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An in-depth exploration of qualitative analysis, a laboratory technique used to separate and detect cations in a substance. The procedure involves a series of tests to identify and confirm the presence of specific ions, including sodium, ammonium, silver, and transition metals like iron, nickel, and chromium. The document also covers common reagents used in qualitative analysis and their functions.
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Pre-lab: Pages 230 – 231
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Qualitative analysis is used to separate and detectcations and anions in a sample substance.
Qualitative analysis is the procedure by which onecan determine the nature, but not the amount ofspecies in a mixture.
Lab Techniques
Types of reactions
Ag
, Hg
2
2+
, Pb
2+
Precipitated in 1 M HCl
Bi
3+
, Cd
2+
, Cu
2+
, Hg
2+
, (Pb
2+
), Sb
3+
and Sb
5+
, Sn
2+
and Sn
4+
Precipitated in 0.1 M H
2
S solution at pH 0.
Al
3+
, (Cd
2+
), Co
2+
, Cr
3+
, Fe
2+
and Fe
3
, Mn
2+
, Ni
2+
Zn
2+
Precipitated in 0.1 M H
2
S solution at pH 9
Ba
2+
, Ca
2+
, Mg
2+
, Na
4
Ba
2+
, Ca
2+
, and Mg
2+
are precipitated in 0.2 M
4
2
3
solution at pH 10; the other ions are
soluble
Increases [H
] Decreases [OH
]
Dissolves insoluble carbonates,chromates, and hydroxidesDissolves insoluble sulfides byoxidizing sulfide ionDestroys hydroxo andammonia complexesGood oxidizing agent.
3
Increases [H
] Increases [Cl
]
Decreases [OH
]
Dissolves insoluble carbonates,chromates, hydroxides, somesulfatesDestroys hydroxo and NH
3
complexesPrecipitates insoluble chlorides
HCl
Effect
Reagent
Increases [NH
3
] Increases
[OH
] Decreases [H
]
Precipitates insolublehydroxidesForms NH
3
complexes
Forms a basic buffer withNH
4
NH
3
Increases [OH
] Decreases
[H
]
Forms hydroxo complexesPrecipitates insolublehydroxides
NaOH
Flame Tests
Heat the solution to a moist residue, use theevaporating dish and a bunsen flame.
Solutions of ions, when mixed with concentratedHCl and heated on a nickel/chromium wire in aflame, cause the flame to change to a colorcharacteristic of the atom.
A flame test can be used as a confirmatory test.
Sodium is often an impurity so will almost alwayssee a yellow flame. Therefore careful observationsneed to be recorded.
A brilliant yellow persistent flame indicates thepresence of sodium.