Analytical Chemistry Flashcards
1
Q
What is qualitative analysis?
A
Seeing what components are present in your sample
2
Q
Name methods of carrying out qualitative analysis
A
NMR
Mass Spec
IR Spectrophotometry
3
Q
What is quantitative analysis?
A
Seeing how much of each component is in your sample
4
Q
Name methods of carrying out quantitative analysis
A
Titrations
Gravimetric analysis
Electroanalytical methods
Optical methods
5
Q
Define accuracy
A
Closeness to a standard or true value
6
Q
Define precision
A
Closeness to a series of measurements
7
Q
Define repeatability
A
Same person, same instrument, increased precision
8
Q
Define reproducibility
A
Different person, different instrument, same result
9
Q
What is the British Pharmocopeia?
A
A catalog of over 3000 drug monographs that set the minimum pharmocopeial standards of medicines for human and animal use.
10
Q
Why must healthcare professionals adhere to the standards in the BP?
A
Compliance with the standards assures quality and provides a level of protection to the public
11
Q
What must you do before carrying out any analytical technique?
A
Sampling
12
Q
Describe the process of sampling
A
- obtain a representative bulk sample
- extract a smaller homogeneous sample
- convert sample into a form suitable for analysis
- remove/mask species that may interfere with analysis
- measure concentration of analyte in several aliquots
- interpret the results
13
Q
Define stoichiometry
A
The quantitative study of reactants and products in chemical reactions.
14
Q
What is the unit for amount of a substance?
A
Molar Mass (g/mol)
15
Q
What is a mole ratio?
A
A mole ratio converts moles of one compound in a balanced chemical equation into moles of another compound eg. 2Mg + O2 —-> 2MgO
2 : 1 : 2
16
Q
Advantages of gravimetric analysis
A
- accurate and precise
- possible sources of error are readily checked
- Absolute method
- cheap
17
Q
Disadvantages of gravimetric analysis
A
- only single element
- slow
- tricky
- many external factors can influence the reaction
18
Q
Advantages of volumetric analysis
A
- High purity- precision 0.1%
- stability toward air
- Absence of hydrate water
- moderate cost
- can analyse soluble substances
- can be automated
19
Q
What type of reaction occurs during Acid-Base titrations?
A
Neutralisation reaction
20
Q
What is the generic neutralisation reaction?
A
Acid + Base —> Salt + Water
21
Q
A strong acid + a strong base will result in a solution with what pH?
A
Neutral pH 7
22
Q
A strong acid + a weak base will result in a solution with what pH?
A
Acidic pH
23
Q
A weak acid + a strong base will result in a solution with what pH?
A
Basic >7
24
Q
What is a polyprotic acid and what is it neutralised by?
A
A polyprotic acid is an acid that can donate more than one proton and they are neutralised by strong bases.
25
What are precipitation titrations that are carried out with silver nitrate (AgNO3) called?
Argentimetric titrations
eg. AgNO3 + Cl- ---> AgCl + NO3-
(potassium chromate used as indicator- Red in excess Ag+)
26
What are precipitation titrations that are carried out with silver nitrate (AgNO3) called?
Argentimetric titrations
eg. AgNO3 + Cl- ---> AgCl + NO3-
(potassium chromate used as indicator- Red in excess Ag+)
27
Describe how you would carry out a back titration
- Add AgNO3 to sample
- Excess AgNO3 titrated with NH4SCN and ammonium ferrous sulphate (indicator) - AgNO3 + NH4SCN ---> AgSCN + NH4NO3
- AgCl has to be filtered off beforehand
- SCN- causes dissociation
28
Describe how you would carry out a back titration
- Add AgNO3 to sample
- Excess AgNO3 titrated with NH4SCN and ammonium ferrous sulphate (indicator) - AgNO3 + NH4SCN ---> AgSCN + NH4NO3
- AgCl has to be filtered off beforehand
- SCN- causes dissociation
29
What are compleximetric titrations used for?
Used in the estimation of metal salts - EDTA used as titrant as it forms 1:1 complex with most metals.
30
What occurs during a redox titration?
Transfer of electrons between the titrant and analyte
31
What is the reduction potential?
The measure of how thermodynamically favourable it is for something to gain electrons
32
Elements with high positive E0 values are said to be..?
