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Dr. W M TSUI Department of Chemistry, HKUST Workshop for Secondary School Teachers 1 Experiments of Microscale Analytical Chemistry

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Experiments of Microscale Analytical Chemistry. Workshop for Secondary School Teachers. Dr. W M TSUI Department of Chemistry, HKUST. Outline. Introduction: Analytical Chemistry Analysis of organic compounds Qualitative Analysis ; Quantitative analysis Accuracy and precision: use of balance - PowerPoint PPT Presentation

TRANSCRIPT

Page 1: Workshop for Secondary School Teachers

Dr. W M TSUI

Department of Chemistry, HKUST

Workshop for Secondary School Teachers

1

Experiments of Microscale Analytical Chemistry

Page 2: Workshop for Secondary School Teachers

Department of Chemistry

Outline2

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

Page 3: Workshop for Secondary School Teachers

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?

Page 4: Workshop for Secondary School Teachers

Department of Chemistry

Introduction: Analytical Chemistry

4

“includes any aspect of the chemical characterization of a sample material.”

Quantitation; Detection; Identification; Separation

Page 5: Workshop for Secondary School Teachers

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.”

Page 6: Workshop for Secondary School Teachers

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?

Page 7: Workshop for Secondary School Teachers

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 >C=O + DNPH → orange precipitate

NH

NH2

NO2

NO2

tests for the presence of carbonyl compounds using 2,4-dinitrophenylhydrazine (DNPH) solution

NH

N

NO2

NO2

CCH3

CH3

OC

CH3

CH3

Page 8: Workshop for Secondary School Teachers

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 >C=C< + Br2 → rapid decolorization

Br BrCC

CH3

CH3

CH3

H

CC

CH3

CH3

CH3

HBr Br

Br BrCH3 C

H2

CH2

C C CH3 → CH3 CH2

CH2

C C CH3

Br

Br Br

Br

Page 9: Workshop for Secondary School Teachers

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

Paper chromatogram

Analysis of components in ball-pen inks

Page 10: Workshop for Secondary School Teachers

Department of Chemistry

Qualitative analysis of organic compounds

10

Infra-red spectroscopy

infrared absorption

Tra

nsm

itta

nce

(%)

OHO

Page 11: Workshop for Secondary School Teachers

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. OHO

O

O

CH3

Page 12: Workshop for Secondary School Teachers

Department of Chemistry

Quantitative analysis of organic compounds

e.g. measuring the amount of aspirin in analgesic tablets, by gravimetric method

e.g. measuring the amount of aspirin in analgesic tablets, by volumetric method

12

OHO

O

O

CH3

OHO

O

O

CH3

OO

O

O

CH3

Na+

NaOH(aq)+ H2O+

Page 13: Workshop for Secondary School Teachers

Department of Chemistry

Accuracy and Precision13

Measurement in Science: In science, we want measurements to

be both accurate and precise.

What is the difference between accuracy and precision?

Page 14: Workshop for Secondary School Teachers

Department of Chemistry

Accuracy and Precision14

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?)

Page 15: Workshop for Secondary School Teachers

Department of Chemistry

Accuracy and Precision15

What sort of measurements do we have?

Page 16: Workshop for Secondary School Teachers

Department of Chemistry

Accuracy and Precision16

Results may be reproducible, but wrong. 5.0 grams of sample

Balance “A” 5.2; 5.4; 5.3; 5.3

Balance “B” 5.0; 4.9; 5.1; 5.0

Balance “A”

Balance “B”

sample

Page 17: Workshop for Secondary School Teachers

Department of Chemistry

Accuracy and Precision

Systematic errors are constant and

always of the same sign and thus may not be reduced by averaging over a lot of data

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.

17

Weighing bottle

Page 18: Workshop for Secondary School Teachers

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)

Page 19: Workshop for Secondary School Teachers

Department of Chemistry

Datalogger – Introduction19

Datalogging device: also know as datalogger or data recorder

Datalogging: a process of measuring variables in a laboratory or outside using electronic sensors

Page 20: Workshop for Secondary School Teachers

Department of Chemistry

Datalogger – Introduction20

The measured data will be stored to computer/ hardwares

Data of experiment can be displayed in form of charts, graphs and tables

Page 21: Workshop for Secondary School Teachers

Department of Chemistry

Datalogger – components21

Electronic device that records data retrieved from electronic sensors

Page 22: Workshop for Secondary School Teachers

Department of Chemistry

Datalogger – components

Typical hardware setup

22

Page 23: Workshop for Secondary School Teachers

Department of Chemistry

Datalogger – components23

Electronic device that records data retrieved from electronic sensors

Stand-Alone datalogger

Page 24: Workshop for Secondary School Teachers

Department of Chemistry

Sensors24

Light sensor

Sound sensor Temperature

sensor

Various electronic sensors, e.g. Temperature sensor, humidity sensor, pressure sensor, & light sensor.

Page 25: Workshop for Secondary School Teachers

Department of Chemistry

Advantages25

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

Page 26: Workshop for Secondary School Teachers

Department of Chemistry

Advantages26

Suitable for both prolonged & short experiments.

How temperature, particle size, and concentration affect the rate of reaction between magnesium metal and HCl(aq)?

