Contents pmd

Analytical Chemistry

Dhruba Charan Dash
Formerly, Professor and Head
Postgraduate Department of Chemistry
Sambalpur University
Orissa

New Delhi-110001
2011

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ANALYTICAL CHEMISTRY
Dhruba Charan Dash

© 2011 by PHI Learning Private Limited, New Delhi. All rights reserved. No part of this book
may be reproduced in any form, by mimeograph or any other means, without permission in
writing from the publisher.
ISBN-978-81-203-4077-0
The export rights of this book are vested solely with the publisher.
Published by Asoke K. Ghosh, PHI Learning Private Limited, M-97, Connaught Circus,
New Delhi-110001 and Printed by Mohan Makhijani at Rekha Printers Private Limited,
New Delhi-110020.

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To My Parents

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Contents
Preface

xvii

UNIT 1
1.

Qualitative Analysis
1.1
1.2
1.3

1.4

3–53

Introduction
3
1.1.1
Solubility Product Principle
3
1.1.2
Common Ion Effect
4
Separation of Cations into Groups

4
Detection and Separation of Cations of Each Group
10
1.3.1
Separation and Detection of Group I (Silver Group) Cations
10
1.3.2
Separation of Group IIA from Group IIB Cations
(by Yellow Ammonium Sulphide)
11
1.3.3
Separation and Detection of Group IIA Cations (Copper Group)
11
1.3.4
Separation and Detection of Group IIB Cations (Arsenic Group)
14
1.3.5
Separation and Detection of Group IIIA Cations (Iron Group)
16
1.3.6
Separation and Detection of Group IIIB Cations (Zinc Group)
18
1.3.7
Separation and Detection of Group IV Cations
20
1.3.8
Separation and Detection of Group V Cations
21
Separation and Detection of Acid Radicals (Anions)
23

1.4.1
Detection of Group I Anions
23
1.4.2
Detection of Group II Anions
27
1.4.3
Group III Anions (Precipitation Group)
38

Group A
A.
B.
C.

Questions on Qualitative Analysis of Basic Radicals (Cations)
Objective Type Questions
44
Short Answer Type Questions
46
Long Answer Type Questions
48

Group B Questions on Qualitative Analysis of Acid Radicals (Anions)
D. Multiple Choice Questions
49
E. Short Answer Type Questions
50
F. Long Answer Type Questions
53

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49


vi

Contents

UNIT 2
2.

Quantitative Analysis—Volumetric (Titrimetric) Analysis

57–100

2.1
2.2

Introduction
57
Volumetric (Titrimetric) Calculation
59
2.2.1
Calculation Based on Normality (N) of the Solution
60

2.2.2
Calculation Based on Molarity (M) of the Solution
60
2.3 Conditions for Volumetric (Titrimetric) Analysis
61
2.4 Types of Titrimetric Analysis
62
2.5 Acid-base Titration and Ways of Locating End Point
62
2.5.1
Theory of Acid-base Titration
62
2.5.2
Ways of Locating the End Point of an Acid-base Titration
63
2.5.3
Titration of Strong Acid with Strong Base
65
2.5.4
Titration of Weak Acid with Strong Base
65
2.5.5
Titration of Weak Base with Strong Acid
67
2.5.6
Titration of Weak Acid with Weak Base
69
2.5.7
Factors Determining the Exact Form of a pH Curve
70

2.6 Oxidation Reduction (Redox) Titration and Ways of Locating End Point
71
2.6.1
Theory of Redox Titration
71
2.6.2
Study of Redox Titration by Electrochemical Potential Method
72
2.6.3
Ways of Locating the End Point for Redox Titration
73
2.7 Complexometric Titration and Ways of Locating End Point
79
2.7.1
Theory of Complexometric Titration Involving EDTA
79
2.7.2
Study of EDTA Complex Formation Taking Disodium Salt of EDTA and Effect
of pH
83
2.7.3
Ways of Locating the End Point
84
2.7.4
Estimation of Calcium and Magnesium by Complexometric
Titration by EDTA
85
2.8 Problems Involved in Titrimetric Methods
86
2.8.1

Problems on Acid-base Titration
86
2.8.2
Problems on Redox Titration
93
A. Objective Type Questions
97
B. Very Short Answer Type Questions
98
C. Short Answer Type Questions
99
D. Long Answer Type Questions
100

3.

Quantitative Analysis—Precipitation Gravimetry
3.1

3.2

Introduction
101
3.1.1
Precipitation Gravimetry
101
3.1.2
Gravimetric Calculation and Gravimetric Factor
3.1.3
Requirements for Successful Gravimetry

102
3.1.4
Steps Involved in Gravimetric Analysis
102
Precipitation
103
3.2.1
Definition of Precipitation
103
3.2.2
Conditions of Precipitation
103

