Chelating agent:- It may be defined as ligands which are able to form metal chelate are called as chelating agent. eg. i) H2N-CH2-CH2-NH2, ii) DMG, iii) EDTA
The term metal chelate may be defined as the formation of heterocyclic ring by attachment of polydentate ligand or ligands with metal atom or ion.
i.e. Metal ion + Chelating agent chelation Metal chelate
Cu+2 + 2 H2N-CH2-CH2-NH2
The process of formation of a heterocyclic ring containing metal atom or ion which being linked by either coordinate or covalent bond or by both type of bond with two or more non metal atoms in same species is known as chelation.
eg. chlorophyll the green pigment of plant is a magnesium chelate with hemin. Hemoglobin is iron chelate , myoglobin, Vit. B12 are other examples.
* Structural requirements for chelate formation.
1. The metal ion must have a small size, appreciable charge & vacant 'd' orbital.
2. The chelating agent must have at least two donor groups, which can easily donate lone pair of electron to metal. Generally N, O, S, P etc are donor groups.
3. Chelating agent must contain donor group which facilitates the formation of stable ring. eg. –NH2 (amide), =NOH (oxime), =NH (imino), -COOH (carboxylic) SO3H (sulphonyl), -OH (enol), -SH (thiol).
4. Formation of heterocyclic ring between metal & chelating agent is essential. More the no. of rings more stable is the chelate. Hence metal must have high coordination number.
5. Metal chelates should be free from ring starin. Five member rings are practically strain free & more stable.
6. The donor atom should not posses substitution of bulky groups.
Chelate containing group a) will be comparatively more stable than chelate
containing group b) & c) as there is no bulky group substitution.
7. Size of chelating agent should be proper to avoid steric hinderence.
* Distinction between Metal chelate & Metal complex.
Metal chelate Metal complex
1. These are essentially special kind of 1. These are noncyclic mononuclear
heterocyclic ring compounds. compounds.
2. Approaching ligands are always 2. Approaching ligands are always
3. Bonding in metal chelate is either 3. bonding in metal complex is
covalent, coordinate or both type. essentially coordinate.
4. Heterocyclic ring formation is essential. 4. Ring system is absent.
5. It has high stability. 5. Relatively it has low stability.
6. They may or may not have regular shape. 6. They have generally regular shape.
i.e. octahedral, tetrahedral.
7. They negligibly dissociates in solution. 7. Comparatively they dissociate more.
8. They may or may not be represented in 8. They are represented in square bracket.
9. e.g. H2C-NH2 NH2-CH2 9. e.g. H3N NH3
H2C-NH2 NH2-CH2 H3N NH3
* Classification of chelating agents:-
According to H. Diehl chelating agents are classified on the basis of number & nature of donor groups present in it. Depending upon no. of donor group it is classified as bidentate, tridentate, quadridentate, pentadentate, hexadentate etc. further each group is classified on the basis of relative no. of acidic & coordinating group involved in ring closure. i.e. for bidentate chelating agent containing A] two acidic groups. B] one acidic & one coordinating group. C] two coordinating groups.
A] Two acidic group
i) Oxalic acid
ii) Malonic acid
iii) Pthalic acid
iv) Salicylic acid
B] One acidic one coordinating group
ii) 8-Hydroxyquinoline (oxine)
C] Two coordinating group
i) Ethylene diamine
* Application of chelation:
The chelation has wide application in the field of water softening, food preservation, chemical analysis, dying, medicine, detergent, agriculture etc. It plays an important role in body metabolism & plant process. Chelting agents are important in qualitative & quantitative analysis.
A] Use of EDTA as chelating agent:
EDTA has very wide application in analytical chemistry. Its disodium salt is always used due to its high solubility in water. It has very strong chelating ability, forms two coordinating & four covalent bonds. It forms stable chelate with almost all metals.
1) Removal of dissolved salts of iron from water supply.
Dissolved salts of iron in water supply form sediment on standing. Iron spoils the taste of water, tea, it decolorizes water tubes, bath tubs etc. Use of EDTA prevents these bad effects of iron without its removal from water. EDTA forms stable five memebered with iron.
