Colour within the Chemistry Lab

World of Colour Menu

Frontpage
Origin of Colour
Colour within the Chemistry lab
The physical measurement of colour
Measuring Concentrations using absorbance,
Transition Metal Solutions,
Rhodopsin and the eye
Basics behind Dyes and Pigments
Phosphorescence and Fluorescence
A Thermochromic example
Colour Therapy
Detection using Optical Sensors
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Action 3: Book turning. This image is taken from gifs (ref. 14) and is copyright restricted according to the source given (i.e. it is not the authors' own work).

 

The physical measurement of colour:  The primary colours are those that cannot be made by the mixing of other colours and yet between them make up all the colours of the spectrum. Red, blue and green are known as the additive primaries but yellow is thought of as the third primary instead of green in most contexts. There are three properties of colour that need to be considered:

The hue - the percentage of primary colours 

The chroma – richness of the colour

The lightness – this refers to the amount of light 

                                              reflected.

 

 

Figure 2: Colour wheel. The idea for this image is taken from Christie (ref. 3) and is copyright restricted according to the source given (i.e. it is not the author’s own work).

Using the colour wheel given, all three attributes can be quantified: the hue is within the plane of the colour wheel itself; the chroma increases with distance from the centre; and the lightness is the third dimension with black (no light reflected) and white (all light reflected) as the extremes.

 

Measuring concentrations using absorbance readings: In kinetics particularly, it is often essential to determine the concentration of reactant remaining or product produced by time t after the start of a reaction. When monochromatic light is passed through the sample, a percentage is absorbed if the energy gap DE of the compound corresponds to the frequency of the source and therefore if a sample is more concentrated less light passes to the detector.  The in-built reference helps in the conversion of light detected to actual sample absorbance.

 

Figure 3: Inside a spectrometer. This image is taken from  Atkins (ref. 3) and is copyright restricted according to the source given (i.e. it is not the author’s own work).

 

 

Using the Beer-Lambert Law given below, a calibration graph of known concentrations vs. absorbance can be drawn up and e determined. Therefore under the same standard conditions if absorbance is measured for a sample whose concentration is unknown, it can be read from the chart.

 

where A = absorbance of the solution, c = concentration of sample, l = path length  

 

 

Transition metal solutions: Why are they so colourful? The non-degeneracy of the five 3d orbitals in transition metal complexes is governed by their dissimilar orientations: the 2 “axial” orbitals (d(x2–y2) and d(z2) – i.e. those pointing directly at the ligands) are relatively de-stabilised compared to the 3 other orbitals (d(xy), d(xz) and d(yz)) which point between them. This is explained fully in the Crystal Field Theory of transition metal complexes and also has great bearing on what is currently under discussion.  For example, Titanium (III) is a colourless gas as a free ion however, due to an electronic d-d transition between the two now, non-degenerate d-orbital sets, the Ti3+ hexaaqua complex appears as a deep purple solution. Remember, objects which are exposed to white light appear coloured as they have absorbed the energy corresponding to the wavelength of the complimentary colour.  Other transition metal solutions are different colours due to the variation of Δoct which is itself dependent on the nature of the ligands within the complex. Simply click on the empty beaker below to see just a few transition metal solutions:  

Action 5: Some typical transition metal solution

 

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