Experiments Involving Complexometric Titrations

An Introduction to Complexometric Titration

Complexometric titration is defined as the conversion of a simple metal ion to a complex ion by the addition of a reagent. The complex produced is soluble in water and stable in nature.

In a complexometric titration, the metal ion receives electrons while the species donates electrons to the ligand. The ligand EDTA (ethylene diamine tetra acetic acid) or Na EDTA (disodium salt of EDTA) is commonly employed in complexometric titration.

A metal indicator is a type of indicator used in complexometric titration. In the presence of metal ions, they emit one colour, but they emit a different colour in the absence of metal ions.

Table of Contents

• What is Complexometric Titration?
• Complexometric Indicators
• Complexometric Titration Experiment Using EDTA
• Types and Applications of Complexometric Titration
• Frequently Asked Questions – FAQs

What is Complexometric Titration?

Complexometric titration, also known as chelatometry, is a method of volumetric analysis in which the endpoint of the titration is determined using a coloured complex. Titration is a typical laboratory technique for determining the unknown concentration of a previously defined analyte. It is a quantitative chemical analysis approach.

It is often called volumetric analysis, as volume measurements are essential. Titrants are solutions that use reagents as a standard solution. The reaction quickly reaches equilibrium when each drop of titrant is introduced. A complexometric titration can be used to identify the equivalent point very accurately. The use of EDTA as a titrant is quite well established.

Complexometric Indicators

When conducting metal cation titrations with EDTA, a complexometric indicator is nearly always required to determine when the endpoint has been achieved. Organic colours like Fast Sulphon Black and Eriochrome Black T are common indicators. When the endpoint is reached, the indicator is displaced (typically by EDTA) from the metal cations in the solution, resulting in a colour shift. As a result, the endpoint indicator is the free indicator rather than the metal complex.

Complexometric indications include calcein, curcumin, Eriochrome Black T, fast sulphon black, hematoxylin, etc.

The detection of the endpoint in a complexometric titration can be observed in two ways.

Visual Method

Because of its simplicity, low cost, and precision, it is one of the most often used methods for determining endpoints. Some visual methods for finding the endpoint of complexometric titrations are listed below.

• Metallochromic or PM indicators
• pH indicators
• Redox indicators

Instrumental Method

The use of visual methods to determine the endpoint has drawbacks, such as inaccuracy and human visual errors. The following are some of the instrumental approaches used in endpoint determination:

• Photometry
• Potentiometry
• Miscellaneous methods

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Complexometric Titration

Complexometric Titration Experiment Using EDTA

We describe an experiment for estimating the hardness of water using complexometric titration for a better understanding of complexometric titration.

Aim

By utilising EDTA in a complexometric titration, the quantity of overall hardness in a particular water sample can be estimated.

Requirements

Burette, pipette, spatula, buffer solution, Eriochrome Black T indicator, standard EDTA Solution (0.01M), and inhibitor.

Preparation of a 0.01M EDTA Solution

In a drying oven at 80°C for one hour, dry about 2 g of EDTA dihydrate, Na₂H₂Y₂ 2H₂O. Then weigh out approximately .95 g ± 0.lmg. Transfer the EDTA to a 250 mL volumetric flask, mix in the distilled water, and dilute to the desired concentration with distilled water. By inverting and shaking the tightly stoppered flask, thoroughly mix the ingredients. This solution should be labelled “Standard EDTA.”

Principle

When associated with metal ions, the indicator Eriochrome Black T turns wine red, but it remains blue when free of metal ions. EDTA (ethylene diamine tetra acetic acid) is colourless, regardless of whether or not it is linked to metal ions. As Eriochrome Black T binds to metal ions, adding the EBT indicator to the sample (water containing metal) turns it wine red. Eriochrome Black T has weak binding to metal ions, whereas EDTA has a significant binding. When all metal ions are bonded to EDTA, the indicator EBT in the sample remains free, and the solution appears blue.

In an aqueous solution, the complex-forming ion of EDTA can be written as H2Y^2-. It can be written as follows when reacting with metal ions:

M⁺² + H₂Y^2- → MY^2- + 2H⁺

M⁺³ + H₂Y^2- → MY^– + 2H⁺

M⁺⁴ + H₂Y^2- → MY + 2H⁺

Procedure

1. In a conical flask, put 25ml of the provided sample.
2. A 1ml buffer solution has now been added.
3. Add 1ml of the inhibitor to the sample and buffer solution now.
4. Add 2 drops of Eriochrome Black-T solution to the mixture, rendering the solution a bright red colour.
5. An EDTA solution should be poured into the burette. Starting with a regular EDTA solution, titrate the combination.
6. A change in colour from wine red to blue can be used to identify an endpoint.
7. To get three concordant readings, repeat the titration three times.

Observation Table

Calculation

Total hardness (mg/l) by calcium carbonate = (A×B×1000)/Volume of sample in ml

Here, A denotes the amount of EDTA required by the sample or the amount of EDTA used in the titration.

B = 1*(Molarity of EDTA/0.01M)

Result

Thus, the overall hardness level in the provided water sample is……mg/l.

Precautions

• There are a few precautions to take in this experiment. To begin with, perform the titration process at least twice to obtain more accurate and precise findings.
• Second, make sure the EDTA solution is filled with the burette tip. Before starting the titration, make sure there are no air bubbles in the solution between the tip and the stopcock or the solution in the burette.
• To avoid parallax errors when reading the meniscus in the burette, place a piece of white paper behind it. The line of sight must be parallel to the meniscus’s bottom.
• Furthermore, the burette’s key must be carefully operated to ensure that one drop of EDTA solution falls into the conical flask containing the analyte at a moment when the endpoint, where blue colouration would be detected, is reached.
• Then, throughout the experiment, only use deionised water rather than distilled water.
• Do not add extra indication (EBT) to make the solution darker since this may cause endpoint issues.
• Finally, use safety goggles, gloves, and a lab coat throughout the experiment to protect yourself and the environment.

Types and Applications of Complexometric Titration

Types of Complexometric Titration

In various ways, EDTA can be employed as a chelating titrant. Titrations with EDTA can be done in a variety of methods. The following are a few of them:

• Direct Titration is the most practical and most straightforward method of EDTA complexometric titration. The EDTA solution is added to this titration standard to give a sample containing metals, then burette until the endpoint is reached.
• Back Titration – It’s called “back titration” because we’re back titrating for the extra amount of EDTA in this type of titration.
• Replacement Titration – This method is used when direct titration or back titration fails to produce sharp endpoints.
• Indirect Titration – With metal cations, some anions produce a precipitate. EDTA does not affect these anions. As a result, indirect titration with EDTA can be used to examine these.

Applications of Complexometric titration

• The total hardness in water is estimated via complexometric titration.
• It’s commonly used in the pharmaceutical industry to determine how much metal is in medicine.
• Many cosmetic products contain titanium dioxide. Complexometric titration can be used to investigate this.
• It’s a device that analyses urine samples.
• It’s a standard tool in analytical chemistry.