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Stories About Chemistry


82. It's Only a Drop of Mercury that Does It

From hoary antiquity comes the saying: “All things of genius are simple.”

Only once was the Nobel Prize awarded for a discovery in the field of chemical analysis. The discovery was made in 1922 by Yaroslav Heyrovskı, the famous Czech scientist. Since then Prague has become a kind of Mecca. Numerous pilgrims have travelled to Prague to learn Heyrovskı’s new method, polarography.

Today over a thousand papers are published yearly all over the world on polarographic analysis. The ABC of the method is as follows. Some mercury is placed at the bottom of a glass beaker containing the solution in which the concentration of a given substance has to be determined. The mercury layer serves as an electrode. Drops of mercury falling at definite intervals from a capillary into the beaker are the other electrode.

A source of electricity is connected to the electrodes. This should cause electrolysis in the solution. But electrolysis will occur only if the potential on the mercury drop is high enough. If the potential is small, no current will flow through the circuit. It is increased until the ions contained in the solution begin to discharge. Then current begins to flow through the circuit.

If there are ions of different elements in the solution, each species will be discharged at a different potential value characteristic of those particular ions.

The chemists plot graphs, marking off the potential values along the abcissa axis, and the currents that appear along the ordinates. The resulting curve resembles a staircase, each step corresponding to the discharge of a definite species of ions.

The staircase obtained is compared with a standard curve, one which has been plotted beforehand for a solution containing known concentrations of known substances.

Thus the solution can be analysed qualitatively and quantitatively at the same time. By means of special devices the analysis can be made automatic.

The first epithet that occurs to you respecting the polarographic method is elegant. But elegance is not the only thing. Polarography is simple, rapid, and precise; moreover, it surpasses most other methods of analysis in these qualities. For example, zinc can be determined polarographically with as little as a millionth of a gram of zinc chloride in one cubic centimetre of solution. This analysis takes less than ten minutes.

Heyrovskı’s original idea has now been perfected and many new versions have been suggested. One of these is adsorption polarographic analysis which has a very high sensitivity. It can readily be used to determine organic substances in concentrations as low as thousand-millionths of a gram per cubic centimetre of solution.

Where is polarography needed? Why, practically everywhere: for automatic production control, and for analysis of minerals and alloys. Polarography gives an idea of the contents of vitamins, hormones and poisons in the organism. Physicians are even thinking of using polarography for early diagnostics of cancer.

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by Ian Ellis
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