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Acids and Anhydrides, Organic

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Organic acids and their derivatives cover a wide range of substances. They are used in nearly every type of chemical manufacture. Because of the variety in the chemical structure of the members of the organic acid group, several types of toxic effects may occur. These compounds have a primary irritant effect, the degree determined in part by acid dissociation and water solubility. Some may cause severe tissue damage similar to that seen with strong mineral acids. Sensitization may also occur, but is more common with the anhydrides than the acids.

For the purpose of this article, organic acids may be divided into saturated monocarboxylic and unsaturated monocarboxylic acids, aliphatic dicarboxylic acids, halogenated acetic acids, miscellaneous aliphatic monocarboxylic acids and aromatic carboxylic acids. Many carboxylic acids are of importance because of their use in food, beverages, drugs and a range of manufacturing processes. The following are among the most common: adipic acid, azelaic acid, fumaric acid, itaconic acid, maleic acid, malic acid, malonic acid, oxalic acid, pimelic acid, sebacic acid, succinic acid, tartaric acid and thiomalic acid.

The long-chain saturated monocarboxylic acids are the fatty acids and are in the main derived from natural sources. Synthetic fatty acids may also be manufactured by air oxidation of paraffins (aliphatic hydrocarbons) using metal catalysts. They are also produced by the oxidation of alcohols with caustic soda.


Organic acids are employed in the plastics, tanning, textile, paper, metal, pharmaceutical, food, beverage and cosmetics industries. Organic acids are also found in perfumes, herbicides, dyes, lubricants and cleaners.

Formic acid and acetic acid are the major industrial chemicals in the group of saturated monocarboxylic acids. Formic acid is primarily used in the textile and leather industries. It acts as a dye-exhausting agent for a number of natural and synthetic fibres and as a reducing agent in chrome dyeing. Formic acid is used as a deliming agent and a neutralizer in the leather industry, and as a coagulant for rubber latex. It also finds use in the manufacture of fumigants and insecticides. Acetic acid serves as a chemical intermediate, a deliming agent during leather tanning, a solvent, and an oil-well acidizer. In addition, it is an additive for various foods and glazes as well as a catalyst and a finishing agent in the dye-stuff and textile industries.

Weak concentrations of acetic acid (vinegar contains about 4 to 6%) are produced by aerobic fermentation (Acetobacter) of alcohol solutions. Acetic acid is one of the most widely used organic acids. It is employed in the production of cellulose acetate, vinyl acetate, inorganic acetates, organic acetates and acetic anhydride. Acetic acid itself is used in the dyeing industry, pharmaceutical industry, the canning and food preserving industry and pigment production.

Chloroacetic acid is used in the pharmaceutical, dye-stuffs and chemical industries as a chemical intermediate. Salicylic acid acts as another chemical intermediate used in the synthesis of aspirin and in the rubber and dye-stuffs industries. Benzoic acid, nonanoic acid, ascorbic acid and oleic acid (9-octadecenoic acid) are other useful compounds found in the food, beverage and pharmaceutical industries.

Palmitic acid and stearic acid have a wide application in soaps, cosmetics, detergents, lubricants, protective coatings and intermediate chemicals. Propionic acid is used in organic synthesis. It is also a mould inhibitor and a food preservative. Acrylic acid, methacrylic acid and crotonic acid are employed in the manufacture of resins and plasticizers in the paper, plastics and paint industries. In addition, acrylic acid is an ingredient in floor-polish formulations. Crotonic acid finds use in the manufacture of softening agents for synthetic rubber. Lactic acid, butyric acid and gallic acid are employed in the leather-tanning industry. Lactic acid is also used in adhesives, plastics and textiles. It serves as a food acidulant and as an agent in oil-well acidizing. Glycolic acid is used in the leather, textile, electroplating, adhesives and metal-cleaning industries.

The dicarboxylic acids (succinic acid, maleic acid, fumaric acid, adipic acid) and the tricarboxylic acid (citric acid) are useful in the food, beverage and pharmaceutical industries. Succinic acid is also used in the manufacture of lacquers and dyes. Maleic acid is used in the manufacture of synthetic resins and in organic syntheses. Maleic acid acts as a preservative for oils and fats; its salts are used in the dyeing of cotton, wool and silk. Fumaric acid is used in polyesters and alkyd resins, plastics surface coatings, food acidulants, inks and organic syntheses. The majority of adipic acid is utilized for nylon production, while smaller quantities are used in plasticizers, synthetic lubricants, polyurethanes and food acidulants.

