Chemistry of Colour Flashcards
Forensic Chemistry
How many colour cones do we have and what colours are they?
- Red
- Blue
- Green
How do we perceive colour?
- When we see the colour white all our are cones are stimulated equally.
- If you see a mix colour, only two cones will be stimulated.
- One cone can be more stimulated compared to another cone.
What electromagnetic range can humans see colour?
380-700nm
What does CIE mean?
Commission Internationale de L’Eclariage
What are the roles of receptor cells in colour?
- The receptor cells are located in the retina and they can be divided into cone and rod cells.
- The rod cells are responsible for night vision so they have a low affinity for light intensity.
- Cones cells are used for colour sensing and they have a high affinity for light intensity.
What are the tristimulus values for our coloured cones?
Blue= X
Green= Y
Red= Z
All equal values produce the colour white and this is due to all of our cones being stimulated.
The coordinates produce colours on the spectrum and some cones may be stimulated more than others.
What is a drawback of the CIE system?
- It can describe colour but it can not quantify colour as it is not represented equally on the diagram.
- Does not distinguish light and darkness of colour.
What can the tristimulus values be represented as?
- The values can be converted into two coordinates which can be located on the X and Y axis.
- The diagram they are plotted on is called a chromaticity diagram.
What is the shape of the chromaticity diagram called?
Hyper parabola shape
What is the relationship between the chromaticity diagram and saturation?
- The more saturated a colour is the closer to the edge of the parabola it will be.
- The more saturated a colour is means the more intense the colour is.
- Saturation is the strength of the dominant wavelength or hue.
What is the difference between the Munsell system and the chromaticity diagram.
- The Munsell system describes and quantifies colour.
- The chromaticity diagram just describes colour.
What is the Munsell system?
- It is a system that can describe and quantify colour.
- Can help in describing the colour of soil in forensics.
- Has three variables which are the Hue, Chroma and Value.
Define chroma
The intensity of the colour, how strong or weak it is.
Define hue
The actual colour.
Define value
- How light or dark a colour is.
- The depth of colour.
What are colorants?
- They are chromophores (particles that absorb light).
- They impart colour to an object.
- Dyes and pigments are the main colorants.
Describe the differences between dyes and pigments.
-Dyes are soluble in the solvent as they have a high affinity for the substrate it is deposited on.
-The pigment particles are suspended in the solvent and it is insoluble as the particles have not affinity for the solvent. Will not penetrate the solvent and will form a layer on the top of the surface.
Define dyes
- They can be natural or synthetic, organic, inorganic.
- It is soluble and it has a high affinity for its solvent.
What are the five types of dyes and what are they used for?
- Acid- used for the detection of fingerprints.
- Basic- used for the detection of greases or latent fingerprints.
- Azo- product of fast BB test.
- Natural
- Direct
What are the three types of pigments?
- Natural
- Inorganic
- Organometallic- contains a transition metal that is surrounded by organic molecules.
What does high and low opacity mean?
High opacity is when applying pain to a surface and not seeing the background colour and low opacity is applying a later of paint and seeing the background colour.
What does transparent mean?
Light being allowed to be past through so objects can be seen distinctly seen.
What is paint composed of?
- Water
- Pigment- add colour and opacity.
- Binder- binds the pigment to the wall.
- Surfactant and antifoam- reduces air bubbles.
- Thickener- thickens the paint when its on the wall.
What does the refractive index mean between the binder and pigment?
The pigment needs to have a higher refractive index compared to the binder.
What does refractive index mean?
The measure of light being bent from one matter to another.
What is the most commonly used white pigment?
Titanium dioxide
What are the three crystal structures found in titanium dioxide?
- Rutile
- Anatase
- Brooktite
What shape does rutile and anatase have?
Tetragonal
Out of rutile, anatase and brooktite, which one has the highest opacity?
- Rutile as it has a higher refractive index compared to the binder.
What are some key features of titanium dioxide?
- It is photocatalytic
- Self cleaning
- Oxidises dirt on wall, but will wash off when it rains.
Describe luminescent pigments.
- They are used in fluorescent inks and paints.
- They absorb photons (particles of light) at different wavelengths.
Describe pearlescent pigments.
- Can be described as having a ‘shimmering effect’.
- The ‘shimmering effect’ is due to the titanium dioxide having a high refractive index, so the pigment has a higher refractive insect compared to the binder.
Describe thermochronic pigments.
- Pigments that change colour when they are heated up; change in temperature.
What does vehicle mean?
It means solvent.
What is an azo dye?
- It is a large class of synthetic dyes.
- The molecules contain two adjacent nitrogen atoms between carbon atoms.
What is a solvent black dye?
- It is an azo dye.
- Is used to detect grease contaminated fingerprints.
What is a gentian violet dye?
- It is a basic dye.
- Used to detect sebaceous fingerprints that are found on tape.
What are the types of acid dyes?
- Yellow, black violet are acid dyes.
- Used to detect bloody fingerprints.
- Binds to the haem group.
What is an indigo dye?
- A carbonyl dye.
- Consists of two conjugated carbonyl groups.
Where can methyl blue be found?
- Ball point pens.
What is basic red used for in forensic chemistry?
- To enhance white superglue fume fingerprints to produce better enhancement.
What is copper phthalocyanines used for?
- Pigments in paints.
What are the different ‘colours’ used in ball point pens?
- Methyl violet.
- Victoria violet.
- Rhodamine red.
- Victoria green.
What dyes are used for rolling ball markers?
- Water soluble.
- Acidic dyes and salts.
Name three solvents that are ink oil based.
- Coconut fatty acids.
- Linseed oil.
- Mineral oil.
Name three solvents that are ink glycol based.
- Benzyl alcohol.
- Ethylene glycol.
- Glycerin.
What are fatty acids used for in ball point pens?
- To lubricate the ball point pen.
What are resins used for in ball point pens?
To help to dissolve the ink in the solvent.
What is surface acting agent used for in ball point pens?
-To reduce surface tension.
What are organic additives used for in ball point pens?
- To prevent corroding.
What is deforming agents used for in ball point pens?
- To prevent bubbles from forming in the ink.
What are glycols and formaldehydes used for in rolling ball pens?
- To prevent the pen from drying out.
What are the three types of techniques used for the analysis of inks and dyes?
- Raman spectroscopy.
- IR spectroscopy- identifying the different components.
- UV-vis analysis- differentiation between different inks.
What three types of analysis are used in pigments in paints?
- FTIR.
- Raman spectroscopy.
- XRF
what three types of analysis is used in dyes in fibres?
- FTIR.
-HPLC and TLC. - UV-Vis
