Redox Reactions and Volumetric Analysis

Introduction

We live in a world where oxidation-reduction (redox) reactions and processes are very much a part of our everyday lives. These activities or reactions span from the combustion of fossil fuels to the action of sodium hypochlorite, the active ingredient in home bleach, which has an oxidising power. It is highly possible that each and every one of us has come across at least one process or characteristic that is the outcome of a redox process at some point in our lives. In our everyday lives, it is highly likely that we have witnessed numerous examples of corrosion, such as the formation of rust on iron surfaces, the tarnish of silverware, and the greening of copper or brass surfaces or materials. All of the changes that we see on a nearly daily basis without giving any thought to their chemical origin or nature are examples of processes that emerge from redox reactions that we can notice. The majority of metallic and nonmetallic elements, which have vital applications in today’s world, are derived from their ores by the process of oxidation or reduction processes, respectively. All of these examples are examples of the results of oxidation-reduction reactions in their various manifestations. These are essentially processes that involve the transfer of electrons. Over the years, oxidation-reduction analysis has been employed as an alternative way of assessing materials that exhibit a variety of oxidation states. This unit will, in part, assist us in making the connection between what is happening at the submicroscopic level and the macroscopic electrochemical events that we observe.

Redox Reactions

Redox Titration is a laboratory technique for measuring the concentration of a particular analyte by inducing a redox reaction between the titrant and the analyte in the presence of a standard. In some cases, the use of a potentiometer or a redox indicator will be required for these types of titrations.

It is possible to do redox titration by using an oxidation-reduction reaction between the titrant and the analyte. In the laboratory, it is one of the most commonly used procedures for determining the concentration of unknown analytes.

In order to analyse redox titrations, it is necessary to first determine the form of the related titration curve. When doing these types of titrations, it is more convenient to monitor the reaction potential rather than the concentration of a reactive species in the sample solution.

In redox reactions, as previously established, both oxidation and reduction are involved.

Example – 2NaH → 2Na + H2

This is the example of decomposition redox reaction in which the compound is broken down into smaller compounds after reaction.

Oxidation numbers

Using the example of iron rusting, we could have deduced that the process is a redox process by just noting that it involves the creation of ions (Fe3+ and O2+) from free elements (Fe and OX2 ). When it comes to other situations, however, it is less evident, particularly when the reaction in question only involves nonmetallic substances.

In order to aid in the identification of these less visible redox processes, chemists invented the idea of oxidation numbers, which provides a mechanism to track electrons before and after a reaction takes place. The oxidation number (also known as the oxidation state) of an atom is the imaginary charge that the atom would have if all of the bonds that connect it to the rest of the universe were entirely ionic. One thing to keep in mind is that oxidation numbers are always preceded by a plus or minus sign (+ or ). This is in contrast to the charges on ions, which are denoted by the addition of a minus sign after the numerical value.

Redox Titrations

It is possible to conduct an oxidation/reduction titration using an analyte and a titrant in the presence of an oxidation/reduction process. Redox titrations, like acid-base titrations, are typically performed with an indicator that changes colour dramatically. When large concentrations of the reducing agent are present, the colour of the indicator becomes characteristic of the reduced form of the indicator. When the indicator is present in an oxidising media, it will often take on the colour of its oxidised state. An abrupt shift in the indicator’s colour will occur at or near the equivalence point as it transitions from one form to another, making the equivalence point easily distinguishable from other points on the graph.

Because all redox titrations involve the movement of electrons, the electrical potential of the solution can be used to monitor the progress of all redox titrations. To monitor the potential of a solution, only two electrodes are required: a reference electrode and an inert electrode. Although the specifics of how such a setup operates are outside the scope of this section, they will not be discussed in length here. Although the applicable expression that makes use of the experimentally measured electrochemical potential, E as a function of titrant volume will be described later, the relevant expression will be discussed first.

Techniques that are based on the concepts of redox reactions have been widely employed in the determination of metals that have two well-defined oxidation states, such as titrimetric techniques. The process of analysis frequently entails one or more of the following:

1. Increasing the oxidation state of all metal ions to be analysed (analyte) through the use of an oxidising agent such as sodium peroxide or sodium bismuthate, or 2. Increasing the oxidation state of all metal ions to be analysed (analyte) through the use of an oxidising agent such as sodium peroxide or sodium bismuthate
2. using a reducing agent such as sulphur dioxide or sodium bisulphite to reduce the oxidation state of all of the analyte metal ions to a lower oxidation state

In both cases, an excessive amount of reagent is required, which must be destroyed or eliminated before the sample can be titrated properly.

There are a variety of different approaches to doing quantitative reduction experiments that are outside the scope of this Unit and will not be explored in this section.

Volumetric Analysis

Titrimetric analysis is a term used to describe any method of quantitative chemical analysis in which the amount of a substance is determined by measuring the volume that the substance occupies or, in more general usage, the volume of a second substance that combines with the first in known proportions, which is more correctly referred to as titrimetric analysis (see titration).

The first approach, developed by a French scientist named Jean-Baptiste-André Dumas, is exemplified by a mechanism for detecting the amount of nitrogen coupled with other elements in organic compounds that is still in use today. A weighted sample of the chemical is burned in a furnace under conditions that assure the complete conversion of all of the nitrogen to elemental nitrogen gas (N2) is achieved. Initially, the nitrogen is transported from the furnace in a stream of carbon dioxide that is then passed through a strong alkali solution, which absorbs the carbon dioxide while allowing the nitrogen to collect in a graduated tube. The mass of nitrogen can be calculated from the volume it fills under known conditions of temperature and pressure, and the proportion of nitrogen in the sample can be calculated from the mass of nitrogen in the sample under known parameters of temperature and pressure. Additionally, the volumetric approach is used in the examination of nitrates, which are capable of being transformed into the gas nitric oxide (NO). The amount of carbon dioxide produced or consumed during biological activities is frequently quantified in volumetric units. In order to evaluate the composition of fuel gases and combustion products, it is necessary to measure the changes in volume that occur when a sample is treated successively with reagents that particularly absorb components such as carbon dioxide, carbon monoxide, oxygen, and other components.

Example – pouring lye into a combination of vegetable oil and alcohol to determine the amount of acid in the vegetable oil that will be utilised as biodiesel

Conclusion

Oxidation-reduction reactions, also known as redox reactions, are chemical processes that occur when electrons are transferred from one substance to another. In this unit, we will look at the class of reactions known as oxidation-reduction reactions, or redox reactions, and examine the oxidation-reduction process. We will also learn how to use the oxidation state and oxidation number concepts to not only identify redox reactions, but also keep track of the electrons transferred in a chemical reaction as well. These sections provide an introduction to galvanic cells, thermodynamics of electrochemical reaction, and other essential aspects of electrochemical reaction. It is also addressed how the concept of redox equilibria can be used to explain oxidation-reduction titrations, which are a tool for volumetric chemical analysis.