Qualitative Analysis II

Before the advent of modern instrumentation, a plethora of 'wet chemical' techniques had been devised to determine if a given element was present in a sample.  One classic example was the grizzled old prospector in the wild west.  He would take his sample of 'gold' or 'silver' ore to the assay office were they would determine if any precious elements were present (qualitative analysis).  If precious elements were found, then further testing (quantitative analysis) would determine how much of the element was present in the ore.   Another classic example (if you're a murder mystery fan) is the analysis of various body parts to determine if those lonely, elderly gentlemen actually died of 'yellow fever', or had they been helped along with small doses of arsenic or some other poison.

In both of these case, the most difficult and time consuming part of the analysis is the separation of the element(s) of interest from the rest of the sample (the so-called matrix).  For these wet chemical techniques, this normally means 'digesting' the sample with strong acid to dissolve the matrix and make the element(s) of interest soluble.  Then based on the general solubility guidelines below (remember them from the first semester?), various reagents are added to produce compounds that are insoluble.  Remember that if either the cation or the anion is considered soluble, then the whole compound is soluble.  A compound is only considered to be insoluble when both the cation and the anion portions are insoluble.  

The trick, of course, is to pick a set of conditions so that only a single element or a small subset of elements precipitate while leaving all of the other element(s) in solution.  To accomplish this we need to consider several variables:  concentration, pH, temperature, charge density, and health risks.  Luckily, over the last several hundred years, these variables have been studied extensively and fairly standardized schemes now exist to analyze almost any element in the presence of almost any other element(s).  Together these procedures are know as Qualitative Analysis.

Purpose:

The purpose of this experiment is to use Qualitative Analysis to positively determine the presence or absence of a specific group of ions in an unknown mixture, namely:   Mn²⁺, Ni²⁺, Fe³⁺, Al³⁺, and Zn²⁺.  You will first have to separate a given ion from the rest of the mixture, and then perform a series of tests to confirm the identity of that ion.  As discussed above, most of the separation and identification procedures are based on manipulating the solubility of these ions.  The actual procedures for conducting this experiment are well understood.  The actual purpose of this lab is more an exercise in organizational/laboratory skills and attention to detail:  scrupulous cleaning of glassware, careful observation, labeling all containers, separating liquids and solids, etc.

Lab Tips:

Procedure:

During this experiment, you will be performing specific ion separation and confirmation procedures with two samples.  One will be a reference solution labeled "Qual II Known' and contains all of the ions of interest: Mn²⁺, Ni²⁺, Fe³⁺, Al³⁺, and Zn²⁺.  The second sample will be a solution that contains any or all of these ions.  The most effective method for determining the ions in your unknown involves performing all of the procedures simultaneously on the known and unknown solutions.  This practice provides you with invaluable feedback on proper ion separation as well as both positive and negative confirmation test results.  NOTE:  It is very important to understand the chemical reactions that are taking place as you analyze your samples.  If you try to 'cookbook' these procedures, or skip the known solution, you WILL fail to properly identify the contents of your unknown.

Unlike the Ag⁺ and Bi³⁺ ions in the Qual I experiment, the chlorides and sulfides of the Qual II ions are soluble at low pH.  However, they can be precipitated as sulfides (Mn²⁺, Ni²⁺, Fe³⁺, and Zn²⁺) or hydroxides (Al³⁺) at high pH.  It is this difference in solubility that defines these two groups of ions.

Separation of Qual II ions:

The addition of a strong base, such as NaOH, to a solution containing Mn²⁺, Ni²⁺, Fe³⁺, Al³⁺, and Zn²⁺ will cause the Mn²⁺, Ni²⁺, and Fe³⁺ ions to precipitate as hydroxides while the Al³⁺, and Zn²⁺ ions form hydroxides that are still soluble:

1. Add approximately 2 mL of your original solution to a small test tube.
2. Add 15 drops of 6 M NaOH.
3. Centrifuge and save the precipitate.
4. Test for complete precipitation by adding several more drops of 6 M NaOH to the supernatant.
5. Save this solution for later testing (Al³⁺, Zn²⁺).
6. Dissolve the precipitate with a minimum amount of concentrated nitric acid.
7. If necessary, heat the solution in a hot water bath.

