The Importance of Oxidation- Reduction

OBJECTIVES:

To understand the significance of the oxidation and reduction

To balance the oxidation-reduction reactions

To become familiar with some breath tests

Oxidation Numbers

For ionic compounds, the oxidation number, sometimes called the charge of an element, is equal to the number of electrons lost or gained and, therefore, is the same as the charge on the ion. That is, in sodium chloride, Na+CI‾, the oxidation number of sodium is +1 and that of chlorine is -1. In the compound MgBr², where the magnesium ion has a charge of 2+ and each bromide ion a charge of 1-, the oxidation number of magnesium is +2 and that of each bromine -1.

For covalent compounds, where electrons are shared and not transferred, oxidation numbers are assigned to elements using the following rules.

All elements in their free state have an oxidation numbers of zero.

The oxidation number of oxygen is -2 (except in peroxides, where it is -1).

The oxidation number of hydrogen is +1 (except in metal hybrids, where it is -1).

The sum oxidation numbers in all compounds must equal to zero. (That is, all compounds are electrically neutral.)

All elements in group IA have an oxidation number of +1.

All elements in group llA have an oxidation number +2.

Reduction

Oxidation is defined as a loss of electrons and also as an increase in oxidation number. Reduction is the opposite of oxidation- the gain of electrons, and, therefore, a decrease in oxidation number. Oxidization can never take place without the reduction because something must be able to pick up electrons lost by the oxidized atom, ion, or compound. Free electrons cannot exist by themselves for very long.

Antiseptic Effects

Because they are oxidizing agents, many *antiseptics have the property of killing bacteria. Among these is chlorine, which oxidizes organic matter and bacteria and so is used in the treatment of water to make it potable. Calcium hypochlorite, Ca (OCI)², another commonly used the oxidizing agent and bleaching powder, is used as a disinfectant with for clothes and hospital beds.

Formaldehyde and sulfur dioxide are two reducing agents used in disinfecting rooms formerly occupied by patients with contagious diseases.

Effects on Hair Protein

Oxidizing and reducing agents denature protein by affecting the disulfide bonds of the amino acid cysteine. Use is made of this effect in “home permanents.” Hair protein is primarily keratin, and keratin contains the large amount of the cysteine. During the treatment, a reducing agent is used first. This substance breaks the disulfide bonds in the hair protein. The hair is then shaped with rollers. The new shape is “set” by an oxidizing agent, which forms new disulfide bonds in the desired places. The hair will retain its new shape only until new hair grows out. The entire process has to be repeated.

Black and White Photography

In black and white photography (as in making of x-ray films), oxidation-reduction reactions occur.

Photographic film has an emulsion containing silver bromide (AgBr), which is highly sensitive to light. Exposure to light (or radiation) activates some of the silver ions to the emulsion.

When the film is developed, the activated silver ions react with the developers, hydroquinone, which is a reducing agent. The activated silver ions are reduced to black metallic silver.

Ag+ → Ag↓

The next step in the process, called fixing, sodium uses thiosulfate (Na²S²O³) to dissolve the inactivated silver bromide. If the remaining AgBr were not removed, it would gradually darken when later exposed to the light.

After washing and drying, the negative is ready for use. Areas of the film that were exposed to the greatest amount of light (or radiation) are darkest, whereas those exposed to the least amount of light are the lightest.

That is, in, a negative, the dark and light areas of the object are reversed.

A print, called positive, is made by shining light through the negative onto a piece photographic paper containing a silver bromide emulsion. The same processes as in developing negatives are used. But this time, the dark areas of the negative (which come from the light area of the object) appear light and the light areas on the negative appear dark

Breath-Alcohol Analyzer

Reactions involving the oxidation-reduction are used to measure the amount of alcohol in the driver’s breath. A sample of the driver’s breath is blown through an orange colored solution of acidified potassium dichromate.

The chromic sulfate thus produces is green. The greater the amount of alcohol in a driver’s breath can be determined by comparing the color produced with that of the standardized chart.

Other Breath Tests

Analysis of human breath has confirmed the presence of nearly 400 gaseous compounds. One of these tests, for alcohol, as indicated, in the previous paragraph, has been widely used. However, the detection of the other gases in the breath can be of diagnostic value.

The detection of acetone on the breath is an indication of uncontrolled diabetes mellitus.

Some breath analysis tests require an individual to consume large quantities of a specific precursor to a volatile compound. The disease may reveal itself by the presence of a certain breakdown products that appear in the breath.

In the test of malabsorption syndrome, a patient is given an oral dose of xylose, a carbohydrate. The appearance of large quantities of hydrogen in the breath in the succeeding few hours is the confirmation of this syndrome.

Damage to the pancreas can be detected by the administration of the rich starch orally, and then testing the breath for hydrogen.

Pancreatic disease can be detected by using the radioactive carbon. The amount of the radioactive carbon dioxide can be an indication of pancreatic malfunction.

The use of the galactose can be used to test for a liver damage by testing for the amount of the level of the carbon dioxide.

Another test for the liver disease involves the presence of the dimethyl sulfide in the breath.

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