Analytical Chemistry Lab: Qualitative Analysis
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Questions and Answers

What color precipitate forms when As³⁺ is present in a solution?

reddish brown

Which cations are part of Group III?

  • Fe³⁺
  • Co²⁺
  • Ni²⁺
  • All of the above (correct)

    • Adding excess Na₂O₂ should be avoided in the analysis of Group III cations.

    True

    To identify the presence of Zn²⁺, a white precipitate is formed using ____________.

    <p>CH3CSNH2</p> Signup and view all the answers

    Match the confirmatory results with the correct cations:

    <p>Al³+ = red ppt; 3F NH₃ Cr³+ = blue color that fades rapidly; H₂O₂ Zn²+ = white ppt; CH3CSNH2</p> Signup and view all the answers

    What are the two possible outcomes when a white precipitate forms after treatment with ammonia during the Analysis of Group I Cations?

    <p>lead oxychloride or silver chloride</p> Signup and view all the answers

    Which cations form the sulfides that precipitate in Group II as sulfides?

    <p>Hg</p> Signup and view all the answers

    Aqua regia dissolves HgS due to the complexing action of the nitrate ion.

    <p>False</p> Signup and view all the answers

    If acidifying and diluting is properly carried out, the solution will produce a concentration of ___.

    <p>0.3F</p> Signup and view all the answers

    What is the purpose of making the solution strongly basic?

    <p>remove the highly colored chromate or dichromate ions</p> Signup and view all the answers

    Which of the following cations are present in the soluble Group V?

    <p>All of the above</p> Signup and view all the answers

    Ammonium ion will always occur in Group V.

    <p>True</p> Signup and view all the answers

    The separation of strontium ions from calcium ions is not ________.

    <p>complete</p> Signup and view all the answers

    What is the scale of operations for volume in semimicro analysis?

    <p>1 drop to 1 mL</p> Signup and view all the answers

    What apparatus is used for heating a solution in semimicro analysis?

    <p>Water bath</p> Signup and view all the answers

    PbCl2 is more soluble than AgCl.

    <p>False</p> Signup and view all the answers

    The process of filtration is replaced by the use of a ________ in the laboratory work of qualitative analysis.

    <p>centrifuge</p> Signup and view all the answers

    Match the Group I cations with their precipitation color result and last reagent used:

    <p>Pb2+ = Yellow precipitate Ag+ = White precipitate Hg22+ = White/gray precipitate</p> Signup and view all the answers

    What confirmatory result indicates the presence of CO3^2- ions?

    <p>vigorous effervescence with KClO3</p> Signup and view all the answers

    How are SO3^2- ions confirmed?

    <p>white precipitate with H2O2</p> Signup and view all the answers

    What color precipitate indicates the presence of AsO2^- ions?

    <p>yellow precipitate with HCl</p> Signup and view all the answers

    How are AsO4^3- ions identified?

    <p>pink color in the CCl4 layer with CCl4</p> Signup and view all the answers

    Which ion causes the etching of glass upon its presence?

    <p>F^- ions</p> Signup and view all the answers

    Study Notes

    Identification of Cations

    • Cd2+ produces a yellow precipitate with CH3CSNH2
    • Excess sodium nitrate will also produce the same color change
    • Magnesium metal is added to reduce Sb3+ to metal and stannic to stannous ions
    • Magnesium ribbon must be dissolved before making the confirmatory test for tin

    Confirmatory Results

    • As3+ produces a reddish-brown precipitate of Ag3AsO4 with 2.5F NaAc
    • Sb3+ produces a lavender shade with C28H31ClN2O3
    • Sn2+ produces a white/gray precipitate with saturated HgCl2

    Group III Cations

    • Group III cations are precipitated in a solution containing ammonia, ammonium chloride, and ammonium sulfide
    • The cations precipitate as a mixture of sulfides and hydroxides
    • 7 cations are present: Fe3+, Co2+, Ni2+, Zn2+, Mn2+, Al3+, and Cr3+
    • Theoretical Discussion:
      • If the hydronium-ion concentration of the centrifugate from Group II is decreased, the sulfide ion concentration will increase
      • Five sulfides are permitted: Zn, Fe, Co, Ni, and Mn
      • Zinc is identified as white sulfide
      • The high concentration of sulfide ion is obtained in an ammoniacal solution (basic)

