Complexometric Titration

Introduction

Titration or titrimetry is an analytical method that is used for the detection of an unknown amount of concentration of the unknown analyte. It is a volume measurement so it is also called a volumetric method. A standard solution of a reagent or titrant is prepared and it will then react with the analyte to obtain its concentration. The volume in which the reagent or titrant reacted is called the titrant volume. There are various titrimetry methods. Complexometric titration is one among them. It is a titration in which the formation of a colored coordination complex is indicated as the endpoint is complexometric titration. It is also called chelatometry since it involves the formation of metal chelates.

What is Complexometric Titration?

A type of volumetric analysis that involves the formation of the colored complex at the endpoint is complexometric titration. It is particularly used for the detection of metal ions in solutions. For this, the metal ions or cations react with complex forming reagents. And these reagents are commonly anions or neutral molecules. So they share the unpaired electrons present on them with the cations or metal ions. Complexometric indicators are used for the detection of the endpoint by forming a colored complex. For example, the metal Silver reacts with Cyanide to form a complex as,

$$\mathrm{Ag^{+}\:+\:2CN^{-}\:\longleftrightarrow\:[Ag(CN)_{2}]^{-}}$$

The central metal atom is called lewis acid while the ligands are called lewis bases. A bond formed between the central metal atom or the acceptor and the donor or ligands is a coordinate bond. The titrant commonly used in titration is EDTA. It is a hexadentate ligand since it can donate six electron pairs. The full form of EDTA is Ethylene diamine tetra acetic acid. So it contains two amine and four carboxyl groups. The structure of EDTA is,

It is shortly represented as 𝐻4𝑌. It can form complexes with almost all the metal ions and the complex formed are enough stable too. The stoichiometry of almost all the complex ions formed irrespective of the metal ion is 1:1. Except for 𝐶𝑟3+ ions, all the reactions are rapid and form a colorless, water-soluble compound. A general reaction between the metal ion and EDTA is shown below.

$$\mathrm{M^{n+}\:H_{4}Y\:\longleftrightarrow\:MY^{(4-n)-}\:+\:4H^{+}}$$

To obtain the endpoint detectors are used.

Complexometric Indicators

The detection of the endpoint is an important step in the complex metric titration. As it is the step in which we can determine the unknown concentration of the analyte. There are mainly two types of detection methods visual method or metal indicators or metallochromic indicators and the instrumental method

Visual method

One of the commonly used methods in which its accuracy is great and the cost is affordable is the visual method. And is a traditional method. Metal ion indicators for complexometric indicators are used for the detection of endpoints in complexometric titration. Organic dyes are commonly used for this purpose as they can give color to metal ions during the titration reaction. In which the titrant, EDTA replaces the indicator and forms a stable complex with the metal ion and a color change is visible at the endpoint. The new color developed at the end of the reaction is due to the free ligands present in the indicator. An example of an indicator uses is Eriochrome Black T. The indicator is a weak acid too. The reaction that takes place with the metal ion, $\mathrm{Mg^{2+}}$, and indicator is shown below.

$$\mathrm{Mg^{2+}\:Hlnd^{2-}(blue)\:\rightarrow\:Mglnd^{-}(red)+\:H^{+}}$$

$$\mathrm{Mglng^{-}(red)\:+\:H_{2}Y^{2-}\rightarrow\:Mgy^{2-}(colourless)\:+\:Hlnd^{2-}(blue)\:+\:H^{+}}$$

The change in color from red to blue is an endpoint.

Instrumental Method

The instrumental method involves the measurement of the redox potential of the metal complex with the help of potentiometric titration. Platinum electrodes can be used for this purpose. Another indicator that can be used is the mercury indicator in which potential change is measured during the reaction of metal and Hg-EDTA complex. The reaction is,

$$\mathrm{Mg^{2+}\:+\:Hg\:-\:EDTA^{2-}\:\rightarrow\:Hg^{2+}\:+\:M\:-\:EDTA^{2-}}$$

Another method is photometric titration in which the absorbance is measured. The value of the complex formed in the absorbance spectrum is zero and is the basis of using this method.

Complexometric Titration using EDTA

The titrant using EDTA is commonly found in the complex metric titration. Since the complex formation with EDTA is stable enough and its endpoint detection using a metallochromic indicator is also affordable and accurate. For example, the estimation of metal ions in water for detecting the hardness of water is done with the help of EDTA. The ions present in the water form a complex with metal as,

$$\mathrm{M^{+2}\:+\:H_{2}Y^{2-}\:\rightarrow\:MY^{2-}\:2H^{+}}$$

$$\mathrm{M^{+3}\:+\:H_{2}Y^{2-}\:\rightarrow\:MY^{2-}\:2H^{+}}$$

The indicator used is Eriochrome black T. The color of Eriochrome black T becomes blue from wine red when it is free from metal ions. This way the endpoint is detected.

Types of Complexometric Titration

Different types of complexometric titrations are discussed below.

• Direct titration − Like acid-base titration, the cations or metal ions are titrated against EDTA directly. And the endpoint is detected using an indicator.

• Back titration − In this titration a back titration with EDTA is present. When the reaction between the analyte and EDTA is almost complete this type of titration involves the addition of a further amount of EDTA and performing titration again. These are applicable for titrations that can't form enough stable compounds with EDTA.

• Replacement titration − When both the direct and back titration is ineffective this is used. And it involves the displacement of Magnesium or Zinc ions from the EDTA complex with an equal amount of metal ions. Then the liberated ions are again titrated with EDTA solution. The indicator used is mordant black. It is commonly used for the determination of calcium salts.

• Indirect titration − For the anions that can form a precipitate with cations are estimated using this titration. They cannot react with EDTA so they are analyzed using indirect titration. Barium ions are commonly estimated using this method.

Applications of Complexometric Titrations

• It is used for the estimation of the hardness of the water.

• It is used for interpreting urine samples.

• It is used for the estimation of metal concentration in drugs.

• It can be used for the estimation of a weak acid in a solution.

Conclusion

Titration is important for estimating the unknown concentration of analytes present in a solution. Complexometric titration is important among them. The estimation of metal ions can be done with the help of this technique. The commonly employed titrant in this process is EDTA. And the indicators used are organic dyes. Eriochrome Black T is the commonly employed one. And some other indicators are also present and it has varying titration types. There are mainly four types of complexometric titrations. They are direct titration, indirect titration, back titration, and replacement titration. The application of this technique is also wide since the estimation based on this technique is economically benign and efficient.

FAQs

1. Who discovers complexometric titration?

Gerold Schwarzenbach discovered complexometric titration during the discovery of visual color indicators.

2. is the buffer used in complexometric titration?

To resist the change in pH during reaction buffer is used. And all the reactions involved in this titration are pH-dependent. So it is very important to add buffer during this titration.

3. How chelating agents are used in complexometric titration?

The organic reagents that can form chelates with metal are chelating agents. These compounds contain two or more groups that can form a complex with the metal ions thus a chelated complex is formed. And is a type of complexometric titration called chelatometric titration.

4. What are the limitations of complexometric titration?

The limitations are,

• A bulk amount of substance is needed for conducting analysis.

• The chance of human error during the process is high.

• A large amount of chemical waste is produced as a byproduct.

5. What are masking and Demasking?

The reagents that will interfere during the analysis process by reacting with the chemical components is a masking agent. Demasking is a process in which the masked substance recovers its ability to enter a chemical reaction. Manganese, cyanides, etc. are examples of masking agents. Triethanolamine is a demasking agent.