HYDROQUINONE

OH CASRN: 123-31-9; DOT: 2662; molecular formula: C6H6O2; FW: 110.11; RTECS: MX3500000; Merck Index: 12, 4853 Physical state, color, and odor: Colorless to pale brown, odorless, hexagonal crystals Melting point (°C): 173-174 (Weast, 1986) 174.00 (Martin et al., 1979) Boiling point (°C): 285 at 750 mmHg (Weast, 1986) 285-287 (Windholz et al., 1983) Density (g/cm3): 1.328 at 15 °C (Weast, 1986) 1.358 at 20 °C (Sax and Lewis, 1987) Diffusivity in water (x 10-5 cm2/sec): 0.83 at 20 °C using method of Hayduk and Laudie (1974) Dissociation constant: At 25 °C: pK1 = 10.0, pK2 = 12.0 (Dean, 1973) Flash point (°C): 166 (molten, NIOSH, 1997) Entropy of fusion (cal/mol⋅K): 14.6 (Andrews et al., 1926) Heat of fusion (kcal/mol): 6.48 (Tsonopoulos and Prausnitz, 1971) Henry’s law constant (x 10-9 atm⋅m3/mol): <2.07 at 20 °C (approximate - calculated from water solubility and vapor pressure) Ionization potential (eV): 7.95 (NIOSH, 1997)

1.81 (Chlorella fusca, Geyer et al., 1981, 1984) 2.94 (activated sludge), 1.60 (algae), 1.60 (golden ide) (Freitag et al., 1985) Soil organic carbon/water partition coefficient, log Koc: 0.98 using method of Kenaga and Goring (1980) Octanol/water partition coefficient, log Kow: 0.59 (quoted, Leo et al., 1971) 0.50 at pH 5.62 (Umeyama et al., 1971) 0.55 (Geyer et al., 1984) 0.54 (shake flask-HPLC, Nahum and Horvath, 1980) 0.46 (Janini and Attari, 1983) Solubility in organics: In mole fraction: 0.3645 in acetic acid at 21.5 °C, 0.1177 in methanol at 20.65 °C, 0.1914 in

ethanol at 24.5 °C, 0.1469 in 2-propanol at 21.20 °C, 0.1017 in ethyl acetate at 25.65 °C, 0.1173 in butyl acetate at 24.05 °C (shake flask/laser monitoring, Li et al., 2006)

Solubility in water: 80,135 mg/L at 25 °C (Korman and La Mer, 1936) In mg/L: 59,000 at 15 °C, 70,000 at 25 °C, 94,000 at 28 °C (quoted, Verschueren, 1983) 0.1025 at 75.3 °C (mole fraction, Walker et al., 1931) In mole fraction (x 10-2): 0.663 at 7.35 °C, 0.795 at 12.85 °C, 0.965 at 17.90 °C, 1.163 at 23.8 °C,

1.515 at 30.30 °C, 2.000 at 37.70 °C (shake flask/laser monitoring, Li et al., 2006) Vapor pressure (mmHg): <10-3 at 20 °C (Sax and Lewis, 1987) 4 at 150 °C (quoted, Verschueren, 1983) Environmental fate: Biological. In activated sludge, 7.5% mineralized to carbon dioxide after 5 d (Freitag et al., 1985). Under methanogenic conditions, inocula from a municipal sewage treatment plant digester degraded hydroquinone to phenol prior to being mineralized to carbon dioxide and methane (Young and Rivera, 1985). In various pure cultures, hydroquinone degraded to the following intermediates: benzoquinone, 2-hydroxy-1,4-benzoquinone, and β-ketoadipic acid. Hydroquinone also degraded in activated sludge but no products were identified (Harbison and Belly, 1982). Heukelekian and Rand (1955) reported a 5-d BOD value of 0.74 g/g which is 39.2% of the ThOD value of 1.89 g/g. In activated sludge inoculum, following a 20-d adaptation period, 90.0% COD removal was achieved. The average rate of biodegradation was 54.2 mg COD/g⋅h (Pitter, 1976). Photolytic. A carbon dioxide yield of 53.7% was achieved when hydroquinone adsorbed on silica gel was irradiated with light (λ >290 nm) for 17 h (Freitag et al., 1985). Chemical/Physical. Ozonolysis products reported are p-quinone and dibasic acids (Verschueren, 1983). Moussavi (1979) studied the autoxidation of hydroquinone in slightly alkaline (pH 7 to 9) aqueous solutions at room temperature. The oxidation of hydroquinone by oxygen followed first-order kinetics that yielded hydrogen peroxide and p-quinone as products. At pH values of 7.0, 8.0, and 9.0, the calculated half-lives of this reaction were 111, 41, and 0.84 h, respectively (Moussavi, 1979). Chlorine dioxide reacted with hydroquinone in an aqueous solution forming p-benzoquinone (Wajon et al., 1982). Kanno et al. (1982) studied the aqueous reaction of hydroquinone and other

pyrocatechol, resorcinol, and 1-naphthol) with hypochlorous acid in the presence of ammonium ion. They reported that the aromatic ring was not chlorinated as expected but was cleaved by chloramine forming cyanogen chloride. As the pH was lowered, the amount of cyanogen chloride formed increased (Kanno et al., 1982). At influent concentrations of 1.0, 0.1, 0.01, and 0.001 mg/L, the GAC adsorption capacities were 160, 90, 51, and 29 mg/g, respectively (Dobbs and Cohen, 1980). Exposure limits (mg/m3): NIOSH REL: 15-min ceiling 2, IDLH 50; OSHA PEL: TWA 2; ACGIH TLV: TWA 2 (adopted). Toxicity: Acute oral LD50 for cats 70 mg/kg, dogs 200 mg/kg, guinea pigs 550 mg/kg, mice 245 mg/kg, pigeons 300 mg/kg, rats 320 mg/kg (quoted, RTECS, 1985). Source: Hydroquinone occurs naturally in strawberry tree leaves, pears, blackberries, Chinese alpenrose, bilberries, blackberries, hyacinth flowers, anise, cowberries, and lingonberries (Duke, 1992). Uses: Antioxidant; photographic reducer and developer for black and white film; determination of phosphate; dye intermediate; medicine; in monomeric liquids to prevent polymerization; stabilizer in paints and varnishes; motor fuels and oils.