Iodometric Titration: Principle, Example, Advantages, and How Does it Work

Learn about the principle of iodometric titration, reaction, indicators, examples, advantages, and how the iodometric titration works.

Titration, often known as titrimetry, is a volumetric analysis used to measure analyte concentration in a sample solution. It consists of a burette filled with titrant and a pipette used to introduce the titrand into the conical flask where the reaction occurs.

A titrant of known concentration is added until the reaction is complete i.e. the chemical equivalence point. A titrant of known concentration is applied, until the reaction is complete, i.e. the chemical equivalence or endpoint. Titration is mainly classified into four types: acid and base, redox, complexometric, and precipitation titration.

Table of Contents: What is redox titration? What is iodometric titration? Principle of iodometric titration Reaction involved in iodometric titration Examples of iodometric titration How does iodometric titration work? Indicator is used in iodometric titration Advantages of iodometric titration

What is redox titration?

Redox titration is a type of titration; it is an oxidation-reduction reaction that occurs between an oxidizing and a reducing agent. There are several types of redox titration, including bromatometry, cerimetry, iodometry, iodimetry, permanganometry, and dichrometry are classified as direct titration, and back titration.

It is a laboratory method used to determine the concentration of a sample analyte by causing a redox reaction between the titrant and the analyte. A potentiometer or a redox indicator may be required for this type of titration.

What is iodometric titration?

Iodometry, known as iodometric titration, is a type of redox titration in which a solute (oxidizing agent) is added to excess iodide to obtain iodine, and the amount of iodine produced is determined by titration with a sodium thiosulfate solution.

It's an indirect titration in which starch indicator is used to determine the endpoint. At the end of the experiment, the blue color of the starch-iodine complex will fade.

What is the principle of iodometric titration?

Iodometric titration works on the principle of determining the concentration of an oxidizing agent in a sample solution. Iodometry involves the indirect titration of iodine liberated by reaction with the analyte.

In which starch solution is used as an indicator as it can absorb the I2 that is released. When titrated with a standardized sodium thiosulphate (Na2S2O3) solution, absorption causes the color to change from bright yellow to dark blue. This indicates the endpoint of the titration.

Reaction involved in iodometric titration:

The reaction between iodine and the thiosulfate ion is as follows.

Examples of iodometric titration:

• To standardize the sodium thiosulfate (Na2S2O3) using potassium dichromate (K2Cr2O7)
• Iodometric estimation of Cu (II) (Copper (II) oxide ) using sodium thiosulfate solution
• Estimation of vitamin C by the iodometric method

How does iodometric titration work?

To use this iodometric titration procedure will require using a material that has oxidizing properties. This oxidizing property indicates that it has the ability to remove electrons that are owned by other compounds.

This oxidizing substance is later dissolved in the solvent and then the solution is acidified. We can use sulfuric acid (H2SO4), hydrochloric acid (HCl), or acetic acid (CH3COOH) to acidify the solution. Following that, add chlorine and allow the mixture to settle in a dark room for a few minutes.

The iodide will then be released as a result of this procedure and then titrated with a standardized solution of sodium thiosulfate (Na2S2O3). As the solution becomes yellow and becomes more dilute, this solution of sodium thiosulfate will act as a reducing agent. The starch indicator will then be added and will be titrated until the color of the solution becomes clear.

Indicator is used in iodometric titration:

In an iodometric titration, starch is commonly used as an indicator since it can absorb the I2 that is released. It causes the absorption color to change from dark blue to pale yellow when the solution is titrated with a standardized thiosulfate solution. Due to the formation of a starch-iodine complex, the endpoint in iodometric titration is the disappearance deep blue-black color.

Advantages of iodometric titration:

• Iodometry can be used to determine the concentration of both reducing and oxidizing agents.
• It has the ability to precisely determine the concentration of the analyte.
• It allows you to visually detect reactivity at equilibrium point(s).
• The presence of iodine in starch causes a visible blue color change that occurs simultaneously, which is one of the two most important criteria for titration.
• It requires only a small amount of chemicals or substances.