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Contents

3.2.3
Theories of Precipitation
103
3.2.4
Homogeneous Precipitation
105
3.2.5
Contamination of the Precipitate
107

3.2.6
Errors in Precipitation
111
3.3 Digestion (Aging)
111
3.3.1
Reasons for Digestion
111
3.4 Filtration
111
3.5 Washing of the Precipitate
113
3.5.1
Ideal Qualities of a Washing Liquid
113
3.5.2
Types of Wash Solution
113
3.5.3
Mode of Washing
114
3.6 Drying And/Or Incineration of the Precipitate
114
3.6.1
Conditions of Drying
114
3.6.2
Purpose of Ignition
115
3.6.3

Ignition Temperature
115
3.7 Weighing
115
3.8 Specific and Selective Precipitation
116
3.9 Organic Precipitants
116
3.9.1
Types of Organic Precipitants
116
3.9.2
Advantages of Using Organic Precipitants
119
3.9.3
Disadvantages of Using Organic Precipitants
120
3.10 Sequestering (or Masking) Agent
120
3.11 Problems Involved in Precipitation Gravimetry
121
3.11.1 Problems on Gravimetric Factor (GF)
121
3.11.2 Problems on Determination of Elements and Percentage of Purity
3.11.3 Determination of Sample Size
126
3.11.4 Analysis of Alloy
130
A. Objective Type Questions
131

B. Very Short Answer Type Questions
133
C. Short Answer Type Questions
133
D. Long Answer Type Questions
135

vii

123

UNIT 3
4.

Statistical Methods of Analysis
4.1
4.2
4.3

Introduction
139
Significant Figures
139
4.2.1
Definition of Significant Figure
139
4.2.2
Rules for Determining Significant Figures
Errors and Their Causes
142

4.3.1
Definition of Errors
142
4.3.2
Classification of Errors
142
4.3.3
Determinate Errors
142
4.3.4
Causes of Determinate Errors
143
4.3.5
Indeterminate Errors
145

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4.4

Propagation of Errors

146
4.4.1
Uncertainty Involving Addition and Subtraction
146
4.4.2
Uncertainty Involved in Multiplication and Division
146
4.5 Accuracy and Precision
147
4.5.1
Accuracy
147
4.5.2
Methods of Expressing Accuracy
148
4.5.3
Precision
149
4.5.4
Comparison between Accuracy and Precision
149
4.5.5
Methods of Expressing Precision
150
4.6 Test of Significance
155
4.6.1
Comparing a Mean Value with a True Value (The Student’s t Test)
4.6.2
Comparing Two Experimental Means

157
4.6.3
Comparison of Two Standard Deviations (F Test)
158
4.6.4
Chi-square Test (l2 Test)
159
4.7 Rejection of a Result
159
4.7.1
Rule Based on Average Deviation
159
4.7.2
Rule Based on the Range (Q Test)
160
4.8 Problems Involved in Data Analysis
161
4.8.1
Problems on Significant Figures
161
4.8.2
Problems on Rounding off Number
162
4.8.3
Problems on Uncertainties
163
4.8.4
Problems on Errors and Uncertainty
164
4.8.5

Problem on Relative Error
165
4.8.6
Problems on Expressing Precision
166
4.8.7
Problems on Propagation of Errors
169
4.8.8
Problem on Confidence Level
171
4.8.9
Problem on Rejection of Data
171
4.8.10 Problem on Student t Test
171
A. Objective Type Questions
172
B. Short Answer Type Questions
174
C. Long Answer Type Questions
175

156

UNIT 4
5.

Estimation of Organic Compounds
5.1

5.2
5.3

Introduction
179
Detection of Elements (Principles Only)
179
5.2.1
Detection of Carbon and Hydrogen
179
5.2.2
The Preparation of Sodium Extract (Lassaigne’s Test)
180
Estimation of Elements
181
5.3.1
Estimation of Carbon and Hydrogen (Liebig’s Combustion Method)
Principle
181
5.3.2
Estimation of Nitrogen
185
5.3.3
Estimation of Sulphur (By Carius Method)
190
5.3.4
Estimation of Halogens (By Carius Method)
190
5.3.5
Estimation of Phosphorus (By Carius Method)

191

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Contents

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5.4
5.5
5.6
5.7
5.8

Estimation of Glucose
192
Estimation of Phenol
193
Estimation of Aniline
195
Estimation of Keto Group
196
Analysis of Oils and Fats
197
5.8.1
Determination of Iodine Value
197

5.8.2
Determination of Saponification Values
199
5.8.3
Determination of Reichert–Meissel Value (RM Value)
200
5.9 Problems Involved in Estimation of Organic Compounds
201
5.9.1
Problems on Estimation of Carbon and Hydrogen
201
5.9.2
Problems on Estimation of Nitrogen
202
5.9.3
Problems on Estimation of Halogens and Sulphur
203
5.9.4
Problems on Estimation of Sugar
204
5.9.5
Problems on Saponification Value and RM Value
204
5.9.6
Problems on Estimation of Phenol and Aniline
205
A. Objective Type Questions
207
B. Very Short Answer Type Questions
208

C. Short Answer Type Questions
209
D. Long Answer Type Questions
210

UNIT 5
6.

