Pharm.analysis: Complexometric Titrations

      A complexometric titration is one in which a soluble, undissociated and stoichiometric complex is^-formed during the addition of titrant to the sample solution (usually solution of a metal ion).

      It is a method of volumetric analysis developed after the introduction of the analytical reagent, commonly known as ethylene diaminetetraacetic acid disodium salt (E.D.T.A).

      Complexometric titrations are those reactions in which simple metal ion are transformed into complex by the addition of a reagent which is known as ligand or complexing agent.

      The complex formed is stable and water-soluble.

      A complexing agent in its widest sense includes any electron-donating system, which by its ability to form a bond or bonds (Covalent or dative), with a metal ion, produces with it a complex which has different properties from that of the free metal ion.

      The simplest complexing agents are elementary anions, e.g., the fluoride ion in K+ (BF₄)^—,the iodide ion in 2K+ (HgI₄)^--2 and complex anions, e.g., the cyanide ion in potassium ferri-and ferrocyanides.

      The link with the metal in the complex is covalent involving either the sharing of two electrons, one from metal and one from the ligand or donation of a lone pair of electrons from the ligand.

      Once the complex is formed, there is hybridization and equalization of all the bonds.

      There are also neutral groups involving lone pairs of electrons forming a covalent bond, usually from a nitrogen atom (e.g., NH₃ in the complex Co(NH₃)³₆⁺).These are called simple ligands.

      There are several organic compounds which possess more than one donating group which may be neutral or charged and thus form bi-, tri-, or polydentate ligand. For example, ethylenediamine is a bidentate ligand and E.D.T.A. can act as a hexadentate ligand. The term sequestering agent is generally applied to ligands which form water-soluble complexes with metal ions (e.g., EDTA).

      The term chelating agent is generally applied to ligands which form water-insoluble complexes with metal ions (e.g., salicylaldoxime).

EDTA disodium salt is the most widely used titrant in complexometric titrations. There are several reasons for the versatility of EDTA. They are:

1.       EDTA forms stable, soluble, stoichiometric 1:1 complexes with metal ions.

2.       Certain amount of selectivity can be obtained because of differences in stability constants and through the proper control of the pH of the solution.

3.       All the metal-EDTA complexes are soluble and most complexes form rapidly.

4.       The end-point is readily detected by chemical or instrumental methods.

5.       The titration is suitable for a semi micro to macro concentration range.

If EDTA (can be generally represented as Na₂H₂Y) is used, the reaction with a metal ion and the formation constant for the complex formed are represented by

From the above reaction it is evident that complexometric titrations will be very sensitive to pH and all procedures in which EDTA is used as a titrant must include a buffer with sufficient capacity to take care of the hydrogen ion produced during the titration.

DETECTION OF END-POINT

In complexometric titrations, the end-point is detected by using (i) visual indicators or (ii) instrumental methods. Amongst the visual category are important complexometric indicators (Metal-ion indicators. Metallochromic indicators or indicators).

The complexometric indicator is a dye which is capable of acting as ligand to form dye-metal complex. This dye-metal complex is different in colour from the dye itself, and also has a lower stability constant than the chelate-metal complex. The colour of the solution, therefore, remains that of dye-metal complex until the end-point. As soon as there is slight excess of EDTA, the metal-dye complex decomposes to produce free-dye, this is accompanied by a change in colour. It is essential in a complexometric titration to use a buffer solution to maintain the required pH during the titration. Some examples of commonly used complexometric indicators are:

1.     Murexide or Ammonium purpurate

2.     Solochrome Black T or Eriochrome black T or mordant black II

3.   Catechol violet

4.   Xylenol organge

5.   Methyl thymol blue

6.   Alizarin complexone

7.   Sodium alizarine sulphonate

8.   Diphenyl carbazone

9.  Tiron

In addition to the visual indicators, instrumental methods like potentiometric titration, photometric titration and amperornetric titration are also employed to detect the end-point.

TYPES OF COMPLEXOMETRIC TITRATIONS

1.       Direct Titrations: To the metal ion solution a suitable buffer solution and indicator are added and the solution is titrated with standard EDTA solution until the indicator just changes colour. A blank titration may be performed omitting the sample as a check on the presence of trace of metallic impurities in the reagents.

2.       Indirect or Back Titrations : This procedure is necessary for metals which precipitate as hydroxides    

from solution at the pH required for titration, for insoluble substances (like lead sulfate and calcium oxalate) for substances which do not react instantaneously with EDTA, and for those metal ions which form more stable complexes with EDTA than with the desired indicator.

