Redox Titrations

Redox titration is a laboratory method used in analytical chemistry to determine the concentration of an analyte. This method is based on a redox reaction, which involves the transfer of electrons between the titrant and the analyte. One compound undergoes oxidation (loses electrons), while the other undergoes reduction (gains electrons). The concentration of the analyte can be accurately calculated by carefully measuring the amount of reagent required to complete the reaction. These types of titrations sometimes require the use of a potentiometer or a redox indicator. In this article we will study concept of oxidation and reduction and certain types of redox reactions.

Oxidation

Oxidation is a chemical process that involves the loss of electrons or an increase in the oxidation state of a molecule, atom, or ion. This process can occur in various ways, such as the addition of oxygen or any electronegative element, or the removal of hydrogen or any electropositive element. Here are some examples of oxidation.

Combustion of Carbon

When carbon (C) combines with oxygen (O2), it forms carbon dioxide (CO2). In this process, carbon is oxidized as it gains oxygen.

C + O2 → CO2

Oxidation of Iron

Iron (Fe) reacts with sulfur (S) to form iron sulfide (FeS). Here, iron is oxidized as it gains an electronegative element, sulfur.

Fe + S → FeS

Oxidation of Hydrogen Sulfide

Hydrogen sulfide (H2S) reacts with bromine (Br2) to form hydrogen bromide (HBr) and sulfur (S). In this reaction, hydrogen sulfide is oxidized as it loses hydrogen.

H2S + Br2 → 2HBr + S

Oxidation of Potassium Iodide

When potassium iodide (KI) reacts with hydrogen peroxide (H2O2), it forms iodine (I2) and potassium hydroxide (KOH). Here, iodide is oxidized as it loses the electropositive element, potassium.

2KI + H2O2 → I2 + 2KOH

In all these reactions, the substance undergoing oxidation acts as a reducing agent.

Reduction

Reduction is a chemical process that involves the gain of electrons or a decrease in the oxidation state of a molecule, atom, or ion. This process can occur in various ways, such as the addition of hydrogen or any electropositive element, or the removal of oxygen or any electronegative element. Here are some examples of reduction.

Reduction of Nitrogen

Nitrogen (N2) combines with hydrogen (H2) to form ammonia (NH3). In this process, nitrogen is reduced as it gains hydrogen.

N2 + 3H2 → 2NH3

Reduction of Mercuric Chloride

Tin(II) chloride (SnCl2) reacts with mercuric chloride (HgCl2) to form tin(IV) chloride (SnCl4) and mercury(I) chloride (Hg2Cl2). Here, mercuric chloride is reduced as it gains the electropositive element, tin.

SnCl2 + 2HgCl2 → SnCl4 + Hg2Cl2

Reduction of Zinc Oxide

Zinc oxide (ZnO) reacts with carbon (C) to form zinc (Zn) and carbon monoxide (CO). In this reaction, zinc oxide is reduced as it loses oxygen.

ZnO + C → Zn + CO

Reduction of Ferric Chloride

Iron(III) chloride (FeCl3) reacts with hydrogen (H2) to form iron(II) chloride (FeCl2) and hydrochloric acid (HCl). Here, ferric chloride is reduced as it loses the electronegative element, chlorine.

2FeCl3 + H2 → 2FeCl2 + 2HCl

In all these reactions, the substance undergoing reduction acts as an oxidizing agent.

Types of redox titrations

Redox titrations can be classified based on the titrant used and the method employed. Each type serves specific purposes in analyzing different analytes. We will see principles and applications of each type.

Cerimetry

Cerimetry, also known as cerimetric titration or cerate oximetry, is a type of redox titration that involves the use of cerium (IV) as the oxidizing agent. It is based on the principle that cerium (IV) ions can be reduced to cerium (III) ions by a variety of reducing agents, including iodides, thiosulfates, and oxalic acid. The endpoint of the titration is determined by the disappearance of the yellow color of the cerium (IV) ion, which is due to its reduction to the colorless cerium (III) ion.

