An inorganic acid is a compound of hydrogen and one or more other element (with the exception of carbon) that dissociates or breaks down to produce hydrogen ions when dissolved in water or other solvents. The resultant solution has certain characteristics such as the ability to neutralize bases, turn litmus paper red and produce specific colour changes with certain other indicators. Inorganic acids are often termed mineral acids. The anhydrous form may be gaseous or solid.
An inorganic anhydride is an oxide of metalloid which can combine with water to form an inorganic acid. It can be produced by synthesis such as: S + O2 → SO2, which can be transformed into an acid by the addition of a water molecule (hydration); or by eliminating water from an acid, such as:
2HMnO4 → Mn2O7 + H2O
Inorganic anhydrides share in general the biological properties of their acids, since hydration can readily occur in watery biological media.
Inorganic acids are used as chemical intermediates and catalysts in chemical reactions. They are found in a variety of industries, including metal- and woodworking, textile, dye-stuff, petroleum and photography. In metalworking they are often used as cleaning agents before welding, plating or painting. Sulphamic acid, sulphuric acid and hydrochloric acid are used in electroplating, and perchloric acid is used in metal plating.
Hydrochloric acid, sulphuric acid, perchloric acid and sulphamic acid are widely used in industry. Hydrochloric acid, or hydrogen chloride in aqueous solution, is used for industrial acidizing, for refining ores of tin and tantalum, for converting cornstarch to syrup, and removing scale from boilers and heat-exchange equipment. It is also a tanning agent in the leather industry. Sulphuric acid is used in parchment paper and in various processes including purification of petroleum, refining vegetable oil, carbonization of wool fabrics, extraction of uranium from pitchblende, and iron and steel pickling. Sulphuric acid and perchloric acid are used in the explosives industry. Sulphamic acid is a flame retardant in the wood and textile industries and a bleaching agent and bactericide in the pulp and paper industry. It is also used for chlorine stabilization in swimming pools.
Nitric acid is used in the manufacture of ammonium nitrate for fertilizer and explosives. In addition, it is used in organic synthesis, metallurgy, ore flotation, and for reprocessing spent nuclear fuel.
The specific hazards of the industrially important inorganic acids will be found below; however, it should be noted that all these acids have certain dangerous properties in common. Solutions of inorganic acids are not flammable in themselves; however, when they come into contact with certain other chemical substances or combustible materials, a fire or explosion may result. These acids react with certain metals with the liberation of hydrogen, which is a highly flammable and explosive substance when mixed with air or oxygen. They may also act as oxidizing agents and, when in contact with organic or other oxidizable materials, may react destructively and violently.
Health effects. The inorganic acids are corrosive, especially in high concentrations; they will destroy body tissue and cause chemical burns when in contact with the skin and mucous membranes. In particular, the danger of eye accidents is pronounced. Inorganic acid vapours or mists are respiratory tract and mucous membrane irritants, although the degree of irritation depends to a large degree on the concentration; discolouration or erosion of the teeth may also occur in exposed workers. Repeated skin contact may lead to dermatitis. Accidental ingestion of concentrated inorganic acids will result in severe irritation of the throat and stomach, and destruction of the tissue of internal organs, perhaps with fatal outcome, when immediate remedial action is not taken. Certain inorganic acids may also act as systemic poisons.
Safety and Health Measures
Wherever possible, highly corrosive acids should be replaced by acids which present less hazard; it is essential to use only the minimum concentration necessary for the process. Wherever inorganic acids are used, appropriate measures should be instituted concerning storage, handling, waste disposal, ventilation, personal protection and first aid.
Storage. Avoid contact with other acids and combustible or oxidizable materials. Electrical installations should also be of the acid-resistant type.
