5: Inorganic Qualitative Analysis (Experiment)

Pre-Lab Questions

1. Separate the following cations into groups based on their reactivity: \( \ce{Na ^{+}} \), \( \ce{K^{+}} \), \( \ce{Cu^{2+}} \), \( \ce{Pb^{2+}} \), \( \ce{Ag ^{+}} \), and \( \ce{Ca ^{2+}} \). Label each group based on the anion used to precipitate the cations belonging to that group. We will not be using Group III. Identify which belong to group III in your pre-lab.
2. The cations in Group V are \( \ce{Na ^{+}} \) and \( \ce{K^{+}} \). They might more properly be called "group zero," because the unknown is tested for \( \ce{Na ^{+}} \) and \( \ce{K^{+}} \) before any other operations are performed on it. Why are these cations tested first?
3. Write a balanced chemical equation for the removal of sulfide with the chlorate anion. What type of reaction is occurring?

Introduction

Qualitative analysis is a branch of analytical chemistry that involves the separation and identification of components of a mixture using a systematic chemical method based on the differences in reactivity and properties of the components. In our experiment, the components are cations of different valences and reactivity, such as those listed in pre-lab Question A. A set of precipitating agents is used to separate the cation mixture into groups of cations with similar reactivity. For example, Group I (\( \ce{Ag ^{+}} \), \( \ce{Pb^{2+}} \)) cations precipitate as chlorides.

The first goal of this experiment will be to develop a flow chart based on the systematic grouping of cations by chemical reactivity. The strategy for analyzing a solution (i.e. your unknown) containing all the cations is summarized by this flow chart and is the key to successfully identifying the cations in your unknown, so construct it with care. A process of elimination can be used, and having an accurate flow chart will help to guide you through the process of elimination to correctly identify the cations in your unknown solution.

If you are unsure of what a product should look like or what reactions occur, then take the time to find a Qualitative Analysis (QA) text book in the library. QA is a classic general chemistry experiment, and therefore an explanation of each reaction can be found in detail in a QA textbook. Also, there are many good references on the web if you search using the keywords "qualitative analysis."

Safety

Bottles of concentrated reagents are to be kept and used in the fume hoods. This includes dropper bottles of concentrated \( \ce{HCl} \), \( \ce{HNO3} \), \( \ce{H2SO4} \), glacial acetic acid, and thioacetamide.

Thioacetamide produces hydrogen sulfide gas, \( \ce{H2S} \), which is highly toxic and can cause serious injury and death. Handle with care. Use the hood for all thioacetamide procedures. Discard sulfide-containing solutions in the sink in the hood. Cover any \( \ce{H2S} \) containing glassware before removing it from the fume hood.

When working with the sulfide reactions at your lab bench (after the initial acid + thioacetamide reaction in the hood), use a vacuum umbrella to trap unpleasant odors from the test tubes. Connect a long stem funnel to an aspirator with a trap with a piece of suction tubing, and invert it over (and close to) the top of your container. Use a clamp to hold the funnel in position.

Methods of Separation

At some point during the analysis of the unknown sample, the precipitate (ppt) will have to be separated from the solution above it (called the supernatant). The supernatant may contain any cation that hasn’t been removed by a precipitation reaction. There are several ways to carry out this separation: decanting, centrifugation, and filtration.

Decanting involves carefully pouring the supernatant into another test tube without losing any of the solid. If necessary, leave some of the supernatant with the solid. The left-over supernatant can be removed by washing the solid with deionized water several times— decanting each time. Only the first supernatant needs to be saved for further testing provided there is a significant quantity (1-2 mL); subsequent washes can be discarded as these will be very dilute solutions of any remaining precipitated cations.

Decanting is useful when the ppt is heavy and settles quickly at the bottom of the test tube. This will not be the case for all ppts. Sometimes the ppt might form a cloudy suspension or a flocculent. In this case, centrifugation is necessary before the supernatant liquid can be decanted. **One useful tip: if you aren’t sure whether a ppt has formed, centrifuge the sample and see if any ppt forms at the bottom of the test tube. When using the centrifuge, make sure a counter-balance is positioned directly across from the sample tube in the centrifuge. A counter-balance is a test tube that contains a volume of water equivalent to the sample volume. The counter-balance serves to evenly distribute the weight in the centrifuge. Failure to use a counter-balance could result in a serious accident or injury.