Cerimetry has a wide range of applications in analytical chemistry as follows.

Determination of Iodine: Cerimetry can be used for the determination of iodine in a variety of samples, including pharmaceuticals, food products, and environmental samples.

Determination of Sulfite: Cerimetry can also be used for the determination of sulfite, which is commonly used as a preservative in food products.

Determination of Oxalic Acid: Cerimetry can be used for the determination of oxalic acid, which is commonly used as a reductant in a variety of chemical reactions.

Determination of Iron: Cerimetry can also be used for the determination of iron in a variety of samples, including ores, soils, and water samples.

Overall, cerimetry is a versatile and widely used analytical method in the determination of various analytes, particularly those that can be reduced by cerium (IV) ions.

Iodimetry

Iodimetry is a type of redox titration that involves the use of iodine (I2) as the titrant. The principle of iodimetry involves the oxidation of the reducing agent by iodine under acidic conditions to produce iodide ions. The iodine is added to the sample containing the reducing agent, and the excess iodine is then titrated with a standard solution of a reducing agent, such as sodium thiosulfate, until the iodine is completely consumed. The endpoint of the titration is indicated by a color change, typically from brown to colorless, or by the use of a starch indicator, which forms a blue-black complex with excess iodine.

Iodimetry has a wide range of applications in various fields as follows.

Analytical Chemistry: Iodimetry is used for the quantitative determination of various reducing agents in analytical chemistry, including sulfite, thiosulfate, and ascorbic acid.

Food and Beverage Industry: The method is widely used in the food and beverage industry for the determination of sulfite in wine, beer, and other beverages.

Pharmaceutical Industry: Iodimetry is used for the determination of the concentration of reducing agents, such as ascorbic acid, in pharmaceuticals. It is also used for the determination of the purity of certain drugs, such as vitamin C.

Water and Wastewater Treatment: Iodimetry is used for the determination of the concentration of thiosulfate and other reducing agents in water and wastewater treatment processes.

Industrial Processes: Iodimetry is used for the determination of the concentration of reducing agents in various industrial processes, including the pulp and paper industry and the textile industry.

Overall, iodimetry is a useful analytical method for the determination of the concentration of reducing agents, and its applications span various industries and fields.

Iodometry

Iodometry, also known as iodometric titration, is a method of volumetric chemical analysis. It’s a type of redox titration where the appearance or disappearance of elementary iodine indicates the endpoint. The principle of iodometry involves the oxidation of iodide ions (I-) to iodine (I2) by an oxidizing agent in the sample. The iodine formed is then titrated with a standard solution of a reducing agent, such as sodium thiosulfate. The endpoint of the titration is indicated by a color change from deep blue to light yellow when titrated with standardized thiosulfate solution.

Iodometry has a wide range of applications as follows.

Determination of Vitamin C: Iodometry can be used to determine the concentration of vitamin C in fruit juices and other food products.

Determination of Sulfite: Iodometry can also be used to determine the concentration of sulfite in food products, such as wine and beer.

Determination of Thiosulfate: Iodometry can be used to determine the concentration of thiosulfate in photographic processing solutions.

Determination of Iodide: Iodometry can also be used to determine the concentration of iodide in solutions.

Overall, iodometry is a useful analytical method that has a wide range of applications in the food, pharmaceutical, and photographic industries.

Bromatometry

Bromatometry is a type of volumetric analysis used for the determination of the concentration of an oxidizing agent in a solution, such as the amount of bromine present. The principle of bromatometry is based on the reaction of the oxidizing agent with a reducing agent, which is added in excess and then titrated with a standard solution of bromine. The standard solution of bromine is prepared by dissolving a known amount of potassium bromide (KBr) in water and then adding an excess of chlorine water (Cl2) to produce a solution containing a known concentration of bromine.

Bromatometry has many applications in various industries as follows.

