Accepting electrons. Reduction in the oxidation state of the substance. A substance undergoes oxidation in the following ways:. Addition of oxygen. Removal of hydrogen. Loss or donation of electrons. An overall increase in the oxidation state of the substance. Hence, we can say that redox titrations consist of a transfer of electrons between the titrant and the analyte. For example, many drugs and supplements contain iron in potentially different forms. To determine the concentration of iron in a substance, an ionic solution of the substance can be prepared and then titrated with a particularly reactive compound such as permanganate.
Given the many complex chemical reactions that take place in the human body, knowing the concentration of nutrients you are ingesting helps determine the best course of dietary action. Vitamin C, for example, undergoes a redox reaction with iodine. Methane reacts with oxygen to form carbon dioxide and two water molecules. In some redox reactions, substances can be both oxidized and reduced. These are known as disproportionation reactions. One real-life example of such a process is the reaction of hydrogen peroxide, H 2 O 2 , when it is poured over a wound.
At first, this might look like a simple decomposition reaction, because hydrogen peroxide breaks down to produce oxygen and water:. The key to this reaction lies in the oxidation states of oxygen, however.
Notice that oxygen is present in the reactant and both products. In H 2 O 2 , oxygen has an oxidation state of In H 2 O, its oxidation state is -2, and it has be en reduced.
In O 2 ,however, its oxidation state is 0, and it has been oxidized. Oxygen has been both oxidized and reduced in the reaction, making this a disproportionation reaction. It is usually used to determine medium and high concentrations of elements. Furthermore, titration gives reliable results even in field conditions.
Redox titrimetry is used to analyse a wide range of inorganic analytes. A redox titration also called an oxidation-reduction titration can accurately determine the concentration of an unknown analyte by measuring it against a standardized titrant.
It is used for the analysis of organic analytes. One important example is the determination of the chemical oxygen demand COD of natural waters and wastewaters. Redox titration is used in pharmaceutical analysis like in the determination of valganciclovir hydrochloride VLGH in pure drugs and tablets.
Two simple, selective and sensitive spectrophotometric methods were developed and validated. In the second method method B , permanganate was reduced by VLGH to bluish green manganate in alkaline medium and the absorbance was measured at nm. The absorbance measured in each case was related to VLGH concentration.
The determination of dissolved oxygen. In natural waters, such as lakes and rivers, the level of dissolved O 2 is important for two reasons: it is the most readily available oxidant for the biological oxidation of inorganic and organic pollutants; and it is necessary for the support of aquatic life.
Several reagents are used as auxiliary oxidizing agents , including ammonium peroxydisulfate, NH 4 2 S 2 O 8 , and hydrogen peroxide, H 2 O 2. Peroxydisulfate is a powerful oxidizing agent. Excess peroxydisulfate is destroyed by briefly boiling the solution. The reduction of hydrogen peroxide in an acidic solution. Excess H 2 O 2 is destroyed by briefly boiling the solution.
Because a titrant in a reduced state is susceptible to air oxidation, most redox titrations use an oxidizing agent as the titrant. Which titrant is used often depends on how easily it oxidizes the titrand. A titrand that is a weak reducing agent needs a strong oxidizing titrant if the titration reaction is to have a suitable end point. An aqueous solution of permanganate is thermodynamically unstable due to its ability to oxidize water.
A moderately stable solution of permanganate is prepared by boiling it for an hour and filtering through a sintered glass filter to remove any solid MnO 2 that precipitates. Potassium dichromate is a relatively strong oxidizing agent whose principal advantages are its availability as a primary standard and its long term stability when in solution.
Its reduction half-reaction is. Iodine is another important oxidizing titrant. This apparent limitation, however, makes I 2 a more selective titrant for the analysis of a strong reducing agent in the presence of a weaker reducing agent.
The reduction half-reaction for I 2 is. Because iodine is not very soluble in water, solutions are prepared by adding an excess of I —. The complexation reaction. If the titrand is in an oxidized state, we can first reduce it with an auxiliary reducing agent and then complete the titration using an oxidizing titrant.
Alternatively, we can titrate it using a reducing titrant. Standardization is accomplished by dissolving a carefully weighed portion of the primary standard KIO 3 in an acidic solution that contains an excess of KI. Although thiosulfate is one of the few reducing titrants that is not readily oxidized by contact with air, it is subject to a slow decomposition to bisulfite and elemental sulfur. If used over a period of several weeks, a solution of thiosulfate is restandardized periodically.
Several forms of bacteria are able to metabolize thiosulfate, which leads to a change in its concentration. This problem is minimized by adding a preservative such as HgI 2 to the solution. Periodic restandardization with K 2 Cr 2 O 7 is advisable. One of the most important applications of redox titrimetry is evaluating the chlorination of public water supplies. Representative Method 9. The efficiency of chlorination depends on the form of the chlorinating species. There are two contributions to the total chlorine residual—the free chlorine residual and the combined chlorine residual.
The free chlorine residual includes forms of chlorine that are available for disinfecting the water supply. The combined chlorine residual includes those species in which chlorine is in its reduced form and, therefore, no longer capable of providing disinfection. When a sample of iodide-free chlorinated water is mixed with an excess of the indicator N , N -diethyl- p -phenylenediamine DPD , the free chlorine oxidizes a stoichiometric portion of DPD to its red-colored form.
The oxidized DPD is then back-titrated to its colorless form using ferrous ammonium sulfate as the titrant. The volume of titrant is proportional to the free residual chlorine. Having determined the free chlorine residual in the water sample, a small amount of KI is added, which catalyzes the reduction of monochloramine, NH 2 Cl, and oxidizes a portion of the DPD back to its red-colored form.
The amount of dichloramine and trichloramine are determined in a similar fashion. Chlorine demand is defined as the quantity of chlorine needed to react completely with any substance that can be oxidized by chlorine, while also maintaining the desired chlorine residual.
It is determined by adding progressively greater amounts of chlorine to a set of samples drawn from the water supply and determining the total, free, or combined chlorine residual. Another important example of redox titrimetry, which finds applications in both public health and environmental analysis, is the determination of dissolved oxygen. In natural waters, such as lakes and rivers, the level of dissolved O 2 is important for two reasons: it is the most readily available oxidant for the biological oxidation of inorganic and organic pollutants; and it is necessary for the support of aquatic life.
In a wastewater treatment plant dissolved O 2 is essential for the aerobic oxidation of waste materials. If the concentration of dissolved O 2 falls below a critical value, aerobic bacteria are replaced by anaerobic bacteria, and the oxidation of organic waste produces undesirable gases, such as CH 4 and H 2 S.
One standard method for determining dissolved O 2 in natural waters and wastewaters is the Winkler method.
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