Complexometric Titrations

Complexometric titration, also known as chelatometry, is a form of volumetric analysis. It’s particularly useful for determining a mixture of different metal ions in solution. The formation of a colored complex is used to indicate the end point of the titration. The reaction reaches equilibrium rapidly after each portion of titrant is added, and there’s no chance for any interfering situations. A complexometric indicator capable of locating equivalence point with fair accuracy is available. EDTA (ethylenediaminetetraacetic acid) is commonly used as a titrant. It’s a hexadentate ligand, capable of donating its six lone pairs of electrons for the formation of coordinate covalent bonds to metal cations.

Table of Contents

The endpoint detection in complexometric titration can be done by two methods as follows.

Visual Method: This is one of the most common methods for determination of endpoint owing to its simplicity, least cost and accuracy.
Instrumental Method: Use of visual methods in determining the endpoint is not free from limitations including inaccuracy or human visual errors.

Classification

Direct Titration

This is the most straightforward method where the metal ion is directly titrated with a complexing agent like EDTA. The process of titration involves the preparation of a titrant, which is a standard solution whose volume and concentration are predetermined. This titrant is then made to react with the analyte until some endpoint or equivalence point is reached. At that stage, the concentration of the analyte can be determined by measuring the amount of titrant consumed.

Back Titration

A back titration is a titration method where the concentration of an analyte is determined by reacting it with a known amount of excess reagent. The remaining excess reagent is then titrated with another, second reagent. The second titration’s result shows how much of the excess reagent was used in the first titration, thus allowing the original analyte’s concentration to be calculated.

Replacement Titration

This type of titration is used for metal ions that do not react with a metal ion indicator or for the metal ions which form EDTA complexes that are more stable than those of other metals, such as Mg and Ca. Upon the introduction of a substantial or equivalent amount of the chelate of a metal that is less stable than that of the metal being determined, a substitution occurs, and the metal ion displaced can be titrated by the chelate in the same solution.

Indirect Titration

Also known as back titration, it allows the concentration of an unknown solution to be determined by reacting it with a known excess of a reagent. The quantity of the excess reagent is determined by titration with a second reagent. The indirect titration includes the process of reacting the analyte with moles of a particular excess reagent and thereby titrating this reagent with a second reagent.

Metal ion indicators

Metal ion indicators in complexometric titrations are known as complexometric indicators. These are organic compounds that undergo a color change when they form a complex with a metal ion. They are used to signal the endpoint of a complexometric titration, where the formation of a stable complex indicates the completion of the reaction.

Some examples of complexometric indicators include,

Calcein
Eriochrome Black
Curcumin
Hematoxylin
Fast Sulphon Black

These indicators are also known as pM indicators or metallochromic indicators. They are usually used to detect the endpoint of EDTA titration. The indicator changes its color when the metal ion is bound with the indicator. Different types of metal indicators are used for different metal ions.

These indicators are very sensitive to pH and are used to detect the endpoint in complexometric titrations. Most of the EDTA titrations belong to complexometric titrations.

Masking and demasking reagents

The use of masking and demasking agents can help to improve the accuracy and reliability of the titration, particularly in samples containing multiple metal ions. However, it’s important to carefully consider the potential effects of the masking or demasking agent on the titration and to select appropriate concentrations to prevent any unwanted effects.

Masking Reagents: These are chemicals that prevent interference from certain ions during the titration. They form stable complexes with interfering ions, preventing their reaction with the titrant. For example, EDTA (Ethylenediaminetetraacetic acid) is a common masking agent used to mask calcium, magnesium, and other metal ions. Other masking agents include citric acid, tartaric acid, and oxalic acid.
Demasking Reagents: These are used to remove the masking agent from the metal ion of interest, allowing the metal ion to be titrated. Demasking agents typically form a more stable complex with the masking agent than the metal ion of interest, causing the masking agent to be displaced from the metal ion. Common demasking agents include sodium cyanide, which is used to demask copper from EDTA complexes, and potassium iodide, which is used to demask mercury from EDTA complexes.

Estimation of magnesium sulphate

A step-by-step process for the estimation of magnesium sulphate (MgSO4) by complexometric titration is given below.

