Trihydroxybenzene or more commonly known as benzenetriol are compounds that have three hydroxyl groups attached to a benzene ring. Because of the presence of three hydroxyl groups they are classified as polyphenols. These three hydroxyl groups can be present at different positions such 1,2,3 of the benzene ring, positions 1,2,4 on the benzene ring, and positions 1,3,5 on the benzene ring. Thus, there are three types of trihydroxybenzene, and each of them is the isomer of the other. The three types of trihydroxybenzene are: Hydroxyquinol - Benzene-1,2,4-triol, Phloroglucinol - Benzene-1,3,5-triol and Pyrogallol-1,2,3-triol. Thus, pyrogallol is one of the three benzenetriol with hydroxyl groups attached to the benzene ring in the pyrogallol structure at positions 1, 2, and 3 of the ring.
General Properties of Pyrogallol
It is clear that Pyrogallol is a trihydroxybenzene compound with three hydroxyl groups present instead of hydrogen. The molecular formula of Benzene is C6H6 with a resonating double bond. Hence, hydroxyl groups are substituted with hydrogen in pyrogallol structure, the molecular formula for Pyrogallol is given as C6H6O3 or C6H3(OH)3. The IUPAC name of Pyrogallol is Benzene-1,2,3-triol. Thus it is an aromatic ring compound and shares properties inclusive of both the functional hydroxyl groups and the benzene ring. For example, benzene is insoluble in water, but because of the presence of hydroxyl groups, Pyrogallol is a water soluble compound. Of the other names of Pyrogallol included are 1,2,3-Trihydroxybenzene and Pyrogallic acid.
Pyrogallol is a white crystalline organic compound. Yet some of the samples of the compounds can be seen to be typically brownish in colour. This is because of the sensitivity of Pyrogallol to oxygen. The pyrogallol structure is given in the image below:
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Some of the general properties of the compound pyrogallol or pyrogallic acid are mentioned as follows:
The molecular weight of pyrogallic acid or pyrogallol is 126.11 g/mol.
The density of the compound is 1.45 g/cm3.
The melting point of the compound is 131℃ - 134℃ and the boiling point is 309℃.
Reactions of Pyrogallol
The most important of the reactions of the compound pyrogallol is the reaction of its production. It was first prepared by Scheele in 1786. It was then prepared by the decarboxylation reaction of gallic acid as the reactant. It is this same method of preparation from gallic acid that is used till today for the production of pyrogallol. Due to the preparation from gallic acid, it is also known as pyrogallic acid. In the method, gallic acid is heated which induces decarboxylation of the carboxyl group present at position 1 of gallic acid. Hence, after the decarboxylation pyrogallol is produced. The reaction for the production of pyrogallol is given below:
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The source of gallic acid used for the production of pyrogallol is tannin. Owing to the high prices of tannin, many other preparation strategies and reactions have been devised for the production of pyrogallic acid. One such method utilizes para-chloro phenol disulfonic acid. The preparation method involves the treatment of para-chloro phenol disulfonic acid with potassium hydroxide. This reaction is a variant of the most commonly used reaction of production of phenols from sulfonic acid. Since, the production of phenols from sulfonic acids is a well established one, the production of pyrogallol from para chloro phenol sulfonic acid is also a more accurately handled procedure. In nature, the aquatic plant Myriophyllum spicatum is the one that is known to produce pyrogallic acid.
Uses of Pyrogallol
There are many uses of pyrogallol. One of the simplest ones being the measurement of oxygen content present in the surrounding air. Due to the presence of hydroxyl groups, Pyrogallol is reaction sensitive to oxygen. It can readily absorb oxygen from the air when it is in an alkaline environment. The alkaline pyrogallol absorbs the oxygen from the surrounding air or environment causing the colourless solution to turn brown. With the help of Orsat analyser, an alkaline pyrogallol solution can be used to estimate the amount of oxygen present in the air in the surroundings. Because of this property of oxygen absorption, it is also used for the isolation of inert gases from a mixture of gases.
Other common use of pyrogallol includes its use in hair dyeing and dyeing of suturing materials. But its recent use in hair dye formulation is known to have decreased because of the concerts regarding its toxicity. Pyrogallol has also been known and has been used for its antiseptic properties. It has been used in the past as a developing agent for black and white developers. Under special application requirements it can still be used for the same. For the purpose of developing, pyrogallol has been largely replaced by hydroquinone in common use. But a generalist revival led to its more common use than before alongside hydroquinone in the developing industry.
One of the most common applications of pyrogallol has been in the photographic industry. It had been a notable photographic developing agent in the 19th and early 20th centuries. Its use was then drastically reduced in the 1920's and the following years because of its property of oxidising itself by absorption of oxygen which in turn caused pyrogallol to behave unpredictably and erratically. Even then it had been continuously used by notable photographers like Edward Weston. It again experienced prominence in its use as a photographic developing agent in 1980’s because of the extensive efforts put in by the photographic industry experimenters Gordon Hutchings and John Wimberly.
Some of the well known variants of pyrogallol and mixtures containing pyrogallol that are now being used extensively for the purposes of photographic development are given below:
PMK: PMK is a photographic developing agent developed by Gordon Hutchings. Htuchings experimented with Pyrogallol for over a decade working on different formulas to provide consistency to the developing agent. The successful one was named as PMK. The name PMK stands for the main ingredients that it uses which are - Pyrogallol, Metol and Kodalk. The third ingredient, Kodalk, is the trade name used by Kodak (the well known and historic analogue photographic company) for sodium metacarbonate. The PMK mixture resolved the long standing issues of consistency that existed for pyrogallol because of its property of absorbing oxygen. Hutchings also found that the interaction between the greenish stain that is given by the pyrogallol developers and the colour sensitivity of the modern variable contrast photographic papers provided an extreme compensating developer.
WD2D: During the years from 1969 to 1977, John Wimberly also experimented with pyrogallol functioning as a developing agent. The pyrogallol developing formula named WD2D was published by him in Petersen's Photographic. This pyrogallol formulation is also widely used by modern photographic developers alongside PMK. It is also used by many of the black-and-white photographic developers.
510-Pyro: The 520-Pyro developer was formulated by Jay DeFehr. It is also a formulation based on the use of pyrogallol as a developing agent. It is a concentrate mixture that is known to use Triethanolamine as an alkali reagent and use of pyrogallol and phenidone as combined developers. This formulation is said to have both staining and tanning properties. The negatives that have been developed using this formulation are observed to be immune to the callier effect (an effect known to cause variation in contrast to the images that are produced on a photographic film by different manners of illumination). The formulation can be used for both, small and large negative film formats.
Two common books that can find use of pyrogallol as a developing agent are The Film Developing Cookbook and The Darkroom cookbook. They are amongst the primary books that are used for referring to the trends and well established procedures in the photographic developing industry.