Complexometric Titrations

Chapter 8 

Complexometric Titrations

Introduction

• As the name indicates, this type of titrations involves formation of complex during titration. These titrations are used mainly for the analysis of pharmaceuticals which contains metals and also for their salts, certain anions and indirectly some drugs. The titrant used in this titration is usually called as ligands. There are many such ligands known, among which, ethylenediamine tetra - acetic acid disodium salt (EDTA) or sodium edetate is most widely used.

• The principle involved in these titrations is that the metal ion presents in the sample (titrate) react with the ligand (titrant) to produce a complex. The complex formed is stable and water soluble. Further, like acid - base titrations, these titrations also follow the concept of proton acceptance and donation. In complexometric titration, metal ion accepts electrons. and ligand donates it.

Classification 

Complexometric titrations are classified into the following categories:

• Direct titrations: This is a simple direct complexometric titrations in which a metal ion present in a solution is titrated with ligand (standard disodium edentate) solution in a suitable buffer medium using a metal indicator. 

• Back titrations:  In some cases, the direct titration is not feasible due to the problems such as the substances are insoluble; the complex formed may not be stable and metal hydroxide in alkaline solution of buffer may get precipitated. These conditions affect the titration.

• Replacement titrations:  This titration is carried out when difficulties arise with direct and back titration methods. In this method, the analyte metal ion is initially allowed to react with a complex of another W Pharmaceutical Analysis Complexometric Titrations. metal and a complexing agent or ligand. The analyte metal ion replaces the metal ion from the complex

• Alkalimetric titration of metals: In this method, an acid - base titration is utilized. When the metal - EDTA complex is formed, there will be the liberation of protons from disodium edetate resulting in formation of acid. M ++ HAY MY + 4H + The liberated acid is used for titration with standard alkali.

• Indirect titrations: The indirect titration method involves the determination of certain organic substances using complexometric method. In general, these organic substances are precipitated by reaction with excess metal ion solution. The unreacted excess metal in solution is determined by titrating with EDTA solution.

Metal Lon Indicators 

These indicators are basically dyes. They exhibit color in the solution depending on the presence of metal ion. With some metal ions they form some color and with other metal ions they form different color or become colorless. 

1. The stability of the metal ion indicator is important because during reaction it should not be dissociated which affects the cooler change.

2. The metal ion indicator should have the capability to produce strong color change at the endpoint.

3. All the above prerequisites must be satisfied inside the pH extend at which the titration is performed. The mechanism of the indicators in forming complex with metals is explained as below 

Masking and Demasking Reagents

One of the important aspects of complexometric titration is its adoptability in estimation of two or more metals in the same solution. This can be done either by selection of suitable pH at which one metal forms a complex without the involvement of other metal ion or by using a suitable masking agent.

Demasking Agent: It is the substance which releases the masked metal ion. The cyanide complexes of zinc and cadmium can be demasked by formaldehyde - acetic acid solution. Demasking can also be possible by use of chloral hydrate. [ Zn (CN)] + 4H + + 4HCHO → Zn ++ + 4HO CH₂ - CN Thus, the use of masking and selective demasking agent permits the estimation of many metals.

 Classical Separations: Various precipitates may be used for the separation. They are redissolved and cations from this solution can be determined complexometric ally. Example, CaC₂O4 nickel. dimethylglyoxime, Mg ₂) PO4, 6H₂O and CuSCN.

Control of pH: This particularly is based on different stabilization of EDTA complexes. Fe (III) is acid solution (pH 3) with variamine blue as indicator in presence of divalent cations (divalent cations does not interfere). Determination of calcium in the presence of magnesium is strongly alkaline medium by using Murexide or calcon as indicato.

Solvent Extraction: Zinc can be separated from copper and lead by addition of excess of ammonium thiocyanate and extracting zinc thiocyanates with isobutyl methyl ketone. Extract is diluted with water and the zinc content is estimated by EDTA titration.

Applications 

Estimation of Magnesium Sulphate

Molecular weight of MgSO4.7H₂O = 246.5. Magnesium sulphate can be directly. determined by titrating with a complexing agent or chelating agent such as EDTA using a metal ion indicator, such as Mordent black or Solo chrome black or Eriochrome black T (EBT).

Principle: MgSO4.7H₂O solution is made alkaline by using ammonium buffer (aq. NH3 and NH4CI) (to maintain pH in range of 7-11) and a pinch of indicator. This solution is then titrated with EDTA solution as titrant. 

Estimation of Calcium Gluconate 

Assay of calcium gluconate has been described below as per IP method. 

Principle: Weigh accurately about 0.5 g of calcium gluconate and dissolve in 50 ml of warm water. Cool the solution and then add to it 5.0 ml of 0.05 M magnesium sulphate and 10 ml of strong ammonia solution. After mixing well, the solution is titrated with 0.05 M disodium edetate using mordant black II mixture as indicator until the colour changes from red to purple.