Sky, blue colour

Figure 11.13 X-feed system, (a) Central feed dog (gold colour) is up and transporting the fabric while needle is down, (b) The side feed dog (sky blue colour) is up and transporting the material while needle is up.

Rubidium was discovered as a minor constituent of lepidolite by R. W. Bunsen and G. R. Kirchhoff in 1861 only a few months after their discovery of caesium (1860) in mineral spa waters. These two elements were the first to be discovered by means of the spectroscope, which Bunsen and Kirchhoff had invented the previous year (1859) accordingly their names refer to the colour of the most prominent lines in their spectra (Latin rubidus, deepest red caesius, sky blue). 

The blue colour of the sky is, of course, due to Rayleigh scattering and not to electronic absorption by O2 molecules. 

Such efforts have met with limited success, and the reason usually advanced is our lack of understanding of the frequency dependence of molecular NLO properties. In classical electromagnetism, we refer to properties that depend on the frequency of radiation as dispersive and we say that (for example) dispersion is responsible for a rainbow. The blue colour of the sky is a dispersion effect, as is the red sky at night and morning. There is more to it than that, and you might like to read a more advanced text (Hinchliffe and Munn, 1985). 

Caesium (Cs, [Xe]6.vl), name and symbol from the Latin caesius (sky blue, the colour of two bright lines in its spectrum). Discovered (1860) by Robert Bunsen and Gustav Kirchhoff. 

Potassium iodide is coloured sky-blue when heated in a sealed tube with the vapours of potassium or sodium. The salt is also coloured by cathode rays.41 The decomposition of soln. of the alkali iodides by exposure to radium radiations, and ultra-violet light increases with increasing cone. A. Kailan supposed the radiations decompose the undissociated iodide liberating iodine and hydrogen both in acid and in alkaline soln. Aq. soln. of the alkali iodides are neutral, but, as 0. Loew 42 has shown, the soln. gradually acquires a yellow colour and an alkaline reaction when kept for say 8 to 10 days if air be excluded, the soln. remained colourless for 4 months. A. Houzeau attributed the effect to the presence of traces... 

The blue colour of sky, hazy illumination of the light beam from the film projector in a smoke filled theatre or light beams from the head-lights of a car on a dusty road are common examples of Tyndall effect. 

Lord Rayleigh 3 attributes the blue colour of the sea to that of the sky, seen by reflection. 

Liquid water, also in the form of droplets in clouds, have a much broader type of vibrational bands than the gaseous molecule. H20 shows many overtones and combination frequencies unexpectedly strong, compared with the three fundamental frequencies. The best known effect is perhaps the blue colour of liquid water, mainly due to a weak band close to the sodium lines at 17000 cm"1 (and absent in D20). With a spectroscope, it is easy to see the narrow lines of gaseous H20 and a broad band of the liquid in the daylight from the sky. 

Elliot also wrote a Medical Pocket Book, London, 1781, and later eds. (e.g. 1800) An Account of the Nature and Medicinal Virtues of the PrincipcU Mineral Waters 0/ Great Britain and Ireland, London, 1781, 2 ed. 1789 Observations on the Affinities of Substances in Spirit of Wine, Phil. Trans., 1786, Ixxvi, 155 a paper On the Blue Colour of the Sky, submitted to the Royal Society in December 1786, was not published. 

An alternative approach is to use hydrophobic indicators, which remain in the chloroform layer throughout the titration and give a very sharp colour change. Eppert and Liebscher [15] used dimethyl yellow (cationic) in titrations with carbethoxypentadecyltrimethylammonium bromide (Septonex). The colour change in the chloroform layer is from pink to yellow and is very sharp. Tsubouchi and Matsuoka [16] used the potassium salt of the ethyl ester of tetrabromophenolphthalein (anionic) for titration with tetradecyldimethylbenzylammonium chloride (Zephir-an) in a solution buffered at pH 6. The chloroform layer starts off yellow, becomes green near the end-point and turns sky blue when one drop of excess titrant is added. 

Titrate with 5 x 10 M NaTPB, shaking after each addition. The colour remains in the organic layer throughout, and changes sharply from sky blue to pale yellow at the end-point. 

---