Chemlab: Chemistry 3/5


Qualitative Analysis of Cations

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Techniques
In this lab, you will perform simple test tube experiments and make careful observations. To separate ions of each group, you will form an insoluble precipitate and centrifuge to remove solid from the ions that remain in solution. You will need to carefully wash the precipitate and decant the solution to remove all soluble ions. You will use pH paper when you acidify and basify your samples. Finally, you will heat in a water bath to perform some reactions. Be sure to use clean glassware to avoid contamination of your sample. Wash glassware with soap and rinse with distilled water.

Each of the techniques listed above are described and illustrated on the ChemLab website. Use the information on the website to learn each boldfaced technique before coming to lab. The technique information on the website will be helpful in answering Prelab Problem 1.

Procedure
First start heating your water bath, on a hotplate. It should contain distilled water. The bath should be boiling gently when used.

You are strongly advised to retain the test tubes of all confirmation tests until you complete the experiment. This way, you can reinterpret your results, if necessary.

Unknown mixture of cations
Obtain your unknown mixture, which contains a subset of the six possible ions. Note its number in your notebook and note as well any simple physical characteristics of the mixture such as color, pH (from indicator paper), etc. Your task is to determine the constituents of the mixture. Use your flowchart and observations to confirm the presence or absence of each cation. Check your flowchart with your TA before beginning your analysis.

To confirm your analysis of the unknown mixture, use the known solutions of single cations to observe positive confirmation tests. Record an observation for each cation's confirmation test in your notebook, either from your unknown or from a known solution.

When you have completed the analysis of your unknown, repeat it with a new sample from your unknown vial, if time permits. This will confirm your initial conclusions.

1. Separation and Analysis of Group I Ions
The solubility products (Ksp) of silver chloride and lead chloride are 1.8 x 10-10 and 1.7 x 10-5, respectively. This means that neither salt is soluble and that AgCl is even less soluble than PbCl2. These two Group I ions (Ag+ and Pb2+) are separated from the other cations in the sample by addition of HCl, precipitation of the Group I metal chlorides, and separation of the solid precipitate from the remainder of the solution, which contains the dissolved Group II and III ions. Be careful to avoid a large excess of HCl, since both cations form soluble complex cations with excess chloride ions.

Add 6 drops of 6 M HCl to no more than 1 mL of the solution to be analyzed. If a precipitate forms, centrifuge the sample, and save the liquid solution for further analysis in Section 2.

Image 4A sample after centrifugation is shown. Make sure the solution is completely clear. Add an additional drop or two of HCl and let the solution sit, to see if more AgCl or PbCl2 precipitate forms. If so, remove by centrifugation. Carefully wash the precipitate and test it for lead and/or silver.

To make the most efficient use of time, you should begin the separation of Group II ions, in section 2 at this point. While that solution is heating in the water bath, you can return to your Group I ions and perform the confirmation tests for lead and silver. Careful labeling and notebook records will help you to keep track of your test tubes.

Image 5A. Confirmation of Lead (Pb2+)
Lead chloride is almost three times more soluble in hot water than cold. One may use this as a basis for separating it from silver chloride. The presence of lead is then confirmed by precipitation of yellow lead chromate.

Add ~20 drops of hot distilled water to the solid precipitate from above. Centrifuge while hot, decant, and save both the solution and the solid. Add 2-3 drops of 1 M K2CrO4 to the solution. A bright yellow precipitate confirms the presence of Pb2+.

B. Confirmation of Silver (Ag+)
Silver forms a soluble complex ion with aqueous ammonia. The presence of silver is confirmed by dissolving any remaining solid residue in 6 M NH3 (aq) and then re-precipitating the chloride by freeing the silver ion from the complex ion using 6 M acid.

Image 6Add ~5 drops of 6 M NH3 (aq) to the solid from A, keeping your test tube near the inlet of the fume exhaust vent. The solid should dissolve, but if any precipitate remains, centrifuge and proceed using only the centrifugate. Add 6 M HNO3 to the solution until the solution is acidified, using pH indicator paper to test for acidification. A white precipitate (AgCl) confirms the presence of Ag+. The Cl- needed for precipitation will be present from the prior dissolution of AgCl.

To perform this test on a known solution containing silver ions, add HCl to the solution to observe the AgCl precipitation.

2. Separation and Analysis of Group II Ions
The Group II ions are separated from those in Group III by adding hydrogen sulfide to the mixture in acidic solution. We use an organic precursor, thioacetamide (CH3CSNH2), which decomposes to hydrogen sulfide in the presence of acid according to the net reaction
CH3CSNH2 + 2 H2O CH3CO2- + NH4+ + H2S
The Group II ions precipitate as sulfides on reaction with S2- from H2S.

Image 7Add 2 drops of 6 M HCl to the solution from Section 1 and dilute to ~2.5 mL. Confirm acidity with pH paper. With the sample solution placed near the fume exhaust vent, add 10 drops 5% thioacetamide solution, stir, and heat in a water bath for at least 10 minutes. The precipitate contains copper sulfide. Centrifuge, decant, and save the centrifugate for Group III identification. Wash the precipitate by stirring it with ~10 drops of 1 M HCl, centrifuge, and combine the centrifugate with that from the previous centrifugation. It is essential to remove all solid group II ions, so they will not interfere with the group III analysis later on.

