Transcript k 1 - Department of Chemistry

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

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Introduction: Analytical Chemistry
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
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
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

“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.”

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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?

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Qualitative analysis of organic compounds
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
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
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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
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Qualitative analysis of organic compounds
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

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
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Qualitative analysis of organic compounds
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Infra-red spectroscopy
O
Transmittance(%)

infrared absorption
OH
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Quantitative analysis of organic compounds
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

“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
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Quantitative analysis of organic compounds
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
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
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
Measurement in Science:

In science, we want measurements to be both
accurate and precise.

What is the difference between accuracy and
precision?
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Accuracy and Precision
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
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?)
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Accuracy and Precision
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
What sort of measurements do we have?
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Accuracy and Precision
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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
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
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.
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Instrumental Quantitative Analysis
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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
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Datalogging device: also know as datalogger or
data recorder
 Datalogging: a process of measuring variables in a
laboratory or outside using electronic sensors

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Datalogger – Introduction
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The measured data will be stored to computer/
hardwares
 Data of experiment can be displayed in form of
charts, graphs and tables

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Datalogger – components
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
Electronic device that
records data retrieved
from electronic sensors
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Datalogger – components
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
Typical hardware setup
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Datalogger – components
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
Electronic device that
records data
retrieved from
electronic sensors
Stand-Alone datalogger
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Sensors
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Various electronic sensors,
e.g. Temperature sensor,
humidity sensor, pressure
sensor, & light sensor.
Light sensor
Sound sensor
Temperature
sensor
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Advantages
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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

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Advantages
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
Suitable for both prolonged & short experiments.
How temperature, particle
size, and concentration
affect the rate of reaction
between magnesium metal
and HCl(aq)?
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Applications of Datalogging Experiments
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
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 …
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Datalogger Set up
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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
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Neutralization Reaction
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
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
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Neutralization Reaction
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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
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Spectrophotometry
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Intensity of
electromagne
tic radiation:
IR;
Visible;
UV;
X-ray.
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UV-Vis spectrophotometry
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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

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UV-Vis spectrophotometry
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
Department of Chemistry
UV-Vis spectroscopy
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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
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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]
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Transmittance and Absorbance
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


Transmittance (T) = I1/I0
(i.e. %T = T  100)
Absorbance = -log10(%T/100)
= -log10(I1/I0)
= -log10 T
T≠1-A
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Instrumentation
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



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
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Instrumentation
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
Colorimeter
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Experiment: 1
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Determine the order of reaction of
phenolphthalein in alkaline solution
Department of Chemistry
Determine the order of reaction of
phenolphthalein in alkaline solution
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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
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Determine the order of reaction of
phenolphthalein in alkaline solution
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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
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UV-Vis Spectrophotometry
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
Color of a solution:
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Determine the order of reaction of
phenolphthalein in alkaline solution
UV spectrum of phenolphthalein
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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
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Determine the order of reaction of
phenolphthalein in alkaline solution
A plot of In A against t
Department of Chemistry
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Determine the order of reaction of
phenolphthalein in alkaline solution
Half-life (t1/2) determination
Department of Chemistry
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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
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Procedure of using colorimeter
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



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-]
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Experiment: 2
50
Thin layer chromatography analysis and
purification of aspirin by recrystallization
O
OH
C H3
O
O
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Thin Layer Chromatography Analysis and
Purification of Aspirin by Recrystallization
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
Recrystallization
 For
purification of solid samples
Dissolution  Filter  Crystallization  Collection  Drying
solvent,
supersaturated,
heating
Cooling
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Thin Layer Chromatography Analysis and
Purification of Aspirin by Recrystallization
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
Thin layer chromatography (TLC)
 For
determination of purify
Chromatography:
Separation of compounds by the
distribution between two phases –
Mobile phase & Stationary phase
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Thin Layer Chromatography Analysis and
Purification of Aspirin by Recrystallization
stationary
phase
(TLC plate:
Silica gel)
Sample mixture
mobile phase (solvent)
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Thin Layer Chromatography Analysis and
Purification of Aspirin by Recrystallization
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Thin Layer Chromatography Analysis and
Purification of Aspirin by Recrystallization
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
TLC visualization

UV lamp (254 nm)
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Thin Layer Chromatography Analysis and
Purification of Aspirin by Recrystallization
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
TLC visualization

Bromocresol green stain