Page 27: Workshop for Secondary School Teachers

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 …

Page 28: Workshop for Secondary School Teachers

Department of Chemistry

Datalogger Set up

1. Log onto the computer2. Choose an electronic

detector 3. Connect it with interface

hardware (USB-link)4. Connect the assembly into

the USB port on computer5. Start the software in the

computer

28

Page 29: Workshop for Secondary School Teachers

Department of Chemistry

Neutralization Reaction29

pH measurement:1. Transfer 30 mL of

HCl(aq) into a beaker2. Put the probe into the

solution3. Start collecting the

data4. Add equimolar

NaOH(aq) slowly until you have added 35 mL

5. Stop collecting the data6. Print your graph

Page 30: Workshop for Secondary School Teachers

Department of Chemistry

Neutralization Reaction Mark in red where the graph is acidic. Mark in blue where the graph is basic. Make in green where neutralisation

happens.

30

Time

pHpH

Time

Page 31: Workshop for Secondary School Teachers

Department of Chemistry

Spectrophotometry31

Intensity of electromagnetic radiation:

IR;Visible; UV;X-ray.

Page 32: Workshop for Secondary School Teachers

Department of Chemistry

UV-Vis spectrophotometry32

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 Carbonyl ~ 300 nm

C-C=C-C

C=C-C=C

C=C-C-C=C

Page 33: Workshop for Secondary School Teachers

Department of Chemistry

UV-Vis spectrophotometry33

Page 34: Workshop for Secondary School Teachers

Department of Chemistry

UV-Vis spectroscopy

Two sources are required to scan the entire UV-VIS band:

Deuterium (D2) lamp – covers UV: 200-400 nm

Tungsten (W) lamp – covers visible: 400-800 nm

34

Page 35: Workshop for Secondary School Teachers

Department of Chemistry

Beer-Lambert Law35

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]

Page 36: Workshop for Secondary School Teachers

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

Page 37: Workshop for Secondary School Teachers

Department of Chemistry

Instrumentation37

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

Page 38: Workshop for Secondary School Teachers

Department of Chemistry

Instrumentation

Colorimeter

38

Page 39: Workshop for Secondary School Teachers

Department of Chemistry

Experiment: 1

Determine the order of reaction of phenolphthalein in alkaline solution

39

Page 40: Workshop for Secondary School Teachers

Department of Chemistry Determine the order of reaction of phenolphthalein in alkaline solution

40

Acid base titration Phenolphthalein in alkaline solution

C

C

O

O

HO OH

2 OH -

- H2OC

COO

O

O

(fast)

(Colourless)H2Ph Ph2-

(Pink)

(pH = 0 - 8.2) (pH = 8.2 - 12)

C

O O

OH

COO

OH -

(slow)

POH3-

(Colourless)(pH > 12)

Page 41: Workshop for Secondary School Teachers

Department of Chemistry Determine the order of reaction of phenolphthalein in alkaline solution

41

Reaction: Ph2- + OH- POH3-

Rate law: Rate = k [OH-]m [Ph2-]n

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-]oAccording to Beer’s Law, A

= bcA plot of In A against t

Page 42: Workshop for Secondary School Teachers

Department of Chemistry

UV-Vis Spectrophotometry42

Color of a solution:

Page 43: Workshop for Secondary School Teachers

Department of Chemistry Determine the order of reaction of phenolphthalein in alkaline solution

43

UV spectrum of phenolphthalein

Page 44: Workshop for Secondary School Teachers

Department of Chemistry

Colorimeter44

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

Page 45: Workshop for Secondary School Teachers

Department of Chemistry Determine the order of reaction of phenolphthalein in alkaline solution

45

A plot of In A against t

Page 46: Workshop for Secondary School Teachers

Department of Chemistry Determine the order of reaction of phenolphthalein in alkaline solution

46

Half-life (t1/2) determination

Page 47: Workshop for Secondary School Teachers

Department of Chemistry Determine the order of reaction of phenolphthalein in alkaline solution

47

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]

Page 48: Workshop for Secondary School Teachers

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

Page 49: Workshop for Secondary School Teachers

Department of Chemistry

Procedure of using colorimeter49

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-]

Page 50: Workshop for Secondary School Teachers

Department of Chemistry

50

OHO

O

O

CH3

Experiment: 2

Thin layer chromatography analysis and purification of aspirin by recrystallization

Page 51: Workshop for Secondary School Teachers

Department of ChemistryThin Layer Chromatography Analysis and Purification of Aspirin by Recrystallization51

Recrystallization For purification of solid samples

solvent,supersaturated, heating

Cooling

Dissolution Filter Crystallization Collection Drying

Page 52: Workshop for Secondary School Teachers

Department of ChemistryThin Layer Chromatography Analysis and Purification of Aspirin by Recrystallization

Thin layer chromatography (TLC) For determination of purify

52

Chromatography: Separation of compounds by the distribution between two phases – Mobile phase & Stationary phase

Page 53: Workshop for Secondary School Teachers

Department of ChemistryThin Layer Chromatography Analysis and Purification of Aspirin by Recrystallization53

stationary phase(TLC plate: Silica gel)

Sample mixture

mobile phase (solvent)

Page 54: Workshop for Secondary School Teachers

Department of ChemistryThin Layer Chromatography Analysis and Purification of Aspirin by Recrystallization54

Page 55: Workshop for Secondary School Teachers

Department of ChemistryThin Layer Chromatography Analysis and Purification of Aspirin by Recrystallization55

TLC visualization UV lamp (254 nm)

Page 56: Workshop for Secondary School Teachers

Department of ChemistryThin Layer Chromatography Analysis and Purification of Aspirin by Recrystallization56