    Precipitation of Group III Cations

    • The hydroxide-ion concentration must be sufficiently large to permit Al(OH)3 and Cr(OH)3 but not large enough to permit Mg(OH)2
    • Group III is divided into two subgroups: IIIA (Al3+, Cr3+, and Zn2+) and IIIB (Fe3+, Co2+, Ni2+, and Mn2+)

    Notes on the Precipitation of Group III Cations

    • The reaction of Group III with H2S may produce hydronium-ion concentration sufficient to prevent complete precipitation of sulfides
    • Complete precipitation is assured by keeping the solution alkaline
    • To prevent precipitation of Mg(OH)2:
      • Ammonia neutralizes hydronium ions
      • Ammonium ions from NH4CL repress the hydroxide-ion concentration
    • Gelatinous reddish-brown ppt may indicate Fe3+ while uncolored gelatinous ppt indicates Al3+

    Identification of Group IIIA

    • Al3+ produces a red ppt with 3F NH3
    • Cr3+ produces a blue color that fades rapidly with H2O2
    • Zn2+ produces a white ppt with CH3CSNH2

    Identification of Group IIIB

    • Mn2+ produces a reddish-purple in the supernatant liquid with NaBiO3
    • Fe3+ produces a deep red with 3F NH4CNS
    • Co2+ produces a blue-green color at the junction with alcoholic NH4CNS
    • Ni2+ produces a red ppt with C4H8O2N2

    Group IV Cations

    • Group IV is composed of the ions of the alkaline earth metals and magnesium
    • The only anion which will produce satisfactory precipitation of all four cations in aqueous solution is the phosphate
    • The alkaline earth ions are precipitated as tertiary phosphates and the magnesium ion as magnesium ammonium phosphate### Group I Cations Precipitation
    • Group I cations are precipitated with dilute HCl as insoluble chlorides
    • The resulting precipitate is washed with cold water
    • PbCl2 is removed by hot water
    • A white precipitate may be lead oxychloride or silver chloride, both undissolved by ammonia
    • If Hg22+ is present, the residue from ammonia treatment must be black or greatly discolored due to the precipitation of colloidal mercury

    Group II Cations Precipitation

    • Group II cations are composed of eight cations that precipitate as sulfides from a solution that is 0.3F with respect to hydronium ions
    • The sulfides of cupric, mercuric, lead, bismuth, cadmium, arsenic, antimony, and tin ions are precipitated
    • The sulfides are separated into two groups: those that will precipitate in 0.3F HCl solution and those that are soluble in 0.3F HCl
    • The group is subdivided into two subgroups (IIA and IIB) by the action of KOH solution
      • Group IIA sulfides are insoluble in KOH
      • Group IIB sulfides dissolve in KOH

    Precipitation of Group II

    • Precipitation of Group II is accomplished in two steps:
      1. Hydrogen sulfide is bubbled into hot, concentrated solution of hydronium ions to precipitate arsenious sulfide
      2. Sulfides of mercury and copper may precipitate to the concentration of hydronium ions but to precipitate the other sulfides of the group, it is necessary to dilute the solution
    • The color of the precipitate may indicate the cations present
      • Black precipitate: Hg2+, Bi3+, Cu2+, Pb2+
      • Yellow precipitate: As3+, Cd2+, Sn2+
      • Orange precipitate: Sb3+
      • White precipitate that turns black: Hg

    Separation of Group II into Subgroups

    • Notes on the Analysis of Group IIA:
      • CuS, Bi2S3, CdS, PbS are dissolved due to the oxidation of the sulfide ion by the nitrate ion to free sulfur
      • Treatment of HgS with 3F HNO3 may result in the formation of white Hg(NO3)2·2HgS
      • Aqua regia dissolves HgS by oxidation of sulfide ion to free sulfur and complexing action of chloride ion to produce HgCl4-
    • Notes on the Analysis of Group IIB:
      • Make the solution acid with HAc and add a drop of K4Fe(CN)6 if Cu2+ is small in concentration
      • The solution is heated to expel H2S, and the residue will be washed to remove chloride ion

    Analysis of Group IV Cations

    • The flame tests are sometimes difficult to obtain, especially from the insoluble precipitates of BaCrO4 and SrSO4
    • The presence or absence of alkaline earth metals should be judged not only from flame tests but also from the amounts of precipitates of the individual separations of Group IV
    • Notes on the Precipitation of Group IV:
      • The purpose of making the solution strongly basic is to remove the highly colored chromate or dichromate ions
      • SrSO4 and CaC2O4 are precipitated as white precipitate which are not discerned in a colored solution
      • The separation of strontium ions from calcium ions by SrSO4 is not complete, and CaSO4 may coprecipitate with SrSO4