Finally, a Barber tube may be used to separate precipitates from solutions instead of the centrifuge. A Barber tube is prepared from a small plug of glass wool that is placed into a disposable glass Pasteur pipet. Use another Pasteur pipet to deliver the solution containing the precipitate into the top of the Barber tube. Then place a rubber bulb on top of the tube and blow the solution through the glass wool. (Never use your mouth!!) The precipitate will remain behind, caught in the glass wool. Use deionized water to wash the precipitate. The precipitate may then be redissolved using an appropriate reagent, such as an acid or base.

Methodology

Qualitative analysis uses the differences in solubility of the salts of cations to separate them from one another. Once isolated, each cation can be identified based on its specific
reactivity and color with various anions. For example, both \( \ce{Ag ^{+}} \) and \( \ce{Pb^{2+}} \) form ppt with \( \ce{Cl ^{-}} \). However, the solubility product constant, \( \ce {K_{sp}} \), is much larger for \( \ce {PbCl2} \) relative to \( \ce {AgCl} \). After these two cations are precipitated and separated from the supernatant, the cations can be separated from one another and identified with further testing. This will be explained in the section describing the analysis of Group I cations.

After the insoluble chlorides have been removed from the solution, the \( \ce{H^{+}} \) concentration of the supernatant solution is adjusted to about 0.3 M, and hydrogen sulfide is introduced into the solution. Hydrogen sulfide will be generated by the hydrolysis of thioacetamide and proceeds by the following reaction:

\[ \ce {CH3CSNH2 \xrightarrow{heat} H2S + NH4^{+} + CH3CO2^{-}}\]

The acid dissociation-constant expressions for each equilibrium reaction

\[ \ce {H2S \leftrightarrow HS^{-} + H^{+} }\]

\[ \ce {HS^{-} \leftrightarrow S^{2-} + H^{+} }\]

may be combined to yield the result

\[ \ce { K1K2 \cong 5 \times 10^{-23} \cong \frac { [H^{+}]^{2}[S^{2-}] }{H2S} } \]

A saturated solution of \( \ce {H2S} \), hydrogen sulfide, is approximately 0.1 M in \( \ce {H2S} \) concentration so that in the presence of 0.3 M \( \ce {H^{+}} \) the solution will contain \( \ce {[S^{2-}] \simeq 10^{-21}} \). Cations will be present at a concentration of approximately 0.05 M under these conditions. Thus, those divalent cations whose solubility products are smaller than about \( \ce{5 \times 10^{-23}} \) will precipitate, while those with solubility products larger than about \( \ce{5 \times 10^{-23}} \) will remain in solution. The proper solubility product form must be worked out to determine whether or not precipitation will occur for cations other than the divalent cations.

The following procedures will teach you how to separate and identify each cation within a group, such as the chlorides (GROUP I) or sulfides (GROUP II). Known cations will be available such as 0.05 M solutions of the nitrate salts (i.e. \( \ce{ AgNO3_{(aq)}} \), \( \ce{ Cu(NO3)2_{(aq)}} \), etc.) Record all observations, such as color and type of ppt, in your laboratory notebook. (Note: the procedure for the separation of each group should be read completely and understood before beginning the experiment.)

Group V Cations

The ions included in Group V are \( \ce{Na^{+}} \) and \( \ce{K^{+}} \). These cations could also be classified as Group 0 because the unknown can be tested for \( \ce{Na^{+}} \) and \( \ce{K^{+}} \) before any other analyses are performed on it. The tests for these cations are not affected by the presence of other cations in the solution.

Diagnostic Tests: To test for \( \ce{Na^{+}} \) and \( \ce{K^{+}} \) obtain a length of Nichrome wire, form a small loop in one end by bending it around the tip of a forceps. Heat the loop using a Bunsen burner just above the inner blue cone until no coloration of the flame occurs. Remove the last traces of flame color due to the presence of impurity ions on the wire by dipping the wire into concentrated \( \ce{HCl} \) and reheating. It may be necessary to repeat this procedure several times. Once clean, use the loop to perform flame tests for \( \ce{K^{+}} \), \( \ce{Na^{+}} \), and a mixture of \( \ce{K^{+}} \) and \( \ce{Na^{+}} \). Can you detect \( \ce{K^{+}} \) if \( \ce{Na^{+}} \) is present?

