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Qualitative Analysis of Cations: Identifying and Separating Ions, Slides of Statistics

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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2021/2022

Uploaded on 01/21/2022

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Qualitative Analysis
Pre-lab: Pages 230 – 231
No Post-Lab Questions
Pre-lab and Lab report 50%
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Qualitative Analysis

Pre-lab: Pages 230 – 231

No Post-Lab Questions

Pre-lab and Lab report 50%

Introduction to Qualitative Analysis

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.

Note

  • Part II: Separation of anions will not be

performed.

  • Read pages 241 – 242, experimental

techniques.

Lab Techniques

  • Ensure all glassware is clean, but it does not

need to be dry.

  • Use distilled water at all times • Label test tubes to avoid confusion • Mix solutions by flicking the test tube • Estimate volumes by assuming 20 drops by

pipette

1ml

Experimental Procedure

  • You will be provided with 2 samples. • One will contain all of the cations to be

detected. This is your reference solution

  • The other test-tube will contain a number of

unknown cations that you will need toidentify by experimental observations.

  • Make careful notes of your observations. • Look closely for gases, and note colors of

solutions and precipitates.

  • Recap on your solubility rules

Types of reactions

  • Precipitation • Complex ion formation • Redox reactions • Acid-base reactions

Some means of identifying ions by

qualitative analysis are:

  • Color changes or color of the solution. • Evolution of gas • Change in pH (acidity or basicity) • Or ability to redissolve a precipitate by

addition of a complexing ligand.

Common ways to test for ions

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.

Common grouping of cations

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+

, K

, Mg

2+

, Na

, NH

4

Ba

2+

, Ca

2+

, and Mg

2+

are precipitated in 0.2 M

(NH

4

2

CO

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.

HNO

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.