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107

Stories About Chemistry

INDEX

81. Waves and Substance

The number of colour hues in nature is infinite. Chemists know that as well as anybody else. And not infrequently the fantastic gamut of colours leaves them nonplussed.

“What colour is, say, a solution of neodymium nitrate?”

“Pink,” replies the chemist.

“And what colour will a solution of trivalent iron turn if potassium thiocyanate is added to it?”

“Red.”

“And what colour does phenolphthalein turn if a solution of alkali is added to it?”

“Crimson.”

We could continue almost indefinitely: very many chemical reactions involve colour of a definite hue. Probably if we were to name a dozen more compounds whose solutions are coloured some shade of red, we should get entirely confused. They say that artists and textile workers who dye fabrics can distinguish about two dozen shades of red. Now that is what acquiring an eye for colour means!

But such an “intuitive” way of distinguishing hues and shades is of little use to chemists. Even a solution of the same substance may have an infinite number of shades depending on its concentration. How could they all be remembered?

There appear to be people on Earth who can distinguish colours with their eyes blindfolded by using their fingertips. Doctors say that such people have highly developed cutaneous vision. The famous writer Jonathan Swift wrote sarcastically of the Laputan Academy of Sciences that the blind mixed various colours there to ironize on the “scientific” subjects they studied.

Today the sarcasm of the English satirist is no longer appropriate. Now chemists can name the colour of a solution without seeing it. They do it with the aid of what is known as spectrophotometry. This distinctive method of analysis derives its name from an instrument called the spectrophotometer with which the colour of a chemical compound or its solution can be analysed.

Isaac Newton passed a narrow sunbeam through a glass prism and discovered that white light is complex. We have all of us probably seen a rainbow. The colours of the rainbow are the components of white light. Newton observed such a rainbow when he passed the sunbeam through the prism. This rainbow is called a spectrum.

But what is light? It is electromagnetic vibrations or waves. Each wave has a definite length (usually denoted by the Greek letter “lambda”). The wavelength exactly characterizes any colour or shade. For example, chemists say: “a red colour of the wavelength 620 millimicrons” or: “a red colour of the wavelength 637 millimicrons” (a millimicron is one thousandth of a micron, or one millionth of a millimetre). This eliminates the need to give definite names to the separate shades, such as “crimson,” “red,” “bordeau,” “scarlet,” “vermilion”, etc. Just name the wavelength and all the scientists in the world will know quite definitely the colour and shade meant. Each compound has now received a kind of “certificate” with “Lambda equals so much” filled in under the item “colour”. Believe us, it is a very reputable document.

But this is only half the matter. The colour of a compound depends on the wavelengths of the rays it absorbs and those of the rays it transmits. For instance, if a solution of a nickel salt is green, it absorbs light of all the wavelengths except those which correspond to the green hue. A yellow solution of potassium chromate is transparent only to yellow rays.

The spectrophotometer gives beams of light rays of definite wavelengths and makes it possible to determine how they are absorbed by various substances. A great number of compounds, organic and inorganic, have been investigated with spectrophotometers.

Besides visible light there is light that is invisible, that the human eye does not see. These kinds of light “from the beyond” lie outside the limits of the visible light spectrum, and are called ultraviolet and infrared rays. Chemists have learned to use them too. They studied the spectra of various chemical substances in the ultraviolet and infrared ranges and discovered a very interesting phenomenon. They found that each chemical compound (or ion) has its own distinctive absorption band spectrum. Each substance has its own “colour (infrared or ultraviolet) certificate.”

Absorption spectra can be used not only for qualitative, but for quantitative analysis as well. This is due to the fact that in many cases the higher the concentration of a chemical compound in a solution, the more intense its colour, i.e., the stronger it absorbs colour of a definite wavelength. Hence, by determining the absorption of light by a solution (one usually says, “by determining its optical density”), one can easily find the concentration of any particular element.


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by Ian Ellis
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