Oxalic acid is a scouring agent in textile finishing, stripping and cleaning, and a component of household formulations for metal cleaning. It also finds use in the paper, photography and rubber industries. Oxalic acid is used in calico printing and dyeing, bleaching straw hats and leather, and cleaning wood. Aminoacetic acid is used as a buffering agent and in syntheses. Peracetic acid is used as a bleach, catalyst and oxidant.

Commercial naphthenic acid is usually a dark-coloured malodourous mixture of naphthenic acids. Naphthenic acids are derived from cycloparaffins in petroleum, probably by oxidation. Commercial acids are usually viscous liquid mixtures and may be separated as low- and high-boiling fractions. The molecular weights vary from 180 to 350. They are used principally in the preparation of paint dryers, where the metallic salts, such as lead, cobalt and manganese, act as oxidizing agents. Metallic naphthenic acids are used as catalysts in chemical processes. An industrial advantage is their solubility in oil.

Organic acid anhydrides

An anhydride is defined as an oxide which, when combined with water, gives an acid or a base. Acid anhydrides are derived from the removal of water from two molecules of the corresponding acid, such as:

2HMnO4 → Mn2O7 + H2O

Industrially, the most important anhydrides are acetic and phthalic. Acetic anhydride is used in the plastics, explosives, perfume, food, textile and pharmaceutical industries, and as a chemical intermediate. Phthalic anhydride serves as a plasticizer in vinyl chloride polymerization. It is also used for the production of saturated and unsaturated polyester resins, benzoic acid, pesticides, and certain essences and perfumes. Phthalic anhydride is employed in the production of phthalocyanine dyes and alkyd resins used in paints and lacquers. Maleic anhydride has a significant number of applications as well.

Propionic anhydride is used in the manufacture of perfumes, alkyd resins, drugs and dyes, while maleic anhydride, trimellitic anhydride and acetic anhydride find use in the plastics industry. Trimellitic anhyide (TMA) is also utilized in the dye-stuff, printing and automotove upholstery industries. It is used as a curing agent for epoxy and other resins, in vinyl plasticizers, paints, coatings, dyes, pigments and a wide variety of other manufactured products. Some of these products find applications in high-temperature plastics, wire insulation and gaskets.


Monocarboxylic acids

The low-molecular-weight monocarboxylic acids are primary irritants and produce severe damage to tissues. Strict precautions are necessary in handling; suitable protective equipment should be available and any skin or eye splashes irrigated with copious amounts of water. The most important acids of this group are acetic acid and formic acid.

The long-chain saturated monocarboxylic acids (the fatty acids) are non-irritant and of a very low order of toxicity. They appear to pose few problems in industrial use.

Unsaturated monocarboxylic acids are highly reactive substances and are recognized as severe irritants of the skin, eye and respiratory tract in concentrated solution. Hazards appear to be related to acute rather than cumulative exposures.

The majority of these acids appear to present minimal hazard from low-level chronic exposure, and many are normally present in human metabolic processes. Primary irritant effects are present with a number of these acids, however, particularly in concentrated solutions or as dusts. Sensitization is rare. As the materials are all solids at room temperature, contact is usually in the form of dust or crystals.

Acetic acid. Acetic acid vapour may form explosive mixtures with air and constitute a fire hazard either directly or by the release of hydrogen. Glacial acetic acid or acetic acid in concentrated form are primary skin irritants and will produce erythema (reddening), chemical burns and blisters. In cases of accidental ingestion, severe ulceronecrotic lesions of the upper digestive tract have been observed with bloody vomiting, diarrhoea, shock and haemoglobinuria followed by urinary disorders (anuria and uraemia).

The vapours have an irritant action on exposed mucous membranes, particularly the conjunctivae, rhinopharynx and upper respiratory tract. Acute bronchopneumonia developed in a woman who was made to inhale acetic acid vapours following a fainting attack.

Workers exposed for a number of years to concentrations of up to 200 ppm have been found to suffer from palpebral oedema with hypertrophy of the lymph nodes, conjunctival hyperaemia, chronic pharyngitis, chronic catarrhal bronchitis and, in some cases, asthmatic bronchitis and traces of erosion on the vestibular surface of the teeth (incisors and canines).

The extent of acclimatization is remarkable; however, such acclimatization does not mean that toxic effects will not also occur. Following repeated exposure, for example, workers may complain of digestive disorders with pyrosis and constipation. The skin on the palms of the hands is subject to the greatest exposure and becomes dry, cracked and hyperkeratotic, and any small cuts and abrasions are slow to heal.

Formic acid. The principal hazard is that of severe primary damage to the skin, eye or mucosal surface. Sensitization is rare, but may occur in a person previously sensitized to formaldehyde. Accidental injury in humans is the same as for other relatively strong acids. No delayed or chronic effects have been noted. Formic acid is a flammable liquid, and its vapour forms flammable and explosive mixtures with air.

