Download ANION ANALYSIS and more Lecture notes Chemistry in PDF only on Docsity!
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 meth- ods of analysis. While the procedures you will use do not use fancy and expen- sive 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:
PO 4 3-^ , phosphate Cl -^ , chloride SO 4 2-^ , sulfate NO 3 -^ , 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 BaCl 2 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 PO 4 3-^ , phosphate 2 SO 4 2-^ , sulfate 3 Cl -^ , chloride 4 NO 3 -^ , nitrate Next, make each solution slightly basic by adding 5 M ammonia (NH 3 ) 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 BaCl (^2) to form precipitates between Ba 2+^ and some of the anions.
Ba 2+^ (aqueous) + anion(aqueous) → Ba(anion) Record observations on the color and texture or appearance of the pre- cipitates in your notebook. It is best to draw a table in your notebook some- thing like that below.
Some of the precipitates you have formed will dissolve in acid. In each case where a precipitate has formed with BaCl 2 , make the solution acidic with 6 M HCl (blue litmus paper should turn red in acid). Be sure to mix the solu- tion 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
Be sure to record the results of your tests in your notebook.
To test for a basic solu- tion, use red litmus paper. It will turn blue if the solution basic. Just remember: blue = base.
The ammonia bottle may be labeled either with the formula NH 3 or, less cor- rectly, as NH 4 OH.
acid solubility to this table.) Discard the solutions from the tests above and clean the test tubes thoroughly.
2. The AgNO 3 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, AgNO 3 , solution. Now you should see some of the anions combine with silver ion to again produce insoluble precipitates.
Ag +^ (aq) + anion(aq) → Ag(anion) 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, HNO 3 , rather than hydrochloric acid, HCl. (Why?) Add 4 drops of 3 M HNO 3 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 you may have observed in your tests thus far, the nitrate ion does not form precipitates with either Ba2+^ or Ag+. Therefore, we have to have some indepen- dent way of testing for the ion. The test that has been used for many years is called the brown ring test , and it is specific for NO 3 -^. Place 10 drops of the solution to be tested in a clean, well-rinsed test tube. Make the solution acidic by adding 3 M H 2 SO 4 as needed. Next, add 5 drops of a freshly prepared, saturated solution of iron(II) sulfate (FeSO 4 ) and mix gently. Incline the test tube at a 45o^ angle, and, as shown in the sketch, carefully add 5 drops of concentrated H 2 SO 4 so the drops roll down the side of the test tube and slide gently onto the top of the solu- tion. DO NOT MIX the solutions! Two separate liquid layers will be observed in the test tube. If NO 3 -^ is present, a very faint brown ring will be observed near the bottom of the test tube, thereby confirming the pres- ence of nitrate ion. Record your observations in your notebook.
Anion PO 4 3-^ SO 4 2-^ NO 3 -^ Cl -
Color and texture of
precipitate
Precipitates that dissolve
in HCl
Remember to use blue litmus paper to test for acidity.
Blue litmus paper turns red in acid solution.
- Write the balanced, net ionic equation for the reaction occurring when (a) Ag+^ is added to the Cl-^ containing solution (b) Ag+^ is added to the PO 4 3-^ containing solution
- When some precipitates dissolve in HCl or HNO 3 , the reason is that a more “stable” or less strong acid than either HCl or HNO 3 is formed. For example, barium phosphate, which is normally insoluble in water, dissolves in HCl because the weaker acid H 3 PO 4 (phosphoric acid) is formed. Ba 3 (PO 4 ) 2 (s) + 6 HCl(aq) → 2 H 3 PO 4 (aq) + 3 BaCl 2 (aq) Write a balanced equation to show how insoluble silver(I) phosphate, Ag 3 PO 4 , can dissolve in the strong acid HNO 3 for a similar reason.