Storage areas should be separated from other premises, well ventilated, sheltered from sunlight and sources of heat; they should have a cement floor and contain no substances with which an acid might react. Large stocks should be surrounded by kerbs or sills to retain the acid in the event of leakage, and provisions for neutralization should be made. A fire hydrant and a supply of self-contained respiratory protective equipment for emergency or rescue purposes should be provided outside the storage premises. Spillages should be dealt with immediately by hosing down; in the event of a large leakage, personnel should vacate the premises and then, having donned emergency equipment, return to neutralize the acid with water or calcined sand. Electrical equipment should be of the waterproof type and resistant to acid attack. Safety lighting is desirable.
Containers should be kept tightly closed and should be clearly labelled to indicate the contents. Decompression measures should be taken where necessary. Piping, couplings, gaskets and valves should all be made of material resistant to nitric acid. Glass or plastic containers should be adequately protected against impact; they should be kept off the floor to facilitate flushing in the event of leakage. Drums should be stored on cradles or racks and chocked in position. Gas cylinders of gaseous anhydrous acid should be stored upright with the cap in place. Empty and full containers should preferably be stored apart. Maintenance and good housekeeping are essential.
Handling. Wherever possible acids should be pumped through sealed systems to prevent all danger of contact. Wherever individual containers have to be transported or decanted, the appropriate equipment should be employed and only experienced persons allowed to undertake the work. Decanting should be done by means of special syphons, transfer pumps, or drum or carboy tilting cradles and so on. Cylinders of anhydrous acid gas require special discharge valves and connections.
Where acids are mixed with other chemicals or water, workers must be fully aware of any violent or dangerous reaction that may take place. For example, a concentrated acid should be slowly added to water, rather than vice versa, in order to avoid the generation of excessive heat and violent reactions which can cause splashes and skin or eye contact.
Ventilation. Where processes produce acid mists or vapours, such as in electroplating, exhaust ventilation should be installed.
Personal protection. Persons exposed to dangerous splashes of inorganic acids should be required to wear acid-resistant personal protective equipment including hand and arm protection, eye and face protection and aprons, overalls or coats. Provided safe working procedures are adopted, the use of respiratory protective equipment should not be necessary; however, it should be available for emergency use in the event of leakage or spillage.
When workers are required to enter a tank that has contained inorganic acids in order to carry out maintenance or repairs, the tanks should first be purged and all precautions for entry into enclosed spaces, as described elsewhere in the Encyclopaedia, should be taken.
Training. All workers required to handle acids should be instructed about their hazardous properties. Certain work activities, such as those involving enclosed spaces or handling of large quantities of acids, should always be done by two persons, one being ready to come to the other’s aid in case of need.
Sanitation. Personal hygiene is of utmost importance where there is contact with inorganic acids. Adequate washing and sanitary facilities should be provided and workers encouraged to wash thoroughly before meals and at the ends of shifts.
First aid. Essential treatment for inorganic acid contamination of skin or eyes is immediate and copious flushing with running water. Emergency showers and eyewash fountains, baths or bottles should be strategically located. Splashes in the eye should be treated with copious irrigation with water. Contaminated clothing should be removed and other appropriate emergency skin treatment procedures should be in place and personnel trained in their administration. Neutralization of the acid in the affected area with an alkaline solution such as 2 to 3% sodium bicarbonate, or 5% sodium carbonate and 5% sodium hyposulphite, or 10% triethanolamine is a standard procedure.
Persons who have inhaled acid mists should be removed immediately from the contaminated zone and prevented from making any effort. They should be put in the care of a physician immediately. In the event of accidental ingestion, the victim should be given a neutralizing substance, and gastric lavage should be carried out. In general, vomiting should not be induced since this may make the injury more widespread.
Medical supervision. Workers should receive pre-employment and periodic medical examinations. The pre-employment examination should be particularly directed at the detection of chronic respiratory, gastro-intestinal or nervous diseases and any eye and skin diseases. Periodic examinations should take place at frequent intervals and should include a check on the condition of the teeth.
Water pollution. This should be prevented by ensuring that wastewater containing spent acid is not emptied into watercourses or sewage systems until the pH (acidity) has been brought to a level that is between 5.5 and 8.5.