    Identification

    • Confirmatory Results:
      • Hg2+ - white/gray ppt; SnCl2
      • Pb2+ - yellow ppt; 1F K2CrO4
      • Bi3+ - blackening of ppt; 1F K2SnO2
      • Cu2+ - deep blue solution; conc.NH3
    • Group Precipitation:
      • Ba2+ - yellow green flame; conc.HCl
      • Sr2+ - crimson red flame; conc.HCl
      • Ca2+ - brick red flame; conc.HCl
      • Mg2+ - blue ppt; 3F KOH

    Group V Cations

    • The centrifugate form Group IV contains the cations which are not precipitated by the chloride, sulfide, and phosphate ions

    • This group is called the soluble group (Na+, K+, NH4+)

    • Theoretical Discussion:

      • The ammonium ion will always occur in Group V
      • If a strong base such as KOH and NaOH is added to a solution containing ammonium ions and the solution is warmed, the escape of ammonia gas results

    • Confirmatory Results:

      • Na+ - intense yellow flame persisting for 5 seconds; conc.HCl
      • K+ - reddish violet flame; conc.HCl### Introduction to Cation Analysis

    • Qualitative analysis involves identifying constituents of materials using systematic methods.

    • Scale of operations:

    • Macro analysis: 5-100 mL

    • Semimicro analysis: 1 drop to 1 mL (less than one-tenth of macro analysis amounts)

    Apparatus

    • Micropipettes: at least 12 cm long, 1.5 mm inside diameter
    • Four glass stirring rods: 125 x 3 mm, fire-polished
    • H2S generator: 200 x 25 mm test tube with rubber tubing and delivery tip
    • Aluminum bath rack: designed to fit a 250mL beaker, for suspending test tubes in boiling water

    Reagents

    • Frequently used solutions: acids, bases, and reagent solutions
    • Solutions less frequently used (e.g., test solutions) are utilized from 250mL bottles with dropper assemblies

    Semimicro Analysis Manipulations

    • Precipitations:
    • Carried out in test tubes (ordinary or tapered)
    • Precipitating agent added dropwise
    • Stirring accomplished using a glass rod
    • Heating a solution:
    • Cannot be heated over direct flame due to bumping
    • Water bath is the most satisfactory method for heating solutions in small test tubes
    • Evaporation:
    • Frequently needed to reduce solution volume
    • Evaporation done by heating solution in a small porcelain casserole over an open flame
    • Centrifuge:
    • Replaces filtration process
    • Used to speed up rate of settling of a precipitate
    • Test tube balanced by another inside the centrifuge
    • Removal of supernatant liquid:
    • Removed by micropipette without disturbing the precipitate
    • Washing of precipitate:
    • Ions of centrifugate may cause interference, so removed by washing with distilled water
    • Washing accomplished by adding distilled water, mixing, and centrifuging
    • Transferring of precipitates:
    • Accomplished using a nickel microspatula (not used on precipitates wet with a solution that reacts with nickel)

    Group I Cations

    • Composed of common cations whose chlorides are relatively insoluble in dilute acids
    • Theoretical discussion:
    • Insoluble chlorides of Group I: PbCl2, Hg2Cl2, AgCl
    • Solubility products:
      • PbCl2: 1 x 10-4
      • Hg2Cl2: 2 x 10-18
      • AgCl: 1.56 x 10-10
    • PbCl2 is one thousand times more soluble than AgCl and one hundred thousand times more soluble than Hg2Cl2
    • Notes on the analysis of Group I:
    • Solution may contain white precipitate (presence of chlorides of Group I or oxychlorides of antimony and bismuth)
    • 1 drop of HCl will dissolve the oxychlorides of antimony and bismuth (SbOCl and BiOCl); Group I cations will remain
    • Slight excess of HCl causes more complete precipitation of the chlorides of Group I due to common-ion effect
    • Wash water should contain HCl to reduce the solubility of lead chloride by common-ion effect
    • PbCl2 is soluble in hot water, but precipitates when cooled
    • AgCl is separated from Hg2Cl2 by its solubility in ammonia (forms Ag(NH3)2+)

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    Description

    This lab introduces students to qualitative analysis, identifying constituents of materials through systematic methods. It covers the scale of operations, including cation analysis and preparation of solutions.

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