The way a coloured substance is used determines whether it is called a dye or a pigment. Dyes are soluble substances whereas pigments are insoluble.
Pigments can be spread in a surface layer or mixed into the bulk of a material. Dyes are incorporated into the bulk of a material and attach themselves to the molecules of the substance they colour. Most dyestuffs tend to be organic whereas pigments tend to be inorganic.
Monastral blue is one of the best blue pigments ever made. It consists of large flat molecules, each of which contains a 16-membered ring of alternating carbon and nitrogen atoms, with an iron atom at the centre.
The 16-membered ring consists of alternating single and double bonds, an arrangement known as a conjugate system of bonds. For each double bond, one pair of electrons is not confined to linking two particular carbon atoms as in a single bond, but is delocalised over the whole conjugated system. The lone pairs on two of the nitrogen atoms and four benzene rings are also involved, forming a huge delocalised structure. The more delocalisation, the more stable the structure. Molecules with extended conjugate systems tend to be coloured.
Chrome yellow is the pigment lead chromate which can be made by ionic precipitation.
Pb(NO3)2 (aq) + Na2CrO4 (aq) PbCrO4 (s) + 2NaNO3 (aq)
or Pb2+ (aq) + CrO42- (aq) PbCrO4 (s)
Other shades of yellow can be produced by making different metal chromates, e.g. zinc, barium, strontium.
Chrome yellow exists in more than one solid form in which the ions are arranged differently. It is said to be polymorphic. This results in different shades of yellow.
A paint is made up of two components:
In watercolours the medium is water whereas in spray paints the medium is an organic solvent e.g. methyl benzene, as this evaporates more quickly. In oil paints the medium is oil which dries more slowly.
The medium used to bind particles of pigment together to form a paint must be viscous enough to prevent the paint from running as it is applied. It must not be too sticky otherwise the artist's freedom would be restricted during painting. Once the paint has been applied, the medium must then dry and become hard in order to produce a durable painted surface. Natural oils, such as linseed or walnut oil are suitable.
Most oils (and fats) are esters of propane-1,2,3-triol (commonly called glycerol) with long chain carboxylic acids, such as palmitic acid and stearic acid.
As glycerol has three hydroxyl groups in each molecule, three carboxylic acid molecules can form ester linkages with each glycerol molecule to form a triester. The triester formed from glycerol and palmitic acid is shown below.
Most natural oils are mixed triesters in which the three acids groups are not the same. The carboxylic acids are usually unbranched containing 16 to 18 carbon atoms. They are sometimes called fatty acids. The alkyl groups (R groups) often contain one or more double bonds. Stearic acid and palmitic acids contain no double bonds and are said to be saturated. Linoleic acid contains two double bonds and is unsaturated.
Like all esters, oils and fats can be split up by heating with concentrated sodium or potassium hydroxide solution in a hydrolysis reaction. Glycerol and the sodium or potassium salt of the fatty acid is produced.
This is how soap is formed. Soaps are sodium or potassium salts of fatty acids.
Chromatography is an analytical technique used to separate and identify the components of a mixture. There are a number of different types, all of which depend on the equilibrium set up when a compound distributes itself between two phases. One phase stands still - the stationary phase, while the other moves over it - the mobile phase. In paper chromatography and thin-layer chromatography a small sample of the mixture is spotted onto the paper or plate which is supported in a suitable solvent. The solvent rises up the paper or plate. The distance travelled by a substance depends on:
The Rf value for a substance is the distance moved by the spot divided by the distance moved by the solvent. Rf values are always less than one, as the spot cannot move further than the solvent.
In gas-liquid chromatography the principle is the same, however, the mobile phase is an unreactive (carrier) gas such as nitrogen. The stationary phase is a finely divided powder coated with oil. This material is packed into a long thin tube called a column which is coiled inside an oven. Different compounds have different affinities for the stationary phase when compared with the mobile phase. The components leave the column at different times, known as retention times, after injection. Various detectors are used, including those which measure the thermal conductivity of the emerging gas. The results are usually presented as a graph where the area under each peak is proportional to the amount of that component.
This technique is very sensitive and has been used for testing athletes for drugs, analysing the alcohol content of blood and determining the oils used for old paintings when they are restored.
Part of this site was last updated on 21st January 2009.
This work is licensed under a Creative Commons Attribution-NonCommercial 2.0 England & Wales Licence.
Copyright © Article Gems 2006-09.