Workshop for Secondary School Teachers

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

2. Outline • 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

3. Introduction: Analytical Chemistry • 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?

4. Introduction: Analytical Chemistry • “includes any aspect of the chemical characterization of a sample material.” • Quantitation; Detection; Identification; Separation

5. Introduction: Analytical Chemistry • 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.”

6. Qualitative analysis of organic compounds • “An analysis in which we determine the identity of the constituent species in a sample” • Does it contain …? • Type? • Binding state?

7. Qualitative analysis of organic compounds • development of chemical tests to identify the presence of organic functional groups. >C=O + DNPH → orange precipitate • tests for the presence of carbonyl compounds using 2,4-dinitrophenylhydrazine (DNPH) solution

8. Qualitative analysis of organic compounds • tests for the presence of >C=C< / -CΞC- using bromine solution >C=C< + Br2 → rapid decolorization → →

9. Qualitative analysis of organic compounds • 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

10. Qualitative analysis of organic compounds • Infra-red spectroscopy Transmittance(%) infrared absorption

11. Quantitative analysis of organic compounds • “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.

12. 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

13. Accuracy and Precision • Measurement in Science: • In science, we want measurements to be both accurate and precise. • What is the difference between accuracy and precision?

14. Accuracy and Precision • 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?)

15. Accuracy and Precision • What sort of measurements do we have?

16. Accuracy and Precision • 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 sample Balance “A” Balance “B”

17. 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. Weighing bottle

18. Instrumental Quantitative Analysis • 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)

19. Datalogger – Introduction • Datalogging device: also know as datalogger or data recorder • Datalogging: a process of measuring variables in a laboratory or outside using electronic sensors

20. Datalogger – Introduction • The measured data will be stored to computer/ hardwares • Data of experiment can be displayed in form of charts, graphs and tables

21. Computer interface Sensor Datalogger – components Sensor 2,3 Electronic device that records data retrieved from electronic sensors

22. Datalogger – components • Typical hardware setup

23. Datalogger Display Storage Sensor Datalogger – components • Electronic device that records data retrieved from electronic sensors Sensor 2,3 Stand-Alone datalogger

24. Sensors Various electronic sensors, e.g. Temperature sensor, humidity sensor, pressure sensor, & light sensor. Light sensor Sound sensor Temperature sensor

25. Advantages • 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

26. Advantages • Suitable for both prolonged & short experiments. How temperature, particle size, and concentration affect the rate of reaction between magnesium metal and HCl(aq)?

27. Applications of Datalogging Experiments • 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 …

28. Datalogger Set up • 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

29. Neutralization Reaction • pH measurement: • 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

30. pH pH Time Time Neutralization Reaction • Mark in red where the graph is acidic. • Mark in blue where the graph is basic. • Make in green where neutralisation happens.

31. Spectrophotometry Intensity of electromagnetic radiation: IR; Visible; UV; X-ray.

32. UV-Vis spectrophotometry • 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

33. UV-Vis spectrophotometry

34. 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

35. Beer-Lambert Law • Also known as Beer’s Law: • A = -log10(I1/I0) = ε c l where • ε = molarabsorptivity; or molar extinction coefficient • [cm-1·M-1 or cm-1·mol-1·dm3] • c = concentration [mol-1·dm3] l = path length [cm]

36. Transmittance and Absorbance • Transmittance (T) = I1/I0 (i.e. %T = T  100) • Absorbance = -log10(%T/100) = -log10(I1/I0) = -log10 T •  T ≠ 1 - A

37. Instrumentation • 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 spectraONLY

38. Instrumentation • Colorimeter

39. Experiment: 1Determine the order of reaction of phenolphthalein in alkaline solution

40. Determine the order of reaction of phenolphthalein in alkaline solution Acid base titration Phenolphthalein in alkaline solution

41. Determine the order of reaction of phenolphthalein in alkaline solution • 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-]o According to Beer’s Law, A = bc A plot of In A against t

42. UV-Vis Spectrophotometry • Color of a solution:

43. Determine the order of reaction of phenolphthalein in alkaline solution UV spectrum of phenolphthalein

44. Colorimeter • 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

45. Determine the order of reaction of phenolphthalein in alkaline solution A plot of In A against t

46. Determine the order of reaction of phenolphthalein in alkaline solution Half-life (t1/2) determination

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]

48. Determine the order of reaction of phenolphthalein in alkaline solution Data logger – for repeating measurements – for storage of the change of absorbance against time

49. Procedure of using colorimeter • 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 k1vs In [OH-]

50. Experiment: 2Thin layer chromatography analysis and purification of aspirin by recrystallization