1860

Oxidation States

Johann Rudolf Glauber (1604–1668), Henry Bollmann Condy (1826–1907)

Chemists often speak of the oxidation state of a given element. The convention is that the plain element, not part of any chemical compound, is in the zero oxidation state, and as electrons are removed or added (each electron being a single minus charge), the oxidation state moves up or down, respectively. The elements on the left-hand side of the periodic table lose electrons very easily and jump to a higher oxidation state at almost any opportunity, while the elements toward the right-hand side tend to pick up electrons. This is why sodium (all the way on the left-hand side of the periodic table) is usually in its +1 oxidation state and chlorine (nearly as far to the right as sodium is to the left) in its −1 state, which is how they balance out in table salt (sodium chloride).

Elemental sodium (in the zero oxidation state), on the other hand, is a soft, silvery metal that bursts into flame on contact with water, and elemental chlorine is a greenish gas that’s fiercely reactive and poisonous—both of them are dangerously ready to move out of their original oxidation states. Exposing sodium metal to chlorine gas would give you salt, but you’d have trouble retrieving it from the salty debris of the resulting explosion.

High-oxidation state metals are often ready to move to a lower-energy lower oxidation state by oxidizing something else in turn. This process can decolorize pigments, kill bacteria, clean impurities from surfaces, and perform a lot of useful chemistry. German chemist Johann Rudolf Glauber first described potassium permanganate, a brilliant purple compound with its manganese metal in the high +7 oxidation state, in 1659, but it wasn’t until 1860 that English chemist Henry Bollmann Condy, commercialized it as a disinfectant he called Condy’s fluid. (As the manganese atoms drop in oxidation state, they turn brown or pink.)

Many other metals in the middle of the periodic table (the “transition metals”) show similar color changes across their oxidation states, with chromium being another standout. These changes can be used to monitor their chemical reactions and also account for the vivid rainbow colors of the salts.

When dissolved in water, potassium permanganate has a vivid, unmistakable purple color that is instantly recognizable to any chemist who’s ever worked with it.