Propionic acid in solution has corrosive properties towards several metals. It is irritant to eye, respiratory system and skin. The same precautions recommended for exposure to formic acid are applicable, taking into account the lower flashpoint of propionic acid.

Maleic acid is a strong acid and produces marked irritation of the skin and mucous membranes. Severe effects, particularly in the eye, can result from concentrations as low as 5%. There are no reports of cumulative toxic effects in humans. The hazard in industry is of primary irritation of exposed surfaces, and this should be averted where necessary by the provision of appropriate personal protective equipment, generally in the form of impermeable gloves or gauntlets.

Fumaric acid is a relatively weak acid and has a low solubility in water. It is a normal metabolite and is less toxic orally than tartaric acid. It is a mild irritant of skin and mucous membranes, and no problems of industrial handling are known.

Adipic acid is non-irritant and of very low toxicity when ingested.

Halogenated acetic acids

The halogenated acetic acids are highly reactive. They include chloroacetic acid, dichloroacetic acid (DCA), trichloroacetic acid (TCA), bromoacetic acid, iodoacetic acid, fluoroacetic acid and trifluoroacetic acid (TFA).

The halogenated acetic acids cause severe damage to the skin and mucous membranes and, when ingested, may interfere with essential enzyme systems in the body. Strict precautions are necessary for their handling. They should be prepared and used in enclosed plant, the openings in which should be limited to the necessities of manipulation. Exhaust ventilation should be applied to the enclosure to ensure that fumes or dust do not escape through the limited openings. Personal protective equipment should be worn by persons engaged in the operations, and eye protective equipment and respiratory protective equipment should be available for use when necessary.

Fluoroacetic acid. Di- and trifluoroacetic acids have a lower level of toxicity than monofluoroacetic acid (fluoroacetic acid). Monofluoroacetic acid and its compounds are stable, highly toxic and insidious. At least four biological plants in South Africa and Australia owe their toxicity to this acid (Dichapetalum cymosum, Acacia georginae, Palicourea marcgravii), and recently more than 30 species of Gastrolobium and Oxylobrium in Western Australia have been found to contain various amounts of fluoroacetate.

The biological mechanism responsible for the symptoms of fluoroacetate poisoning involves the “lethal synthesis” of fluorocitric acid, which in turn blocks the tricarboxylic acid cycle by inhibiting the enzyme aconitase. The resultant deprivation of energy by stopping of the Krebs cycle is followed by cellular dysfunction and death. It is impossible to be specific about the toxic dose of fluoroacetic acid for humans; a likely range lies between 2 and 10 mg/kg; but several related fluoroacetates are even more toxic than this. A drop or two of the poison by inhalation, ingestion and absorption through skin cuts and abrasion or undamaged skin can be fatal.

From a study of hospital case histories, it is apparent that the major toxic effects of fluoroacetates in humans involve the central nervous system and cardiovascular system. Severe epileptiform convulsions alternate with coma and depression; death may result from asphyxia during a convulsion or from respiratory failure. The most prominent features, however, are cardiac irregularities, notably ventricular fibrillation and sudden cardiac arrest. These symptoms (which are indistinguishable from those frequently encountered clinically) are usually preceded by an initial latent period of up to 6 h characterized by nausea, vomiting, excessive salivation, numbness, tingling sensations, epigastric pain and mental apprehension; other signs and symptoms which may develop subsequently include muscular twitching, low blood pressure and blurred vision.

Chloroacetic acid. This material is a highly reactive chemical and should be handled with care. Gloves, goggles, rubber boots and impervious overalls are mandatory when workers are in contact with concentrated solutions.

Other acids

Glycolic acid is stronger than acetic acid and produces very severe chemical burns of the skin and eyes. No cumulative effects are known, and it is believed to be metabolized to glycine. Strict precautions are necessary for its handling. These are similar to those required for acetic acid. Concentrated solutions can cause burns of the skin and eye. No cumulative effects are known. Personal protective equipment should be worn by persons handling concentrated solutions of this acid.

Sorbic acid is used as a fungicide in foods. It is a primary irritant of the skin, and individuals may develop sensitivities to it. For these reasons contact with the skin should be avoided.

Salicylic acid is a strong irritant when in contact with skin or mucous membranes. Strict precautions are necessary for plant operatives.