Separation Techniques
6.1
6.2

6.3
6.4

6.5
6.6

213–262

Introduction
213
Solvent Extraction Method
213
6.2.1
Introduction
213
6.2.2
Principle of Solvent Extraction
214

6.2.3
Comparison between Single and Multiple Extraction
217
6.2.4
Separation Factor
220
6.2.5
Methods of Solvent Extraction
221
Application of Solvent Extraction
223
6.3.1
Solvent Extraction of Metal Ions by Chelation
223
6.3.2
Conclusions on Extraction of Metal Chelates
224
Chromatographic Methods And Their Classification
225
6.4.1
Introduction
225
6.4.2
Definition of Chromatography
225
6.4.3
Classification of Chromatographic Methods
226
General Theory and Principle of Column or Adsorption Chromatography
Ion-Exchange Chromatography

234
6.6.1
Principle
234
6.6.2
Cation Exchange Resin
235
6.6.3
Anion Exchange Resin
236
6.6.4
Mechanism of Ion Exchange
236
6.6.5
Ion Exchange Capacity
237
6.6.6
Factors Affecting Ion Exchange Equilibria
237

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Contents

6.6.7

Experimental Set-up
238
6.6.8
Packing of Column
239
6.6.9
Applications of Ion Exchange Chromatography
6.7 Paper Chromatography
240
6.7.1
Principle
240
6.7.2
Theory of Paper Chromatography
240
6.7.3
Technique of Paper Chromatography
242
6.7.4
Applications of Paper Chromatography
246
6.8 Thin Layer Chromatography (TLC)
247
6.8.1
Principles
247
6.8.2
Choice of Adsorbent for TLC
247
6.8.3

Choice of Solvent
247
6.8.4
Experimental Techniques
248
6.8.5
Sample Application
248
6.9 Development of the Chromatogram
248
6.10 Gas Chromatography
251
6.10.1 Introduction
251
6.10.2 Principle of Gas Chromatography
251
6.10.3 Applications of Gas Chromatography
255
A. Objective Type Questions
256
B. Short Answer Type Questions
260
C. Long Answer Type Questions
261

7.

239

Purification Techniques

263–284

7.1
7.2

Introduction
263
Purification Techniques for Solid Organic Compounds
263
7.2.1
Crystallization
264
7.2.2
Fractional Crystallization
267
7.2.3
Sublimation
267
7.2.4
Sublimation under Reduced Pressure
268
7.2.5
Solvent Extraction
269
7.3 Purification Techniques for Liquids
270
7.3.1
Simple Distillation
270

7.3.2
Fractional Distillation
271
7.3.3
Distillation under Reduced Pressure
273
7.3.4
Steam Distillation
275
7.4 Chemical Method of Separation and Purification
278
7.5 Criteria of Purity
281
A. Objective Type Questions
282
B. Very Short Answer Type Questions
283
C. Short Answer Type Questions
283
D. Long Answer Type Questions
284

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xi

UNIT 6

8.

Electroanalytical Techniques—Electrogravimetry
Introduction
287
Classification of Electroanalytical Techniques
287
Electrical Components
288
8.3.1
Electrodes and Electrode Potential
288
8.3.2
Electrochemical Cell
290
8.3.3
Electrical Circuit
291
8.3.4
Galvanostat and Potentiostat
292
8.4 Electrogravimetry
292
8.4.1
Introduction
292
8.4.2
Theory and Principle of Electrogravimetry
292
8.4.3

Types of Electrogravimetry
293
8.5 Electrolysis in a Simple Cell
294
8.6 Electrolysis in a Non-galvanic Cell
294
8.6.1
Concept of Decomposition Potential
295
8.6.2
Ohmic Potential or IR Drop
296
8.6.3
Overpotential (Overvoltage)
297
8.6.4
Causes of Overvoltage
297
8.6.5
Expression for Total Potential Applied to Cause Electrolysis
8.7 Electrolysis in a Galvanic Cell
298
8.8 Electrolysis at Constant Current
299
8.9 Electrolysis at Constant Voltage
302
8.10 Electrolysis at Controlled Potential
302
8.11 Spontaneous or Internal Electrolysis
304

8.11.1 Electrolysis at the Anode
304
8.12 Problems Involved in Electrogravimetry
305
A. Objective Type Questions
312
B. Very Short Answer Type Questions
313
C. Short Answer Type Questions
314
D. Long Answer Type Questions
315

287–315

8.1
8.2
8.3

9.

298

Electroanalytical Techniques—Coulometry
9.1
9.2
9.3
9.4

9.5

9.6

Introduction
316
Coulometric Calculation
316
Determination of Charge, Q
317
Coulometers
318
9.4.1
Silver Coulometer
318
9.4.2
Iodine Coulometer
319
9.4.3
Gas Coulometer (Hydrogen-Oxygen Coulometer)
319
Constant Current Coulometry
320
Comparison of Constant Current Coulometry with Conventional Volumetric
Titration
322

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Contents

9.7

Coulometric Titration
322
9.7.1
Primary Coulometric Titration
322
9.7.2
Secondary Coulometric Titration
323
9.7.3
End Points in Coulometric Titration
324
9.8 Applications of Coulometric Titration
324
9.8.1
Neutralization (Acid-Base) Titration
324
9.8.2
Precipitation Titration
325
9.8.3
Redox Titration
326
9.8.4
Complexometric Titration

