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PM1B > Analytical Chemistry > Flashcards

Analytical Chemistry Flashcards

1
Q

What is qualitative analysis?

A

Seeing what components are present in your sample

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2
Q

Name methods of carrying out qualitative analysis

A

NMR
Mass Spec
IR Spectrophotometry

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3
Q

What is quantitative analysis?

A

Seeing how much of each component is in your sample

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4
Q

Name methods of carrying out quantitative analysis

A

Titrations
Gravimetric analysis
Electroanalytical methods
Optical methods

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5
Q

Define accuracy

A

Closeness to a standard or true value

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6
Q

Define precision

A

Closeness to a series of measurements

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7
Q

Define repeatability

A

Same person, same instrument, increased precision

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8
Q

Define reproducibility

A

Different person, different instrument, same result

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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.

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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

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11
Q

What must you do before carrying out any analytical technique?

A

Sampling

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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
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13
Q

Define stoichiometry

A

The quantitative study of reactants and products in chemical reactions.

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14
Q

What is the unit for amount of a substance?

A

Molar Mass (g/mol)

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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

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16
Q

Advantages of gravimetric analysis

A
  • accurate and precise
  • possible sources of error are readily checked
  • Absolute method
  • cheap
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17
Q

Disadvantages of gravimetric analysis

A
  • only single element
  • slow
  • tricky
  • many external factors can influence the reaction
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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
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19
Q

What type of reaction occurs during Acid-Base titrations?

A

Neutralisation reaction

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20
Q

What is the generic neutralisation reaction?

A

Acid + Base —> Salt + Water

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21
Q

A strong acid + a strong base will result in a solution with what pH?

A

Neutral pH 7

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22
Q

A strong acid + a weak base will result in a solution with what pH?

A

Acidic pH

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23
Q

A weak acid + a strong base will result in a solution with what pH?

A

Basic >7

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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.

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25
Q

What are precipitation titrations that are carried out with silver nitrate (AgNO3) called?

A

Argentimetric titrations
eg. AgNO3 + Cl- —> AgCl + NO3-
(potassium chromate used as indicator- Red in excess Ag+)

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26
Q

What are precipitation titrations that are carried out with silver nitrate (AgNO3) called?

A

Argentimetric titrations
eg. AgNO3 + Cl- —> AgCl + NO3-
(potassium chromate used as indicator- Red in excess Ag+)

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27
Q

Describe how you would carry out a back titration

A
  • 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
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28
Q

Describe how you would carry out a back titration

A
  • 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
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29
Q

What are compleximetric titrations used for?

A

Used in the estimation of metal salts - EDTA used as titrant as it forms 1:1 complex with most metals.

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30
Q

What occurs during a redox titration?

A

Transfer of electrons between the titrant and analyte

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31
Q

What is the reduction potential?

A

The measure of how thermodynamically favourable it is for something to gain electrons

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32
Q

Elements with high positive E0 values are said to be..?

A

Strong oxidising agents (removes e-)

33
Q

What is the iodiometry equation?

A

I2 + 2e- —> 2I-

Iodine is a moderately strong oxidising agent

34
Q

describe what occurs in an iodine displacement reaction

A

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
Q

What are the components of an electrochemical cell?

A
  • wires
  • ion solutions
  • junctions
  • complete electrical circuit
  • conduction electrodes
36
Q

What are the components of an electrochemical cell?

A
  • wires
  • ion solutions
  • junctions
  • complete electrical circuit
  • conduction electrodes
  • voltmeter
  • salt bridge
37
Q

What is the other name for an electrochemical cell?

A

Galvanic cell

38
Q

What occurs at the anode?

A

Oxidation (-ve)

39
Q

What occurs at the cathode?

A

Reduction (+ve)

40
Q

Which direction do the electrons flow around the circuit?

A

From anode to cathode (-ve to +ve)

41
Q

What is an electrolytic cell?

A

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
Q

What is electrogravimetric analysis?

A
  • 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
Q

What are the advantages of electrogravimetric analysis?

A

Filtration avoided and co-deposition unlikely if correct conditions are established first.

44
Q

Explain what coulometry is.

A

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
Q

What analytical technique uses a suitable electrode to detect an end point?

A

Potentiometry

46
Q

What are the advantages of potentiometry?

A
  • can be used in coloured solutions or solutions with an unclear end point
  • can be automated
47
Q

What are the disadvantages of potentiometry?

A
  • slow

- expensive

48
Q

What is the most common type of electrode used in potentiometry?

A

Glass membrane pH electrode

49
Q

Explain how a glass membrane pH electrode works.

A
  • 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
Q

Give a simple definition of how UV/VIS spectroscopy works.

A

It is the measurement of how organic compounds interact with electromagnetic radiation.

51
Q

Outline the steps that occur withing a UV/VIS spec machine.

A
  • 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
Q

Outline the steps that occur withing a UV/VIS spec machine.

A
  • 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
Q

How would you calibrate the absorbance scale on a UV/VIS spec machine?

A

Use potassium dichromate solution (0.0065% w/v in 0.005M H2SO4) at select wavelengths (235,257,313 and 350nm)

54
Q

How would you calibrate the wavelength scale on a UV/VIS spec machine?

A

Use a specific wavelength maxima of 5% w/v holmium perchlorate solution.

55
Q

How would you calibrate the resolution on a UV/VIS spec machine?

A

Use 0.02% w/v toluene in hexane. Ratio of absorbance at 269nm to that at 266nm is approximately 1.5

56
Q

How would you assess the optics on a UV/VIS spec machine?

A

Use 1.2% KCl solution assessed at 200nm. Absorbance must be

57
Q

Give a simple definition of how UV/VIS spectroscopy works.

A

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
Q

Outline the steps that occur within a UV/VIS spec machine.

A
  • 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
Q

Give a simple definition of how UV/VIS spectroscopy works.

A

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
Q

What do you call a functional group that absorbs UV/VIS light?

A

Chromophore

61
Q

What happens when an electron is excited?

A

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
Q

State the Beer-Lambert law

A 
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
Q

How would you assess the optics on a UV/VIS spec machine?

A

Use 1.2% KCl solution assessed at 200nm. Absorbance must be

64
Q

What is an auxochrome?

A

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
Q

How do you determine pKa using UV/VIS spec?

A

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
Q

How do you determine pKa using UV/VIS spec?

A

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
Q

What does IR spec most commonly identify?

A

Functional groups, not complete molecules.

68
Q

What region would you find OH, NH and NH2 groups?

A

3100-3600 cm-1

69
Q

What region would you find triple bonds; alkynes and nitriles?

A

2000-2400 cm-1

70
Q

What region would you find C=0, N=O, esters, ketones, aldehydes, carboxylic acids, amides and nitro groups?

A

1550-1800 cm-1

71
Q

What is the region below 1500 cm-1 called?

A

Fingerprint region which is ignored during interpretation of IR spectra.

72
Q

State the simple theory behind IR spec

A
  • 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
Q

What influences IR spec signal intensity?

A
  • 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
Q

Describe how you would prepare a KBr disc

A
  • 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
Q

Describe how you would prepare a mull

A

-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
Q

Describe how you would prepare a thin film

A
  • 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
Q

What is used to calibrate an IR spec machine?

A

A thin film of polystyrene used as standard in the BP

78
Q

Name two applications of IR spec

A

1) Qualitative analysis of drugs

2) Identity check for drugs

PM1B (2 decks)

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