molecules which have several atoms with one free lone paid e.g.ethylenediamine (bidentate ligand) H2N - CH2 - CH2 - NH2and 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 donoratoms to form a bend simultaneously with the same metal atom, which is thus enclosedin a ring structure. Ligands of this type are called chelating agents and their complexesare called metal chelates.-OOC---CH2 CH2---COO- / N ---CH2 ----CH2---N / -OOC---CH2 CH2---COO- Ionized EDTA
2 CO / CH2 O- | | | CH2--CO | / | N----------O------- / | CH2 | / Fe3+-----OH / CH2 | | N-------------O---- | |/ | CH2-- CO | | CH2 O- / / CO EDTA Ferric Chelate Fig. 1This definition draws its origin from Greek word, where the term chele, which is thescientific name for the claws of crabs and similar. The term chelate is expressive of thenotion of enveloping the ion, consequently hiding its most evident characteristics.A complexing agent is thus a compound, which forms complexes of any type withmetal ions. A sequestering agent is a compound which forms water soluble complexeswith such ions. Generally speaking, the most stable complexes are the ones where themetal is enclosed is a 5 or 6 atoms ring structure. In conclusion, sequestering agents arealways 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 andtherefore ceases to exist as such. Obviously the reactions to which the pre-existing ionsgive rise can no longer take place. Among the best known and widely usedsequestering agents one may count amino carboxylic acids and especially ethylenediamine tetra acetic acid.Owing to the great and not only historic importance of amino carboxylic acids, recentlythey have been replaced in many application by other types of sequestering agents,among them, to quote an instance, phosphonic sequestering agents. EDTA chelatecompound with the Fe+3 ion is shown in Fig. 1.
3Stabilizers for Hydrogen Peroxide :Hydrogen peroxide is commercially available as a 35% solution in water. Thesesolutions are stable in the presence of sulphuric acid or phosphoric acid. Traces ofheavy metals like gold, silver, platinum, iron, copper, manganese etc. Catalyticallydecompose 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+ + HOO-Perhydroxyl ions are supposed to be the active bleaching agent. In the presence of analkali, 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 perhydroxylions. On the other hand, in the acidic medium, the backward reaction is favoured andthe 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 fasterthan sodium silicate at the same pH. Whereas sodium silicate has a strong stabilizingeffect 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 inhard water or upon acidification has resulted in its replacement by non-silicatestabilizers.Sodium silicate is available in various forms, such as the following :1. Sodium orthosilicate (2Na2O . SiO2)2. Sodium pyrosilicate (3Na2O . SiO2)3. Sodium metasilicate (Na2O . SiO2)4. Sodium disilicate (Na2O . 2SiO2)5. Sodium trisilicate (Na2O . 3SiO2)6. Sodium tetrasilicate (Na2O . 4SiO2)
4Mechanism of bleaching :Earlier, it was thought that during bleaching with hydrogen peroxide under alkalineconditions nascent oxygen is first produced, a part of which combines with itselfproduces molecular oxygen in the gaseous form and escapes into the atmosphere. Thisoxygen is not available for bleaching purposes. The other part attacks the colouredpigment 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 -----> | 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 theperhydroxy ion (HOO-). This decomposes into the more stable hydroxy (OH-) ion andsinglet oxygen. This active form of oxygen reacts with the double bonds of thechromophore (e.g., carotenoid pigments) that impart the characteristic brown colour toraw 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 ofhydrogen peroxide, which then competes with the bleaching reaction. These metals cancause the formation of free radicals. The characteristic property of these metals is thatthey can exhibit in several valencies (Fe, Co, Mn, Cu, etc.). The free radicals can attackthe pigments as well as cotton cellulose, leading to damage and can form “catalystholes” in the cotton fabric.
5Mechanism of peroxide stabilisation :When bleaching textile materials (cotton, wool, silk etc.) with hydrogen peroxide underalkaline conditions, bleach stabilizers must be used. These inhibit the decomposition ofbleach-active perhydroxy anions (H-O-O-) and ensure a high oxidation potential overthe whole bleaching time. The residual hydrogen peroxide content on the fabrics afterbleaching in amounts of 15-40% of the original hydrogen peroxide content indicates thatthe bleaching process was satisfactory and that spontaneous catalytic decompositionhas not occurred to a large extent.Some stabilizers contain water soluble magnesium salts (producing Mg++ ions inaqueous solution and oxidation --- stable costabilisers. The magnesium cations stabilisethe 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, bythe 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+ + HOOO- Mg++ + 2HOO- ------> Mg (OOH)2The stabilisers, available commercially, have various compositions. Apart frommagnesium ions, responsible for direct stabilisation, there are complexing agents likeEDTA, DTPA. Gluconic acid, phosphonic acid, poly (acrylic acid) derivatives. Theorganic stabilisers do not contain sodium silicate. The silicate-containing stabilisersinclude sodium metasilicate in aqueous solution at a concentration of 38° - 40° Be.Organic stabilisers containing surfactants are also marketed.In pad-steam process with reaction items of upto 30 min. Silicate-free bleaching in thepresence of organic stabilisers has been established. Stabilisation of bleach liquors withsodium silicate and magnesium ions has the disadvantages that silicate encrustations(scaling) form in the bleaching equipment. These scales are difficult to remove anddamage the surface of the fabric. Silicate can also get deposited on the fabric and thisspoils the hand of the fabric and reduces its absorbency. Good stabilizers should have the following properties1. Good stabilizing action2. Good resistance towards oxidants3. Prevention of silicate build-up on rollers in the steamer4. Inactivation of catalysts, like heavy metal ions, and5. Good metering and pumping properties.
