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107

Stories About Chemistry

INDEX

83. A Chemical Prism

By a strange coincidence, this scientist’s name and profession are consonant with his discovery.

He was a botanist and his name Mikhail Tsvet. Tsvet is the Russian for colour. The botanist Tsvet was interested in chlorophyll which, as we know already, is the colouring matter of green leaves.

But Professor Tsvet also knew many chemical procedures. In particular, he knew that there were substances (adsorbents) which could retain (adsorb) many gases and liquids on their surfaces.

He ground a leaf into a green paste and treated it with alcohol. The paste lost its colour, which meant that all its colouring matter had been extracted by the alcohol.

Then he filled a glass tube with chalk moistened slightly with benzene; into this tube he poured the solution containing the chlorophyll.

The upper layer of the chalk powder turned green.

The scientist washed the chalk in the tube with benzene, adding it drop by drop. The green ring budged and then began to move downwards And then—oh wonder!—it separated into several bands of different colours. There were a yellow-green, a green-blue and three yellow bands of different shades. It was a curious sight that the botanist Tsvet observed. And this sight proved a most important find for chemists.

It indicated that chlorophyll was a complex mixture of several compounds, different, though close, in molecular structure and properties. What is now called chlorophyll was only one of them, true, the most important one. All these substances had now been separated from one another by a very simple method.

All of them had been adsorbed by the chalk, but each in its own way. The strength of retention of each compound by the surface of the chalk powder was different. And when the benzene (the eluant) passed through the tube, it carried off the substances in a definite sequence. First those that were held less strongly, and then those that were retained more strongly. This caused the separation.

Like a prism resolves solar light into the spectrum colours, so did the column of adsorbent (the “chemical prism”) split up the complex mixture of substances into its component parts. This new method of analysis discovered by Tsvet in 1903 was christened chromatography by its author. This word comes from the Greek for “colour writing.”

Today the method of chemical “colour writing” is one of the most important instruments in all analytical laboratories the world over.

But the fates of many scientific discoveries are inscrutable. Some are forgotten, sometimes for many years, only to shine afterwards on the scientific horizon like stars of the first magnitude. So was it with chromatography. It was actually remembered only in the forties, and nobody ever regretted this.


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