Frequently Asked Questions (FAQ):

Why is KI used in iodometric titration?

Because iodide will oxidize iodine in the presence of an oxidizing agent, KI (potassium iodide) is employed in iodometric titrations.

Why excess of KI is used in iodometric titration?

Excess KI is added to help in the solubilization of free iodine, which is insoluble in water under normal conditions.

What do you mean by iodometric and iodimetric titration?

Iodometry and iodimetry both are common volumetric analysis titration methods. Iodometric titration is an indirect titration method used to quantify oxidizing agents, whereas iodometric titration is a direct titration method used to quantify reducing agents.

References:

1. Wikipedia contributors. (2022, February 3). Iodometry. In Wikipedia, The Free Encyclopedia. Available Here:
2. Hyprowira, and PT Hyprowira Adhitama. Iodometric Titration Functions and How It Works. Available Here:
3. ‘Iodometric Determination of Cu in Brass’. Chemistry LibreTexts, 18 Jan. 2017, Available Here:

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What is end point in iodometric titration?

Iodometry, also known as iodometric titration, is a volumetric chemical analysis method based on a redox titration in which the presence or disappearance of elementary iodine is used to determine the endpoint of the titration. It is a type of titration in which the Iodide solution is titrated with an oxidizing agent. The oxidation of iodide to iodine is the basis of iodometry.

End point of iodometric titration:

A starch solution is used as an indicator in an iodometric titration as it can absorb the I2 that is released. This absorption causes the color to change from deep blue to pale yellow when the solution is titrated with a standardized thiosulfate solution. This indicates the endpoint of the iodometric titration.

It is a technique for determining the amount of oxidizing chemicals in a solution. The concentration of the oxidizing agent is measured by titration of iodine with thiosulfate in which starch is used as an indicator. The detection of the endpoint is based on the formation of the blue starch complex.

The following is the chemical reaction:

I2+Na2S2O3→2NaI+Na2S2O4

The reduction of iodine shows by this principle.

Due to thiosulphate decomposition, iodometric titrations are performed in a weak-acidic medium. Because iodine disproportionates at basic pH, iodimetric titrations are carried out in neutral or moderately alkaline (pH 8.0) or weakly acidic solutions.

Frequently Asked Question (FAQ):

Why the endpoint in iodometric titration is colorless?

Starch is a branched molecule in which iodine (I2) is trapped; therefore, gives it a blue color. However, at the end of the titration, when all of the iodine (I2) has transformed into iodide (I-) ions (after the thiosulphate ion has reduced it), there is no more iodine (I2) to be trapped in starch, so the blue color fades.

Is starch solution an indicator?

To detect the presence of iodine, a starch solution is commonly used as an indicator. A deep-blue starch–iodine complex is formed when starch and iodine are present together.

Why iodometric titrations are done in dark?

Because light accelerates a side reaction in which atmospheric oxygen oxidizes iodide ions to iodine, the reaction mixture should be kept in the dark before titration.

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Iodometric Titration: Principle, Example, Advantages, and How Does it Work

Why do we need to standardize NaOH solution?

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Why starch is added towards the end of iodometric titration?

Starch is added near the endpoint of an iodine titration when the iodine concentration is low, as the starch-iodide combination is not highly soluble in water. This keeps I2-related errors from remaining adsorbed on the complex and going undetected.

Iodometry is one of the most significant redox titration methods. Many organic and inorganic compounds react immediately, quickly, and quantitatively with iodine. 

Iodometry is a volumetric analysis that can be used to determine the amount of both reducing and oxidizing substances, through direct iodine titration or by titration of iodine with sodium thiosulfate (Na2S2O3).

Why is a starch added at the end of the reaction of iodometric titration and not in the initial stage?

When the iodine solution is lower, starch is added at the end. The trapped molecules of iodine do not react with Na S O, the endpoint is diffuse. If the starch was added earlier in the titration, an iodine-starch complex would develop, resulting in a reddish-brown endpoint.