Anhydrous hydrogen chloride is not corrosive; however, aqueous solutions attack nearly all metals (mercury, silver, gold, platinum, tantalum and certain alloys are exceptions) with release of hydrogen. Hydrochloric acid reacts with sulphides to form chlorides and hydrogen sulphide. It is a very stable compound, but at high temperatures it decomposes into hydrogen and chlorine.
Hazards. The special hazards of hydrochloric acid are its corrosive action on skin and mucous membranes, the formation of hydrogen when it contacts certain metals and metallic hydrides, and its toxicity. Hydrochloric acid will produce burns of the skin and mucous membranes, the severity being determined by the concentration of the solution; this may lead to ulcerations followed by keloid and retactile scarring. Contact with the eyes may produce reduced vision or blindness. Burns on the face may produce serious and disfiguring scars. Frequent contact with aqueous solutions may lead to dermatitis.
The vapours have an irritant effect on the respiratory tract, causing laryngitis, glottal oedema, bronchitis, pulmonary oedema and death. Digestive diseases are frequent and are characterized by dental molecular necrosis in which the teeth lose their shine, turn yellow, become soft and pointed, and then break off.
Safety and health measures. In addition to the general measures described above, the acid should not be stored in the vicinity of flammable or oxidizing substances, such as nitric acid or chlorates, or near metals and metal hydrides which may be attacked by the acid with the formation of hydrogen. (The explosive limits of hydrogen are 4 to 75% by volume in air.) Electrical equipment should be flameproof and protected against the corrosive action of the vapours.
Nitric acid is highly corrosive and attacks a large number of metals. Reactions between nitric acid and various organic materials are often highly exothermic and explosive, and reactions with metals may produce toxic gases. Nitric acid will cause skin burns, and the vapours are highly irritant to the skin and mucous membranes; inhalation of significant quantities will produce acute poisoning.
Fire and explosion. Nitric acid attacks most substances and all metals except the noble metals (gold, platinum, iridium, thorium, tantalum) and certain alloys. The rate of reaction varies depending on the metal and the concentration of the acid; the gases produced during the reaction include the nitrogen oxides, nitrogen and ammonia, which may have a toxic or asphyxiating effect. When in contact with sodium or potassium, the reaction is violent and dangerous, and nitrogen is released. However, in the case of certain metals, a protective oxide film is formed which prevents further attack. Nitric acid may react explosively with hydrogen sulphide. Nitrates obtained by the action of the acid on various bases are powerful oxidizing agents.
Even in dilute concentrations, nitric acid is a powerful oxidizing material. Solutions of a concentration higher than 45% may cause the spontaneous ignition of organic materials such as turpentine, wood, straw and so on.
Health hazards. Solutions of nitric acid are highly corrosive and will produce lesions of the skin, eyes and mucous membranes, the severity of which will depend on the duration of contact and the acid concentration; the lesions range from irritation to burns and localized necrosis following prolonged contact. Nitric acid mists are also corrosive to the skin, mucous membranes and dental enamel.
Nitric acid vapours will always contain a certain proportion of other gaseous nitrogen compounds (e.g., nitrogen oxides), depending on the concentration of the acid and the type of operation. Inhalation may produce acute poisoning and peracute poisoning. Peracute poisoning is rare and can be fatal. Acute poisoning generally comprises three phases: the first consists of irritation of the upper respiratory tract (burning in the throat, cough, feeling of suffocation) and of the eyes with tearing (lacrimation); the second phase is misleading, since pathological signs are absent for a period of up to several hours; in the third phase, the respiratory disorders reappear and may develop rapidly into acute pulmonary oedema, often with serious outcome.
Accidental ingestion will produce severe damage in the mouth, pharynx, oesophagus and stomach, and may have serious sequelae.
