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

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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.