Colorimetry

= =

Colorimetry
In chemistry, **colorimetry** is the measurement of the concentration of a solution of a coloured substance by reference to (comparison with) standard solutions. The instrument used for this purpose is called a**colorimeter**. The concentration of the solution under investigation may be changing during the course of a chemical reaction, such as in a reaction kinetics experiment. Reference to the above colour wheel can be made to determine a suitable wavelength of light to be used in the instrument. A colorimeter is used to determine the absorbance of the solution at intervals of time. The concentration corresponding to each absorbance measurement is read from a 'calibration curve'. A 'concentration against time' graph can now be plotted.

=More about Colour =

It does..., subjectively in our minds.
Our perception of colour is a complex neurophysiological process; the retina in the eye produces a physiological response to light, which in turn is subject to neural processing by the brain. Though this might be helpful, it is, of course, a huge over-simplification hardly doing justice to the complexity of colour perception.

The Visible Spectrum...
The eye is sensitive to a broad band of wavelengths within the approximate range 350-750 nm of the spectrum of electromagnetic radiation. We perceive as colours only those wavelengths within this range. Light with a wavelength of 350 nm is subjectively perceived as violet, and light with a wavelength of 750 nm as red. The wavelengths in-between are perceived as blue, green , yellow and orange. The colour of light of a single wavelength or of a narrow band of wavelengths is known as a **pure spectral colour**. The wide variety of colours we see, however, are mixtures of light of various wavelengths.

Primary Colours...
Mixing varying quantities (intensities) of red, green , and blue lights can produce almost any **colour sensation**. This is why these colours are known as **primary colours** or **additive primary colours**. When they are mixed in equal intensities, white light is produced.

Cyan, magenta , and yellow are also **primary colours**, producing white light when mixed in equal intensities.

> Or, when an object removes blue light from white light it appears yellow. > Or, when an object removes green light from white light it appears magenta. > Or, when an object removes red light from white light it appears cyan. As illustated below, mixing yellow, magenta and cyan lights amounts to combining red , green and blue lights.
 * A mixture of red and green lights produce yellow light.
 * A mixture of red and blue lights produce magenta light.
 * A mixture of green and blue lights produce cyan light.

Why is an object a particular colour?
Remember, in most circumstances, the colour an object appears is a consequence of the wavelengths of light arising from it and entering the eyes. With reference to the three primary colours in white light, a solution of potassium chromate(VI) appears yellow because it is absorbing blue light. Also, because the eye is not a machine the same colour sensation can result in different ways. For example, a mixture of red and green light of the right intensities appears exactly the same as spectral yellow even though it does not contain light of wavelengths corresponding to yellow. > Move mouse over picture... ||= ||  Yellow, magenta and cyan are also known as **subtractive primary colours** (or pigment primaries), because pigments of these colours, mixed in suitable proportions, can match almost any colour. However, a mixture is always darker than the component pigments. Mixing all three subtractive primaries in equal amounts will produce a very dark colour, though it is never completely black. Colour (CMYK) inkjet printers, for example, make use of this. A colour cartridge may contain the three ink colours, **C**yan, **M**agenta and **Y**ellow. Although 'nearly black' can be produced by mixing these three, most printers also contain a blac**K** ink cartridge for better quality and economy. > === Complementary Colours... === Also, there are certain pairs of pure spectral colours which when added produce the sensation of white light. These are called **complementary colours**. When only one colour is absorbed, the substance has the complementary colour. For example, the manganate(VII) ion absorbs green (typical wavelength 524 nm) and appears purple (typical wavelength 430 nm). These are shown in the table below, and illustrated by the **colour wheel**. > wavelength (nm) absorbed ||||= Complementary colour > observed by eye (nm) || > ||= Violet ||= 410 ||= Yellow ||= 573 || > ||= Blue ||= 481 ||= Orange ||= 604 || > ||= Green ||= 524 ||= Purple ||= 430 || > ||= Red ||= 650 ||= Blue-green ||= 500 || >
 * A solution of potassium chromate(VI) appears yellow because it absorbs blue light.
 * = Colour and typical