Complexometric Titration: EDTA, Types & Applications

Complexometric Titration, also known as chelatometry, is a volumetric analysis in which the endpoint of the titration is determined using a coloured complex. Titration is a common technique used in labs to determine the concentration of an unknown analyte.  

• Complexometric Titration involves the formation of a complex between a chelating agent and a metal ion.
• The complex is then titrated with a standardized solution of EDTA.
• Complexometric titration is used in the fields of environmental science and medicine for precise measurements of metal ion concentrations.
• Some commonly used chelating agents include ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), and nitrilotriacetic acid (NTA).

It is a powerful analytical technique that enables researchers to accurately measure metal ion concentrations in a wide range of applications.

Also Check: Analytical Chemistry

Key terms: Complexometric Titration, Complex, Indicators, Titrant, EDTA, Chelatometry, Metal ion, Volumetric Analysis 

What is Complexometric Titration?

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Complexometric titration is a volumetric titration or analysis during which the endpoint is marked by a coloured complex. In this sort of titration, an indicator is utilized that can produce a clear colour shift in the titration, indicating the titration's endpoint. The concentration of metal ions in a solution is determined using complexometric titrations. 

• The volume of the analyte, titrant, and even indicator all play a role in titration, hence it's a volumetric analysis.
• In the complexometric titration, indicators like calcein and Eeriochrome black T are used.
• The chelating agent is added to the sample solution containing the metal ion. 
• Then a titrant solution with a known concentration of a chelating agent, usually ethylenediaminetetraacetic acid (EDTA), is added gradually until the endpoint is reached.
• At the endpoint, all metal ions react with a chelating agent, forming a stable complex that is easily detected using a colour indicator or by monitoring the change in pH.
• The concentration of the metal ion in the sample can be calculated from the known concentration of the titrant and the volume of titrant required to reach the endpoint.

Complexometric titration is widely used to determine the concentration of metal ions in various samples, including biological fluids, environmental samples, and industrial products. 

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Complexometric Indicators

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Calmagite and Eriochrome BlackT (EBT) are examples of indicators that change colour from blue to pink when calcium or magnesium is present. When the colour changes from pink to blue, the endpoint of a complexometric EDTA titration utilising either Calmagite or EBT as the indicator has been reached. Some commonly used complexometric indicators include Eriochrome Black T, Calmagite, Xylenol Orange, Murexide, and Ferroin. In a complexometric titration, there are two approaches for detecting the endpoint.

Visual Technique

​Due to its simplicity, low cost, and precision, the visual technique is one of the most often used methods for determining endpoints. Some of the visual approaches for finding the end point of complexometric titrations are listed below.

• PM indicators (metallochromic)
• Indicators of pH
• Redox Indicators

Instrumental Techniques

The visual methods for determining the endpoint have limitations, such as inaccuracy and human visual errors. The following are some of the instrumental approaches used in endpoint determination.

• Photometry
• Potentiometry
• Miscellaneous methods.

The choice of the complexometric indicator depends on the metal ion being analyzed. This is because different indicators have different sensitivity and selectivity for different metal ions. A proper selection of the indicator is necessary to ensure accurate results in complexometric titration.

EDTA Complexometric Titration

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EDTA (ethylenediaminetetraacetic acid) complexometric titration is used to determine the concentration of metal ions in a sample. EDTA (ethylenediaminetetraacetic acid) is a complexometric indicator made up of two amino groups and four Lewis bases (carboxyl groups). EDTA is a selective chelating agent forming strong complexes with metal ions, including calcium, magnesium, zinc, and iron.

• Because of its ability to signify six pairs of lonely electrons due to the formation of covalent bonds, EDTA is a hexadentate ligand.
• Even the presence of small metal ions would cause a noticeable colour change leading to the formation of a weak complex.
• The majority of complexing agents are free acids, which are less soluble in water.
• In volumetric solutions, they're transformed into sodium salts that can be used in water before they can be used.
• They are used for titration as they have a lower solubility in water.

Simple titration methods are sometimes used to determine the presence of simple metal ions in water. However, complexometric titration with EDTA is performed to identify the exact number of metal ions present.

Also read: Non-Aqueous Titration

Types of Complexometric Titration

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Complexometric titration is an analytical technique to determine the concentration of various metal ions in a wide range of samples. EDTA can be used as a chelating titrant. Titrations with EDTA can be done in a variety of methods. The following are a few of the complexometric titration types:

Direct Titration

Direct titration is the most practical and straightforward form of EDTA complexometric titration.

• It is comparable to the acid–base titration method.
• The EDTA solution is added to this titration standard to give a sample containing metals, which is then burette until the endpoint is reached.
• Metals that can be determined by direct complexometric titration include copper, barium, zinc, mercury, aluminium, lead, bismuth, chromium, and others.

Back Titration

Back Titration is used when the metal ion of interest reacts slowly with the chelating agent.

• In this method, an excess of EDTA  is added to the sample solution.
• The unreacted chelating agent, EDTA is then back-titrated with the standard solution of a metal ion. 
• From this, the concentration of the metal ion in the sample solution can be calculated.

Replacement Titration

When direct titration or back titration fails to provide sharp endpoints, replacement titration is utilised.

• This type of titration involves the displacement of a metal ion from a complex by a stronger chelating agent.
• An excess of the added metal ion is then titrated with a solution of another chelating agent of known concentration to determine the amount of the added metal ion. 

Indirect Titration

Some anions react with metal cations to generate a precipitate. EDTA does not affect these anions. As a result, indirect titration with EDTA can be used to examine these.

• Indirect titration, for example, can be used to determine barium ions. 
• Indirect titration is used to determine the concentration of an analyte by reacting it with a reagent that forms a product, which is then titrated with a standardized solution.

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Applications of Complexometric Titration

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Complexometric titration is a widely used analytical technique that has multiple applications such as:

• The amount of total hardness in water is estimated via complexometric titration. This is important in water treatment for domestic and industrial purposes.
• It determines the concentration of metal ions in food samples, which can be harmful to human health if present in excess.
• It's commonly used in the pharmaceutical sector to determine the amount of metal ions present in medicines which affect their safety.
• Complexometric titration is used to determine the concentration of metal ions in environmental samples that has adverse effects on human health and the environment.
• Many cosmetic products contain titanium dioxide. Complexometric titration can be used to examine this.
• It is employed in the examination of urine samples.
• It is used in biological samples such as blood and urine to determine the concentration of metal ions, indicative of certain diseases and disorders.
• In analytical chemistry, it is commonly used.

Also read: Conductometric Titration

Things to Remember

• Complexometric titration is a technique to determine the concentration of metal ions in a sample.
• As volume measurements are important, complexometric titration is also known as volumetric analysis. 
• The volume of a titrant that has been reacted is known as titrant volume. 
• This technique involves the use of chelating agents to form complexes with metal ions facilitating their detection.
• The choice of chelating agent depends on the metal ion being analyzed, and suitable indicators are used to detect the endpoint of the titration.
• This technique has a wide range of applications in various fields, including water analysis, environmental analysis, and pharmaceutical analysis.