Lecture 5 - Chemistry of colour Flashcards
What are Chromaphores?
Light pigments that impart colour to an object
What wavelength on the electromagnetic spectrum shows visible light?
400-750nm
How do we see white?
We see white when an object is reflecting all wavelengths of visible light
How do we see black?
We see black when an object is absorbing all wavelengths of visible light
How do we see colour?
Humans have 3 congruent cones in our retinas, that allow us to see colour when they are stimulated
What is the red cone in our retina?
Stimulated by red light, optimum wavelength 560nm
What is the blue cone in our retina?
Stimulated by blue light, optimum wavelength 420nm
What is the green cone in our retina?
Stimulated by green light, optimum wavelength 530nm
What does it mean when we see a mixed colour, e.g. yellow?
2 of our cones are stimulated, e.g red and blue
What is the shade of the colour we see dependant on?
Which cone is stimulated most
Why are there systems in place to identify colour?
Colour is a difficult concept to quantify
What are the ISO systems in place to quantify colour?
- CIE system
- Munsell system
How does the CIE system characterise colour?
- Our coloured cones: red, green, blue
- Mathematical number
- Tristimulus value
What are the tristimulus values for the CIE system?
Blue: X
Green: Y
Red: Z
What are the properties of the tristimulus values in the CIE system?
- Always produce a positive value
- Produces any colour in the spectrum
- Equal values of X, Y, Z produce white
- Y coordinates determine luminescence
What’s the equation to determine the X coordinate on the chromaticity diagram?
X= X/(X+Y+Z)
What’s the equation to determine the Y coordinate on the chromaticity diagram?
Y= Y/(X+Y+Z)
What shape is the chromaticity diagram?
A parabola shape
What colour does the chromaticity diagram show a large representation of?
Green
Why can the CIE system not be used to quantify colour?
All colours are not represented equally
What can the CIE system not do when describing colour?
distinguish how light or dark it is
Where do paler colours fall on the chromaticity diagram?
closer to the interior
What is the white point on the chromaticity diagram?
- achromatic
- When all cones are stimulated equally
What is Munsell system used for in forensic science?
To describe and quantify the colour of soil
What are the 3 variables the munsell system is composed of?
- Hue
- Value
- Chroma
What is the hue variable in the munsell system?
The actual colour
What is the value variable in the munsell system?
Brightness of the colour
What is the chroma variable in the munsell system?
The strength of the color (saturation)
How are the hues arranged in the munsell system?
In a circle
Which hues have the largest spots in the munsell system?
Red, Blue, Yellow, Green, Purple
How are the values arranged in the munsell system?
A vertical axis, ranging from 0-10
*Each shade has a designated number
Where does the chroma come from in munsell system?
Comes from the value pole and reaches maximum saturation as it gets further away from the pole.
What does each colour on the munsell system have?
A unique identifying number
*First identifier is the hue, then the value
What are the main colourants in forensic science?
Dyes and pigments
What is a dye?
- Dyes are soluble in the solvent there in
- They have an affinity for the substrate it’s deposited on
What is a pigment?
- 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
What are acidic dyes?
Used for the detection of bloody fingerprints
e.g. Acid red 33
What are basic dyes?
Used for detection of greasy fingerprints
e.g. Basic violet 3
What are azo dyes?
Created by coupling reactions, e.g. the end product of a fast blue BB salt test.
e.g. Solvent red 1
What are natural dyes?
Used for colouring textiles, food etc.
e.g. Natural yellow 26 (B carotene)
What are direct dyes?
Applied directly to cellulose in clothes fibres
e.g. Direct red 28
What are solvent dyes
molecular dyes rather than ionic, they dissolve in non-polar solutions
e.g. solvent red 1
What are inorganic pigments?
Transition metal oxides or synthetic chemicals
e.g. Titanium dioxide in white paint
What are organometallic pigments?
A transition metal surrounded by organic molecules
e.g. copper thiocyanite in paints and inks
What are natural pigments?
Coloured substances derived from natural substances
e.g. lapis lazuli, red and yellow ochre
What is pigment opacity?
The ability of paint to stop the transmission of light
What is high pigment opacity ?
Less coats of paint are needed to stop the transmission of light of the previous paint
How to optimise pigment opacity?
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
What is paint?
A liquid that oxidises in the air
What is paint composed of?
- Water
- A pigment to give colour and opacity
- Binder to bind paint to the wall
- Surfactant; Reduces surface tension
- Anti-foam
What is titanium dioxide?
An organic pigment, existing in 3 crystal structures
What are the 3 crystal structures of titanium dioxide?
Rutile, Anatase and brookite
What is titanium dioxide used in?
White paint production
What is the refractive index of rutile?
2.7
What is the refractive index of Anatase?
2.55
Why does rutile have a better opacity than anatse or brookite?
It has a higher RI
What does photocatalytic mean?
Self-cleaning: The paint particles will combine with sunlight and will oxidise any dirt
What must be considered to maximise the scattering of light of pigments?
-strength of the particles
-size of the particles
-RI of the particles
What are luminescent pigments?
Used in fluorescent paints
- They absorb photons of light of a specific wavelength and emit photons of a longer wavelength
What are pearlescent pigments?
Used in pearlescent paints, e.g on cars
How do pearlescent pigments produce a shimmering effect?
- 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
What are thermochromic pigments?
Sensitive to temperature and will change colour
What is solvent black used for in forensics?
- An azo dye (Due to N=N group)
- Detects grease contaminated
fingerprints
What is Gentian violet used for in forensics?
- A basic dye
- Detects sebaceous (oil based) fingerprints
What are acid dyes used for in forensics?
- Acid yellow, black and violet.
- Used to detect bloody fingerprints, as they bind to the haem group in blood.
How is the Indigo dye used in forensics?
- A carbonyl dye (C=O) group
- Found in blue fibres
What is methyl violet used for in forensics?
- Dye used in ball point ink
What is basic red 40 dye used for in forensics?
- Enhances white superglue fumed fingerprints
What are the components of an ink?
- Colourant: dye or pigment
- Solvent: dyes dissolve or pigment suspended
- Additives: Chemicals to adjust ink properties
What are examples of ballpoint pen inks?
-Methyl violet
-Victoria blue
-Victoria green
-Copper phthalocyanine
-Luxol fast
What are oil based solvents in inks?
-Mineral oil
-Linseed oil
-Fatty acids
-Isophorone
-Methyl ethyl ketone
What are glycol based solvents in inks?
-Ethylene glycol
-Glycerin
-Benzyl alcohol
What are examples of additives in inks?
- Fatty acids (lubricate)
- Natural resin (Dissolves the ink)
- Defoaming agents
- Organic additives (prevents corroding)
- Glycols (prevents drying)
How are dyes and inks analysed in forensics?
- Uv-vis spectroscopy: Differentiates between inks but can’t tell composition
- Infra-red spectroscopy: Differentiates and shows composition of inks.
- Raman spectroscopy: can differentiate
How are paint pigments analysed in forensics?
- FTIR: IR light scans samples and detects chemical properties
- Raman & FTIR: Can be combined if one doesn’t provide enough info.
- Xray fluorescence: Can confirm presence of a pigment
How are dyes in fibres analysed in forensics?
- FTIR: Has a fibre attachment, if not dye must be extracted.
- HPLC & TLC: dye extracted into solvent and analysed
3.UV-VIS: attached to a microscope, if not dye extracted into solvent.
