Question

What turns alkaline pyrogallol solution dark brown?
A.${H_2}$
B.${H_2}O$
C.${O_2}$
D.None of the above

Answer 1

Pyrogallol is an organic compound. It is a white, water-soluble solid although samples are typically brownish because of its sensitivity toward oxygen. It is one of three isomeric benzenetroils. The aquatic plant Myriophyllum spicatum produces pyrogallic acid. When in alkaline solution, it absorbs oxygen from the air, turning brown from a colourless solution. It can be used in this way to calculate the amount of oxygen in air, notably via the use of the orsat apparatus.

Complete step by step answer:
In alkaline solution, pyrogallol absorbs oxygen rapidly from the air and turns brown due to the formation of a complex mixture of oxidation products.
Oxygen is absorbed in colorless alkaline solution of pyrogallol and turns dark brown.
Pyrogallol is the common name for 1,2,3-trihydroxybenzene. Hydroxyl groups are strong activators of aromatic systems. By the time 3 hydroxyl groups have been added to a benzene nucleus you have a very reactive molecule, reactive enough to react with oxygen in the air. The reaction is very complex and free radicals are thought to be involved. Free radical reactions involving activated aromatic rings often lead to colored, high molecular weight products that are not easy to characterize. Initial products are probably the ortho-quinone and 2-pyrone pictured below. Certainly the quinone will be colored, and as it reacts further to produce the higher molecular weight products, they will be colored too. Some of the higher molecular weight products contain at least 20 carbon atoms.
Hence the correct option is (C).

Note:
One can find its uses in hair dyeing, dyeing of suturing materials and for oxygen absorption in gas analysis. It also has antiseptic properties. Pyrogallol was also used as a developing agent in black-and-white developers, but its use is largely historical except for special purpose applications. Hydroquinone is more commonly used today. It is also used in isolation of inert gases from a mixture of gases, which requires absorption of oxygen from the mixture.