Qualitative Analysis of a Liquid Solution Study Notes

QUALITATIVE ANALYSIS OF A LIQUID SOLUTION

INTRODUCTION

  • Analytical Chemistry

    • Definition: The branch of chemistry focusing on separating and identifying components of a mixture (qualitative analysis) and sometimes quantifying them (quantitative analysis).

    • Usefulness: Critical in environmental contexts, such as detecting toxic chemicals in water sources (e.g., lead contamination in Flint, MI).

  • Separation of Common Cations

    • Based on the selective solubilities of cation compounds.

    • Analytical Group 1 includes:

    • Silver (I), Ag+

    • Mercury (I), Hg2²+

    • Lead (II), Pb²+

    • Characteristic: These cations form insoluble (or nearly insoluble) chlorides in cold water.

    • Separation Process:

    • Other cations, forming soluble chlorides, will remain in solution.

    • AgCl, Hg2Cl2, and PbCl2 precipitate out, allowing them to be separated from the mixture.

    • Post-Separation: Filtration or centrifugation used to isolate solid precipitates and confirm the identity of the cations based on their chemical properties.

YOUR TASK

  • Objective:

    • Organize the differing solubilities of various cations into a flow chart to guide selective precipitation and separation.

    • Follow the flow chart and protocol to precipitate and confirm the presence of cations in known and unknown samples.

SEPARATIONS

  • Process Overview:

    • Cations are separated by selective formation of insoluble precipitates or selectively dissolving specific precipitates.

    • Separation achieved by:

    • Filtration

    • Centrifugation

      • Centrifugation Technique:

      • Solid-liquid mixture spun at high speeds in small tubes, packing solids at the bottom.

      • The supernatant is carefully decanted.

      • A successful separation has no solid remaining in the liquid.

CAUTION

  • Centrifuge Usage Guidelines:

    • Use only small test tubes (13 X 100 mm).

    • Balance tubes in the centrifuge to prevent severe vibrations. Place similarly weighted tubes opposite to each other in the holder.

WASHING A PRECIPITATE

  • Purpose: Eliminate residual supernatant liquid clinging to the precipitate.

  • Process:

    • After decantation, washing liquid is added, and the precipitate is stirred/mixed.

    • Followed by centrifugation and decantation again.

    • Stirring Method: Swirling the tube while gently tapping the side with a finger or a pencil.

TESTING FOR COMPLETENESS OF PRECIPITATION

  • Initial Precipitation:

    • A precipitant is introduced to a solution with cations that will form a solid.

  • Testing Method:

    • Add one drop of precipitant to the supernatant after centrifugation.

    • If no new precipitate forms, the initial precipitation is complete.

    • If new precipitate forms, repeat the centrifuge and testing process until completion.

PROCEDURE

  • Analysis Process:

    • Begin with a known sample containing all cations to be analyzed.

    • Purpose: Familiarize with techniques and recognize positive tests.

    • Follow the same steps for an unknown sample which may contain any of the analyzed cations.

Preparation of the Known Sample (Analytical Group 1)

  1. Combine 1 mL each of the following solutions in a clean 100 mm test tube:

    • 0.1 M AgNO3

    • 0.1 M Pb(NO3)2

    • 0.1 M Hg2(NO3)2

  2. Follow the plan outlined below.

  3. Consider developing a flow chart to track performed tests and results.

Preparation of the Unknown Sample

  • Obtain an unknown sample and note its number.

  • Follow the same analysis plan and document the flow chart for found ions.

Outline of Analysis (Analytical Group 1)

  • Notation:

    • Decantates (saved liquid) marked with “D.”

    • Precipitates marked with “P.”

    • Use the following abbreviations:

    • ppt = precipitate

    • s = solid

    • w = white

    • y = yellow

    • blk = black

  • Add three drops of 6 M HCl to 3 mL of test solution and mix.

  • If a precipitate forms, continue to add 6 M HCl dropwise until complete.

  • Centrifuge and decant the supernatant liquid (2D), which contains cations from analytical groups 2, 3, 4, etc., and should be saved.

  • Wash the ppt (2P) twice with 1-mL portions of cold water and save washings to add to the (2D) decantate.

  • Precipitate (2P) may contain:

    • AgCl(s,w)

    • Hg2Cl2(s,w)

    • PbCl2(s,w)

  • Add 2 mL of water to the ppt, heat to near boiling, stir, centrifuge, and decant (3D).

  • Wash remaining ppt (3P) with an additional 2 mL of water, centrifuge, and discard decantate.

  • Decantate (3D): possible Pb2+ ion presence.

  • Divide into two equal parts:

    • Part 1: Add a couple drops of 6 M H2SO4; a white ppt of PbSO4(s,w) confirms Pb2+ presence.

    • Part 2: Add a couple drops of 1 M K2CrO4; a yellow ppt of PbCrO4(s,y) also confirms Pb2+ presence.

  • Confirm lead chromate presence by adding 6 M NaOH; stir and check for dissolution.

  • Remaining residue from wash (3P) may contain:

    • AgCl(s,w)

    • Hg2Cl2(s,w)

  • Add 1 mL of 6 M ammonium hydroxide, stir, centrifuge, and decant (4D).

  • Residue should be black, indicating the presence of mercury (I) ion.

  • In decantate (4D): possible presence of Ag(NH3)2+.

  • Carefully add 6 M HNO3 dropwise until the solution is acidic to litmus (turns pink).

  • A white ppt of AgCl(s,w) confirms the presence of the silver ion, which may just appear as a milky white suspension if not fully formed.