Here is a complete explanation:

Green or dark urine in carbolic acid poisoning is called carboluria. The urine may appear normal or have a greenish hue when first passed, but turns dark or olive green on standing and exposure to air.

1. Absorption of Phenol Carbolic acid (phenol) is readily absorbed through all routes - ingestion, inhalation, and even intact skin - and rapidly enters the bloodstream.

2. Hepatic Metabolism In the liver, phenol is partially oxidized to two key metabolites:

The rest is excreted unchanged, partly free and partly conjugated with sulphuric acid or glucuronic acid.

3. Renal Excretion These metabolites (pyrocatechol and hydroquinone) are filtered by the kidneys and excreted in the urine. Freshly voided urine may look nearly normal.

4. Oxidation in Air - The Key Step When the urine is exposed to air (oxygen), pyrocatechol and hydroquinone undergo oxidation and polymerization, forming dark-colored quinone polymers (melanin-like pigments). This is why the color develops on standing - it is not visible immediately.

The reaction:

The same metabolites (hydroquinone and pyrocatechol), in chronic poisoning, can deposit in cartilage, cornea, and venous complexes, causing bluish-black pigmentation - a condition called ochronosis (normally associated with alkaptonuria, but also seen here).

The green color of urine in carbolic acid poisoning is due to the air-oxidation of pyrocatechol and hydroquinone - the hepatic oxidation products of phenol - which are excreted in urine and polymerize into dark quinone pigments when exposed to oxygen. The urine color progresses from greenish to olive-green to almost black on prolonged standing.