Starch and iodine produce a deep blue complex, adding just a few drops of reducing agent will transform the solution from nearly black to clear. This is considerably easier to observe than the fading yellow color of the iodine alone.

The color change becomes noticeable only at the equivalence point, which is why we don't add the starch earlier. Iodine color fades more slowly on its own, giving a better idea of whether we are close to (pale yellow) or far from the equivalence point (red).

It means we can titrate more rapidly without the starch if we know we're not close there yet, and then adding the starch near the end makes it simpler to observe if any unreacted iodine remains.

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Purpose of back titration

How does iodometric back titration work?

How does iodometric back titration work?

Back titration is a titration in which a known excess of standard reagent is added to the solution and the excess is titrated instead of the original sample. Back titration is used when direct titration is not possible due to the slow reaction between analyte and titrant.

Titrate an iodine solution with a thiosulfate solution or another compound that will be reduced by iodine in direct iodometric titration. Alternatively, we can use an iodide (I) solution that has been oxidized by a stronger oxidant than iodine.

Back titration involves allowing excess iodine to react with the analyte, such as acetone, and then determining the excess amount of iodine used by titration with thiosulfate. Because we know how many moles of iodine we've added and how many haven't reacted, determining how many moles have reacted and, ultimately, determining how many analytes were present is simple.

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Why starch is added towards the end of iodometric titration?

How does starch work as an indicator?

A starch solution is widely used as an indicator, particularly in an iodometric titration, because it can absorb the I2 that is emitted. While titrated with a solution of standardized Na2S2O3, this absorption causes the color of the solution to change from dark blue to pale yellow. It represents the endpoint of the titration.

Compounds that change color when exposed to acidic/basic medium are known as indicators. The indicator is a weak acid or weak base that forms ions when dissociated in a solution. Color indicators are widely used to detect pH and can be added to the reaction mixture to determine the titration endpoint/equivalence point.

Table of Contents: 1. Starch as an indicator 2. How does starch work as an indicator 3. Why does iodine turn starch blue

In titrations such as acid-base, redox, complexometric, and precipitation there are three types of indicators: natural, artificial, and olfactory indicators are used to identify whether a substance is acidic or basic.

Phenolphthalein, bromothymol blue, methyl red/orange are often used in the laboratory or for practical purposes. A pH paper, universal indicator, litmus paper (blue or red), and pH meter are commonly used to determine the pH of a substance.

Starch as an indicator:

In titrations of titrations involving Iodine solutions, starch solutions are used as indicators to detect the endpoint or equivalence point of the reaction. In the presence of iodide ion, starch combines with iodine to form a strongly blue-colored complex that is visible at very low iodine concentrations, making it an excellent indicator in both direct and indirect iodometric titrations. 

It forms a very dark blue-black complex with triiodide. However, If only iodine or iodide (I) is present, the complex does not form.

Iodine test:

It is often used to use iodine to test for the presence of starch. A solution of I2 and KI in water has a light orange-brown color. When it's mixed with a sample that contains starch, it turns a dark blue color. However exactly, how does this color change work? Let’s check it.

How does starch work as an indicator:

Starch is a carbohydrate found in plants, consists of 2 types of polysaccharides, each of which is composed of glucose units connected in one of two ways. The linear amylose and the branched amylopectin are the two types of amylose. The blue color is caused by a chemical called amylose. Its chain is formed like a helix, and iodine can be bound inside it.

Charge-transfer complexes:

Charge transfer complexes are responsible for the color. Potassium iodide (KI) is used because molecular iodine (I2) is not easily soluble in water, combined, they form polyiodide ions. In these compounds, the negatively charged iodide works as a charge donor, while the neutral iodine acts as a charge acceptor.

Light can easily excite electrons in such charge-transfer complexes to a higher energy level. The light is absorbed in the process, and we can see its complementary color. The absorptions of the various species result in the Charge transfer a brownish color in the aqueous solution of polyiodides. 

