Experiments of Microscale
Analytical Chemistry
1
Workshop for Secondary School Teachers
Dr. W M TSUI
Department of Chemistry, HKUST
Department of Chemistry
Outline
2
Introduction: Analytical Chemistry
 Analysis of organic compounds

 Qualitative
Analysis ; Quantitative analysis
Accuracy and precision: use of balance
 Instrumental quantitative analysis
 Introduction to data-logger
 Experiments highlight

Department of Chemistry
Introduction: Analytical Chemistry
3

What is chemical analysis?

Questions to answer:
 How
much of substance X is in the sample?
 Does the sample contain substance X?
 What is the identity of the substance in the sample?
 How can the species of interest be separated from the
sample matrix for better quantitation and identification?
Department of Chemistry
Introduction: Analytical Chemistry
4


“includes any aspect of the
chemical characterization
of a sample material.”
Quantitation; Detection;
Identification; Separation
Department of Chemistry
Introduction: Analytical Chemistry
5

What is analytical chemistry?
“The science of chemical measurement…”
 “…Its object is the generation, treatment and
evaluation of signals from which information is
obtained on the composition and structure of
matter.”

Department of Chemistry
Qualitative analysis of organic compounds
6

“An analysis in which we determine the identity of the
constituent species in a sample”
Does it contain …?
 Type?
 Binding state?

Department of Chemistry
Qualitative analysis of organic compounds
7

development of chemical tests to identify the
presence of organic functional groups.
>C=O +
DNPH → orange precipitate
H
N
C H3
C H3
C
H3C
H2 N
C
H3 C
N O2
N O2
N
O
N O2

H
N
N O2
tests for the presence of carbonyl compounds using
2,4-dinitrophenylhydrazine (DNPH) solution
Department of Chemistry
Qualitative analysis of organic compounds
8

tests for the presence of >C=C< / -CΞC- using
bromine solution
>C=C< +
Br2 → rapid decolorization
CH3
C
H3C
C
CH3
Br
Br
→
CH3
C
H3C
C C C
H2 H2
Br
CH3
Br
H
H
H 3C
C
C
CH3
Br
Br
→
Br Br
H 3C
C C C C CH3
H2 H2
Br Br
Department of Chemistry
Qualitative analysis of organic compounds
9


Qualitative analysis – chromatography and
identification by measuring physical property (e.g.
mass spectrometry, infra red spectroscopy)
Paper chromatography
Analysis of components
in ball-pen inks
Paper chromatogram
Department of Chemistry
Qualitative analysis of organic compounds
10
Infra-red spectroscopy
O
Transmittance(%)

infrared absorption
OH
Department of Chemistry
Quantitative analysis of organic compounds
11


“An analysis in which we determine how much of a
constituent species is present in a sample”
Developing methods to determine the concentration of
targeted species in complex samples.
O
OH
C H3
O
O
Department of Chemistry
Quantitative analysis of organic compounds
12

e.g. measuring the
amount of aspirin in
analgesic tablets, by
gravimetric method
O

O
OH
e.g. measuring the
amount of aspirin in
analgesic tablets, by
volumetric method
OH
CH3
O
C H3
O
O
+
O
Na
O
O
O
+
N a O H (a q)
CH3
O
+
H 2O
Department of Chemistry
Accuracy and Precision
13

Measurement in Science:

In science, we want measurements to be both
accurate and precise.

What is the difference between accuracy and
precision?
Department of Chemistry
Accuracy and Precision
14

Accuracy
 is
a measure of how close a measured value to
the true value (is it the correct value?)

Precision
 is
a measure of the reproducibility of a result (is
it exactly the value?)
Department of Chemistry
Accuracy and Precision
15

What sort of measurements do we have?
Department of Chemistry
Accuracy and Precision
16
Results may be reproducible, but wrong.
 5.0 grams of sample


Balance “A”
 5.2;
sample
5.4; 5.3; 5.3
Balance “A”

Balance “B”
 5.0;
4.9; 5.1; 5.0
Balance “B”
Department of Chemistry
Accuracy and Precision
17

Systematic errors
 are
constant and always
of the same sign and thus
may not be reduced by
averaging over a lot of
data
Weighing bottle