Strong oxidising agents (removes e-)
33
What is the iodiometry equation?
I2 + 2e- ---> 2I-
| Iodine is a moderately strong oxidising agent
34
describe what occurs in an iodine displacement reaction
Iodine is displaced from iodide by a stronger oxidising agent. The displaced iodine is then titrated with sodium thiosulphate.
1) Cl2 + 2I- ---> 2Cl- + I2
2) 2S2O32- + I2 ---> S4O62- + 2I-
35
What are the components of an electrochemical cell?
- wires
- ion solutions
- junctions
- complete electrical circuit
- conduction electrodes
36
What are the components of an electrochemical cell?
- wires
- ion solutions
- junctions
- complete electrical circuit
- conduction electrodes
- voltmeter
- salt bridge
37
What is the other name for an electrochemical cell?
Galvanic cell
38
What occurs at the anode?
Oxidation (-ve)
39
What occurs at the cathode?
Reduction (+ve)
40
Which direction do the electrons flow around the circuit?
From anode to cathode (-ve to +ve)
41
What is an electrolytic cell?
Similar to galvanic cell but this cell requires a potential difference greater than in the galvanic cell in order to drive the reaction away from equilibrium.
-In electrolytic the anode and cathode are opposite to what you would expect in a galvanic cell.
42
What is electrogravimetric analysis?
- easy, accurate method for determining metal concentrations
- electric potential applied to drive the reaction
- element to be determined is deposited as a solid on a preweighed electrode via a reduction process - metal conc. determined by measuring the weight gained by the electrode
- selectively achieved by using a particular voltage
43
What are the advantages of electrogravimetric analysis?
Filtration avoided and co-deposition unlikely if correct conditions are established first.
44
Explain what coulometry is.
Measuring an unknown concentration of an analyte in solution by converting it from one oxidation state to another. This is an absolute measurement.
- You measure the amount of electricity (in coulombs) consumed or produced during electrolysis.
45
What analytical technique uses a suitable electrode to detect an end point?
Potentiometry
46
What are the advantages of potentiometry?
- can be used in coloured solutions or solutions with an unclear end point
- can be automated
47
What are the disadvantages of potentiometry?
- slow
| - expensive
48
What is the most common type of electrode used in potentiometry?
Glass membrane pH electrode
49
Explain how a glass membrane pH electrode works.
- Only allows H3O+ ions to become incorporated in its inner and outer layer
- Inside there is a fixed concentration of H3O+
- If the concentration of H3O+ is higher outside, then a positive charge will build up relative to the inside.
- This difference is the membrane potential. (in order to measure this potential a reference electrode and meter are required)
50
Give a simple definition of how UV/VIS spectroscopy works.
It is the measurement of how organic compounds interact with electromagnetic radiation.
51
Outline the steps that occur withing a UV/VIS spec machine.
- 2 lamps (deuterium and tungsten)
- through focusing slit
- to monochromator which disperses light into constituent wavelengths to pass one frequency through the sample at a time
- through another focusing slit
- through sample
- to detector
52
Outline the steps that occur withing a UV/VIS spec machine.
- 2 lamps (deuterium and tungsten)
- through focusing slit
- to monochromator which disperses light into constituent wavelengths to pass one frequency through the sample at a time
- through another focusing slit
- through sample
- to detector
53
How would you calibrate the absorbance scale on a UV/VIS spec machine?
Use potassium dichromate solution (0.0065% w/v in 0.005M H2SO4) at select wavelengths (235,257,313 and 350nm)
54
How would you calibrate the wavelength scale on a UV/VIS spec machine?
Use a specific wavelength maxima of 5% w/v holmium perchlorate solution.
55
How would you calibrate the resolution on a UV/VIS spec machine?
Use 0.02% w/v toluene in hexane. Ratio of absorbance at 269nm to that at 266nm is approximately 1.5
56
How would you assess the optics on a UV/VIS spec machine?
Use 1.2% KCl solution assessed at 200nm. Absorbance must be
57
Give a simple definition of how UV/VIS spectroscopy works.
It is the measurement of how organic compounds interact with electromagnetic radiation.
Irradiating organic compounds with UV/VIS light can promote electrons to higher energy levels. This causes incident photons at that specific frequency to be absorbed.