326
9.9 Controlled Potential Coulometry
327
9.10 Applications of Controlled Potential Coulometry
329
9.11 Problems Involved in Coulometry
330
A. Objective Type Questions
335
B. Very Short Answer Type Questions
337
C. Short Answer Type Questions
337
D. Long Answer Type Questions
338

10. Electroanalytical Techniques—Polarography
10.1 Introduction
339
10.2 Principle of Polarography
339
10.2.1 Factors Affecting Current Flow in a Polarographic Cell
339
10.2.2 Explanation for Residual Current
340
10.2.3 Elimination of Convection Current
340
10.2.4 Elimination or Suppression of Migration Current
340
10.2.5 Measurement of Diffusion Current

341
10.2.6 Ilkovic Equation
342
10.2.7 Derivation of Ilkovic Equation
343
10.2.8 The Concept of Half-wave Potential (E1/2)
344
10.3 Difficulties Encountered in Polarography
346
10.4 Experimental Set-up
348
10.5 Advantages and Disadvantages of DME
349
10.6 Applications of Polarography
350
10.6.1 Qualitative Evaluation of Polarographic Data
350
10.6.2 Quantitative Evaluation of Polarographic Data
350
10.6.3 Determination of Formation Constants of Complexes
353
10.6.4 Analysis of Mixture of Ions
354
10.6.5 Determination of Dissolved Oxygen
355
10.7 Problems Involved in Polarography
355
A. Objective Type Questions
359
B. Very Short Answer Type Questions

360
C. Short Answer Type Questions
361
D. Long Answer Type Questions
362

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xiii

UNIT 7
11. Spectroanalytical Techniques—Ultraviolet and Visible
Spectral Method

365–414

11.1 Introduction
365
11.2 Principle of UV-visible Spectroscopy
366
11.2.1 Origin of UV-visible Spectroscopy
366
11.2.2 Absorption Law
367
11.2.3 Nature of Electronic Spectrum

370
11.2.4 Selection Rules for Absorption
371
11.3 Techniques Involved in UV-visible Spectroscopy
373
11.3.1 Description of UV-visible Spectrophotometer
373
11.3.2 Types of UV-visible Spectrophotometer
374
11.3.3 Working Principle
375
11.3.4 Choice of Solvent
375
11.4 Types of Eletronic Transition
376
11.4.1 Transitions Involving s, p and n (Non-bonding Electrons)
376
11.4.2 Absorbing Species Involving d or f Electrons
380
11.4.3 Charge Transfer Spectral Absorption
380
11.5 Type of Absorptions Bands
381
11.5.1 K-Bands
381
11.5.2 R-Bands
381
11.5.3 B-Bands (Benzeneoid Bands)
381
11.5.4 E-Bands (Ethylenic Bands)

382
11.6 The Concept of Chromophore and Auxochrome
383
11.6.1 Chromophore
383
11.6.2 Auxochrome
384
11.7 Shifting of Absorption Band and Change in Intensity
385
11.7.1 Terminology Used in UV-visible Spectroscopy
385
11.7.2 Effect of Conjugation of Chromophore
386
11.7.3 Additive Characteristics
386
11.7.4 Effect of Aromatic Rings
387
11.7.5 Effect of Substitution of Auxochrome
387
11.7.6 Effect of Solvent Polarity
388
11.7.7 Stereo Chemical Factors
389
11.8 Application of UV-visible Spectral Method
390
11.8.1 Structural Analysis
390
391
11.8.2 Empirical Rules for Calculation of Absorption Maxima (lMax)
11.8.3 Additivity of Absorbance

397
11.8.4 Multiple Analysis
397
11.8.5 Determination of the pK Value of Indicator
398
11.8.6 Composition of the Coloured Complex
400
11.8.7 Quantitative Analysis
404
11.8.8 Detection of Impurities
404
11.8.9 In Tautomeric Equilibria
405

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11.9 Some More Problems Involved in UV-visible Spectral Method
A. Objective Type Questions
408
B. Very Short Answer Type Questions
410
C. Short Answer Type Questions
411

405

12. Spectroanalytical Techniques—Infrared (IR) Spectral Method

415–456

12.1 Introduction
415
12.2 Molecular Vibrations and Vibrational Frequency
415
12.2.1 Vibration of Diatomic Molecules
415
12.2.2 The Vibration of Polyatomic Molecules
418
12.2.3 Types of Molecular Vibrations
418
12.3 Selection Rule for IR Absorption
422
12.4 Breakdown of Selection Rule and Occurrence of Overtones, Combination
Bands and Difference Bands
423
12.5 Symmetries of Vibration and Their IR Activity
424
12.6 Instrumentation
428
12.7 Concept of Group Vibrational Frequencies
429
12.7.1 Factors Influencing Group Vibrational Frequencies
430
12.8 Important Spectral Regions in the Infrared and Presentation IR Spectra
433