Random errors
 produced
by any
unpredictable & unknown
variations in the
experiment, e.g.
fluctuations in room
temperature, fluctuations
in power supply voltage,
mechanical vibrations etc.
Department of Chemistry
Instrumental Quantitative Analysis
18
Employing modern instrumentations for determining
how much of a constituent species is present in a
sample
 Modern instruments play a key role in chemical
analysis nowadays
 Quantitative - measuring property and determining
relationship to concentration (e.g. UV and visible
spectrophotometry)

Department of Chemistry
Datalogger – Introduction
19
Datalogging device: also know as datalogger or
data recorder
 Datalogging: a process of measuring variables in a
laboratory or outside using electronic sensors

Department of Chemistry
Datalogger – Introduction
20
The measured data will be stored to computer/
hardwares
 Data of experiment can be displayed in form of
charts, graphs and tables

Department of Chemistry
Datalogger – components
21

Electronic device that
records data retrieved
from electronic sensors
Department of Chemistry
Datalogger – components
22

Typical hardware setup
Department of Chemistry
Datalogger – components
23

Electronic device that
records data
retrieved from
electronic sensors
Stand-Alone datalogger
Department of Chemistry
Sensors
24
Various electronic sensors,
e.g. Temperature sensor,
humidity sensor, pressure
sensor, & light sensor.
Light sensor
Sound sensor
Temperature
sensor
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Advantages
25
Multi-functional; can be
equipped with different
types of sensors
 Immediate feedback of
results (Real Time!)
 Easy analysis of results
 Easy for the repetition of
experiments

Department of Chemistry
Advantages
26

Suitable for both prolonged & short experiments.
How temperature, particle
size, and concentration
affect the rate of reaction
between magnesium metal
and HCl(aq)?
Department of Chemistry
Applications of Datalogging Experiments
27

Measurements in:
 pH
change caused by chemical reactions
 Temperature change caused by chemical
reactions
 Color change caused by chemical reactions
(colorimetry)
 Pressure change caused by chemical reactions
 etc …
Department of Chemistry
Datalogger Set up
28
1.
2.
3.
4.
5.
Log onto the computer
Choose an electronic detector
Connect it with interface
hardware (USB-link)
Connect the assembly into the
USB port on computer
Start the software in the
computer
Department of Chemistry
Neutralization Reaction
29

pH measurement:
1.
2.
3.
4.
5.
6.
Transfer 30 mL of HCl(aq)
into a beaker
Put the probe into the
solution
Start collecting the data
Add equimolar NaOH(aq)
slowly until you have added
35 mL
Stop collecting the data
Print your graph
Department of Chemistry
Neutralization Reaction
30
Mark in red where the graph is acidic.
 Mark in blue where the graph is basic.
 Make in green where neutralisation happens.

pH
pH
Time
Time
Department of Chemistry
Spectrophotometry
31
Intensity of
electromagne
tic radiation:
IR;
Visible;
UV;
X-ray.
Department of Chemistry
UV-Vis spectrophotometry
32
Corresponds to EM radiation in the ultraviolet (UV)
region---100-400 nm
 Visible (Vis) regions--- 400-800 nm
 Suitable for organic compounds
Unsaturated (conjugated) ~ 180 nm

C-C=C-C
C=C-C=C  C=C-C-C=C
Carbonyl ~ 300 nm

Department of Chemistry
UV-Vis spectrophotometry
33

Department of Chemistry
UV-Vis spectroscopy
34
Two sources are
required to scan the
entire UV-VIS band:
 Deuterium (D2) lamp –
covers UV: 200-400
nm
 Tungsten (W) lamp –
covers visible: 400800 nm

Department of Chemistry
Beer-Lambert Law
35
Also known as Beer’s Law:
 A = -log10(I1/I0) = ε c l

where
ε = molar absorptivity; or molar
extinction coefficient
[cm-1·M-1 or cm-1·mol-1·dm3]
c = concentration [mol-1·dm3]
l = path length [cm]
Department of Chemistry
Transmittance and Absorbance
36



Transmittance (T) = I1/I0
(i.e. %T = T  100)
Absorbance = -log10(%T/100)
= -log10(I1/I0)
= -log10 T
T≠1-A
Department of Chemistry
Instrumentation
37




