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8: Qualitative Analysis of Anions Using Spot Plates (Experiment)

  • Page ID
    94007
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    Objectives
    • To use spots tests to identify common anions in an aqueous solution.

    In the previous two experiments, you have used qualitative analysis to determine the identity of various cations in a sample. In this experiment you will use qualitative analysis to identify the various anions in a sample. Specifically you will test for the presence of each of the following anions: \(\ce{CO3^{2-}}\), \(\ce{SO4^{2-}}\), \(\ce{PO4^{3-}}\), \(\ce{SCN^{-}}\), \(\ce{Cl^{-}}\), \(\ce{NO3^{-}}\).

    The methodology used in identifying the anions will be different than that which was used to identify cations. Here you will use a small portion of the unknown mixture (1 mL) to perform a spot test for each anion individually.

    In some cases the test for a particular ion will be complicated by the presence of other ions in the mixture that will interfere with the test. In these situations the interfering ions must be removed before the test can be performed.

    Procedure

    Chemicals

    1 M \(\ce{Na2CO3}\), 0.5 M \(\ce{Na2SO4}\), 1 M \(\ce{BaCl2}\), 0.5 M \(\ce{Na2HPO4}\), 0.5 M \(\ce{(NH4)2MoO4}\), 0.5 M \(\ce{KSCN}\), 0.1 M \(\ce{Fe(NO3)3}\), 0.5 M \(\ce{NaCl}\), 0.1 M \(\ce{AgNO3}\), 0.5 M \(\ce{NaNO3}\), 1 M \(\ce{CuSO4}\), 6 M \(\ce{HCl}\), 6 M \(\ce{HNO3}\), 6 M acetic acid, 6 M \(\ce{NaOH}\), Aluminum granules

    Equipment

    8 small test tubes, glass stirring rod, small 10-mL graduated cylinder, 250-mL beaker, stand, ring clamp, wire gauze, small watch glass, dropper pipets, blue litmus paper, red litmus paper, wash bottle filled with deionized water, Bunsen burner, centrifuge

    Safety

    There are quite a few concentrated acids and bases in this experiment. Please be careful when handling them.

    Waste Disposal

    All waste generated by this lab is toxic and must be disposed of in the hazardous waste container!

    Unused sample of the unknown should be discarded in the hazardous waste container. Wash the empty test tube and return it to your instructor’s bench.

    General Instructions

    For each anion you will perform a positive control test on 1 mL of solution of the anion, followed by a positive control experiment on 1 mL of a diluted solution of the anion. The diluted anion solutions can be prepare by adding 3 drops of deionized water to 9 drops of the anion solution. This will yield approximately 1 mL of a diluted solution.

    You will then perform the same test on 1 mL of your unknown solution which may or may not contain the anion in the control solution.

    Test for the presence of carbonate ion, \(\ce{CO3^{2-}}\)

    1. Add 1 mL of 1M \(\ce{Na2CO3}\) (or your unknown) and 1 mL of 6M \(\ce{HCl}\) to a small test tube. Effervescence indicates the presence of \(\ce{CO3^{2-}}\). In the concentrated control solution you should see effervescence for at least a few seconds. In the diluted control solution it may be necessary to place the test tube in a hot water bath in order to observe the effervescence. For the unknown if no effervescence is observed, place the test tube in the hot water bath before concluding that \(\ce{CO3^{2-}}\) is not present.

    Test for the presence of sulfate ion, \(\ce{SO4^{2-}}\)

    1. Add 1mL of 0.5 M \(\ce{Na2SO4}\) (or your unknown) and 1mL of 6M \(\ce{HCl}\) to a small test tube. Next add a few drops of 1 M \(\ce{BaCl2}\). A finely divided, white precipitate indicates the presence of the \(\ce{SO4^{2-}}\) ion.

    Test for the presence of phosphate ion, \(\ce{PO4^{3-}}\)

    1. Add 1mL of 0.5 M \(\ce{Na2HPO4}\) (or your unknown) and 1 mL of 6 M \(\ce{HNO3}\) to a small test tube. Next add a 1 mL of 0.5 M \(\ce{(NH4)2MoO4}\) and stir thoroughly. Place the test tube in the hot water bath and continue to stir. A yellow precipitate of indicates the presence of \(\ce{PO4^{3-}}\).

    Test for the presence of thiocyanate ion, \(\ce{SCN^{-}}\)

    1. Add 1 mL of 0.5 M \(\ce{KSCN}\) (or your unknown) and 1 mL of 6 M acetic acid (\(\ce{HC2H3O2}\)) to a small test tube. Now add 2 drops of 0.1 M \(\ce{Fe(NO3)3}\). A dark red solution indicates the presence of \(\ce{SCN^{-}}\).

    Important: If your unknown contains \(\ce{PO4^{3-}}\) it will interfere with the test for the \(\ce{SCN^{-}}\) since it will form a precipitate with \(\ce{Fe^{3+}}\).

    \[\ce{Fe^{3+} (aq) + PO4^{3-} (aq) -> FePO4 (s)} \label{1}\]

    In this case it will be necessary to remove all of the \(\ce{PO4^{3-}}\) before any conclusion can be made concerning the presence of \(\ce{SCN^{-}}\). The \(\ce{PO4^{3-}}\) can be removed by centrifuging the mixture and decanting the supernatant solution. Now add \(\ce{Fe(NO3)3}\) to the supernatant solution. If more precipitate forms, centrifuge and decant a second time. Add the \(\ce{Fe(NO3)3}\) to the supernatant solution. A dark red solution indicates the presence of \(\ce{SCN^{-}}\).

