Acidic solution oxidation number method

Use the oxidation number method to balance each ionic equation in acidic solution. 

The key to the oxidation-number method of balancing redox equations is to realize that the net change in the total of all oxidation numbers must be zero. That is, any increase in oxidation number for the oxidized atoms must be matched by a corresponding decrease in oxidation number for the reduced atoms. Take the reaction of potassium permanganate (KMn04) with sodium bromide in aqueous acid, for example. An aqueous acidic solution of the purple permanganate anion (Mn04 ) is reduced by Br- to yield the nearly colorless Mn2+ ion, while Br- is oxidized to Br2. The unbalanced net ionic equation for the process is... 

To summarize, balancing a redox reaction in acidic solution by the oxidation-number method is a six-step process, followed by a check of the answer (Figure 4.3.)... 

Worked Example 4.10 shows how the oxidation-number method is used to balance a reaction carried out in basic solution. The procedure is exactly the same as that used for balancing a reaction in acidic solution, but OH- ions are added as the final step to neutralize any H+ ions that appear in the equation. This simply reflects the fact that basic solutions contain negligibly small amounts of H+ but large amounts of OH-. 

PROBLEM 4.16 Balance the following net ionic equation by the oxidation-number method. The reaction takes place in acidic solution. 

An alternative to the oxidation-number method for balancing redox reactions is the half-reaction method. The key to this method is to realize that the overall reaction can be broken into two parts, or half-reactions. One half-reaction describes the oxidation part of the process, and the other half-reaction describes the reduction part. Each half is balanced separately, and the two halves are then added to obtain the final equation. Let s look at the reaction of aqueous potassium dichromate (K2Cr2C>7) with aqueous NaCl to see how the method works. The reaction occurs in acidic solution according to the unbalanced net ionic equation... 

Worked Example 4.12 shows how to use the method for balancing a reaction that takes place in basic solution. As in the oxidation-number method, we first balance the reaction for an acidic solution and then add OH- ions in a final step to neutralize H+. 

Use the oxidation-number method to balance this net ionic redox equation for the reaction between the perchlorate ion and the bromide ion in acid solution. 

Use the oxidation number method to balance these net ionic equations, a. Se03 -F 1 —> Se -F I2 (acidic solution)... 

Because of the time and expense involved, biological assays are used primarily for research purposes. The first chemical method for assaying L-ascorbic acid was the titration with 2,6-dichlorophenolindophenol solution (76). This method is not appHcable in the presence of a variety of interfering substances, eg, reduced metal ions, sulfites, tannins, or colored dyes. This 2,6-dichlorophenolindophenol method and other chemical and physiochemical methods are based on the reducing character of L-ascorbic acid (77). Colorimetric reactions with metal ions as weU as other redox systems, eg, potassium hexacyanoferrate(III), methylene blue, chloramine, etc, have been used for the assay, but they are unspecific because of interferences from a large number of reducing substances contained in foods and natural products (78). These methods have been used extensively in fish research (79). A specific photometric method for the assay of vitamin C in biological samples is based on the oxidation of ascorbic acid to dehydroascorbic acid with 2,4-dinitrophenylhydrazine (80). In the microfluorometric method, ascorbic acid is oxidized to dehydroascorbic acid in the presence of charcoal. The oxidized form is reacted with o-phenylenediamine to produce a fluorescent compound that is detected with an excitation maximum of ca 350 nm and an emission maximum of ca 430 nm (81). 

Potassium bromate can also be employed as an oxidizing agent in the assay of a number of pharmaceutical substances, namely mephenesin, phenol, and sodium salicylate. This particular method solely depends upon the formation of iodine monobromide (IBr) in relatively higher concentration of hydrochloric acid solution. 

Other important tests are for acid and alkalinity number and for water content (266), because water content and alkalinity of the polyether glycol can influence the reaction with isocyanates. The standard ASTM test for acid and alkalinity number, ASTM D4662 (267), is not sensitive enough for the low acidity and alkalinity numbers of PTMEG, and special methods have been developed. A useful alkalinity number (AN) has been defined as milliequivalents KOH per 30 kg of PTMEG, as titrated in methanol solution with 0.005 N HC1 (268). Other useful nonstandard tests are for heavy metals, sulfated ash, and peroxide. The peroxides formed initially in oxidations are quickly transformed into carbonyl groups, which are detectable by infrared spectroscopy. On oxidation, a small C—O peak develops at 1726 cm-1 and can be detected in thick (0.5-mm) films. A relative ratio of this peak against an internal standard peak at 2075 C—O is sometimes defined as the carbonyl ratio. 

Of a number of syntheses of cyclohexane-1,2-dione investigated the method preferred is oxidation of 2-hydroxycyclohexanone with ferric chloride in acid solution.""... 

In using the half-reaction method, we usually begin with a skeleton ionic equation showing only the substances undergoing oxidation and reduction. In such cases, we usually do not need to assign oxidation numbers unless we are unsure whether the reaction involves oxidation-reduction. We will find that H (for acidic solutions), OH (for basic solutions), and H2O are often involved as reaaants or products in redox reactions. Unless, ... 

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