Sodium bismuthate, NaBiO₃, is a very strong oxidizing agent.  If it is added to a solution containing Mn²⁺, Ni²⁺, and Fe³⁺ ions, it will only react with the Mn²⁺ ions to produce the intensely purple MnO₄^- ion.  This change from a colorless to a purple solution is the confirmation test for the presence of the Mn²⁺ ion:

1. Add approximately 1/2 of the solution from 'Separation of Qual II ions' Step 6 to a small test tube.
2. Add a small excess of solid NaBiO₃ to the solution and centrifuge.
3. A deep purple solution confirms the presence of the Mn²⁺ ion.

If excess ammonia is added to a colorless solution containing Mn²⁺, Ni²⁺, and Fe³⁺ ions, the Mn²⁺ does not react.  However, the the Ni²⁺ will react to form a blue hexaammine complex, [Ni(NH₃)₆]²⁺, and the Fe³⁺ will form a brownish Fe(OH)₃ precipitate:

The isolated iron hydroxide is then dissolved in hydrochloric acid to regenerate the Fe³⁺ ion.  The presence of the Fe³⁺ ion is confirmed by the addition of SCN^- which forms a blood red complex [FeSCN]²⁺:

1. Add the remaining 1/2 of the solution from 'Separation of Qual II ions' Step 6 above to a small test tube.
2. Add 5 drops of 4 M NH₄Cl to the solution.
3. Now add concentrated NH₃ until the solution is basic to litmus.  Add an additional 3 drops to ensure complete precipitation.
4. Centrifuge and save the supernatant for Ni²⁺ testing.
5. Dissolve the precipitate with 6 M HCl.
6. Add 5 drops of 0.1 M NH₄SCN.  The appearance of a blood red color confirms the presence of Fe³⁺.

Dimethylglyoxime, H₂DMG (C₄H₈N₂O₂), is a complexing agent that reacts specifically with Ni²⁺ ions to form a pinkish red precipitate.  It will not react with any other Qual I or Qual II ions:

1. Add the supernatant from Step 4 of the Fe³⁺ test above to a small test tube.
2. Add 3 drops of dimethylglyoxime solution.
3. The appearance of a pinkish red precipitate confirms the presence of Ni²⁺ ions.

After the initial precipitation of the Mn²⁺, Ni²⁺, and Fe³⁺  ions, your solution contained Al(OH)₄^- and Zn(OH)₃^-.  These ions can be converted back to Al³⁺ and Zn²⁺ ions with the addition of acid:

Subsequent addition of ammonia will precipitate the aluminum as Al(OH)₃, however, the zinc will form a soluble tetraammine complex:

Al(OH)₃ is a translucent, gelatinous, bluish white precipitate that is difficult to see.  To confirm the presence of aluminum, the precipitate is redissolved in nitric acid and aluminon reagent is added.  When ammonia is added to this solution, the Al(OH)₃ reprecipitates however, the red dye (aurin tricarboxylic acid) in the aluminon reagent sticks to the precipitate and gives it a pinkish red appearance:

1. Add 6 M HNO₃ drop wise to the supernatant from Step 5 of  'Separation of Qual II ions' until the solution is acidic to litmus.
2. Add 6 M NH₃ drop wise until the solution is basic to litmus.  Then add 5 more drops.
3. The presence of a white floculent precipitate indicates the presence of Al³⁺.
4. If no precipitate is formed, split the solution in half and proceed with Step 7.  The other half will be saved for Zn²⁺ analysis.
5. Centrifuge and save the supernatant for Zn²⁺ analysis.
6. To confirm the presence of Al³⁺, add 6 M HCl until the precipitate dissolves.
7. Add 2 drops of aluminon reagent and stir.
8. Add 6 M NH₃ drop wise until the solution is basic to litmus.
9. Centrifuge the solution.
10. A pink or red precipitate confirms the presence of Al³⁺.

The addition of potassium hexacyanoferrate(II), K₄[Fe(CN)₆] to an acidic solution containing [Zn(NH₃)₄]²⁺ ions results in the formation of a light green precipitate of K₂Zn₃[Fe(CN)₆]₂(s):

1. Add 6 M HCl to the supernatant from the Al³⁺ analysis above until the solution is acidic to litmus.
2. Add 3 drops of 0.2 M K₄[Fe(CN)₆] solution and stir.
3. A very light green precipitate confirms the presence of Zn²⁺.

Results/Calculations:

• Based on your results, draw a flow chart depicting the steps involved with the separation and confirmation of all five ions.  Include the balanced chemical reactions, precipitates, and all observations.
• Based on your observations, what ions were present in your unknown?

(Updated 2/19/08 by C.R. Snelling)