Complexometry titration with Principle ,types of chelating agent and assay of magnesium sulphate PDF

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notesComplexometry titration with Principle, types of chelating agent and assay of magnesium sulphate notes contain different types of complexometric titration with examples like direct, indirect and replacement titration and description of chellatometric indicator...

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COMPLEXOMETRIC TITRATIONS

PHARMACEUTICAL ANALYSIS-I BP102T

COMPLEXOMETRIC TITRATIONS

Mrs Bhagyashri J.warude ASSISTANT PROFESSOR Pharmaceutical Chemistry Dept

 CONTENT: 1. Complexometric titration 2. Classification of ligands

3. Effect of ph on complex formation 4. Stability of complexes

5. Metal ion indicators 6. Masking and demasking 7. Types of titration 8. Estimation of magnesium sulphate by complexometry 9. Estimation of calcium gluconate by complexometry

RMDIPER Chinchwad Pune

Page 1

 COMPLEXOMETRIC TITRATION: Complexometric titration (sometimes chelatometry) is a form of volumetric analysis in which the formation of a colored complex is used to indicate the end point of a titration. Complexometric titrations are particularly useful for the determination of a mixture of different metal ions in solution. An indicator capable of producing an unambiguous color change is usually used to detect the end-point of the titration. There are various aspects in complex formation and detection, (i) Effect of pH on complexation, (ii) Stability of complexes, (iii) Colouration of complexes, (iv) Titrability of polyvalent metal ions employing disodium edetate, and (v) Usage of pM indicators in complexometric titrations.  CLASSIFICATION OF LIGANDS: 1. Unidentate Ligands: Ligands that are bound to metal ion only at one place are called unidentate ligands (one toothed). NH3, for example, is a unidentate ligand capable of complexing with cupric ions. Halide ions, cyanide ions, H2O and NH3 are common examples of unidentate Ligands. 2. Bidentate and Multidentate Ligands: Many ligands are known that contain more than one group, capable of binding with metal ions. Such ligands are known as multidentate ligands or chelating agents. They include bidentate ligands (2 donar atoms), tridentate ligands (3 donar atoms), quadridentate ligands, etc. 

Ethylene diamine is an example of bidentate ligand.



Ethylene diamine tetra acetic acid (EDTA) is an example of multidentate ligand.



CHELATE COMPOUND OR CHELATE: Complexes involving simple ligands, i.e., those forming only

H2N-CH2-CH2-NH2

one bond are described as coordination compound. A complex of a metal ion with 2 or more groups on a multidentate ligand is called a chelate or a chelate compound. There is no fundamental difference between co-ordination compound and a chelate compound except that in a chelate compound, ring influence the stability of compound. Thus, a chelate can be described as a heterocyclic ring structure in which a metal atom is a member of ring. The stability of a chelate is usually much greater than that of corresponding unidentate metal complex. 

CHELATING AGENT: Ligands having more than one electron donating groups are called chelating agents. The most effective complexing agent in ligands are amino and carboxylate ions. Ethylenediamine

tetra-acetic acid is a typical chelating agent which forms a water

soluble

complex,

whereas

dimethylglyoxime and salicylaldoxime are chelating agents, forming insoluble complexes. EDTA forms chelates with nearly all metal ions and this reaction is the basis for general analytical method for these ions by titration with a standard EDTA solution. Such titrations are called complexometric or chilometric or EDTA titrations. 

NATURE AND STABILITY OF METAL COMPLEXES OF EDTA: Ethylenediaminetetra-acetic acid forms complexes with complexes with most cations in a 1:1 ratio, irrespective of the valency of the ion: M2+ + [H2X]2-

[MX]2- + 2H+

M3+ + [H2X]2-

[MX]-

M4+ + [H2X]2-

[MX]

+

2H+

+ 2H+

2-

Where M is a metal and [H2X] is the anion of the disodium salt (disodium EDTA) which is most frequently used. The structures of these complexes with di-, tri- and tetravalent metals contain three, four and five rings respectively:



EFFECT OF PH ON COMPLEX FORMATION : Ethylenediamine tetracetic acid (H4Y) undergoes ionization at four, different stages, namely : H+ + H3Y–

pKl = 2.07

H+ + H2 Y2–

pK2 = 2.75

H+ + HY3–

pK3 =

6.24 H+ + Y4–

pK4 = 10.34

In reality, the actual complexing species is the tetracetate ion i.e., Y4– ; therefore, complexation will take effect more efficiently and be more stable in an aikaline medium. Hence, it is evident that EDTA complexes of many divalent metals are quite stable in ammoniacal solution. As we have seen earlier that the trivalent metal complexes are normally bound still more firmly due to the formation of four rings (unlike three rings with divalent metal complexes) and stable in strongly acidic solutions, for instance : cobalt (Co2+) EDTA complex is fairly stable in concentrated hydrochloric acid (~ −11.5 N). Though a good number of metalEDTA complexes are found to be quite stable over a wide-spectrum of pH, yet in actual practice solutions are normally buffered for two specific reasons : (a) To stabilize the complex formed, and (b) To achieve the most distinct colour-change of the indicator. 