12.9 IR Characteristics of Some Organic Compounds
435
2.10 IR Characteristics of Some Inorganic Compounds
(Especially Metal Complexes)
448
A. Objective Type Questions
451
B. Very Short Answer Type Questions
453
C. Short Answer Type Questions
455
D. Long Answer Type Questions
456

13. Spectroanalytical Techniques—Nuclear Magnetic Resonance
Spectral Method

457–496

13.1 Introduction
457
13.2 Principle of NMR
457
13.2.1 Magnetic Properties of Nuclei and Their Angular Momentum
457
13.2.2 Magnetic Moments of the Nuclei
458
13.2.3 Effect of External Magnetic Field
459
13.2.4 Potential Energy of a Nucleus in a Magnetic Field

459
13.2.5 Potential Energy of a Proton in a Magnetic Field
460
13.2.6 Classical Description of NMR
462
13.2.7 Intensity of NMR Signals
463
13.3 Technique Involved in NMR Spectroscopy
464
13.4 Prediction of Number of NMR Signals
466
13.5 Position of the Signals and Chemical Shift
468
13.6 Factors Influencing Chemical Shifts
471
13.6.1 Effect of p  Electrons Circulation (Magnetic Anisotropy)
471
13.6.2 Inductive Effect
473
13.6.3 Effect of Electron Withdrawing and Electron Donating Groups
474
13.6.4 Hydrogen Bonding
475

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13.7 Spin-Spin Coupling (or Splitting)

477
13.7.1 Explanation for Spin-Spin Interactions
477
13.8 Multiplicity of NMR Peaks
481
13.9 Problems Involving Chemical Shift and Spin-Spin Splitting
13.10 Coupling Constant (J)
489
A. Objective Type Questions
491
B. Very Short Answer Type Questions
493
C. Short Answer Type Questions
494
D. Long Answer Type Questions
495

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482

14. Spectroanalytical Techniques—Electron Spin Resonance
Spectral Method

497–524

14.1 Introduction
497
14.2 Basic Principle
497

14.2.1 Interaction between Electron Spin and Magnetic Field
497
14.2.2 Potential Energy of Electron When Placed in a Magnetic Field
498
14.2.3 Resonance Condition
499
14.3 Relaxation Process and Line Width in ESR Transition
500
14.4 Techniques of ESR Spectroscopy
502
14.4.1 Instrumentation
502
14.4.2 Sample Concentration and Choice of Solvent
503
14.4.3 Presentation of ESR Spectra
503
14.4.4 Interpretation of Derivative Curve
504
14.4.5 Use of Standards
504
14.5 Hyperfine Splitting
504
14.6 Zero Field Splitting and Kramer’s Degeneracy
511
14.7 Application of ESR
514
14.7.1 Determination of g  Value
514
14.7.2 Shape and Type of Hybridization
515

14.7.3 Study of Free Radicals
515
14.7.4 Study of Internal Motion (Rotation)
516
14.7.5 ESR and Steric Hinderance
516
14.7.6 Analysis of Electron Transfer Reactions through ESR
516
14.7.7 ESR in Polymer Chemistry
517
14.7.8 Spin Labelling of Biomolecules
517
14.7.9 ESR Studies of Inorganic Compounds, Mainly Complexes
517
A. Objective Type Questions
521
B. Very Short Answer Type Questions
522
C. Short Answer Type Questions
523
D. Long Answer Type Questions
523

15. Spectroanalytical Techniques—Mass Spectral Method
15.1 Introduction
525
15.2 Theory (Basic Principle)

525

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Contents

15.3 Instrumentation
527
15.3.1 Sample Introducing System
528
15.3.2 Ion Source and Accelerating Chamber
528
15.3.3 Mass Analyzer and Magnet
528
15.3.4 Ion Collector/Detector and Amplifier
530
15.3.5 Recorder
530
15.4 Interpretation of Mass Spectra
531
15.5 Type of Ions Produced in a Mass Spectrometer
532
15.5.1 Molecular Ion or Parent Ion
532
15.5.2 Isotope Ions
534
15.5.3 Metastable Ions or Peaks

534
15.5.4 Fragmented Ion and Fragmentation Modes
537
15.6 Mass Spectra of Some Organic Compounds
543
15.6.1 Straight Chain Alkanes
543
15.6.2 Branched Chain Alkanes
544
15.6.3 Alkens (Olefins)
545
15.6.4 Cycloalkanes
547
15.6.5 Cyclo Olefins
548
15.6.6 Alkynes (Acetylenes)
548
15.6.7 Aromatic Compounds
548
15.6.8 Alkyl Halides
551
15.6.9 Alcohols, Ethers and Amines
551
15.6.10 Fragmentation Mode of Aromatic Alcohols, Phenols, Aromatic
Amines, Aryl Ethers
557
15.6.11 Aliphatic Aldehydes and Ketones
560
15.6.12 Aromatic Aldehydes and Ketones
563

15.6.13 Carboxylic Acids, Esters and Amides
563
15.6.14 Aromatic Acids
568
15.6.15 Fragmentation Modes of Nitro Compounds
568
15.6.16 Fragmentation Modes of Aliphatic Nitriles R — CH2 — C ºº N
15.6.17 Fragmentation Modes of Aliphatic Nitrites
570
15.6.18 Fragmentation Modes of Aliphatic Nitrates
570
A. Objective Type Questions
570
B. Very Short Answer Type Questions
572
C. Short Answer Type Questions
572
D. Long Answer Type Questions
573

