1. Basic Identification Information

Parameter	English Specification/Value
IUPAC Name	bis(4-(methylamino)phenol); sulfuric acid (also: 4-(Methylamino)phenol sulfate (2:1) salt)
Common Name(s)	p-Methylaminophenol Sulfate; Metol; N-methyl-p-aminophenol sulfate; Bis(4-hydroxy-N-methylanilinium) sulfate; 4-Methylaminophenol hemisulfate
CAS Registry Number	55-55-0
EC/EINECS Number	200-232-0
Molecular Formula	(C₇H₁₀NO)₂·H₂SO₄ (or C₁₄H₂₀N₂O₆S)
Molecular Weight	344.38 g/mol
Chemical Class	Aminophenol sulfate salt; oxidative hair dye coupler; photographic developing agent; reducing agent
Related Compound	4-Aminophenol (CAS 123-30-8) (parent compound without methyl substitution; higher toxicity and sensitization risk)

2. Physicochemical Properties

Property Category	Specific Parameter	Specification/Value
Structural Features	Functional Groups	Two molecules of 4-(methylamino)phenol (each with -OH at C1 and -NHCH₃ at C4) protonated and bound to one sulfate ion (SO₄²⁻); para-substitution enables stable hydrogen bonding and dye coupling
Physical Appearance	State & Color	White to off-white crystalline powder; turns pale pink/brown on prolonged air exposure (slow oxidation); faint phenolic/methylamine odor
	Melting Point	240–245 °C (decomposes; lit.)
	Boiling Point	Decomposes before boiling (releases SO₃ and methylaminophenol vapors above 280 °C)
	Density (25 °C)	1.43 g/cm³ (crystalline form)