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Forensic Chemistry > Lecture 5 - Chemistry of colour > Flashcards

Lecture 5 - Chemistry of colour Flashcards

1
Q

What are Chromaphores?

A

Light pigments that impart colour to an object

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2
Q

What wavelength on the electromagnetic spectrum shows visible light?

A

400-750nm

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3
Q

How do we see white?

A

We see white when an object is reflecting all wavelengths of visible light

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4
Q

How do we see black?

A

We see black when an object is absorbing all wavelengths of visible light

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5
Q

How do we see colour?

A

Humans have 3 congruent cones in our retinas, that allow us to see colour when they are stimulated

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6
Q

What is the red cone in our retina?

A

Stimulated by red light, optimum wavelength 560nm

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7
Q

What is the blue cone in our retina?

A

Stimulated by blue light, optimum wavelength 420nm

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8
Q

What is the green cone in our retina?

A

Stimulated by green light, optimum wavelength 530nm

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9
Q

What does it mean when we see a mixed colour, e.g. yellow?

A

2 of our cones are stimulated, e.g red and blue

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10
Q

What is the shade of the colour we see dependant on?

A

Which cone is stimulated most

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11
Q

Why are there systems in place to identify colour?

A

Colour is a difficult concept to quantify

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12
Q

What are the ISO systems in place to quantify colour?

A
  • CIE system
  • Munsell system
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13
Q

How does the CIE system characterise colour?

A
  • Our coloured cones: red, green, blue
  • Mathematical number
  • Tristimulus value
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14
Q

What are the tristimulus values for the CIE system?

A

Blue: X
Green: Y
Red: Z

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15
Q

What are the properties of the tristimulus values in the CIE system?

A
  • Always produce a positive value
  • Produces any colour in the spectrum
  • Equal values of X, Y, Z produce white
  • Y coordinates determine luminescence
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16
Q

What’s the equation to determine the X coordinate on the chromaticity diagram?

A

X= X/(X+Y+Z)

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17
Q

What’s the equation to determine the Y coordinate on the chromaticity diagram?

A

Y= Y/(X+Y+Z)

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18
Q

What shape is the chromaticity diagram?

A

A parabola shape

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19
Q

What colour does the chromaticity diagram show a large representation of?

A

Green

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20
Q

Why can the CIE system not be used to quantify colour?

A

All colours are not represented equally

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21
Q

What can the CIE system not do when describing colour?

A

distinguish how light or dark it is

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22
Q

Where do paler colours fall on the chromaticity diagram?

A

closer to the interior

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23
Q

What is the white point on the chromaticity diagram?

A
  • achromatic
  • When all cones are stimulated equally
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24
Q

What is Munsell system used for in forensic science?

A

To describe and quantify the colour of soil

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25
Q

What are the 3 variables the munsell system is composed of?

A
  • Hue
  • Value
  • Chroma
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26
Q

What is the hue variable in the munsell system?

A

The actual colour

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27
Q

What is the value variable in the munsell system?

A

Brightness of the colour

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28
Q

What is the chroma variable in the munsell system?

A

The strength of the color (saturation)

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29
Q

How are the hues arranged in the munsell system?

A

In a circle

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30
Q

Which hues have the largest spots in the munsell system?

A

Red, Blue, Yellow, Green, Purple

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31
Q

How are the values arranged in the munsell system?

A

A vertical axis, ranging from 0-10
*Each shade has a designated number

32
Q

Where does the chroma come from in munsell system?

A

Comes from the value pole and reaches maximum saturation as it gets further away from the pole.

33
Q

What does each colour on the munsell system have?

A

A unique identifying number
*First identifier is the hue, then the value

34
Q

What are the main colourants in forensic science?

A

Dyes and pigments

35
Q

What is a dye?

A
  • Dyes are soluble in the solvent there in
  • They have an affinity for the substrate it’s deposited on
36
Q

What is a pigment?

A
  • Pigments are particles that are suspended in a solvent
  • They have no affinity for a substrate, i.e they will lay on top of a substrate rather than penetrate the sample
37
Q

What are acidic dyes?

A

Used for the detection of bloody fingerprints
e.g. Acid red 33

38
Q

What are basic dyes?

A

Used for detection of greasy fingerprints
e.g. Basic violet 3

39
Q

What are azo dyes?

A

Created by coupling reactions, e.g. the end product of a fast blue BB salt test.
e.g. Solvent red 1

40
Q

What are natural dyes?

A

Used for colouring textiles, food etc.
e.g. Natural yellow 26 (B carotene)

41
Q

What are direct dyes?

A

Applied directly to cellulose in clothes fibres
e.g. Direct red 28

42
Q

What are solvent dyes

A

molecular dyes rather than ionic, they dissolve in non-polar solutions
e.g. solvent red 1

43
Q

What are inorganic pigments?

