Formaldehyde formalin/paraformaldehyde

J. A. Kiernan, Formaldehyde, formalin, paraformaldehyde and glutar-aldehyde What they are and what they do, Microsc. Today, 2000, 00-1, 8-12. 

Formaldehyde is a colorless gas that is soluble in water (3). Commercial aqueous preparations of formalin contain 37 0% w/w solubilized gas. They also contain formic acid (<0.05%) and 10-15% methanol, which is added to prevent the polymerization of formaldehyde into paraformaldehyde (3,11). Methanol and formic acid make these solutions an unacceptable fixative for fine structures (9). Paraformaldehyde is a polymerized form of formaldehyde that dissociates at 60°C and neutral pH. Freshly prepared solutions of paraformaldehyde are preferred for most immunochemical procedures because they provide a fixative free of extraneous additives and are usually the conservative fixatives of choice when beginning the development of a fixation protocol (3,5). 

Instead of aqueous formaldehyde (formalin) the required amount of paraformaldehyde is weighed out, dissolved in water followed by addition of the other components. 

Formaldehyde and acetaldehyde are the most common aldehydes. Formaldehyde is a gas at room temperature, so it is often stored and used as a 40% aqueous solution called formalin. When dry formaldehyde is needed, it can be generated by heating one of its solid derivatives, usually trioxane or paraformaldehyde. Trioxane is a cyclic trimer, containing three formaldehyde units. Paraformaldehyde is a linear polymer,... 

Of particular interest for the in situ generation of syngas are formaldehyde and paraformaldehyde. Commercial solutions of formaldehyde in water are called formal or formalin. A saturated water solution of about 40vol% or 37 mass% formaldehyde is commercialized as 100% formalin. Formaldehyde is commonly produced by the metal-catalyzed oxidation of methanol (Formox process) in several million tons per year [10]. Paraformaldehyde is the solid polymerization product (mp = 120 C) of formaldehyde with an average degree of polymerization of 8-100 units. It is depolymerized to formaldehyde upon heating. Up to now, the trimerization product of formaldehyde, 1,3,6-trioxane, does not play a role in hydroformylation. 

In 2010, Morimoto and coworkers showed that a combination of two Rh catalysts [Rh(BIPHEP), Rh(Nixantphos)] can be beneficial, one for the decomposition of formaldehyde and the second for the hydroformylation of olefins (Scheme 3.5) [16]. With this system, aldehydes could be produced with a yield of 95% and a ratio of l/b = 97 3 independent on the formaldehyde source (formalin, paraformaldehyde). 

The manufacture of formaldehyde is described in Section 8.3.2. For the preparation of phenolic resins, formaldehyde is normally used as the aqueous solution, formalin. Paraformaldehyde is occasionally used in the production of resins and as a hardener for adhesives. 

As a preservative for industrial fluids, formaldehyde (formalin and paraformaldehyde) is preferably used in cosmetic products and in polymer dispersions (natural and synthetic latex) often in combination with other microbicides. Formaldehyde is listed in the EC list of preservatives allowed for the in-can protection of cosmetics (maximum authorized concentration 0.2%, except for products for oral hygiene 0.1%, prohibited in aerosol dispensers). Percentage of use in US cosmetic formulations 0.11% paraformaldehyde. Oil, fat and wax emulsions, starch and dextrine glues, adhesive dispersions, pigment and filler slurries, thickening solutions and other aqueous formulations containing no proteins can be preserved by the addition of formaldehyde, too in order to... 

We recommend freshly preparing tlie formaldehyde from paraformaldehyde powder (reagent grade). Liquid 37 % formaldehyde solutions ( formalin ) contain methanol as stabilizing reagent, which quickly leads to a loss of GFP fluorescence even when highly diluted. 

It is marketed as a 35-40 per cent, solution in water (formalin). The rpactions of formaldehyde are partly typical of aldehydes and partly peculiar to itself. By evaporating an aqueous solution paraformaldehyde or paraform (CHjO), an amorphous white solid is produced it is insoluble in most solvents. When formaldehyde is distilled from a 60 per cent, solution containing 2 per cent, of sulphuric acid, it pol5unerises to a crystalline trimeride, trioxane, which can be extracted with methylene chloride this is crystalline (m.p. 62°, b.p. 115°), readily soluble in water, alcohol and ether, and devoid of aldehydic properties ... 