Acid anhydrides have higher boiling points than the corresponding acids. Their physiological effects generally resemble those of the corresponding acids, but they are more potent eye irritants in the vapour phase, and may produce chronic conjunctivitis. They are slowly hydrolyzed on contact with body tissues and may occasionally cause sensitization. Adequate ventilation should be provided and suitable personal protective equipment should be worn. In certain circumstances, particularly those associated with maintenance work, suitable eye protection equipment and respiratory protective equipment are necessary.

There have been reports of conjunctivitis, bloody nasal excreta, atrophy of the nasal mucosa, hoarseness, cough and bronchitis in workers employed in the production of phthalic acid and anhydride. It has been recognized that phthalic anhydride causes bronchial asthma, and skin sensitization has been reported following prolonged exposure to phthalic anhydride; the lesion is usually an allergic dermatitis. A specific IgE to phthalic anhydride has also been identified.

Phthalic anhydride is flammable and constitutes a moderate fire hazard. Its toxicity is comparatively low in relation to other industrial acid anhydrides, but it acts as a skin, eye and upper respiratory tract irritant. Since phthalic anhydride has no effect on dry skin, but burns wet skin, it is probable that the actual irritant is phthalic acid, which is formed on contact with water.

Phthalic anhydride must be stored in a cool, well-ventilated place away from open flames and oxidizing substances. Good local and general ventilation are required where it is handled. In many processes phthalic anhydride is used not as flakes but as a liquid. When so used, it is brought to the plant in tanks and directly pumped into the pipe system, preventing contact as well as contamination of the air with dust. This has led to the complete disappearance of manifestations of irritations among the workers in such plants. However, vapours liberated from liquid phthalic anhydride are as irritating as the flakes; care must, therefore, be taken to avoid any leakage from the pipe system. In case of spillage or contact with the skin, the latter should be washed immediately and repeatedly with water.

Workers who are handling phthalic derivatives must be under medical supervision. Special attention should be paid to asthma-like symptoms and skin sensitization. If any such symptoms are noticed, the worker should be moved to another job. Skin contact is to be avoided under all circumstances. Suitable clothing, such as rubber hand protection, is recommended. Pre-employment examinations are necessary to ensure that persons with bronchial asthma, eczema or other allergic diseases are not exposed to phthalic anhydride.

Acetic anhydride. When exposed to heat, acetic anhydride can emit toxic fumes, and its vapours can explode in the presence of flame. It can react violently with strong acids and oxidizers such as sulphuric acid, nitric acid, hydrochloric acid, permanganates, chromium trioxide and hydrogen peroxide, as well as with soda.

Acetic anhydride is a strong irritant and has corrosive properties on contact with eyes, usually with delayed action; contact is followed by lacrimation, photophobia, conjunctivitis and corneal oedema. Inhalation can cause nasopharyngeal and upper respiratory tract irritation, with burning sensations, cough and dyspnoea; prolonged exposure may lead to pulmonary oedema. Ingestion causes pain, nausea and vomiting. Dermatitis can result from prolonged skin exposure.

When contacts are possible, protective clothing and goggles are recommended and eyewash and shower facilities should be available. Chemical cartridge respirators are appropriate for protection against concentrations up to 250 ppm; supplied air respirators with a full eyepiece are recommended for concentrations of 1,000 ppm; self-contained breathing apparatus is necessary in case of fire.

Butyric anhydride is manufactured by catalytic hydrogenation of crotonic acid. Butyric anhydride and propionic anhydride present hazards similar to those of the acetic anhydride.

Maleic anhydride can produce severe eye and skin burns. These may be produced either by solution of maleic anhydride or by flakes of the material in the manufacturing process coming into contact with moist skin. Skin sensitization has occurred. Strict precautions should be taken to prevent contact of the solution with skin or eyes. Suitable goggles and other protective clothing must be worn by plant operatives; ready access to eye irrigation solution bottles is essential. When suspended in air in a finely divided condition, maleic anhydride is capable of forming explosive mixtures with the air. Condensers in which the sublimed material settles in the form of fine crystals should be situated in a safe position outside an occupied room.

Trimellitic anhydride has been reported to have caused pulmonary oedema in workers after severe acute exposure, and airways sensitization after exposure periods of weeks to years, with rhinitis and/or asthma. Several incidents involving the occupational effects of exposure to TMA have been reported. Multiple inhalation exposures to an epoxy resin containing TMA being sprayed on heated pipes was reported to have caused pulmonary oedema in two workers. Exposure levels were not reported but there was no report of upper respiratory tract irritation while the exposures were being experienced, indicating that a hypersensitive reaction might have been involved.

