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Chemistry LibreTexts

Anion Analysis

  • Page ID
    150655
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    1: INTRODUCTION

    In order to determine the anions in the unknown crystals (any ionic compound must contain both cations and anions) a somewhat different approach that does not require the separation of the anions will be used. The “spot test” procedure for the anions in lab Q-anion will study the common species Cl(aq) , SO4 2-(aq), C2O4 2-(aq) and CO3 2-(aq). For more guidance and useful tables, you should refer back to the pages in your lab manual.

    Known solutions: Place some drops of each of the following known anion solutions into a separate test tube and label appropriately: K2C2O4(aq), Na2CO3(aq), Na2SO4(aq, NaCl(aq).

    Unknown Crystal Analysis: Prepare a solution of the green crystals for analysis as described for the Q1-m experiment. Place some drops of this unknown crystal solution into each of 4 separate test tubes, follow the lab manual for specific instruction.

     

    2: PROCEDURE

     


    2.1: Chloride / Cl(aq) Analysis

    Add some drops of H2SO4(aq) to both the known chloride solution and one of the unknown crystal solutions. After mixing, add 2 or 3 drops of AgNO3(aq) solution to both test tubes and mix well. A white precipitate indicates the presence of chloride. Record observations for each test tube and comment about the presence of chloride in the crystals.

     


    2.2: Sulfate / SO4 2-(aq) Analysis

    Add some amount of HCl(aq) to both the known sulfate solution and one of the unknown crystal solutions. After mixing, add some drops of BaCl2(aq) solution to both test tubes and mix well. The formation of BaSO4(aq) will yield a white, finely divided precipitate. Record observations for each test tube and comment about the presence of sulfate in the crystals.

     


    2.3: Oxalate / C2O4 2-(aq) Analysis

    Add some amount of acetic acid -C2H4O2(aq)- to both the known oxalate - C2O42-(aq) solution and one of the unknown crystal solutions. After mixing, add some amount of CaCl2(aq) solution to both test tubes, mix well and allow to stand for several minutes. Formation of a white precipitate is indicative of oxalate anion.Centrifuge out any precipitate, decant the liquid which can be discarded, and wash the precipitate as indicated in the manual. To the remaining precipitate add some amount of H2SO4(aq) and place in the hot water bath for some time, stirring occasionally to help dissolve the solid. To the hot solution add couple of drops of KMnO4(aq). If the precipitate was indeed oxalate the purple solution will quickly be bleached to colorless. Record observations for each test tube and comment about the presence of oxalate in the crystals.

     


    2.4: Carbonate / CO3 2-(aq) Analysis

    Add some amount of BaCl2(aq) to both the known carbonate solution and one of the unknown crystal solutions. After mixing, let stand for about 1 minute to allow any solids to form. Centrifuge the mixture and discard the liquid. Wash the solid accoding to the manual, centrifuge and discard the wash water. Add some amount of HCl(aq) to the solid, but do not stir the mixture, just wiggle the test tube slightly. The presence of carbonate will result in small gas bubblesrising from the solid surface for at least a minute or so. If there are no bubbles, carbonate is absent. Record the observations for each test tube and state your conclusion about the presence of carbonate in the green crystals.


     

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