Index

569

575–581

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Preface

This book is written exclusively for +3 B.Sc. (Pass and Hons) students of chemistry according to
the recently restructured syllabus prescribed by various Indian universities.
The general objective of this book is to provide a broad understanding of the principles,
applications and limitations of the various techniques involved in analytical chemistry in a
systematic and lucid manner, so that even an average student can grasp the intricacies of the
subject. It includes qualitative and quantitative analysis, data analysis, elemental analysis for
organic compounds, separation and purification techniques, electroanalytical techniques such as
electrogravimetry, coulometry, polarography, spectroanalytical techniques such as ultraviolet and
visible spectral method, infrared spectral method, nuclear magnetic resonance spectral method,
electron spin resonance spectral method and mass spectral method. Each chapter provides a brief
but sufficient overview of the definitions, theoretical principles and instrumentation involved.
These are further elucidated by suitable examples and numerical problems. Different types of
objective type questions (multiple type, true and false type, and fill in the blanks type), short
questions, hints and sets of problems with answers are provided in each chapter, so that a student
can easily judge his/her understanding of the subject.
The book will stimulate the students to face the academic and research challenges in analytical
chemistry for the new millennium. Contributions by several authors referred to in the present
book is gratefully acknowledged.

Dhruba Charan Dash

xvii

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1. Qualitative Analysis

UNIT 1

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CHAPTER

1

Qualitative Analysis
1.1 INTRODUCTION
The word analysis means those chemical reactions by which substances can be identified in the
presence of one another. It is of two types—qualitative analysis and quantitative analysis. Qualitative
analysis deals with the detection of constituents of a substance or a mixture of substances or their
solutions whereas quantitative analysis deals with the estimation of constituents of a substance.
Depending upon the quantity of the sample used to start the analysis, the following methods are
used.
Method of analysis

Quantity of the sample

Macro analysis
Semimicro analysis
Micro analysis
Ultramicro analysis

100 mg–1 gm
10 mg–100 mg
1 mg–10 mg

Less than 1 mg

Generally qualitative analysis involves analysis of metallic parts in the form of cations
(or basic radicals) and non-metallic parts in the form of anions (acid radicals). The common metallic
cations are divided into five groups—group I, group II, group III, group IV and group V based on
solubility product and common ion effect as discussed below.

1.1.1

Solubility Product Principle

If a sparingly soluble salt (like AgCl, BaSO4 etc.) is put in water, a very little amount of it dissolves
in water and thus the solution becomes saturated. But whatever might be the amount dissolved in
water, it gets completely ionized. Then equilibrium is established between the undissolved salt and
the ions in solution.
ZZX
A + + B–
AB
YZZ
Undissolved salt

Ions in solution

Applying the law of mass action, we get

or

[A  ][B ]
k=
[AB]

+
k[AB] = [A ] [B–] where k is equilibrium constant
3

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4

Analytical Chemistry

But since only a little of the salt AB goes into solution, the concentration of undissolved salt nearly
remains constant. Hence,
K ´ constant = [A+] [B–]
Ksp = [A+] [B–]

or

where Ksp is another constant and is known as solubility product of the salt AB. For general study,
let us discuss the sparingly soluble compound AxBy

ZZX xAy+ + yBx–
AxBy YZZ

Hence the solubility product of such salt, AxBy is given by
Ksp = [Ay+]x ´ [Bx–]y
where x and y represent the number of ions in the formula of the compound.
From the above expression of the solubility product, it is obvious that
(i) When the ionic product is equal to the solubility product, the solution is saturated.
(ii) When the ionic product is less than the solubility product, the solution is unsaturated and

more of salt can be dissolved in it.
(iii) When the ionic product exceeds the solubility product, the solution is supersaturated. To
keep the ionic product equal to the solubility product, the excess of the ions will recombine
to form solid and thus precipitation takes place. In other words, precipitation occurs
when the product of the ionic concentration of the salt exceeds its solubility product.

1.1.2

Common Ion Effect

It states that if, to a solution of a weak electrolyte (AB), a solution of a strong electrolyte (AC) is
added, this furnishes an ion common to that furnished by the weak electrolyte (here A +), then the
ionization of the weak electrolyte is suppressed.

ZZX A+ + B–
AB YZZ
K=

[A  ] ¹ [B ]
[AB]

K is dissociation constant of AB

AC being a strong electrolyte gives a large amount of A+ ions; as a result the concentration of A+
ions increases. In order to keep K constant, either the concentration of B– ion should decrease or
the concentration of AB should increase. In other words, this ionization of AB is suppressed due to
a common ion effect.
The separation of cation into various groups and their corresponding group reagents based on
the above principles are discussed below.

1.2

SEPARATION OF CATIONS INTO GROUPS

The common metallic cations are divided into five groups—group I, group II, group III, group IV
and group V. The metallic cations of any group are precipitated by a particular group reagent.