In another report, 14 workers involved in the synthesis of TMA were observed to have respiratory symptoms resulting from sensitization to TMA. In this study three separate responses were noted. The first, rhinitis and/or asthma, developed over an exposure duration of weeks to years. Once sensitized, exposed workers exhibited symptoms immediately after exposure to TMA, which ceased when the exposure was stopped. A second response, also involving sensitization, produced delayed symptoms (cough, wheezing and laboured breathing) 4 to 8 hours after exposure had ceased. The third syndrome was an irritant effect following initial high exposures.

One study of adverse health effects, which also involved measurements of air concentrations of TMA, was conducted by the US National Institute for Occupational Safety and Health (NIOSH). Thirteen workers involved in the manufacture of an epoxy paint had complaints of eye, skin, nose and throat irritation, shortness of breath, wheezing, coughing, heartburn, nausea and headache. Occupational airborne exposure levels averaged 1.5 mg/m3 TMA (range from “none detected” to 4.0 mg/m3) during processing operations and 2.8 mg/m3 TMA (range from “none detected” to 7.5 mg/m3) during decontamination procedures.

Experimental studies with rats have demonstrated intra-alveolar haemorrhage with subacute exposures to TMA at 0.08 mg/m3. The vapour pressure at 20 °C (4 × 10-6 mm Hg) corresponds to a concentration slightly more than 0.04 mg/m3.

Oxalic acid and its derivatives. Oxalic acid is a strong acid which, in solid form or in concentrated solutions, can cause burns of the skin, eyes or mucous membranes; oxalic acid concentrations as low as 5 to 10% are irritating if exposure is prolonged. Human fatalities have been recorded following ingestion of as little as 5 g of oxalic acid. The symptoms appear rapidly and are marked by a shock-like state, collapse and convulsive seizures. Such cases may show marked renal damage with precipitation of calcium oxalate in the renal tubules. The convulsive seizures are thought to be the result of hypocalcaemia. Chronic skin exposure to solutions of oxalic acid or potassium oxalate have been reported to have caused a localized pain and cyanosis in the fingers or even gangrenous changes. This is apparently due to a localized absorption of the oxalic acid and a resultant arteritis. Chronic systemic injury from inhalation of oxalic acid dust appears to be very rare, although the literature describes the case of a man who had been exposed to hot oxalic acid vapours (probably containing an aerosol of oxalic acid) with generalized symptoms of weight loss and chronic inflammation of the upper respiratory tract. Because of the strongly acid nature of the dust of oxalic acid, exposure must be carefully controlled and work area concentrations held within acceptable health limits.

Diethyl oxalate is slightly soluble in water; miscible in all proportions in many organic solvents; a colourless, unstable, oily liquid. It is produced by esterification of ethyl alcohol and oxalic acid. It is used, as are other liquid oxalate esters, as a solvent for many natural and synthetic resins.

The symptoms in rats following ingestion of large quantities of diethyl oxalate are those of respiratory disturbances and muscle twitchings. Large quantities of oxalate deposits were found in the renal tubules of a rat after an oral dose of 400 mg/kg. It has been reported that workers exposed to 0.76 mg/l of diethyl oxalate over a period of several months developed complaints of weakness, headache and nausea together with some slight alterations in the blood count. Because of the very low vapour pressure of this substance at room temperature, the reported air concentrations may have been in error. There was also some use of diamyl acetate and diethyl carbonate in this operation.

Safety and Health Measures

All acids should be stored away from all sources of ignition and oxidizing substances. Storage areas should be well ventilated to prevent the accumulation of dangerous concentrations. Containers should be of stainless steel or glass. In the event of leakage or spillage, acetic acid should be neutralized by application of alkaline solutions. Eyewash fountains and emergency showers should be installed for dealing with cases of skin or eye contact. Marking and labelling of containers is essential; for all forms of transport, acetic acid is classified as a dangerous substance.

To prevent damage to the respiratory system and mucous membranes, the atmospheric concentration of organic acids and anhydrides with high vapour pressure should be kept below maximum permissible levels using standard industrial hygiene practices such as local exhaust ventilation and general ventilation, backed up by periodic determination of atmospheric acetic acid concentrations. Detection and analysis, in the absence of other acid vapours, is by means of bubbling in an alkaline solution and determination of residual alkali; in the presence of other acids, fractional distillation used to be necessary; however, a gas chromatographic method is now available for determination in air or water. Dust exposures should be minimized as well.

Persons working with the pure acid or concentrated solutions should wear protective clothing, eye and face protection, hand and arm protection and respiratory protective equipment. Adequate sanitary facilities should be provided and good personal hygiene encouraged.

Organic acids and anhydrides tables

Table 1 - Chemical information.

Table 2 - Health hazards.

Table 3 - Physical and chemical hazards.

Table 4 - Physical and chemical properties.



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