Safety and health measures. Depending on the quantities and concentrations involved, nitric acid should be stored in stainless steel, aluminium or glass containers. Glass carboys or winchesters should be protected by a metal envelope to provide resistance to impacts. However, nitric acid containing any fluorinated compounds should not be stored in glass. Organic materials such as wood, straw, sawdust and so on, should be kept away from operations involving nitric acid. When nitric acid is to be diluted with water, the acid should be poured into the water, and localized heating should be avoided.
Sulphuric acid is a strong acid which, when heated to above 30 °C, gives off vapour and, above 200 °C, emits sulphur trioxide. When cold, it reacts with all metals including platinum; when hot, reactivity is intensified. Dilute sulphuric acid dissolves aluminium, chromium, cobalt, copper, iron, manganese, nickel and zinc, but not lead or mercury. It has a great affinity for water, absorbs atmospheric moisture, and abstracts water from organic materials, causing charring. It decomposes salts of all other acids except silicic acid.
Sulphuric acid is found in the native state in the vicinity of volcanoes, in particular in the volcanic gases.
Hazards. The action of sulphuric acid on the body is that of a powerful caustic and general toxic agent. Introduced into the body in liquid or vapour form, it causes intense irritation and chemical burns of the mucous membranes of the respiratory and digestive tract, the teeth, eyes and skin. On contact with the skin, sulphuric acid causes violent dehydration. It releases heat in sufficient quantities to produce burns that are similar to thermal burns and may be classified accordingly as first, second or third degree. The depth of the lesions depends on the concentration of the acid and the length of contact. Inhalation of vapours produces the following symptoms: nasal secretion, sneezing, a burning feeling in the throat and retrosternal region; these are followed by cough, respiratory distress, sometimes accompanied by spasm of the vocal cords, and a burning sensation in the eyes with lacrimation and conjunctival congestion. High concentrations may cause bloody nasal secretion and sputum, haematemesis, gastritis and so on. Dental lesions are common; they affect mainly the incisors and present as brown staining, enamel striation, caries and rapid and painless destruction of the tooth crown.
Occupational exposures to strong inorganic acid mists, such as sulphuric acid mists, have been classified by the International Agency for Research on Cancer (IARC) as being carcinogenic to humans.
Chemical burns are the injury most commonly encountered in sulphuric acid production workers. Concentrated solutions cause deep burns of mucous membranes and skin; initially the zone of contact with the acid is bleached and turns brown prior to the formation of a clearly defined ulcer on a light red background. These wounds are long in healing and may frequently cause extensive scarring that results in functional inhibition. If burning is sufficiently extensive, the outcome may prove fatal. Repeated skin contact with low concentrations of acid causes skin desiccation and ulceration of the hands, and panaris or chronic purulent inflammation around the nails. Splashes of acid in the eyes may have particularly serious consequences: deep corneal ulceration, kerato-conjunctivitis and palpebral lesions with severe sequelae.
The general toxic action of sulphuric acid causes alkaline depletion of the body (i.e., an acidosis which affects the nervous system and produces agitation, hesitant gait and generalized weakness).
Safety and health measures. The most effective measures are the total enclosure of processes and the mechanization of handling procedures to prevent all personal contact with sulphuric acid. Particular attention should be devoted to acid storage, handling and application procedures, the ventilation and lighting of workplaces, maintenance and good housekeeping, and personal protective equipment. In addition to the general precautions given above, sulphuric acid should not be stored in the vicinity of chromates, chlorates or similar substances in view of the fire and explosion hazard involved.
Fire and explosion. Sulphuric acid and oleum are not flammable per se. However, they react vigorously with numerous substances, especially organic materials, with the release of sufficient heat to produce a fire or explosion; in addition, the hydrogen released during reaction with metals may form an explosive mixture in air.
Catalysts. Where a vanadium catalyst is used in the contact process, workers should be protected against exposure to emissions of ammonium vanadate or vanadium pentoxide, which are employed on a diatomite or silica gel support.
Inorganic acids, tables
Table 1 - Chemical information.
Table 2 - Health hazards.
Table 3 - Physical and chemical hazards.
Table 4 - Physical and chemical properties.