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Qualitative Analysis

5

The identification of cations within each group is based on specific characteristics of each group
as discussed below.

Group I cations
Pb2+, Ag+, Hg22+ ions are included in this group. By addition of dil HCl to the solution containing
these ions, they are precipitated as PbCl2, AgCl, Hg2Cl2 as the solubility products of their salts are
exceeded. Here dil HCl is group reagent for group I cations. The chlorides of the cations other than
Ag+, Pb+2 and Hg22+ ions are not precipitated because their solubility products are very large
compared to their ionic products.

Group II cations
Hg2+, Pb2+, Cu2+, Cd2+, Bi3+, As3+ or 5+, Sb3+ or 5+ and Sn2+ or 4+ ions are included in this group.
These are precipitated as sulphides by passing H2S to the solution containing these ions in the
presence of 0.3 M HCl. This is explained as follows:
H2S is a weak dibasic acid for which

ZZZX
H 2S YZZ
Z H   HS
K1

[H  ] [HS ]
= K1 = 9.1 ´ 10–8
[H 2S]

HS
[H  ] [S2  ]
[HS ]

(1.1)

K
ZZZ
X H   S2
YZZZ
2

= K2 = 1.2 ´ 10–15

(1.2)

where K1 and K2 are respectively the primary and secondary dissociation constants of H2S at
18°C. On multiplying Eqs. (1.1) and (1.2), we get
[H  ]2 [S2  ]
= K1 ´ K2 » 10–22
[H 2S]

[S2–] »

1022 – [H 2S]
[H  ]2

If H2S gas at 1 atm is bubbled through water forming a saturated solution, the concentration of
H2S » 0.1 mol L–1
[S2–] »
»

10 22 – 0.1
[H  ]2

1023
[H  ]2

(1.3)

Equation (1.3) shows if [H+] increases, [S2–] decreases. In other words, dissociation of H2S is
suppressed. Thus to a solution of a weak electrolyte (here H2S), when a solution of a strong
electrolyte like HCl (HCl ® H+ + Cl–) is added, this furnishes an ion identical to that furnished by

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6

Analytical Chemistry

the weak electrolyte (here H+), the dissociation of the weak electrolyte is suppressed due to a
common ion effect. Here [S2–] is inversely proportional to the square of hydrogen ion concentration.
It can be varied by changing [H+] as exemplified below.
If pH = 0, [H+] = 1 mol L–1
[S2–] » 10–22
But if pH = 12, [H+] = 10–12 mol L–1
[S2] »

1023
 10 mol L–1
(1012 ) 2

In practice, if [H+] of the solution is adjusted to 0.3 M (yellow-green colour to methyl violet
indicator) by adding HCl prior to passing H2S, the group II sulphides are precipitated selectively
because this decreased concentration of S2– ions is sufficient to precipitate the cations of group II
having low Ksp values (~10–22 M) which are given below.
Substances

Solubility product

PbS
HgS
CuS
CdS
Bi2S3

5 ´ 10–29
4 ´ 10–54
1 ´ 10–44
1.4 ´ 10–28

1 ´ 10–48

If the concentration of the acid is much higher than 0.3 M, [S2–] is reduced still further so that
CdS is either not precipitated at all or incompletely precipitated. If the concentration of the acid is
much lower (than 0.3 M), the solubility products of MnS, NiS, CoS, ZnS are exceeded, as a result
Mn2+, Ni2+, Co2+, Zn2+ (ions which are not included in group II) are precipitated as sulphides.
The solubility products of these ions are given below.
Substances

Solubility product

ZnS
NiS
CoS
MnS

1 ´ 10–23
1.4 ´ 10–24
5 ´ 10–22
1.4 ´ 10–15

Explanation for inclusion of Pb2+ in group I as well as group II
Pb2+ is precipitated in group I as PbCl2. However, it is partially soluble in dil HCl, as a result a part
of it goes to the filtrate, which is tested for group II cations. Thus on passing H2S gas, it gives a
black precipitate of PbS.
Pb2+ + H2S ¾® PbS + 2H+

Classification of group II cations into group IIA and IIB
Group II cations are further classified into group IIA and group IIB based on the solubility of their
sulphide with yellow ammonium sulphide. It is a form of ammonium sulphide containing more

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Qualitative Analysis

7

sulphur in it and is represented as (NH4)2 Sx where x varies from 2 to 5. The sulphides of Hg2+,
Bi3+, Pb2+, Cu2+ and Cd2+ ions are not soluble in yellow ammonium sulphide and these ions are
included in group IIA whereas the sulphides of As3+ or 5+, Sb3+ or 5+ and Sn2+ or 4+ are soluble in
(NH4)2Sx forming thiosalts as follows. These ions are included in group IIB.
Sb2S3 + 3(NH4)2S2 ¾® 2(NH4)3SbS4 + S
Ammonium thioantimonate