UV spectra are recorded in solution
Cells (cuvette) can be made of plastic, glass or quartz
Quartz - transparent in 200-700 nm
Plastic and glass - visible spectra ONLY
Department of Chemistry
Instrumentation
38

Colorimeter
Department of Chemistry
Experiment: 1
39
Determine the order of reaction of
phenolphthalein in alkaline solution
Department of Chemistry
Determine the order of reaction of
phenolphthalein in alkaline solution
40
HO
O
OH
O
O
O
2 OH
C
-
OH
C
- H 2O
O
(slo w )
COO
(fast)
C
-
C
OH
COO
O
H 2 P h (pH = 0 - 8.2)
(C olourless)
P h 2- (pH = 8.2 - 12)
(Pink)
Acid base titration
P O H 3 - (p H > 1 2 )
(C o lo u rless)
Phenolphthalein in alkaline solution
Department of Chemistry
Determine the order of reaction of
phenolphthalein in alkaline solution
41
Reaction: Ph2- + OH-  POH3- m
2- n
 Rate law: Rate = k [OH ] [Ph ]

The concentration of OH- is largely excess, therefore,
can be assumed to remain essentially constant
Rate = k1 [Ph2-]n
, where k1 = k [OH-]m
ln[Ph2-] = -k1t + In[Ph2-]o
According to Beer’s Law,
A = bc
A plot of In A against t
Department of Chemistry
UV-Vis Spectrophotometry
42

Color of a solution:
Department of Chemistry
43
Determine the order of reaction of
phenolphthalein in alkaline solution
UV spectrum of phenolphthalein
Department of Chemistry
Colorimeter
44
measure the absorbance at wavelength of 565nm
(green) for each coloured solution using your
colorimeter.
 The measured absorbance is a direct measure of
the intensity of the solution’s colour

Department of Chemistry
45
Determine the order of reaction of
phenolphthalein in alkaline solution
A plot of In A against t
Department of Chemistry
46
Determine the order of reaction of
phenolphthalein in alkaline solution
Half-life (t1/2) determination
Department of Chemistry
47
Determine the order of reaction of
phenolphthalein in alkaline solution
To determine the reaction order with respect to [NaOH]
k1 = k [OH-]m
ln k1 = ln k + m ln[OH-]
Slope = reaction order
with respect to [NaOH]
Department of Chemistry
Determine the order of reaction of
phenolphthalein in alkaline solution
48
Data logger
– for repeating measurements
– for storage of the change of
absorbance against time
Department of Chemistry
Procedure of using colorimeter
49




Select a wavelength filter- Red (660nm),
orange (610nm), green (565nm) or blue
(468nm)
Record the Absorbance of sample at
different time
Plot graph of ln A vs time
Plot graph of In k1 vs In [OH-]
Department of Chemistry
Experiment: 2
50
Thin layer chromatography analysis and
purification of aspirin by recrystallization
O
OH
C H3
O
O
Department of Chemistry
Thin Layer Chromatography Analysis and
Purification of Aspirin by Recrystallization
51

Recrystallization
 For
purification of solid samples
Dissolution  Filter  Crystallization  Collection  Drying
solvent,
supersaturated,
heating
Cooling
Department of Chemistry
Thin Layer Chromatography Analysis and
Purification of Aspirin by Recrystallization
52

Thin layer chromatography (TLC)
 For
determination of purify
Chromatography:
Separation of compounds by the
distribution between two phases –
Mobile phase & Stationary phase
Department of Chemistry
53
Thin Layer Chromatography Analysis and
Purification of Aspirin by Recrystallization
stationary
phase
(TLC plate:
Silica gel)
Sample mixture
mobile phase (solvent)
Department of Chemistry
54
Thin Layer Chromatography Analysis and
Purification of Aspirin by Recrystallization
Department of Chemistry
Thin Layer Chromatography Analysis and
Purification of Aspirin by Recrystallization
55

TLC visualization

UV lamp (254 nm)
Department of Chemistry
Thin Layer Chromatography Analysis and
Purification of Aspirin by Recrystallization
56

TLC visualization

Bromocresol green stain