A. Confirmation of Copper (Cu2+)
The precipitated sulfides are redissolved by addition of conc. HNO3, producing free ions and elemental sulfur. Once the ions are redissolved, one can confirm the presence of the Group II ion, Cu2+. Addition of conc. ammonia (NH3 (aq) ) initially results in the precipitation of copper hydroxide. Excess aqueous ammonia redissolves the copper hydroxide via the complex ion, Cu(NH3)42+. A characteristic blue color confirms the presence of Cu2+ in solution.

Image 8Add ~8 drops of 16M HNO3 to dissolve the precipitate which contains Cu2+. If large black "clumps" remain, centrifuge and discard the solid. It is probably PbS(s) or S(s). If solid floats, remove the solution to a clean test tube with a dropper, leaving the solid behind. Now add conc. NH3 (aq) until the solution is strongly basic according to pH paper. This could take as much as 20 drops. A blue solution indicates the presence of Cu2+ ions.

3. Analysis of Group III Ions
The Group III ions form sulfides that are more soluble than those of Group II, but they may be precipitated as sulfides if the sulfide concentration is sufficiently large. Addition of conc. NH3 (aq) to the thioacetamide insures that the necessary sulfide concentration will be reached.

Your cation solution should contain only Group III ions after the procedure outlined in Section 2 above. Keeping your sample near the fume exhaust vent, add ~5 drops of 5% thioacetamide solution, stire, and heat for ~5 minutes. Add ~5 drops of conc. NH3 (aq), stir and heat for 5 additional minutes. The precipitate contains the sulfides of the Group III ions. Centrifuge and discard the liquid.

A. Separation of Nickel (Ni2+) from Iron (Fe2+) and Manganese (Mn2+)
The sulfides of Fe2+ and Mn2+ are soluble in 1 M HCl, but nickel sulfide is not. This is used as the basis for separating Ni2+ from the remaining two ions.

Add ~10 drops of 1 M HCl to the precipitate prepared above, stir and centrifuge. Decant and wash any solid residue (NiS) with ~5 drops of 1 M HCl, and add the wash to the centrifugate. If present, Fe2+ and Mn2+ should now be in the solution. If solid floats, remove the solution to a clean test tube with a dropper, leaving the solid behind.

B. Confirmation of Ni2+
Nickel sulfide is soluble in a mixture of nitric and hydrochloric acids that converts sulfide to elemental sulfur. In this reaction sulfur is oxidized from the -2 oxidation state (S2-) to the zero oxidation state (S(s)). At the same time, the nitrogen is reduced from a +5 oxidation state (NO3-) to a +2 oxidation state (NO). The free Ni2+ that results is first complexed with ammonia and then detected as an insoluble, scarlet coordination compound of dimethylglyoxime (DMGH2). The structures of DMGH2 and the coordination compound are shown below. Note that DMGH2 is weakly acidic and loses one of its two acidic protons in order to form an electrically neutral complex. This DMGH2 conjugate base anion can be abbreviated DMGH- in your net reactions.
Figure 1
Image 9Add ~6 drops of conc. HCl and 2 drops of conc. HNO3 to any precipitate from A and heat the mixture. Add 6 M NH3 (aq) a few drops at a time. Stir with a clean stirring rod after each addition of NH3 (aq) and test the pH with indicator paper. Add NH3 (aq) until a strongly basic pH is reached. A total of 10-15 drops are typically required. This produces the Ni(NH3)62+ complex ion. Dilute to 1 mL with H2O. Add ~3 drops of dimethylglyoxime to the solution. Ni2+ is confirmed by the formation of a scarlet to strawberry-red precipitate.

C. Confirmation of Mn2+
Manganese is detected by oxidation of Mn2+ to permanganate ion (MnO4-) by sodium bismuthate (NaBiO3) in nitric acid. In this reaction, manganese is oxidized from the +2 oxidation state (Mn+2) to the +7 oxidation state (MnO4-). At the same time, bismuth is reduced from the +5 to the +3 oxidation state (Bi+3). You can tell that reaction has occurred from the distinctive purple or pink color of permanganate.

Image 10Add 5 drops of 6 M HNO3 and ~2 drops of 1 M sodium nitrite (NaNO2) to the solution from part A, and dilute to 1 mL. Heat the solution and, after cooling, divide it into 2 parts, setting aside one part for section D below. Add a spatula tip of NaBiO3 and ~2-3 drops or more of 6 M HNO3. The formation of a transient pink to purple color of MnO4- confirms Mn2+. An additional spatula tip of solid and a gentle shake of the test tube may make this subtle color change more apparent. Bismuthate (BiO3-) in strong acid is predominantly molecular bismuthic acid (HBiO3) and is reduced to Bi3+.

Image 11D. Confirmation of Fe2+/Fe3+
The confirmation test for Fe3+ is simple. Treating the solution with nitric acid in step C, above, oxidizes Fe2+ to Fe3+. The latter ion reacts with thiocyanate ion to produce a well-known "blood red" hexathiocyanato complex ion.

To the solution set aside in part C add 2-3 crystals of NH4SCN. Very little solid is required for the solution to turn a dark blood red, if Fe3+ is present.
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