    Test for the presence of chloride ion, \(\ce{Cl^{-}}\)

    1. Add 1 mL of 0.5 M \(\ce{NaCl}\) (or your unknown) and 1 mL of 6 M \(\ce{HNO3}\) to a small test tube. Next add 2 to 3 drops of 0.1 M \(\ce{AgNO3}\). The formation of a white, curdy precipitate indicates the presence of \(\ce{Cl^{-}}\).

    Important: If your unknown contains \(\ce{SCN^{-}}\) it will interfere with the test for the \(\ce{Cl^{-}}\) since it will form a white precipitate with \(\ce{Ag^{+}}\).

    \[\ce{Ag^{+} (aq) + SCN^{-} (aq) -> AgSCN(s)} \label{2}\]

    In this case put 1 mL of your unknown sample into a small beaker (30 mL or 50 mL) and add 1 mL of 6 M \(\ce{HNO3}\). Boil the solution very gently (directly over the open flame) until the volume has decreased by about one half. Under these conditions the \(\ce{SCN^{-}}\) will decompose. Now pour this solution into a small test tube, add 1 mL of 6M \(\ce{HNO3}\) and 2 to 3 drops of 0.1 M \(\ce{AgNO3}\). A white curdy precipitate indicates the presence of \(\ce{Cl^{-}}\).

    Test for the presence of nitrate ion, \(\ce{NO3^{-}}\)

    1. Add 1 mL of 0.5 M \(\ce{NaNO3}\) (or your unknown) and 1 mL of 6 M \(\ce{NaOH}\) to a small test tube. Then add a few granules of aluminum metal and put the test tube in the hot water bath. The reaction between \(\ce{Al}\) and \(\ce{NaOH}\) will produce \(\ce{H2}\) gas which will reduce the \(\ce{NO3^{-}}\) to \(\ce{NH3}\). The \(\ce{NH3}\) can be detected by placing a piece of moistened red litmus paper directly above (but not in contact with) the mouth of the test tube. If the red litmus paper turns uniformly blue (due to \(\ce{NH3}\) vapor coming out of the test tube) then it can be concluded that \(\ce{NO3^{-}}\) is present in the unknown. Note that small blue spots produced on the red litmus paper are the result of spray from the basic solution in the test tube and do not necessarily indicate the presence of nitrate.

    Important: If your unknown contains \(\ce{SCN^{-}}\) it will interfere with the test for the \(\ce{NO3^{-}}\). In this case add 1 mL of your unknown to 1 mL of 1 M \(\ce{CuSO4}\) in a small test tube. Place the test tube in a hot water bath for about 2 minutes. Centrifuge the mixture and decant the supernatant solution into another small test tube. The solid may be discarded in the waste.

    To the supernatant solution add 1 mL of 1 M \(\ce{Na2CO3}\). Centrifuge the mixture and decant 1 mL of the supernatant solution into another small test tube. The solid may again be discarded in the waste.

    To the supernatant solution in the test tube add 1 mL of 6 M \(\ce{NaOH}\) and a few granules of aluminum metal. Place the test tube in the hot water bath and use red litmus paper to test for the presence of \(\ce{NO3^{-}}\) as described in the previous step.

    Lab Report: Qualitative Analysis of Anions using Spot Tests

    Name: ____________________________ Date: ________________________

    Lab Partner: ________________________ Lab Section: __________________

    Indicate your observations in the table below. Based on your observations indicate which anions are present in your unknown sample.

    Ion

    Control

    Diluted Control

    Unknown

    \(\ce{CO3^{2-}}\)

         

    \(\ce{SO4^{2-}}\)

         

    \(\ce{PO4^{3-}}\)

         

    \(\ce{SCN^{-}}\)

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    \(\ce{Cl^{-}}\)

         

    \(\ce{NO3^{-}}\)

         

    Unknown ID number: __________________

    Ions present in your unknown: ____________________________________

    Pre-Laboratory Assignment: Qualitative Analysis of Anions

    1. Write the net-ionic equation for the chemical reaction that occurs in each of the following spot tests.
    • The production of carbon dioxide gas in the spot test for carbonate.
    • The production of the aqueous \(\ce{Fe(SCN)^{2+}}\) complex ion in the spot test for thiocyanate.
    • The production of the white precipitate in the spot test for sulfate.
    1. Suppose that a series of spot tests on an unknown solution yields the following results.
    • A white precipitate upon the addition of \(\ce{BaCl2}\) (aq)
    • No yellow precipitate upon the addition of \(\ce{(NH4)2MoO4}\).
    • A dark red solution upon the addition of \(\ce{Fe(NO3)3}\).
    • A white precipitate upon the addition of \(\ce{AgNO3}\).

    Indicate whether each of the following is present, not present, or undetermined.

    \(\ce{CO3^{2-}}\)  
    \(\ce{SCN^{-}}\)  
    \(\ce{SO4^{2-}}\)  
    \(\ce{Cl^{-}}\)  
    \(\ce{PO4^{3-}}\)  
    \(\ce{NO3^{-}}\)  

    8: Qualitative Analysis of Anions Using Spot Plates (Experiment) is shared under a CC BY-NC license and was authored, remixed, and/or curated by Santa Monica College.