When amylose is added, a new charge transfer complex is formed, in which the amylose serves as a charge donor and the polyiodide acts as an acceptor. The color changes to dark blue when this compound absorbs light of a different wavelength than that of the polyiodide.

Why does iodine turn starch blue?

Because starch contains amylose, a soluble carbohydrate found in starch and other carbohydrates, that turns a deep shade of blue when combined with iodine. Iodine clings to the beta amylose molecules when introduced to a starch, because of their solubility. 

The iodine is pushed into a line in the middle of the amylose coils by the starch, which results in a charge transfer among the iodine and the starch. The arrangement of electrons and energy level spacings changes as a result of this. The new spacing absorbs visible light differently and produces a deep blue color.

References:

1. Wikipedia contributors. (2021, December 5). Iodine–starch test. In Wikipedia, The Free Encyclopedia. Available Here 
2. Libretexts. (2020, August 11). Starch and iodine. Chemistry LibreTexts. Available Here
3. IAC Publishing. (n.d.). Why does iodine turn starch blue. Available Here

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To perform the assay of copper sulphate by iodometry

Learn about the assay of copper sulphate through a laboratory experiment or practical.

Aim:

Too perform the assay of copper sulphate using standard 0.1 N sodium thiosulphate (Iodometry).

Requirements:

Glasswares: Burette, burette stand, conical flask, volumetric pipette, beaker, volumetric flask, funnel, glass rod, and wash bottle, etc.

Chemicals: LR grade sodium thiosulphate (Na2O3S2), potassium iodide (KI), potassium thiocyanate (KSCN), copper sulphate (CuSO4), acetic acid, dilute sulphuric acid (H2SO4) and starch indicator, etc.

Apparatus: Digital/analytical balance, and Ultrasonicator.

Principle of assay of copper sulphate:

Copper sulphate assay is a type of iodometric titration. It is dependent on the instability of cupric iodide, which is formed when copper sulphate and potassium iodate react with the liberation of free iodine. Cupric iodide was formed when copper sulphate was allowed to react with potassium iodide in the presence of acetic acid.

The cupric iodide formed in the reaction is unstable, so it decomposes to give cuprous iodide with the liberation of iodine. The liberated iodine is titrated with 0.1 N sodium thiosulphate using a starch indicator.

The following is the reaction that is involved in this titration.

Preparation and standardization of sodium thiosulphate:

Click here to get the procedure of preparation and standardization of sodium thiosulphate (0.1 N).

Titration procedure:

• All glassware should be cleaned and dried according to standard laboratory procedures.
• Before filling the burette for the titration, rinse it with distilled water and then pre-rinse it with a portion of the titrant solution. Pre-rinsing is required to make sure that all solution in the burette is the desired solution, not a contaminated or diluted solution.
• Take the unknown stock solution of titrant in a clean and dry beaker then fill the burette using the funnel.
• Remove air bubbles from the burette and adjust the reading to zero.
• Take 01 gm of copper sulphate, pour it into a conical flask and add 50 ml of distilled water.
• After that, add 03 gm potassium iodide and 05 ml acetic acid to the mixture.
• Add few drops of starch indicator.
• Titrate the sample solution with sodium thiosulphate solution. When the solution is swirled for up to 10 seconds, a faint blue color appears and fades, indicates that the endpoint is approaching. The actual endpoint of the titration is indicated by a blue color that lasts longer than 30 seconds.
• To get accurate results, repeat the titration three times.
• Properly record the readings of the burette.
• Take their mean and calculate the molarity of copper sulphate solution.

Observation table:

Sr. No.	Content in conical flask	Burette reading		Volume of titrant used (ml)
		Initial	Final
1
2
3
				Mean:

Calculations:

Percent purity= V x E x AN x 100 / W x RN

Where,

V is a volume of sodium thiosulphate used.

E is an equivalent factor

AN is the actual normality of sodium thiosulphate.