A

Transition metal oxides or synthetic chemicals
e.g. Titanium dioxide in white paint

44
Q

What are organometallic pigments?

A

A transition metal surrounded by organic molecules
e.g. copper thiocyanite in paints and inks

45
Q

What are natural pigments?

A

Coloured substances derived from natural substances
e.g. lapis lazuli, red and yellow ochre

46
Q

What is pigment opacity?

A

The ability of paint to stop the transmission of light

47
Q

What is high pigment opacity ?

A

Less coats of paint are needed to stop the transmission of light of the previous paint

48
Q

How to optimise pigment opacity?

A

To optimise the scattering of the pigment particles, ensure the diameter of the particles is half that of the wavelength of light being directed at the particles

49
Q

What is paint?

A

A liquid that oxidises in the air

50
Q

What is paint composed of?

A
  • Water
  • A pigment to give colour and opacity
  • Binder to bind paint to the wall
  • Surfactant; Reduces surface tension
  • Anti-foam
51
Q

What is titanium dioxide?

A

An organic pigment, existing in 3 crystal structures

52
Q

What are the 3 crystal structures of titanium dioxide?

A

Rutile, Anatase and brookite

53
Q

What is titanium dioxide used in?

A

White paint production

54
Q

What is the refractive index of rutile?

A

2.7

55
Q

What is the refractive index of Anatase?

A

2.55

56
Q

Why does rutile have a better opacity than anatse or brookite?

A

It has a higher RI

57
Q

What does photocatalytic mean?

A

Self-cleaning: The paint particles will combine with sunlight and will oxidise any dirt

58
Q

What must be considered to maximise the scattering of light of pigments?

A

-strength of the particles
-size of the particles
-RI of the particles

59
Q

What are luminescent pigments?

A

Used in fluorescent paints
- They absorb photons of light of a specific wavelength and emit photons of a longer wavelength

60
Q

What are pearlescent pigments?

A

Used in pearlescent paints, e.g on cars

61
Q

How do pearlescent pigments produce a shimmering effect?

A
  • The titanium dioxide is coated in a thin layer of mica
  • The titanium dioxide has a higher RI than the mica, producing the shimmering effect
62
Q

What are thermochromic pigments?

A

Sensitive to temperature and will change colour

63
Q

What is solvent black used for in forensics?

A
  • An azo dye (Due to N=N group)
  • Detects grease contaminated
    fingerprints
64
Q

What is Gentian violet used for in forensics?

A
  • A basic dye
  • Detects sebaceous (oil based) fingerprints
65
Q

What are acid dyes used for in forensics?

A
  • Acid yellow, black and violet.
  • Used to detect bloody fingerprints, as they bind to the haem group in blood.
66
Q

How is the Indigo dye used in forensics?

A
  • A carbonyl dye (C=O) group
  • Found in blue fibres
67
Q

What is methyl violet used for in forensics?

A
  • Dye used in ball point ink
68
Q

What is basic red 40 dye used for in forensics?

A
  • Enhances white superglue fumed fingerprints
69
Q

What are the components of an ink?

A
  • Colourant: dye or pigment
  • Solvent: dyes dissolve or pigment suspended
  • Additives: Chemicals to adjust ink properties
70
Q

What are examples of ballpoint pen inks?

A

-Methyl violet
-Victoria blue
-Victoria green
-Copper phthalocyanine
-Luxol fast

71
Q

What are oil based solvents in inks?

A

-Mineral oil
-Linseed oil
-Fatty acids
-Isophorone
-Methyl ethyl ketone

72
Q

What are glycol based solvents in inks?

A

-Ethylene glycol
-Glycerin
-Benzyl alcohol

73
Q

What are examples of additives in inks?

A
  • Fatty acids (lubricate)
  • Natural resin (Dissolves the ink)
  • Defoaming agents
  • Organic additives (prevents corroding)
  • Glycols (prevents drying)
74
Q

How are dyes and inks analysed in forensics?

A
  1. Uv-vis spectroscopy: Differentiates between inks but can’t tell composition
  2. Infra-red spectroscopy: Differentiates and shows composition of inks.
  3. Raman spectroscopy: can differentiate
75
Q

How are paint pigments analysed in forensics?

A
  1. FTIR: IR light scans samples and detects chemical properties
  2. Raman & FTIR: Can be combined if one doesn’t provide enough info.
  3. Xray fluorescence: Can confirm presence of a pigment
76
Q

How are dyes in fibres analysed in forensics?

A
  1. FTIR: Has a fibre attachment, if not dye must be extracted.
  2. HPLC & TLC: dye extracted into solvent and analysed

3.UV-VIS: attached to a microscope, if not dye extracted into solvent.

Forensic Chemistry (7 decks)

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