6Surfactant-containing stabilisers, used with sodium silicate in the bleach bath, musthave silicate-dispersing properties. The surfactants used in bleaching have emulsifying,dispersing, and wetting properties, which promote the removal of hydrophobicimpurities and soil, and assist in the transport of the reaction products formed by thebleaching process. The wetting properties are necessary to enhance the absorbency ofthe pre-treated goods and to make it uniform.In order to meet these requirements, the surfactants used are usually mixtures ofanionic surfactants, like alkyl sulphonates and alkyl aryl sulphonates, with nonionicsurfactants, such as alkyl phenol ethoxylates, or the bio-degradable fatty alcoholethoxylates. These surfactants must be stable in the bleach bath and must be suitablefor metering equipment.Sequestering / Chelating agentsIn 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 thebleaching 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 outproblems in package dyeing, levelling problems and impairement rubbing and washingfastness. Certain dye molecules (capable of chelating metal ions) can form stablecomplexes with metal ions. Causing changes in shade / tone, accompanied by loss ofbrilliance. In the case of dyeing of cotton with vat dyes, especially blue vat dyes, thepresence of calcium salts like calcium chloride in the water (hardness) producesinsoluble calcium carbonate by reaction with sodium carbonate (formed by contact withstock solution of sodium hydroxide with carbon dioxide of the atmosphere) and getsdeposited in the cotton material. After the oxidation of the leuco vat dyes, thebrightness of the final dyeing is impaired by the presence of calcium carbonate in thefabric. A treatment with dilute hydrochloric acid solution at the room temperature for afew 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 tothe dyebath to form water-soluble complexes with the metal ions, which then lose theirmetallic 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 formand the specific effect on metal cations. Further, the stability of the complex depends onthe pH of the treatment bath.
7Ethylene diamine tetra-acetic acid (as various sodium salts) (EDTA), diethylenetriamine penta acid (as various sodium salts) (DTPA), nitrilo triacetic acid (as sodiumsalts) (NTA), phosphonic acid-based salts are some of the sequestering agents that arevery 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 complexingagents, such as polyphosphates and various polycarboxylic acids. These also have adispersing action on the precipitates from water hardness, which the strong complexingagents do not have.Specific mild complexing agents for heavy metal ions such as copper, iron andmanganese 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 ionare called chelate compounds. The donor molecule or ligand is known as unidentate,bidentate, tridentate, etc. according to whether it forms one, two, three, etc. covalentlinkages with the metal atom. For example, glycine (amino acetic acid) is a bidentateagent, which forms two covalent bonds with a cupric ion, giving five membered, ringstructure A. In this the actual ligand is the glycinate anion, two of which neutralize thepositive charges on the original cupric ion, resulting in an uncharged chelate. 2H2N-CH2-COOH + Cu++ ↓ O=C-O H2N-CH2 | Cu | CH2-NH2 O- C=O (A)Sodium hexametaphosphate sequesters calcium and magnesium ions from hard waterand these metal ions are held in the anion of the complex, thereby losing their metallicproperties
8 (NaPO3)6 --------->Na2(Na4P6O18) <--------- Na2(Na4P6O18) + 2CaCl2 -----> Na2(Ca2P6O18) + 4NaCl Na2(Na4P6O18) + 2MgCl2 -----> Na2(Mg2P6O18) + 4NaClEDTA (tetra sodium salt) holds calcium ions by sequestering : NaOOC-CH2 CH2COONa / N-CH2-CH2-N + 2CaCl2 / NaOOC-CH2 CH2COONa ↓ (-4 NaCl) COO OOC / / CH2 Ca CH2 / N-CH2-CH2-N / CH2 Ca CH2 / / COO COOThe structures of some conventional sequestering agents are given below : Hydroxycarboxylates H H OH H H H OH H H | | | | | | | | |HOCH2 C---C---C---C---CO2H HOCH2 C --- C--- C--- C--- C---CO2H | | | | | | | | | OH OH H OH OH OH H OH OHGluconic Acid Glucoheptonic Acid