Anions: Solution

Topics: Ion, Solubility, Ammonia Pages: 8 (1463 words) Published: February 5, 2014
Chemistry 112: Anion Analysis

Page 7

ANION ANALYSIS

M

uch of the work you will be doing in the Chemistry 112 laboratory will be concerned with identifying positive and negative ions, that is, cations and anions, in solutions whose composition is unknown. This procedure is called QUALITATIVE ANALYSIS.

The modern chemist frequently wishes to identify the constituents in a very small amount of substance, and so he depends heavily on instrumental methods of analysis. While the procedures you will use do not use fancy and expensive instruments, your methods are still very effective in determining the major components of systems containing common inorganic ions. Because anion analysis is somewhat simpler than cation analysis, we shall begin our work in qualitative analysis with methods of identifying four common anions in solution:

PO43-, phosphate

Cl-, chloride

SO42-, sulfate

NO3-, nitrate

After having determined the chemical reactions of the individual ions, you will be asked to identify the ions present in an unknown mixture.

CHEMICAL REACTIONS OF INDIVIDUAL ANIONS
1. The BaCl2 Test
Take a set of four small test tubes. After cleaning them, label them 1 through 4, and place 4-5 drops of one of the known solutions in each tube as follows: Test tube

Known Solution

1

PO43-, phosphate

2

SO42-, sulfate

3

Cl-, chloride

4

Be sure to record the
results of your tests in
your notebook.

NO3-, nitrate

Next, make each solution slightly basic by adding 5 M ammonia (NH3) dropwise. Making sure the solution is thoroughly mixed, test the basicity of the solution with litmus paper as demonstrated by your instructor. When the solutions are basic, note any changes that have occurred, and enter your observations in your lab book. Next, add 2-3 drops of 0.2 M BaCl2 to form precipitates between Ba2+ and some of the anions.

Ba2+(aqueous) + anion(aqueous) → [Ba(anion)](solid)
Record observations on the color and texture or appearance of the precipitates in your notebook. It is best to draw a table in your notebook something like that below. Some of the precipitates you have formed will dissolve in acid. In each case where a precipitate has formed with BaCl2, make the solution acidic with 6 M HCl (blue litmus paper should turn red in acid). Be sure to mix the solution well after adding acid! (The most common error made in qualitative analysis laboratory is to fail to mix solutions completely!) Record your observations. (If you had made a table as described above, you can add your observations on Revised: December 2005

The ammonia bottle may
be labeled either with the
formula NH3 or, less correctly, as NH4OH.
To test for a basic solution, use red litmus
paper. It will turn blue if
the solution basic. Just
remember: blue = base.

Chemistry 112: Anion Analysis

Anion

Page 8

PO43-

SO42-

NO3-

Cl-

Color and texture of
precipitate
Precipitates that dissolve
in HCl
acid solubility to this table.) Discard the solutions from the tests above and clean the test tubes thoroughly.

Blue litmus paper turns
red in acid solution.

2. The AgNO3 Test
Once again prepare four test tubes, each containing 4-5 drops of one of the known solutions. Dilute each solution with about 1 mL of distilled water and then add 2 drops of silver nitrate, AgNO3, solution. Now you should see some of the anions combine with silver ion to again produce insoluble precipitates. Ag+(aq) + anion(aq) → [Ag(anion)](s)

Once again record your observations in a table such as that suggested above.
After observing the precipitates that may form with some anions, attempt to dissolve these precipitates in acid. This time, however, you must use nitric acid, HNO3, rather than hydrochloric acid, HCl. (Why?) Add 4 drops of 3 M HNO3 to each precipitate with silver ion, mix well, and note the results. Again record your observations in the table of results.

3. The Brown Ring Test For Nitrate Ion
As...
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