mst deck

Question 1

what’s the difference between quantitative and qualitative analysis?

Answer 1

qualitative = what is in a sample
quantitative = how much is in a sample

Question 2

6 stages of analytical process

Answer 2

1. definition
2. sampling
3. selecting method of analysis
4. sample preparation
5. quantitation
6. presentation and evaluation

Question 3

define sampling

Answer 3

obtaining a small fraction of material from a large bulk such that the fraction is representative of the bulk in every way

Question 4

3 considerations when deciding method of sampling

Answer 4

1. size of gross sample
   a. heterogeneity
   b. accuracy/precision desired
   c. cost of sampling
2. physical state of fraction to be analysed: solid, liquid, gas
3. chemical composition of material to be analysed

Question 5

4 conditions for storing samples

Answer 5

1. storage container is clean and airtight
2. avoid adsorption effect on walls of containers esp for analysis of trace metals
3. store in rooms not too hot and humid
4. avoid prolonged periods of storage time to prevent decomposition/evaporation

Question 6

define wavelength

Answer 6

distance of one complete cycle

Question 7

wave number formula and unit

Answer 7

1 / λ

unit: cm^-1

Question 8

define frequency

Answer 8

number of cycles passing a fixed point per unit time

Question 9

velocity formula

what is the speed of light

Answer 9

v = f x λ
c = 3.0 x 10^8 m s^-1

Question 10

energy formula

Answer 10

E = hf or hv/λ

Question 11

define absorption

Answer 11

process where atoms or molecules acquire certain discrete quantise amounts of energy from incoming radiation and become excited

Question 12

what is a photon/quanta

Answer 12

minute, discrete amount of energy of electromagnetic radiation

Question 13

difference between atomic and molecular spectrum

Answer 13

when atoms absorb radiation, only electrons get excited = only electronic transition

molecules absorb whole lines of radiation, and have 3 processes of absorbing energy: rotational, vibrational, electronic transition

Question 14

transmittance and absorbance formulae

Answer 14

T = I / I0
A = log10 (1 / T)

Question 15

Beer-Lambert law

Answer 15

A = abc or εbc

Question 16

2 assumptions of Beer-Lambert law

Answer 16

1. monochromatic light is used: only a single wavelength is employed in absorbance measurement
2. there are no concentration-dependent interactions present: each absorbing particle absorbs independently of each other, true in very dilute solution i.e. < 10-3 M

Question 17

2 deviations of Beer-Lambert law

Answer 17

true deviation: when concentration is so high that refractive index changes from that of blank, affecting values of a / ε

apparent deviation: when nature or concentration of absorbing species changes, causing non-symmetrical chemical equilibria and hence nonlinearity.

Question 18

4 major components of a uv-vis spectrometer

Answer 18

1. radiation source (deuterium lamp for ultraviolet, tungsten lamp for visible)
2. monochromator (prism or diffraction grating)
3. sample cell (quartz for both, glass for visible)
4. detector (photocell or photomultiplier)

Question 19

compare pros and cons of single and double-beam uv-vis spectrometer (3)

Answer 19

1. single-beam: risk of analytical error from line voltage changes resulting in fluctuation in intensity of radiation reaching detector; double-beam: beam ratio is obtained and used constantly, minimising analytical error
2. single-beam: risk of analytical error from solvent absorbing some radiation; double-beam: analytical error is corrected
3. both single and double-beam: risk of analytical error from loss of radiation at reflecting surfaces

Question 20

describe procedure for selection of wavelength and obtaining calibration graph in uv-vis spectrometry (2)

Answer 20

1. spectrum scan:
   conduct spectrum scan at from 800 to 200 nm
   take wavelength at highest absorbance = highest sensitivity = ideal choice
2. photometric scan:
   conduct photometric analysis at λmax to obtain calibration graph

Question 21

what are the quantitative applications of uv-vis spectrometry (2)

Answer 21

1. spectrum scan: to determine wavelength that can be absorbed appreciably by sample
2. photometric scan: determine unknown concentration of sample

Question 22

what are the qualitative applications of uv-vis spectrometry (3)

Answer 22

1. detect presence or absence of certain functional groups
2. detect impurities
3. identification by matching spectra of unknown and standard

Question 23

what are the problems with qualitative analysis using uv-vis spectrometer? (2)

Answer 23

1. broad spectrum gives little details

2. spectra of different compounds may overlap to give complex spectrum

Question 24

what are chromophores?

Answer 24

functional groups that can absorb radiation in uv-vis region i.e.
π electrons (multiple bonds) or
n electrons (N, O, S, halogens)

Question 25

4 components of infrared spectrometer

Answer 25

1. radiation source (Nernst Glower or Globar) 2. monochromator (except FT-IR which uses interferometer) 3. sample cell (metal halide) 4. detector (bolometer or thermocouple)

Question 26

3 conditions for radiation source of infrared spectrometer

Answer 26

1. must be able to emit radiation over infrared region i.e. 400-4000 cm^-1 2. must have sufficient intensity such that transmitted energy can be detected at the end of the optical path 3. must be stable (I0 = 100%)

Question 27

3 conditions for radiation source of uv-vis spectrometer

Answer 27

1. must be able to emit radiation over uv-vis region i.e. 800 to 200 nm 2. must have sufficient intensity such that transmitted energy can be detected at the end of the optical path 3. must be stable (I0 = 100%)

Question 28

3 problems with using metal halides as sample cell/prism in infrared spectrometer

Answer 28

1. mechanical instability 2. thermal instability 3. water solubility

Question 29

how to prepare samples for infrared spectrometry for solid (3), liquid (2) and gas (1)

Answer 29

solid sample preparation 1. KBr pellet technique 2. mull technique 3. cast film technique liquid sample preparation 1. KBr disc technique 2. liquid cell technique (CaF2 is used if water is present in sample) gas sample preparation KBr or NaCl sample cell technique *difficult to analyse as concentration is low. we can use a cell with longer path length, which will increase absorbance.

Question 30

theory of infrared absorption (2)

Answer 30

each molecule has a complete absorption spectrum unique to its functional groups. in molecules, atoms are held by chemical bonds and are in continuous synchronised movement.

Question 31

3 conditions for molecule to absorb infrared radiation

Answer 31

1. change in bond length or angle due to vibrational or rotational motion causing net change in dipole moment 2. molecule must be vibrating at a natural frequency equal to frequency of incoming radiation 3. radiation energy must correspond to energy difference between ground state and excited state of molecule

Question 32

4 precautions when doing sample treatment/sample preparation

Answer 32

1. should be done without losing any of the analytes 2. should be done without adding new interferences 3. should bring analyte into appropriate chemical form for method to be used 4. should dilute/concentrate analyte for detectable results by the instrument