2. Two students performing the general unknown lab were told that their unknown

Question

2. Two students performing the general unknown lab were told that their unknown would contain 4 cations. They first tested for the silver and aluminum, but found that another ion was present.

The students were conflicted about whether their unknown contained the iron cation because they did not observe precipitate when they added ammonium chloride and ammonia to make the solution alkaline. (step 5 of page 123.) What suggestion(s) would you give these students to determine if iron (III) is present in their unknowns?

Explanation / Answer

Iron is the most important transition metal in living systems, transporting oxygen in the blood. The average adult requires approximately 10 mg of iron daily. Very high concentrations of iron in the blood apparently renders an individual more susceptible to infections. The lack of iron in the blood causes anemia. Iron is present in the water when there are reddish brown stains in tubs, toilets, sinks, on dishes, glassware, and the laundry.

To test for the presence of iron (III) ion add potassium thiocyanate (KSCN) to the solution to get a blood red color.If 1 M HCl causes bubbling, then carbonate ion must be present. If a clear solution results when barium chloride is added, then sulfate ion must be absent. If Fe(NO3)3 is added and a deep red color results, then SCN- must be present. Since SCN- is present and the solution was not originally red and you had to add iron (III) then iron (III) must have been absent. All the other ions are undetermined because there was no tests run for the other ions

The thiocyanate ion is one of the major constituents of waste water from factories where the gasification of coal occurs. It also occurs where cyanide is used in mining of precious metals. In both of these instances, cyanide which is used is converted to thiocyanate ions by the reaction with sulfur. Thiocyanate ions are toxic but are less toxic than cyanide. Fortunately, there is a bacteria which can convert SCN- ions to a sulfide and ammonia, both of which do not hurt the environment.

Thiocyanate is also found in other things. Some pesticides use thiocyanate as the poison. Your body naturally contains trace amounts of thiocyanate in the blood and body fluids. It has been found that smoking increases the level of thiocyanate in the body. Thiocyanate is the product formed by the bodies metabolism of some drugs used to treat hypertension.

The test for the thiocyanate ion is to add iron (III) nitrate to a solution. If the solution turns blood-red upon the addition of iron(III) nitrate this indicates the presence of the thiocyanate ion.

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