RN is required normality of sodium thiosulphate

W is the weight of the sample

For 1 ml of 0.1 N sodium thiosulphate, the equivalent factor of copper sulphate is 0.02497.

Result:

The percentage purity of the copper sulphate (CuSO4), the sample was found to be_____.

Commonly asked questions on the assay of copper sulphate are as follows.

Why potassium iodide is used in the assay of copper sulphate?

When CuSO4 in aqueous reacts with KI in solid form, cuprous iodide forms as a precipitate liberates iodine gas and forming potassium sulphate in an aqueous state.

Which indicator is used in the estimation of copper sulphate?

The starch indicator is used in the assay of copper sulphate which produces a blue color at the endpoint of the reaction.

Assay of copper sulphate is based on which titration?

The assay of copper sulphate is based on the oxidation-reduction reaction of iodine/thiosulphate.

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Preparation and standardization of 0.1 N sodium thiosulphate

Learn about the preparation and standardization of 0.1 N sodium thiosulphate through a laboratory experiment or practical.

Aim:

To prepare and standardize 0.1 N sodium thiosulphate solution using potassium iodate as primary standard.

Reference: Indian Pharmacopoeia (IP) 2014; Volume-1; Page No. 316

Requirements:

Glasswares: Burette, burette stand, iodine flask, volumetric pipette, beaker, volumetric flask, funnel, glass rod, and wash bottle, etc.

Chemicals: LR grade sodium thiosulphate (Na2O3S2), potassium iodate, potassium iodide, dilute sulphuric acid (H2SO4) and starch indicator, etc.

Apparatus: Digital/analytical balance, and Ultrasonicator.

Principle:

Iodine is liberated when a known concentration of potassium iodate is reacted with excess potassium iodide in an acidic environment. Liberated iodine is directly titrated with sodium thiosulphate to determine its strength. Due to the conversion of iodine into sodium iodide, the endpoint of the titration is determined by the disappearance of the permanent blue color. The following is the reaction that is involved in this titration.

The molecular weight of sodium thiosulphate (Na2S2O3) is 248 g/mol.

Preparation of 0.1 N sodium thiosulphate:

Take 24.8 g of sodium thiosulphate (Na2O3S2) and dissolve in 200 ml of distilled water in a volumetric flask, and properly mixing it. Once it has completely dissolved, make up the volume to 1000 ml.

Preparation of 0.1 N potassium iodate:

Take 356 mg of potassium iodate (KIO3) and dissolve in 50 ml of distilled water in a volumetric flask, and properly mixing it. Once it has completely dissolved, make up the volume up to 100 ml.

Preparation of dilute sulphuric acid:

Take 57.00 ml of sulfuric acid (H2SO4) using a pipette, dilute in 1000 ml of distilled water in a volumetric flask, and properly mixing it.

Preparation of starch indicator solution:

Dissolve 01.00 g of soluble starch in 05 mL distilled water and add to 100 ml boiling water containing 10 mg of mercuric iodide, stirring constantly.

Titration procedure:

• All glassware should be cleaned and dried according to standard laboratory procedures.
• Before filling the burette for the titration, rinse it with distilled water and then pre-rinse it with a portion of the titrant solution. Pre-rinsing is required to make sure that all solution in the burette is the desired solution, not a contaminated or diluted solution.
• Take the unknown stock solution of titrant in a clean and dry beaker then fill the burette using the funnel.
• Remove air bubbles from the burette and adjust the reading to zero.
• Take 10.00 ml of prepared solution of potassium iodate and pour into an iodine flask.
• For 10 minutes, place the flask in the dark (Protect from light.
• Add 2 drops of starch indicator solution.
• Titrate the sample solution with sodium thiosulphate until the endpoint is reached.
• The change in color from blue to colorless indicates that the endpoint is approaching.
• To get accurate results, repeat the titration three times.
• Properly record the readings of the burette.
• Take their mean and calculate the molarity of sodium thiosulphate solution.