STABILITY OF COMPLEXES: Generally, the formation of a 1 : 1 chelate complex (MX) may be designated by the following equation : M+X

MX

Where, M = Metal ion, and X = Chelating ion. Hence, the stability constant, K, may be expressed as: K = [MX] / [M][X] There are two factors which influence the stability constant (K), (a) Elevation in temperature affords a slight enhancement in the ionization of the complex and a slight lowering of K, and (b) Stability constant is decreased on the addition of electrolytes with no common ion ; whereas, ethyl alcohol enhances K, perhaps on account of the suppression of ionization.  METAL ION INDICATORS The success of an EDTA titration depends upon the precise determination of the end point. The most common procedure utilises metal ion indicators. The requisites of a metal ion indicator for use in the visual detection of end points include : (a) The colour reaction must be before the end point, when nearly all the metal ion is complexed with EDTA, the solution is strongly coloured. (b) The colour reaction should be specific or selective. (c) The metal-indicator complex must possess sufficient stability, otherwise, due to dissociation, a sharp colour Change is not attained. The metal-indicator complex must, however, be less stable than the metal-EDTA complex to ensure that, at the end point, EDTA removes metal ions from the metal indicator-complex. The change in equilibrium from the metal indicator complex to the metal-EDTA complex should be sharp and rapid.

(d) The colour contrast between the free indicator and the metal-indicator complex should be readily observed. (e) The indicator must be very sensitive to metal ions (i.e. to PM) so that the colour change occurs as near to equivalence point as possible. (f) The above requirements must be fulfilled within the PH range at which the titration is performed. metal-indicator is a dye that serves as a chelating agent to yield a dye-metal complex, which apparently differs in colour from the original dye, besides possessing a lower stability constant than the corresponding chelatemetal complex. Hence, the colour imparted to the solution is mostly attributed due to the dye-complex formed until the end-point, when an equivalent amount of sodium-EDTA has been incorporated. The critical point at which the metal-dye complex decomposes to yield free-dye on addition of the slightest excess of sodiumEDTA, is distinctly shown by a visible change in colour. M-In

+

M

+

EDTA

M-EDTA

+

In

(Colour of metal-

(original color

indicator complex)

of indicator)

Examples : (i) Mordant Black 2 : (Syn. : Eriochrome Black T ; Solochrome Black T) (ii) Murexide : (Syn. : Ammonium Purpurate) (iii) Catechol-violet (iv) Xylenol orange (v) Calcone (mordant black 17)  MASKING AND DEMASKING: The disodium ethylenediaminetetracetate usually complexes with a wide spectrum of cations, which ultimately renders the selectivity of the titration procedure adversely, thereby providing enough scope for the accompanying metal impurities to be titrated along with the ion it is aimed at for actual estimation. Therefore, in a situation where one or two ions present in a mixture of cations is specifically required to be determined with a view to eliminate completely the possible effects of unwanted impurities that may enhance the titre value, a third substance is added, which is known as the Masking Agent. The following procedures will help to increase the selectivity; 

Use of masking and demasking agents



PH control.



Use of selective metal indicators.



Classical separation



Solvent extraction



Removal of anions

Masking is a reversible process. The important types of masking reactions are masking by precipitation, soluble complex formation, oxidation or reduction, mixed masking, kinetic masking and indirect masking. These agents must fulfill the following requirements, 1. Should form complexes that are definitely more stable than the interfering ion-EDTA complex, and 2. Colour developed by either precipitates or auxiliary complexes should not obscure the end-point. By the use of masking agents, some of the cations in a mixture can often be 'masked' so that they can no longer react with EDTA or with the indicator. An effective masking agent is the cyanide ion; this forms stable cyanide complexes with the cations of Cd, Zn, Hg(II), Cu, Co, Ni, Ag, and the platinum metals, but not with the alkaline earths, manganese, and lead. It is therefore possible to determine cations such as Ca2+, Mg2+, Pb2+, and Mn2+ in the presence of the above-mentioned metals by masking with an excess of potassium or sodium cyanide. A small amount of iron may be masked by cyanide if it is first reduced to the iron(II) state by the addition of ascorbic acid.Titanium(IV), iron(III), and aluminium can be masked with triethanol amine; mercury with iodide ions; and aluminium, iron(III), titanium(IV), and tin(II) with ammonium fluoride (the cations of the alkalineearth metals yield slightly soluble fluorides). The cyanide complexes of zinc and cadmium may be demasked with formaldehyde-acetic acid solution or, better, with chloral hydrate : The use of masking and selective demasking agents permits the successive titration of many metals.  TYPES OF TITRATION: A. Direct titration: The solution containing the metal ion to be determined is buffered to the desired pH (e.g. to pH = 10 with NH:-aq. NH,) and titrated directly with the standard EDTA solution. It may be necessary to prevent precipitation of the hydroxide of the metal (or a basic salt) by the addition of some auxiliary complexing agent such as tartrate or citrate or addition of some auxiliary complexing agent as triethanolamine. At the equivalence point the magnitude of the concentration of the metal ion being determined decreases abruptly. This is generally determined by the change in colour of a metal indicator or by amperometric, spectrophotometric, or potentiometric methods. B. Back-titration: Many metals cannot, for various reasons, be titrated directly; thus they may precipitate from the solution in the pH range necessary for the titration, or they may form inert complexes, or a suitable metal indicator is not available. In such cases an excess of standard EDTA solution is added, the resulting solution is buffered to the desired g pH, and the excess of the EDTA is back-titrated with a standard metal ion solution; a solution of zinc chloride or sulphate or of magnesium chloride or sulphate is often used for this purpose. The