SnS2 + (NH4)2S2 ¾® (NH4)2SnS3 + S
Ammonium thiostannate

As2S3 + (NH4)2S2 ắđ 2(NH4)3AsS4 + S
Ammonium thioarsenate

In case of As2S3, Sb2S3 and SnS2 even ordinary ammonium sulphide may be used as in that case
soluble ammonium thioarsenites are formed.
As2S3 + 3(NH4)2S ắđ 2[NH4]3AsS3
But for SnS, yellow ammonium sulphide is essential as SnS (stannous sulphide) is oxidized first
by the excess of sulphur present in yellow ammonium sulphide into stannic sulphide, which form
soluble thiostannate as given above.
SnS + S ắđ SnS2

Group III cations

This group includes Al3+, Fe3+, Cr3+, Ni2+, Co2+, Mn2+ and Zn2+ cations. The solubility product of
hydroxides of Al3+, Fe3+, Cr3+ are 8.5 ´ 10–23, 3.8 ´ 10–38 and 2.9 ´ 10–29 respectively while
the solubility products of hydroxides of Ni2+, Co2+, Mn2+ and Zn2+ are 8.7 ´ 10–19, 1.6 ´ 10–18,
4.0 ´ 10–14 and 1 ´ 10–17 respectively. All these ions can be precipitated as hydroxides if ammonium
hydroxide is added to the solution containing these ions. NH4OH is a weak base for which the
following equilibrium exists.
ZZX NH 4+ + OH–
NH4OH YZZ
for which

[NH 4 ] – [OH  ]
= Kb = 1.8 ´ 10–5
[NH 4 OH]

Kb is called the base dissociation constant of NH4OH
[NH 4 ] – [OH  ] = 1.8 ´ 10–5 ´ [NH4OH]
But
For 1 M NH4OH
For 0.1 M NH4OH

[NH 4 ] = [OH–]
[OH–]2 = 1.8 ´ 10–5 ´ [NH4OH]
[OH–]2 = 1.8 ´ 10–5
[OH–] = 4.24 ´ 10–3 mol L–1
[OH–]2 = 1.8 ´ 10–5 ´ 10–1 = 1.8 ´ 10–6
[OH–] = 1.34 ´ 10–3 mol L–1

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8

Analytical Chemistry

Hence if a solution of NH4OH alone is added, [OH–] is enough to exceed the requirement to the
solubility product for the hydroxides of all these above ions and hence get precipitated as their
hydroxides. However, Al3+, Fe3+ and Cr3+ cations can be selectively precipitated by addition of
NH4OH in the presence of excess of NH4Cl due to a common ion effect as explained below.

Classification of group III cations into IIIA and IIIB
NH4Cl is a strong electrolyte, which gives a large amount of NH4+ ions as it is strongly dissociated.
NH4Cl ắđ NH 4+ + Cl–
NH 4+ ions furnished by NH4Cl are common to that furnished by weak electrolyte NH4OH. As a
result, the dissociation of NH4OH is suppressed due to common ion effect, so that concentration of
OH– ions falls considerably low. Under such condition, the solubility products of the hydroxides
of Al3+, Fe3+, Cr3+ are exceeded so that they are precipitated as hydroxides by adding NH4OH in
the presence of NH4Cl while those of other ions (like Co2+, Ni2+, Mn2+, Zn2+) which have high
value of solubility products are prevented precipitation. Thus the ions like Al3+, Fe3+, Cr3+ which
get precipitated as hydroxides Al(OH)3 Fe(OH)3, Cr(OH)3 respectively are included in a separate
group IIIA; the NH4OH solution with excess of NH4Cl being its group reagent. The other cations
of the group III (such as Co2+, Ni2+, Mn2+, Zn2+) are precipitated as sulphides by passing H2S
through their ammonical solutions. They form separate group called Group IIIB. At ammonical
medium, dissociation of H2S is favoured

ZZX 2H+ + S2
H2S YZZ
H+ + OH ắđ H2O
As the concentration of H+ decreases due to combination of OH– furnished by NH4OH, the
[S2–] increases in order to maintain its dissociation constant. It is found that the concentration of
S2– in the presence of NH4OH and NH4Cl is large enough to exceed the requirements of the

solubility products for sulphides of Co2+, Ni2+, Zn2+ and Mn2+ ions and thus get precipitated as
their sulphides. Thus H2S gas in the presence of NH4OH and NH4Cl is the group reagent for
group IIIB cations.

Explanation of precipitation of iron in the form Fe3+ instead of Fe2+
Iron forms two important series of salt such as ferrous salt in which the metal is divalent and
ferric salt in which the metal is trivalent. For satisfactory precipitation with the group reagent
(NH4OH + NH4Cl) all of the three cations (Al3+, Fe3+ and Cr3+) must be present as trivalent
cations. It is, therefore, necessary to test the solution for ferrous ion with potassium ferricyanide
which form a dark blue precipitate due to formation of potassium ferro-ferricyanide
Fe2+ + K3[Fe(CN)6] ắđ KFe[Fe(CN)6] + 2K+
Potassium ferricyanide

Potassium ferro-ferricyanide

If Fe2+ ions are present in the mixture under analysis, they must be oxidized to Fe3+ ions (with
concentrated HNO3) prior to the precipitation of group IIIA hydroxides. This is because of the
following reasons:

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