Observation table:-

Sr. No.	Content in conical flask	Burette reading		Volume of titrant used (ml)
		Initial	Final
1
2
3
				Mean:

Calculations:-

N1 V1 = N2 V2

N2      = V1 N1/V2

Where,

N2 is normality of Na2S2O3

V1 is a volume of KIO3 solution used

N1 is a normality of KIO3 solution

N2 is a volume of Na2S2O3 solution used

Result:-

The strength of the prepared sodium thiosulphate solution was found to be_____N.

Commonly asked questions on titration are as follows.

Why do we standardize sodium thiosulphate solution?

Standardization of a prepared sodium thiosulphate solution is performed to determine the exact normality.

Which indicator is used in the standardization of sodium thiosulphate?

The principle of sodium thiosulphate titration is based on redox iodometric titration using potassium iodate as the primary standard in which iodine starch solution is used as the indicator.

What are the different types of titration?

Depending on the chemical reaction that occurs between the titrant and the solute or analyte acid-base, redox, complexometric and precipitation titrations are types of titrations.

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Difference between iodometric and iodimetric titration

The major difference between iodometric and iodimetric titration is that iodometry is an indirect titration process, in which the iodine used before the redox titration is produced through a separate titration, whereas iodimetry is a direct titration process, in which the analyte of interest is supposed to be a reducing agent.

Titrations are based on the reaction between the titrant of known concentration and the sample analyte of unknown concentration, in which an indicator is used to identify the endpoint. Titration has four major types, acid-base, redox, precipitation, and complexometric titration.

Table of Contents What is iodometric titrations? What is iodimetric titrations? Difference between iodometric and iodimetric titration

In analytical chemistry, iodometry and iodimetry are common volumetric analysis titration methods. These two types of titrations depend on oxidation-reduction and can be used to determine redox species quantitatively. Iodine can be used for this redox titration, because of its ability to react quickly with a wide range of species.

What is iodometric titration?

The method is called iodometry when a solute (oxidizing agent) is added to excess iodide to obtain iodine, and the amount of iodine produced is estimated by titration with a sodium thiosulfate solution.

It is an indirect titration process, in which the iodine used before the redox titration is produced through a separate titration. The iodometric titration experiment is performed to find out the concentration of solute is used to produce iodine. When the amount of oxidizing agents in a water body desires to be determined, this testing is more popular.

What is iodimetric titration?

Iodimetry is a method for titrating a compound (reducing agent) directly with a standard iodine solution. It is a direct titration process, in which the analyte of interest is supposed to be a reducing agent. Information on the stoichiometry equations and other required interactions, such as reducing agents and iodine solution, which acts as an oxidising agent, can be used to determine the reaction's endpoint.

Difference between iodometric and iodimetric titration:

• Iodometric titration is an indirect titration method, whereas iodimetric titration is a direct titration method.
• The Iodometric method is used to determine oxidizing agents, whereas the iodimetric method is used to determine reducing agents.
• In iodometric titrations, iodine that has been formed as a result of a prior redox reaction is titrated with a reducing agent (thiosulfate ions), whereas in iodimetric titrations, an iodine solution is directly titrated with a reducing solution.
• In iodometric titration, the total number of redox reactions is two, whereas, in iodimetry titration, the total number of redox reactions is one.
• Iodometry is commonly applied in several experiments, whereas iodimetry is less commonly applied in experiments.

Commonly asked questions on titration are as follows.

Which indicator is used in iodometric and Iodimetric titration?

Generally, the starch indicator is used in iodometric and Iodimetric titration, Iodine with starch produces a dark blue complex. In iodimetry, the endpoint corresponds to a drastic color change from red to blue.

Why iodometric titration is called indirect titration?

In an acidic solution, almost all oxidising agents will quantitatively oxidise the iodide ion to iodine. A normal sodium thiosulfate solution can be used to titrate the iodine generated in the reaction. Iodometry is the name given to this sort of indirect titration.

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