Complexometric Titrations

Titration is a popular method to determine the concentration of various components in a mixture. Titration usually requires an indicator to show it has achieved the result required. These indicators need to have certain properties in terms of reactivity with the component to be measured in the mixture. Complexes are special compounds that are frequently used in titrations. 

The process of titrating with the use of complexes is called complexometric titration. 

What exactly are Complexometric Titrations?

The definition of complexometric titration is that it is a type of volumetric analysis where the end result leads to the formation of colored complexes. The endpoint of any titration is an unambiguous chemical that produces colour in the reaction to indicate the endpoint result. The formation of coloured complexes plays a vital role in determining concentration in a solution with mixed metal ions.

The principle of Complexometric titrations

Here the principle is to create a complex reaction, i.e., the metal ion and ligands are made to interact to form complexes. The metal ion behaves as Lewis acid. The ligand acts as a complexing agent. 

Example: 

The reaction between silver – metal ion and CN- the ligand 

Ag+ + 2CN− → Ag(CN)2−

To understand the process completely, one must have the basic concepts of complex ions covered, such as types of ligands, the concept of ligands, how complex ions work, and their chemical properties.

To detect the magnitude endpoint of the complexometric titrations and to understand the complexometric titration curve, the below factors must be taken into account:

1. Complex stability – the stability of the complex is vital. The higher the constant of stability, the higher its clarity in the endpoint. This is because there is a greater charge in metal concentration during the equivalence point.
2. Ease in formation of complexity – the easier it is to form the complex the faster the breakpoint occurs during the equivalence point in the curve. Hence it becomes easier to detect the endpoint.
3. pH factor – the pH of the dissolved solution or buffer solution must be constant as a change in pH can lead to a change in chelation.

The endpoint is determined by plotting the complexometric titration curve. Chemically this endpoint can be found using an indicator or using instrumental methods. 

Instrumental methods are :

1. Spectrophotometric method – here, the absorption change between metal and the complex is measured.
2. Potentiometric method – uses the electrode potential difference to determine ions.
3. Amperometric method – the titration method uses mercury electrodes to determine the current developed between the metal and the complex.
4. High-frequency titration – uses a basic buffering solution for titration and extraneous electrolytes to reduce the sensitivity of the solution.

Indicators used for identifying the endpoints in complexometric titration:

1. Triphenylmethane dyes.
2. Phthalein and substituted phthalates.
3. Azo dyes.
4. Phenolic compounds.

Types of Complexometric Titrations

Below are the types of complexometric titrations.

Direct Titration

Direct titration is the easiest to understand and implement the types of complexometric titrations. Here the standard complexometric solution is constantly mixed with the metal ion solution till the endpoint is detected. 

Example – acid-base titrations are very good examples of direct titration. The indicator is an acid-base indicator. These indicators are usually colourless. These indicators are usually colourless as the titration proceeds and the pH changes, causing the indicator’s colour to change.

Back Titration

During back titration to the metal solution, an excess standard Ethylenediaminetetraacetic acid solution or EDTA is added, which is then analysed to find the concentration of the metal. Therefore the role of EDTA in complexometric titration is to find the concentration of the metal. A standard solution containing a different metal ion is used to back titrate the excess. Hence the process is called back titration.

E.g., Determination of Mn. EDTA cannot be used to directly detect the presence of Mn. An acidic solution of Mn is prepared using its salt. An EDTA of known volume is taken and is added in excess to the solution. Then the ammonia buffer is mixed to maintain the pH level at 10. After this is achieved, the solution is back titrated with a standard solution of Zinc.

Replacement Titration

In this method, the metal which is to be analysed, is made to displace the central ion in a complex. The drawback of direct and back titrations is that they do not provide an accurate endpoint to the titration, making the detection very hard. A better method of determining the metal in a titration is the calculating the amount of Mg or Zn a certain metal displaces from a less stable edetate complex.

Example:

Mg is displaced by Mn from the Mg EDTA solution. This free Mg metal undergoes direct titration along with a standard EDTA solution. This method requires the addition of an excess quantity of Mg EDTA to the Mn solution.

By this method, Ca, Pb, and Hg can also be determined using EDTA.

Indirect Titration

Indirect titration is also called Alkalimetric titration. It is used mainly to detect ions such as anions that do not react with EDTA. The protons present in disodium edetate get displaced by heavy metals and then undergo titration with sodium alkali.

Applications of complexometric titrations

Complexometric analysis helps us understand and determine various compositions of metal concentration in our day-to-day life, such as in medicines, food chemicals, etc.

• This method is used to determine various complexes of Calcium and Magnesium in water that act as a hardening agent of water.
• It is a very vastly used process in the pharma industry to determine the various concentrations of metals inside medicines or drugs. 
• In the field of biological analysis, it is used for urine sampling, blood sampling, etc 

It is essential to know the concentration of metals as they are very toxic to the human body. In a limited and minute amount, it’s good for one’s health but in excess, it can lead to metal toxicity that damages the functioning of the organs like brain, blood, kidney, etc.

Major diseases caused by metal toxicity are cancer, brain deformities, birth defects, immune system dysfunctionality, and more.

Conclusion

Complexometric Titrations are a form of volumetric analysis that is used in qualitative chemical analysis. It uses complexes formed by metals and ligands as indicators in the titration. It is used due to the ease of observation as it forms a coloured compound. Complexometric Titrations have various applications in real life such as determining the toxicity of metal, the hardness of water, etc. There are various types of complex titrations that can be used depending on the requirement of the situation like direct titration, back titration, replacement titration, and indirect titration.