Preparation of the Sample: A known quantity of the magnesium sulphate sample is dissolved in distilled water.
Addition of the Indicator: A few drops of the indicator, such as Eriochrome Black T, are added to the sample solution. The indicator forms a complex with magnesium ions, which changes color when all the magnesium ions have reacted with the EDTA.
Titration: The EDTA solution is slowly added to the sample solution until the endpoint is reached. The endpoint is determined by observing the color change of the indicator from blue to red or pink. At the endpoint, all the magnesium ions in the sample have reacted with the EDTA.

Mg2⁺ + H2Y2⁻→ MgY2⁻+2H⁺

In this reaction,

Mg2+ represents the magnesium ions present in the magnesium sulphate sample.
H2Y2- represents the EDTA, which acts as the titrant.
MgY2- represents the stable, water-soluble chelate complex formed between the EDTA and the magnesium ions.
Calculation of Results: The amount of EDTA solution added to the sample solution is used to calculate the amount of magnesium sulfate present in the sample, using the stoichiometry of the reaction.

It’s important to standardize the EDTA solution against a standard solution of magnesium sulfate before use to ensure accurate results. Additionally, care should be taken to ensure that the sample solution is free from any interfering substances that may affect the accuracy of the titration.

Overall, complexometric titration using EDTA is a reliable and accurate method for the estimation of magnesium sulfate and is widely used in various industries, including pharmaceuticals and agriculture.

Estimation of calcium gluconate

A detailed explanation of the estimation of magnesium sulphate (MgSO4) by complexometric titration using ethylenediaminetetraacetic acid (EDTA) as a titrant is given as follows.

Preparation of the Sample: A known quantity of the magnesium sulphate sample is dissolved in distilled water.
Addition of the Indicator: A few drops of the indicator, such as Eriochrome Black T, are added to the sample solution. The indicator forms a complex with magnesium ions, which changes color when all the magnesium ions have reacted with the EDTA.
Titration: The EDTA solution is slowly added to the sample solution until the endpoint is reached. The endpoint is determined by observing the color change of the indicator from blue to red or pink. At the endpoint, all the magnesium ions in the sample have reacted with the EDTA.
Calculation of Results: The amount of EDTA solution added to the sample solution is used to calculate the amount of magnesium sulfate present in the sample, using the stoichiometry of the reaction.

The chemical reaction between EDTA and magnesium ions can be expressed as follows.

Mg2⁺+ H2Y²⁻ → MgY²⁻ + 2H⁺

In this reaction:

Mg2+ represents the magnesium ions present in the magnesium sulphate sample.
H2Y2- represents the EDTA, which acts as the titrant.
MgY2- represents the stable, water-soluble chelate complex formed between the EDTA and the magnesium ions.
The 2H+ ions are released as a result of the reaction.

Applications

Complexometric titrations have several applications in the pharmaceutical industry as follows.

Estimation of Metal Ions in Medicines: Complexometric titration is commonly used to determine the amount of metal ions present in medicines, which can affect their safety.
Determination of Hardness in Water: The hardness of water, which can affect the manufacturing process of pharmaceuticals, can be estimated by complexometric titration.
Analysis of Titanium Dioxide: Titanium dioxide is used in many cosmetic products, which can be analyzed by complexometric titration.
Analysis of Biological Samples: In the field of biological analysis, complexometric titration is used for urine sampling, blood sampling, etc.
Determination of Metal Concentration in Environmental Samples: Complexometric titration is used to determine the concentration of metal ions in environmental samples that have adverse effects on human health and the environment.

These applications highlight the importance of complexometric titrations in ensuring the safety and efficacy of pharmaceutical products.

Summary

We discussed complexometric titrations, a form of volumetric analysis used to determine metal ions in solution. We explored its four types: direct, back, replacement, and indirect titration. We also discussed the role of complexometric indicators, which signal the endpoint of a titration. Furthermore, we delved into the use of masking and demasking agents that prevent interference from certain ions during the titration. Lastly, we detailed the process of estimating magnesium sulphate using EDTA in a complexometric titration, including the chemical reaction involved. This method is widely used in industries like pharmaceuticals and agriculture.

Complexometric titrations