Food Industry: Bromatometry is used for the determination of the amount of bromine in water used for food processing, as well as in the measurement of the bromine content in baked goods and other food products.
Pharmaceutical Industry: Bromatometry is used for the determination of the amount of bromine in pharmaceuticals, such as bromides and other anticonvulsant drugs.
Chemical Industry: Bromatometry is used for the determination of the concentration of bromine in solutions used in the production of chemicals, such as dyes and pigments.

Dichrometry

Dichrometry is a type of redox titration that uses potassium dichromate (K2Cr2O7) as the oxidizing agent. It is limited to reducing agents that can be oxidized by dichromate, requires an acidic medium which can be corrosive, and the orange color of the dichromate can mask the color change at the endpoint. The principles of dichrometry are as follows.

Oxidation State Change: Potassium dichromate exists in the +6-oxidation state of chromium. During the titration, it gets reduced to the +3-oxidation state.

Cr2O7²⁻+ 14H⁺ + 6e⁻ →2Cr3⁺ + 7H2O

Acidic Medium: Dichromate titration requires an acidic medium, typically with sulfuric acid (H2SO4). This is because the reduction of dichromate involves the consumption of hydrogen ions.

Self-indicator: Potassium dichromate is a deep orange color, while its reduced form, Cr3+, is green. This color change serves as a self-indicator for the titration, eliminating the need for an external indicator.

Suitable Reducing Agents: Dichromate can oxidize a variety of reducing agents, including ferrous ions (Fe2+), iodides (I−), arsenic (III) (As3+), and antimony (III) (Sb3+).

Standardization of Dichromate Solution: Before using potassium dichromate for titration, its concentration needs to be standardized. This is usually done by titrating it against a primary standard, such as sodium oxalate (Na2C2O4).

Dichrometry has wide applications in analytical chemistry for the quantitative determination of various substances as follows.

Iron Content in Steel and Other Alloys: Dichrometry is used to determine the iron content in steel and other alloys.

Antioxidants in Food and Beverages: It is used to determine the amount of antioxidants in food and beverages.

Vitamin C Content in Fruits and Vegetables: Dichrometry is used to determine the vitamin C content in fruits and vegetables.

Arsenic in Water and Soil: It is used to determine the amount of arsenic in water and soil.

Antimony in Pharmaceuticals: Dichrometry is also used to determine the amount of antimony in pharmaceuticals.

Titration with potassium iodide

Potassium iodide (KI) titration is a type of redox titration, often referred to as iodometric titration. The principle of this titration involves the reaction between an oxidizing agent in the sample and iodide ions (I-) from the potassium iodide. The oxidizing agent oxidizes the iodide ions to iodine (I2). This iodine is then titrated with a standard solution of a reducing agent, such as sodium thiosulfate. The endpoint of the titration is indicated by a color change from deep blue to light yellow when titrated with standardized thiosulfate solution.

Here are some applications of potassium iodide titration as follows.

Determination of Vitamin C: Potassium iodide titration can be used to determine the concentration of vitamin C in fruit juices and other food products.

Determination of Sulfite: It can also be used to determine the concentration of sulfite in food products, such as wine and beer.

Determination of Thiosulfate: Potassium iodide titration can be used to determine the concentration of thiosulfate in photographic processing solutions.

Determination of Iodide: It can also be used to determine the concentration of iodide in solutions.

Summary

Redox titration is a laboratory method used in analytical chemistry to determine the concentration of an analyte. It is based on an oxidation-reduction reaction between the titrant and the analyte. In this process, one compound undergoes oxidation (loses electrons), while the other undergoes reduction (gains electrons). The concentration of the analyte can be accurately calculated by carefully measuring the amount of reagent required to complete the reaction. These types of titrations sometimes require the use of a potentiometer or a redox indicator. Redox titrations are widely used in various fields, including the food, pharmaceutical, and photographic industries, for the determination of the concentration of reducing agents.

Redox Titrations