2) Water softening
Due to dissolved salts of calcium & magnesium water becomes hard. When sequestering agents such as EDTA are added, they combine with Ca++, Mg++ etc. &
forms water soluble stable chelate & water becomes soft.
3) Food preservation
It is our common observation that a cut apple becomes brown, orange juice losses its flavor, fats & oils becomes rancid, vit-c & green vegetable gets spoiled on keeping, all this is because of metal ions present in them. They catalyses atmospheric oxidation & damages food. The food may be preserved by binding the trace of metal by chelation. Very dilute solution of EDTA (0.01%) is found to be quite useful in improving the
keeping qualities of such food.
4) Evaluation of total hardness of water.
Hardness of water is due to Ca++ & Mg++ ions present in it. Total concentration of Ca & Mg ions is measured in terms of CaCO3 by complexometric titration. A known volume of hard water is titrated against std. solution of EDTA by using EBT (erichrome black-T) indicator & hardness of water can be evaluated.
5) It is used in separation & purification of lanthanides & radioactive metals.
6) In qualitative & quantitative analysis it plays an important role. It is widely used for estimation of almost all metals titrimetrically eg. Al, Fe, Cd, Zn, Co, Hg, Pb, Ca, Mg, Th etc. certain anions like F-, CrO4-2, SO4-, [Fe(CN)6]-4, AsO4-3 etc.
7) Most of commercial material such as ores, minerals, slag, alloys, rock, cement, leather, oil, paint, petrol etc. can be analyzed by EDTA.
8) It is used as masking (locking) agent in various analytical separation & extraction.
9) In medicinal field it can be used as medicine to treat copper & lead poisoning, removal of radioactive metals from body fluid. Creams containing EDTA are used to treat skin diseases like dermatitis.
10) In agriculture EDTA complexes are applied with manuare.
B] Use of DMG (dimethyl glyoxime) (diacetyl dioxime):
It is white crystalline solid, sparingly soluble in water. It dissolves freely in alcohol.
1) It is highly specific & selective reagent for nickel. Its 1% alcoholic solution is used in qualitative & quantitative analysis of nickel. It precipitates nickel as scarlet red colored crystalline salt when excess of reagent is added to acidified nickel solution, followed by ammonia. Traces of nickel can be easily detected by DMG. (1 part in 2000000 parts)
Reaction of DMG
2C2H8O2N2 + NiCl2 + NH4OH → C8H14O4N4Ni + 2NH4Cl + 2H2O
i.e. Ni+2 + DMG
Bis (dimethyl glyoximato) nickel (II)
2) DMG as a spot test reagent.
It is specific reagent for nickel. Drop of sample solution is applied on spot paper, drop of DMG is added on it & paper is exposed to ammonia vapours. Appearance of a red spot confirms the presence of nickel.
3) Estimation of Nickel
a) Gravimetric estimation:- Acidified solution of nickel is boiled. It is treated with excess of 1% alcoholic solution of DMG aq. ammonia is added till the precipitation is complete. The ppt. is then digested, filtered & washed with water, dried & weighed.
Amount of nickel is then calculated by following relation.
Mass of Ni in gram. = mass of ppt. × 0.2032
b) Volumetric estimation:- The ppt. of Ni as Ni-DMG chelate is washed & dissolved in 12 N H2SO4. The solution is boiled & treated with 50ml 4% ferric sulphate solution. Then it is heated, cooled, diluted by water & 3ml H3PO4. Then titrated against 0.1N KMnO4 solution & amount of nickel is calculated by using following relation.
1ml of 0.1N KMnO4 = 0.0007335 gm of Ni
c) Colorimetric estimation:- Ni(II) is oxidised to Ni(IV) by bromine water. Its DMG complex is obtained in ammonical solution so as to produce intense scarlet red colored solution, whose optical activity is measured by colorimeter & conc. of Ni is determined by graphical method.
4) Gravimetric estimation of palladium.
Palladium is precipitated as a yellow complex by DMG from dilute mineral acid solution. It is washed, filtered, dried & weighed as [C8H14N4O4Pd] & palladium is estimated. Alternatively the ppt. is ignited & mass of metallic Pd is obtained directly.