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Chemical Identity and Composition
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- Molecular formula CH 2 O Structural formula
- Physical and Chemical Properties
Chemical Identity and Composition
Methanal
Formaldehyde is listed on the Australian Inventory of Chemical Substances (AICS) as formaldehyde. CAS number: 50-00-0 EINECS number: 200-001-8 UN numbers: 2209 for non-flammable formaldehyde solutions (≥25%) 1198 for flammable formaldehyde solutions
Formalith Formol Formic aldehyde Methaldehyde Methyl aldehyde Methylene oxide Morbicid Oxomethane Oxymethylene Paraform CH2O
H | C = O
| H
30.03
Pure formaldehyde is not commercially available. Formaldehyde is generally available as a 37% to 54% (by weight) aqueous solution, known as formalin. To reduce the intrinsic polymerisation of formaldehyde, stabilisers, such as methanol and various amine derivatives, are added to the solution (IPCS, 2002; IARC, 1995). Methanol concentrations can be as high as 15% (by weight). The concentrations of other stabilisers can be in the order of several hundred mg/mL (IPCS, 1989). Formaldehyde is marketed in a solid form as trioxane (CH2O)3 and its polymer paraformaldehyde, with 8 to 100 units of formaldehyde (IPCS, 2002; IARC, 1995).
This section covers physical and chemical properties for both gaseous formaldehyde gas and formalin (37% formaldehyde solution).
At room temperature, formaldehyde is a colourless gas with a pungent, irritating odour. The odour threshold of formaldehyde varies widely, ranging from 0.05 to 1 ppm. However, for most people the odour threshold is in the 0.5 to 1 ppm range (OECD, 2002).
The physical and chemical properties of gaseous formaldehyde and formalin (37% formaldehyde solution) are summarised in Table 5.1. The values in the following text and in Table 5.1 are cited from the CICAD (IPCS, 2002), unless otherwise stated. Gaseous formaldehyde Formaldehyde gas is highly reactive, highly flammable and can form explosive mixtures in air. It presents a fire hazard when exposed to flame or heat. At temperatures greater than 150oC, formaldehyde decomposes to methanol and carbon monoxide (IPCS, 1989). It readily undergoes polymerisation. Formaldehyde polymers or products containing formaldehyde polymers can decompose to release significant amounts of gaseous formaldehyde when overheated. Formaldehyde gas is readily soluble in water, alcohol, and other polar solvents. It can exist as methylene glycol, polyoxymethylene and hemiformal in solutions. Formaldehyde is a reactive aldehyde that undergoes a number of self-association reactions. For example, at concentrations above 30% the polymer precipitates. The chemical species produced when formaldehyde associates with water may have different properties from those of the pure monomolecular substance. These associations tend to be more prevalent at higher concentrations of formaldehyde. Therefore, the properties described at high concentrations may not be relevant for more dilute concentrations. Formalin Formalin without methanol has a flash point of 83 to 85 C and is combustible. Formalin can be a flammable liquid when the formaldehyde or methanol concentration is high. Formalin may become cloudy on standing, especially at cool temperatures, and form paraformaldehyde at very low temperatures. It slowly oxidizes in air to formic acid and is sensitive to light. It is easily hydrated and polymerised if not stabilised (Keith and Walters, 1992). Table 5.1: Physical and chemical properties of formaldehyde gas and 37% formaldehyde solution+ Property Gaseous formaldehyde 37% Formaldehyde solution Relative molecular mass 30.03 30.03# Melting point -118 to -92 oC NA Boiling point -21 to -19 oC 96 oC# (water)
Water solubility (at 25oC) 400 to 550 g/L ≥100 mg/mL at 20.5 oC# Henry’s Law constant (at 25oC) 0.022 to 0.034 NA Pa.m3/mol
Log Kow -0.75 to 0.35 NA Log Koc 0.70 to 1.57 NA Density (at 20oC) 0.82 g/mL* 1.03-1.10 g/mL* Explosivity limits in air Lower (vol %) Upper (vol %) 7%*
73%* 7%# @ 73%# @ Vapour pressure (at 25oC) 516 kPa* 2.26 to 2.66 kPa* Autoignition temperature 300 oC* 430 oC# Flashpoint (closed cup) NA 83 - 85 oC* (for 37% formaldehyde solution without methanol) 50 oC* (for 37% formaldehyde solution with 15% methanol) + The values are cited from the CICAD (IPCS, 2002), unless otherwise stated; Log Kow, Log octanol/water partition coefficient; Log Koc, Log organic carbon/water partition coefficient; NA, not available; *Klasco (2003); # Keith and Walters (1992); @ based on release of formaldehyde from solution. Formalin is a strong reducing agent, especially in the presence of alkalis. It is incompatible with ammonia, alkalis, bisulfides, iron preparations, iodine, phenols, potassium permanganate, tannin and salts of copper, iron, and silver. It combines directly with albumin, casein, gelatin, agar and starch to form insoluble compounds. It reacts violently with hydrogen peroxide, magnesium carbonate, nitromethane, perchloric acid and aniline, and performic acid and also reacts with strong oxidizers and acids. Reactions with nitrous oxides (nitrogen dioxide) become explosive at 180 C. It is corrosive to carbon steel as well as copper and its alloys (Keith and Walters, 1992). Paraformaldehyde emits formaldehyde gas when it is heated to decomposition. It is also hydrolysed by hot water and alkali forming formaldehyde. It behaves like methanol-free formaldehyde of the same concentration once it dissolves in water (Lewis, 1996).
The conversion factors for formaldehyde at 25 C are: 1 ppm = 1.2 mg/m3 1 mg/m3 = 0.83 ppm
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