end point is detected with the aid of the metal indicator which responds to the zinc or magnesium ions introduced in the back-titration. C. Replacement or substitution titration. Substitution titrations may be used for metal ions that do not react (or react unsatisfactorily) with a metal indicator, or for metal ions which form EDTA complexes that are more stable than those of other metals such as magnesium and calcium. The metal cation Mn+ to be determined may be treated with the magnesium complex of EDTA, when the following reaction occurs: Mn+ + MgY-2

(MY)n-4+ + Mg+2

The amount of magnesium ion set free is equivalent to the cation present and can be titrated with a standard solution of EDTA and a suitable metal indicator. An interesting application is the titration of calcium. In the direct titration of calcium ions, solochrome black gives a poor end point; if magnesium is present, it is displaced from its EDTA complex by calcium and an improved end point results.  ESTIMATION OF MAGNESIUM SULPHATE BY COMPLEXOMETRY: Synonyms: Epsom Salts MgSO4, 7H2O Mol. Wt. 246.5 Magnesium Sulphate contains not less than 99.0 per cent and not more than 100.5 per cent of MgSO4, calculated on the dried basis. Description: Colorless crystals or a white, crystalline powder. Preparation And Standardization Of 0.05 M Disodium EDTA 

0.05M Disodium EDTA: Weigh accurately about 18.6g of Disodium EDTA, dissolve it in sufficient quantity of distilled water in 1000 ml volumetric flask and make the volume upto the mark with the help of distilled water.



0.05M Calcium Chloride: Weigh accurately about 3.675g of Calcium Chloride, dissolve it in sufficient quantity of distilled water in 500 ml volumetric flask and make the volume upto the mark with the help of distilled water.



Ammonium Chloride: Weigh accurately about 1.68g of ammonium chloride, add it in 14ml of strong ammonium in 250 ml volumetric flask, shake it properly and make the volume upto the mark with the help of distilled water.



Standardization of 0.05 M Disodium EDTA: Pipette out 25ml of 0.05M calcium chloride solution in conical flask. Add 5ml of ammonium chloride solution to it. Add pinch of modern black indicator and titrate it against prepared disodium EDTA solution till color changes from wine red to pale blue.

Assay procedure: Weigh accurately about 0.3 g, dissolve in 50 ml of water, add 10 ml of strong ammoniaammonium chloride solution and titrate with 0.05 M disodium EDTA, using 0.1 g of mordant black II mixture as indicator, until a blue color is obtained. IP Factor: 1 ml of 0.05 M disodium edetate is equivalent to 0.00602 g of MgSO4. MgSO4

Mg++ + SO4--

 ESTIMATION OF CALCIUM GLUCONATE BY COMPLEXOMETRY:

C12H22CaO14, H2O Mol. Wt. 448.4 Calcium Gluconate is calcium D-gluconate monohydrate. Calcium Gluconate contains not less than 98.5 per cent and not more than 102.0 per cent of C12H22CaO14,H2O. Description. A white, crystalline powder or granules. Preparation and Standardization of 0.05 M Disodium EDTA 

0.05M Disodium EDTA: Weigh accurately about 18.6g of Disodium EDTA, dissolve it in sufficient quantity of distilled water in 1000 ml volumetric flask and make the volume upto the mark with the help of distilled water.



0.05M Calcium Chloride: Weigh accurately about 3.675g of Calcium Chloride, dissolve it in sufficient quantity of distilled water in 500 ml volumetric flask and make the volume upto the mark with the help of distilled water.



Ammonium Chloride: Weigh accurately about 1.68g of ammonium chloride, add it in 14ml of strong ammonium in 250 ml volumetric flask, shake it properly and make the volume upto the mark with the help of distilled water.



Standardization of 0.05 M Disodium EDTA: Pipette out 25ml of 0.05M calcium chloride solution in conical flask. Add 5ml of ammonium chloride solution to it. Add pinch of modern black indicator and titrate it against prepared disodium EDTA solution till color changes from wine red to pale blue.

Assay procedure: Weigh accurately about 0.5 g and dissolve in 50 ml of warm water; cool, add 5.0 ml of 0.05 M magnesium sulphate and 10 ml of strong ammonia solution and titrate with 0.05 M disodium EDTA using mordant black II mixture as indicator. From the volume of 0.05 M disodium EDTA required subtract the volume of the magnesium sulphate solution added. IP factor: 1ml of the remainder of 0.05M disodium EDTA is equivalent to 0.02242 g of C12H22CaO14, H2O. C12 H22 CaO14, H2O

++

Ca

-2

+ (C12 H22 O14)...