While it is easy to see sodium in a flame test, it may not be so easy to detect potassium in the presence of sodium with just a flame test. In addition to the flame test, a precipitation reaction can be performed using a reagent, sodium tetraphenylborate \( \ce {(NaTPB)} \), which is relatively specific for \( \ce{K^{+}} \). Several of the ions in our scheme will interfere. You should add a drop or two of sodium tetraphenylborate solution to test tubes containing 1 mL of each ion in our scheme to determine the interferences. Once you have identified the interfering ions, you can decide where to put the \( \ce{K^{+}} \) ppt test in the flow chart. One other point to consider is that some of the reagents used to ppt other cations may contain potassium; therefore, the addition of these reagents to your unknown would contaminate your sample and possibly give a false positive test.

Group I Cations

Group I contains \( \ce{Ag ^{+}} \) and \( \ce{Pb ^{2+}} \), and according to the solubility rules each ppts with \( \ce{Cl ^{-}} \) to form \( \ce{AgCl} \) and \( \ce{PbCl2} \), respectively. However, \( \ce{PbCl2} \) is only moderately insoluble, and some of the \( \ce{Pb ^{2+}} \) cations may be left in solution owing to incomplete precipitation by \( \ce{Cl ^{-}} \). Beware of this when you are performing separations on your unknown. One side reaction that could occur is

\[ \ce {AgCl_{(s)} + Cl^{-}_{(aq)} \rightarrow AgCl2^{-}_{(aq)}} \]

where the \( \ce{AgCl} \) ppt complexes with another chloride ion to form a soluble species. This could result in the presence of unwanted \( \ce{Ag ^{+}} \) cations in the supernatant if you are analyzing your unknown.

Diagnostic Tests: Place 1 mL of \( \ce{Ag ^{+}} \) into a test tube; likewise, place 1 mL of \( \ce{Pb ^{2+}} \) into a test tube. Also, make a third test tube containing both cations. Add 1 drop of 6 M \( \ce{HCl} \) to each test tube and stir each with a clean stirring rod. Record your observations.

Centrifuge the test tubes. To make sure that all the cations have ppt completely, slowly trickle one drop of \( \ce{HCl} \) down the side of the test tube. Watch to see if any ppt forms. If ppt forms repeat this process; if not continue to the next step. Decant the supernatant [Note: Remember to wash the precipitate once or twice with cold deionized water by resuspending, recentrifuging, and decanting to remove any ions that may have been trapped by occlusion.] When testing the unknown, it is important to save the supernatant for subsequent analyses, then add to each about 2 mL of hot water, stir, and place the tubes in a hot water bath for a few minutes. Record your observations.

Increasing the temperature will increase the solubility of the \( \ce{PbCl2} \) enough to completely redissolve the solid. \( \ce{Pb ^{2+}} \) can be positively identified by two reactions. Divide the supernatant into two parts. To one part, add two drops of 6 M \( \ce{H2SO4} \). What is being formed? To the other part, add a few drops of \( \ce{K2CrO4} \) solution. Observations?

Group II Cations

The cations that will precipitate as sulfides from 0.3 M \( \ce{H^{+}} \) are \( \ce{Pb^{2+}} \) and \( \ce{Cu^{2+}} \). However, there are two caveats. First, the presence of \( \ce{NO3^{-}} \) anions can oxidize the sulfide ion according to the reaction

\[ \ce{ 8H3O^{+}_{(aq)} + 3S^{2-}_{(aq)} + 2NO3^{\bullet} \rightarrow 2NO_{(g)} + 12H2O_{(l)} + 3S_{(s)}} \]

This reaction can be avoided by boiling the solution in the presence of \( \ce {HCl} \) to decompose the nitrate anion and reduce the sample volume. Only then can thioacetamide be successfully used as a source of \( \ce {S^{2-}} \). Second, if the sample with thioacetamide is heated for more than a few minutes other cations may start to ppt with the sulfide ion. So, don’t wait too long after adding the thioacetamide to separate the solid from the supernatant.

