ESGilreath (1954) Cation Group 1

Experimental Procedures

General Laboratory Directions

  • Washing Precipitates:

    • Washing prevents interference from ions in the centrifugate during analysis.

    • Distilled water (about 5 drops) is added and mixed with the precipitate.

    • The mixture is centrifuged, and wash water is removed using a micropipette. Usually, 1-2 washings suffice.

  • Fresenius "Qualitative Analysis":

    • First edition published in 1840; it remains the standard for cation analysis despite modifications.

    • A complete analysis can be complex, involving about 23 common cations systematically separated using group reagents.

    • Two cation groups are precipitated using hydrogen sulfide, one in acid, the other in ammoniacal solution.

    • Hydrogen sulfide is poisonous, complicating analysis due to slower processes.

  • Objectives of Laboratory Course:

    • Familiarize students with identifying tests for ions in qualitative analysis.

    • Conduct analyses on known solutions to build confidence before tackling unknown mixtures.

    • Maintain a neat record of observations with a suggested format for analysis.

    • Emphasizes fundamental inorganic chemistry knowledge, engaging students through unknown analyses.

Laboratory Work

  • Procedures and analysis times vary among institutions. Directions for analysis of known solutions are provided in an appendix.

  • The objective is to prepare students for real-world analytical processes, where methods differ from those taught in class.

Systematic Separation of Cations into Groups

Group I Cations

  • Group Separation:

    • Group I cations are separated based on their relative solubility in dilute acids; chlorides are relatively insoluble.

      • Key Cations: PbCl2, AgCl, Hg2Cl2.

    • Each ion in the group is precipitated successively to simplify identification.

Analysis Procedures

  1. Precipitation of Group I Cations:

    • Add 3 F HCl to the original solution to precipitate chlorides.

    • Centrifuge to obtain precipitates. Check precipitation completeness using HCl.

  2. Separation of Lead Chloride:

    • Lead chloride must be isolated from AgCl and Hg2Cl2 for accurate analysis.

    • Use hot water to leach PbCl2 from precipitate; note that cooling will cause it to reprecipitate.

  3. Confirmation of Silver Ion Presence:

    • AgCl is soluble in ammonia, forming Ag(NH3)2+, which indicates silver's presence upon treatment with HNO3 reprecipitating AgCl.

Theoretical Discussion

  • The process relies on differences in solubility products of the formed compounds during precipitation.

  • Significant numbers include:

    • PbCl2: 1x10^-4, Hg2Cl2: 2x10^-18, AgCl: 1.56x10^-10.

    • PbCl2 is more soluble compared to the other two, affecting how precipitates are handled.

  • The solubility variations lead to effective separation but also potential confusion in identification.

Identification of Cations

  • Mercurous Ion Confirmation:

    • Upon treating mercury compounds with ammonia, a black residue indicates the presence of Hg.

  • Chemical Reactions:

    • Several key reactions support the identification and separation processes, ensuring accurate results through tests with reagents such as stannous chloride.

  • Proper handling of precipitates and correct identification tests are crucial for confirming the presence of cations, especially when color changes occur during reactions.