Qualitative Analysis of Some Common Ions Class Notes

This lab is designed so that you can observe the unique chemistry of some common ions. Using these unique chemical differences you will perform a quantitative analysis of an unknown solution to determine the ions that are present in the unknown solution.


Qualitative Analysis vs Quantitative Analysis


Qualitative analysis is the process of using chemical tests to determine the identity or presence of one or more ions in a mixture, compound or solution.

Quantitative analysis is the process used to determine the amount of an ion in a mixture, compound, or solution.

We are only interested in the information of "Is a certain ion present or not present in the solution?"


Red and Blue Litmus Paper

To test a solution to determine if it is acidic, use BLUE LITMUS PAPER. Blue litmus paper turns red when touched by an acidic solution.

Blue litmus paper turns red = acidic solution


To test a solution to determine if it is basic, use RED LITMUS PAPER. Red litmus paper turns blue when touched by a basic solution.


Red litmus paper turns blue = basic solution.



Carbonate Ion   CO32-

There are a lot of carbonate ions found in our area in the form of calcium carbonate (limestone). Carbonate ions are also found in the common household products of baking soda and baking powder. When the carbonate ion reacts with an acid, gaseous carbon dioxide is produced. These gas bubbles of carbon dioxide causes bread and cakes to rise.

In the test for the carbonate ion the formation of bubbles when an acid is added indicates the presence of the carbonate ion.


Sulfate Ion    SO42-

The sulfate ion is found in a common drugstore product called Epsom Salts, which is used to prepare soothing baths. The sulfate ion is found in the bath salts found in the drugstore. When BaCl2 is added to a solution containing the sulfate ion, a fine white. powdery precipitate of BaSO4 is formed.

In the test for the sulfate ion, when BaCl2 is added, the formation of a white powdery precipitate indicates the presence of the sulfate ion.


Phosphate Ion   PO43-

The phosphate ion is one of the common ingredients found in laundry detergent and wall washing compounds. The phosphate ion is also the principle intracellular anion. It is present in bones where it combines with calcium and it plays an important role in blood plasma.

To test for the phosphate ion, add ammonium molybdate to an acidified solution and upon heating a bright yellow precipitate is formed. This indicates the presence of the phosphate ion.

If you get a bright yellow solution without the precipitate or a pale yellow precipitate you do not have the phosphate ion. You must have the bright yellow precipitate.


Thiocyanate Ion    SCN-

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.


Chloride Ion

We are using sodium chloride, table salt, to provide the chloride ion. The chloride anion is the principle anion of plasma and the fluids in the spaces between the cells of our body. The chloride ion is needed to maintain the electrolyte balance of the body fluids involved in blood transport of oxygen and carbon dioxide.

When a chloride ion reacts with silver nitrate it produces a white (lacy or snowflake looking) precipitate of silver chloride. In the test for chloride ion, a white precipitate is formed upon the addition of silver nitrate indicating the presence of the chloride ion.


Aluminum Ion     Al3+

Aluminum is the most abundant metal and the third most abundant element in the earth's crust after oxygen and silicon. Aluminum is too reactive to exist as a free metal. You cannot walk around a pick up pure aluminum from the ground. The most common form of aluminum is bauxite. Aluminum is found combined in over 270 different minerals. The aluminum ion is used to lower the pH of soils by hydrolysis. Aluminum in the soil is leached out by acid rain, going into the water and then into plants. It has been found that tea grown in acid soil, when mixed with lemon is a significant source of aluminum ingestion and may be the primary cause of osteoporosis ( bone softening, crushing and breaking) and dementia in the elderly. It has also been found that when milk is added to tea, the milk detoxifies the tea by binding with the aluminum ion. There is NO BIOLOGICAL ROLE for aluminum in vertebrate life forms.

To test for the presence of aluminum a gelatinous precipitate of Al(OH)3 is formed when ammonia is added to the aluminum ion. Upon dissolving the precipitate with acetic acid and adding cathecol violet, the solution turns pale blue. You must have the gelatinous precipitate for aluminum to be present.


Copper Ion    Cu2+

Copper is a micronutrient in our body. An average weight man or woman requires approximately 1.5 mg of copper daily. It occurs naturally in water and is found in drinking water when copper pipes are used to transport drinking water in houses. Copper occurs widely in our food, particularly in nuts, raisins, liver, and legumes. A person who is deficient in copper will suffer anemia, skeletal defects and degeneration of nerve cells. Copper is also used to destroy algae and bacteria in water.

In the test for the presence of copper the clear light blue solution of copper(II) sulfate will change to a milky light blue and then to a clear, deep blue when ammonia is added.


Iron Ion     Fe3+

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.





THE ADVANCED STUDY ASSIGNMENT EXAMPLE

An unknown that might contain any of the eight ions studied in this experiment (but no other ions) has the following properties:

a. On addition of 1 M HCl, bubbling occurs.

b. When 0.1 M BaCl2 is added to the acidified unknown, a clear solution results.

c. When 0.1 M Fe(NO3)3 is added to the unknown, a deep red color appears.


On the basis of the preceding information, classify each of the following ions as present (P), absent (A), or undetermined (U) by the tests described:

CO32-      SO42-      PO43-      SCN-     Cl-      AL3+      Cu2+      Fe3+



HINT

What ion do you use the HCl to test for? Is the indicator for that ion present? What ion has a test where you add barium chloride and what is the indicator for that test? What ion uses Fe(NO3)3 to test for it and what is the indicator for that ion to be present? Are you doing any other tests for ions? Is the Fe3+ ion initially present if you had to add Fe(NO3)3?


ANSWER

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.


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last updated: February 10, 2014