Caution

Thioacetamide produces hydrogen sulfide gas, \( \ce {H2S} \), which is highly toxic and can cause serious injury and death. Handle with care. Use the fume hood for all thioacetamide procedures. Discard sulfide-containing solutions in the sink in the fume hood. Cover any \( \ce {H2S} \) containing glassware before removing it from the fume hood.

Diagnostic Tests: Place 1 mL of \( \ce {Cu^{2+}} \) into a test tube, likewise, place 1 mL of \( \ce {Pb^{2+}} \) into a test tube. Again, make a test tube containing both the cations. To each tube add one drop of 6 M \( \ce {HCl} \) and boil to reduce the volume. Next, add 2 drops of thioacetamide solution. Place the test tubes in a boiling water bath IN THE HOOD for 5 minutes.

Centrifuge each test tube and discard the supernatant. [Note: When testing the unknown, remember to wash the precipitate and save the supernatant for subsequent analyses.] Wash each precipitate twice with cold water, followed each time by centrifugation. Add 2 mL of 3 M \( \ce {HNO3} \) to each precipitate. Heat in a water bath, with stirring, until any visible reaction is complete.

Now add 2 drops of 6 M \( \ce {H2SO4} \). Record your observations. What compound have you formed?

Empty the tube with the solution that is unreactive toward sulfuric acid into a small beaker. Very cautiously neutralize the contents of each beaker with 6 M \( \ce{NH3_{(aq)}} \), then add about 10 more drops of ammonia. Record your observations. What compound have you formed?

Based on these tests, construct a flowchart to analyze unknowns that could contain one or more of the elements of groups 0, I, and II. Consult your T.A. for advice when necessary.

Group III Cations

Group III contains cations of the first row of transition metals, namely \( \ce {Mn^{2+}} \), \( \ce {Fe^{3+}} \), \( \ce {Co^{2+}} \), and \( \ce {Ni^{2+}} \). We will not be testing for these cations in this lab.

Group IV Cations

Groups IV and V comprise those ions inert to precipitation by chloride or by sulfide in either acidic or basic solutions. In most qualitative analysis schemes, ammonium carbonate is used to precipitate the group IV ions. In our case, however, only \( \ce{Ca^{2+}} \) will be included in Group IV (\( \ce{Ba^{2+}} \), \( \ce{Mg^{2+}} \), and \( \ce{Sr^{2+}} \) would have been the others), and we can omit the carbonate precipitation step because there is an easy way to identify \( \ce{Ca^{2+}} \).

Diagnostic Tests: Place 1 mL of \( \ce{Ca^{2+}} \) test solution in a test tube. Add 2 drops of 6 M  \( \ce{NH3_{(aq)}} \), then several drops of ammonium oxalate reagent. What ppt was formed in the reaction? Dissolve the ppt in concentrated  \( \ce{HCl} \) and perform a flame test. If  \( \ce{Ca^{2+}} \) is present, the flame should turn brick red or red-orange. The identification of the \( \ce{Ca^{2+}} \) cation should not rest solely on the results of the flame test, rather the flame test should serve to confirm the observations of the ppt reactions.

Unknowns

Your TA will now give you 7 mL of a solution containing 3 cations, each at a concentration of 0.05 M, for you to analyze. Based on your knowledge of the reactivity of the
various cations and your flow chart, carry out experiments that will enable you to identify which cations are present. Use 3 mL of the unknown for your analysis, 3 mL for repeating the analysis, and 1 mL for checks. Note that you should test small portions of your unknown for \( \ce{Na^{+}} \) and \( \ce{K^{+}} \) before any other operations are performed on it. Keep in mind that on many occasions, the supernatant should be saved for future analysis steps in the flow chart. Use the flow chart you have prepared to guide you through the analysis of your unknown.

Data Analysis

Write a balanced chemical equation for each step of the procedure. Your laboratory instructor will answer a properly formulated question, but no information will be volunteered. Put this in the calculations section.

In the results section, remember to include your unknown number and a list of the cations present in your unknown.

In your conclusion section, describe the analysis of your unknown. Please include the net ionic, balance chemical equations for each reaction used to positively identify the cation in question. The description of your analysis should include a discussion of any problems that were encountered while trying to identify the cations. Explain how these problems were resolved.