58
Outline the steps that occur within a UV/VIS spec machine.
- 2 lamps (deuterium and tungsten)
- through focusing slit
- to monochromator which disperses light into constituent wavelengths to pass one frequency through the sample at a time
- through another focusing slit
- through sample
- to detector
59
Give a simple definition of how UV/VIS spectroscopy works.
It is the measurement of how organic compounds interact with electromagnetic radiation.
Irradiating organic compounds with UV/VIS light can promote electrons to higher energy levels. This causes incident photons at that specific frequency to be absorbed.
60
What do you call a functional group that absorbs UV/VIS light?
Chromophore
61
What happens when an electron is excited?
It moves from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The difference in energy is the energy of the absorbed photon.
62
State the Beer-Lambert law
```
A=A(1%,1cm)cI
where:
- A= absorbance (no units)
-A(1%,1cm)= absobance of a 1wt% solution of sample in 1cm cuvette (dLcmg-1) - BP provides this data
-c= concentration of sample (g/100ml)
-I= pathlength (always 1cm)
```
63
How would you assess the optics on a UV/VIS spec machine?
Use 1.2% KCl solution assessed at 200nm. Absorbance must be
64
What is an auxochrome?
They are typically hetroatoms attached to conjugated systems and they modify the ability of chromophores to absorb light, whilst not contributing to absorption themselves
65
How do you determine pKa using UV/VIS spec?
pKa= pH + log Ai-A/A-Au
where:
-A= measured absorbance of a molecule in a buffered solution of known pH at specific wavelength.
-Ai= absorbance of fully ionised molecule
-Au= absorbance of un-ionised molecule
(wavelength selected where there is greatest difference between Ai and Au)
66
How do you determine pKa using UV/VIS spec?
pKa= pH + log Ai-A/A-Au
where:
-A= measured absorbance of a molecule in a buffered solution of known pH at specific wavelength.
-Ai= absorbance of fully ionised molecule
-Au= absorbance of un-ionised molecule
(wavelength selected where there is greatest difference between Ai and Au)
(in rare cases where ionisation leads to a decrease in absorbance the + sign in the equation changes to a -)
67
What does IR spec most commonly identify?
Functional groups, not complete molecules.
68
What region would you find OH, NH and NH2 groups?
3100-3600 cm-1
69
What region would you find triple bonds; alkynes and nitriles?
2000-2400 cm-1
70
What region would you find C=0, N=O, esters, ketones, aldehydes, carboxylic acids, amides and nitro groups?
1550-1800 cm-1
71
What is the region below 1500 cm-1 called?
Fingerprint region which is ignored during interpretation of IR spectra.
72
State the simple theory behind IR spec
- Movement of atoms at either end of a bond causes a change in position of the atoms with time
- this causes a change in the dipole moment of the system
- the change in dipole moment interacts with EM radiation
- energy of this transition coincides with the IR region
- different functional groups vibrate at different, but known, energies therefore each functional group absorbs in a characteristic region.
- Frequency of IR absorbed= bond strength/atomic weight
- as mass increases, frequency decreases
- as bond strength increases, frequency increases
73
What influences IR spec signal intensity?
- Intensity is related to the change in dipole moment of the vibrating bond
- highly polar groups absorb strongly as they enable a greater change in dipole when excited.
74
Describe how you would prepare a KBr disc
- Grind sample into powder (reduces scattering) and mix with KBr
- transfer into a die and compress, KBr liquifies and sets to afford a transparent disc containing KBr and sample
- place in IR spec machine and acquire spectra.
75
Describe how you would prepare a mull
-grind sample into powder
-add a drop of liquid paraffin (nujol) and mix
-place a drop of the mixture on an NaCl disc and sandwich between another NaCl disc to afford a thin film
(IR spectra from paraffin will appear at 3000 and 1500-1400 cm-1 so these need to be subtracted from the final spectra)
76
Describe how you would prepare a thin film
- Solid sample dissolved in low boiling point solvent
- drop placed on KBr disc
- solvent will evaporate and disc can be placed in IR spec
- If sample is liquid, then sandwich between 2 KBr discs
77
What is used to calibrate an IR spec machine?
A thin film of polystyrene used as standard in the BP
78
Name two applications of IR spec
1) Qualitative analysis of drugs
| 2) Identity check for drugs