Alkylation involving formaldehyde in the presence of hydrogen chloride is known as chloromethylation (eq. 3). The reagent may be a mixture of formalin and hydrochloric acid, paraformaldehyde and hydrochloric acid, a chloromethyl ether, or a formal. Zinc chloride is commonly employed as a catalyst, although many other Lewis acids can be used. Chloromethylation of sahcyhc acids yields primarily the 5-substituted product 5-chlotomethylsahcyhc acid [10192-87-7] (4). 

This derivative is stable to TsOH/benzene at reflux and to Cr03/H. It is stable to NBS// . In the formation of this derivative formaldehyde from formalin can react with a C,-hydroxyl group to form a methoxymethyl ether. Paraformaldehyde can be used to avoid formation of the ethers. ... 

Formaldehyde is a gas with a boiling point of -21 °C. It is usually supplied as a stabilised aqueous solution ( 40% formaldehyde) known as formalin. When formalin is used as the source of the aldehyde, impurities present generally include water, methanol, formic acid, methylal, methyl formate and carbon dioxide. The first three of these impurities interfere with polymerisation reactions and need to be removed as much as possible. In commercial polymerisation the low polymers trioxane and paraformaldehyde are convenient sources of formaldehyde since they can be obtained in a greater state of purity. 

The dihydroxyacetone side chain is conveniently protected by forming 17a,20 20,21-bismethylenedioxy compounds (BMD) (92). Formation of llf -ethers as by-products from 11 -hydroxycompounds (91) can be limited by using formalin with a low methanol content, or better with paraformaldehyde as a source of alcohol-free formaldehyde. ... 

Formaldehyde solutions prepared by dissolving and depolymerization of paraformaldehyde (a homopolymer of formaldehyde with empirical formula HO (CH20)nH, where n > 6) are free of admixtures of methanol and formic acid. Depolymerized paraformaldehyde is useful in enzyme histochemistry, when the preservation of the enzyme activity is of crucial importance, but it has no advantage over formalin solutions routinely used in pathology and in immunohistochemistry. 

Properties and handling. Formaldehyde is a colorless, toxic gas at room temperature, with a pungent, irritating odor. It is flammable and explosive in presence of air. Both gaseous and liquid forms of formaldehyde polymerize at room temperature, and because of this, it can only be maintained in the pure state for a very short period. Because of these unhandy conditions, there are two ways formaldehyde gets into commerce, as a water solution called formalin and as a solid called paraformaldehyde or trioxane. 

Paraformaldehyde, (CH20)n where n is between 8 and 100, is a convenient polymer of formaldehyde. The polymer is easily formed by removing water from a 50% formalin solution under reduced pressure. As the formaldehyde concentration increases, crystals of paraformaldehyde form spontaneously. It is available at 91-97% purity. It is more stable than neat formaldehyde but just as useful in applications, where it readily decomposes back to the straight stuff. 

The formaldehyde process is an air oxidation of methanol, CH3OH, which has water as a by-product. Formaldehyde is a gas at room temperature, but is usually handled either as a water solution called formalin or as polymers called paraformaldehyde and trioxane. Both are readily converted back ro formaldehyde. Some uses of formaldehyde are the manufacture of polymer resins and as a germicide. 

The use of distilled formaldehyde, not formalin, which contains alcohol, is recommended. Freshly prepared paraformaldehyde can also be used, especially if large volumes of fixative are needed for perfusion fixation. To prepare an 8% solution of paraformaldehyde, in a fume hood add 2 g of paraformaldehyde (trioxymethylene) powder to 25 mL of deionized glass-distilled water. With constant stirring, heat solution to 60-70°C. Once the solution has reached the proper temperature, continue to stir for 15 min. The solution will be milky. Add one to two drops of 1 VNaOH, with stirring, until the solution clears. A slight miUdness may persist. Cool and filter through Whatman No. 1 filter paper. This solution should be used the same day that it is prepared. 

The formalin contains 35 per cent formaldehyde by weight, as determined by specific gravity. It should contain no suspended paraformaldehyde. 

---