   In this titration excess of standard EDTA solution and a suitable buffer solution to added to the              

   metal solution or suspension. The solution is heated to effect complex formation, cooled and the  

   EDTA not consumed by the sample back-titrated with magnesium or zinc chloride (or sulphate)     

   using a suitable indicator.

3.       Replacement or Substitution Titrations : When direct or back titration do not give sharp end­points, the metal may be determined by the displacement of an equivalent amount of magnesium or zinc from a less stable edetate complex according to the following scheme :

M2+ + _mgx2— m_x2— _mg+2

Calcium, lead and mercury can be estimated satisfactorily by this method using mordant black II as indicator.

4.       Alkalimetric Titration : In this method, protons from EDTA are displaced by a heavy metal and titrated with standard alkali according to the following scheme :

_mn+2 _H,y2—   (m_y),'.-4 _{2 H} +

The titration is carried out in unbuffered solution. A visual pH indicator may be used, but a potentiometric method of locating the end-point is also suggested.

MASKING AND DEMASKING AGENTS

Masking agents: When it is required to estimate selectively one or more ions in a mixture of cations and to .eliminate the effects of possible impurities which would add to the titre, "masking agents" are used. These act either by precipitation or by formation of complexes more stable than the interferring ion-edetate complex.

MASKING BY PRECIPITATION

Many heavy metals (e.g., cobalt copper and lead) can be separated either in the form of insoluble sulphides using Na₂S or as insoluble complexes using thioacetamide. These are filtered, decomposed and titrated with EDTA. Other common precipitating agents are sulphate for lead and Barium; oxalate for calcium and lead, fluoride for calcium, magnesium and lead, ferrocyanide for zinc and copper, and cupferron and 8- hypdroxyquinoline for many heavy metals. Thioglycerol is used to mask copper by precipitation in the assay of lotions containing copper and zinc. Dimercaprol forms precipitates with many cations like mercury, cadmium, zinc, arsenic, antimony, tin, lead and bismuth.

MASKING BY COMPLEX FORMATION

Potassium cyanide reacts with silver, copper, mercury, iron, zinc, cadmium, cobalt and nickel ions to form complexes in alkaline solution which are more stable than corresponding EDTA complexes. Therefore, ions such as lead, magnesiuni and manganese can be determined in their presence. Other examples of masking agents belonging to this category are potassium iodide (He+), Tiron (Aluminium and Titanium) and triethanolamine (Aluminium).

Another simple method of masking is pH control. An example is that the alkaline earth metals do not form EDTA complexes below pH 7.0 and transition elements form EDTA complexes stable down to pH 3.0.

DEMASKING AGENTS

These are substances which release masked metal ion. Formaldehyde or chloral hydrate can be used to release the masked zinc ion by potassium cyanide. These agents are useful in the determination of specifc metal ion from their mixture with other cations.

PREPARATION AND STANDARDISATION OF M/20 EDTA

M/20 EDTA solution: Dissolve 18.6 g of disodium edetate in sufficient quantity of distilled water to produce 1000 ml.

M/20 CaC1₂ solution: Dissolve 5.005 g of AnalaR CaCO₃ in water (25 ml) containing the minimum quantity of dil. HC1. Boil off carbon dioxide, cool and transfer quantitatively to a 1000 ml flask. Dilute with distilled water to 1000 ml.

Buffer Solution: Dissolve 67.5 g of ammonium chloride in 570 ml of strong ammonia solution, dilute to 900 ml with water and add a solution of 0.616 g of magnesium sulphate and 0.93 g of sodium edetate in 50 ml of water. Add sufficient water to produce 1000 ml (the small amount of magnesium edetate complex is included in the buffer to render the end-point sharper in calcium titrations).

Method: Pipette 20 ml of standard calcium chloride solution into a conical flask, add 1 ml of buffer (pH 10) solution and 3 drops of solochrome black T indicator and titrate with EDTA solution until the colour changes from wine-red to blue. A blank titration should be performed on the reagents.

APPLICATIONS

Many pharmacopoeias drugs and chemicals are assayed by complexometric method. These are given the below Table

Drugs Assayed by Complexometry

Drug	Type of titration
Calcium aminosalicylate Calcium carbonate Calcium chloride Dibasic calcium phosphate Tribasic calcium phosphate Magnesium chloride Heavy magnesium carbonate Heavy magnesium oxide Magnesium sulphate Zinc chlroide Zinc stearate Zinc Sulphate Zinc undecylenate Aluminium hydroxide gel Aluminium sulphate Alum Calcium gluconate Calcium lactate Calcium levulinate	Direct Indirect (Back) Indirect (Back) Indirect (Back) Replacement Replacement Replacement