Alkalis are strong organic bases with corrosive properties. This group of compounds includes primarily hydroxides of elements belonging to the first group of the periodic table, so called lithium group. Alkalis also include aqueous solutions of sodium, potassium and ammonium carbonates, as well as ammonia water. From the chemical perspective, alkalis are all bases according to the Arrhenius theory, where an acid is a compound that produces a hydrogen cation (H+) when it dissociates, while a basis is a compound that produces a hydroxyl anion (OH-) when dissociating. Alkalis are characterised by pH values of 7.1 or greater, which means they have a basic reaction. Their concentrated solutions are corrosive, and therefore hazardous to health, so it is necessary to use personal protective equipment during contact with these substances. Furthermore, alkaline solutions are slippery to the touch as they cause saponification of the lipid barrier present on human skin surface.
Alkali reactivity an physicochemical properties
Alkalis are compounds with high reactivity, so they undergo many different chemical reactions and processes. In neutralisation reactions they react with acids, producing various salts and water, while the products of hydroxide reactions with alcohols are alcoholates. Hydroxides easily react with carbon dioxide, forming carbonates or hydrogen carbonates. Combined with hydrogen sulfide, they form sulfides and disulfides, and as a result they can be used to separate thiols from crude oil. They also react with amphoteric oxides, for example oxides of aluminium, zinc and tin. Molten alkalis react with porcelain and glass, and in the presence of air also with platinum. For this reason, the process of hydroxide melting is conducted in iron or silver vessels, which are resistant to their effects.
Hydroxides of elements from the first group of the periodic table release large amounts of heat when they dissolve in water, and their solubility increases with increasing atomic numbers of the lithium group element. They have an ionic structure, so they dissociate completely in aqueous solutions and as a result belong to the strongest bases. They also easily dissolve in alcohols. Alkalis are commonly used as reagents in organic chemistry, for example as basic catalysts, which remove a proton from a weak acid. An intermediate compound thus formed then reacts with another reagent. Strong hydroxides also participate in the fat saponification reaction, or ester hydrolysis, producing a soap and an alcohol. They are also used as nucleophilic reagents (electron donors) in amide hydrolysis, in substitution reactions and in elimination reactions.
Alkalis are commonly used in numerous industries. Among others, they can be used for pH control in various technological processes, for carbon dioxide absorption in breathing gas purification systems (mainly on submarines), for paper, textiles and drinking water production, as well as drainage agents and disinfectants. Solid sodium hydroxide is a popular agent used for pipe cleaning and draining. Its function is to saponify fatty deposits and release hydrogen when reacting with aluminium particles from the drainage pipes. Next, dirt particles dispersed this way can be easily removed, which unclogs the pipe.
In household chemistry, alkalis are frequently used in combination with other surface active agents because they improve cleaning effectiveness. Both dirt particles and natural surfaces have a negative charge. An alkali addition increases the surface potential, and similarly charged structures repel each other. As a result dirt adhesion is reduced, making it easier to remove. Caution is needed, however, when cleaning alkali-sensitive surfaces, such as dispersive pain coats, linoleum or varnished surfaces.
Alkalis in the PCC Group
The best known member of the alkali group is sodium hydroxide. This compound is available from the PCC Group in solid form as caustic soda, and in the form of an aqueous solution, soda lye. Sodium hydroxide can be produced by soda caustification, which is a reaction of sodium carbonate with calcium hydroxide (caustic lime), or by electrolysis of an aqueous solution of sodium chloride. The PCC Group uses the latter method in a membrane electrolysis unit. The product obtained this way is characterised by high quality and chemical purity, so it meets the requirements of the latest revision of the European Pharmacopoeia.
Sodium hydroxide is one of the essential chemical raw materials and finds use in numerous industries, for example as an input in manufacturing of ionic surface active agents, in the pharmaceutical industry for the production of sulfanilamides and salicylic acid, for the production of synthetic dyes, and in water treatment processes.
A chlorinated derivative of sodium hydroxide is sodium hypochlorite, which is also classified as an alkali. In the PCC Group this compound is produced by saturating soda lye with gaseous chlorine. From the chemical point of view, sodium hypochlorite is a sodium salt of hypochloric acid. It is used, for example, in the chemical industry for oxidising organic compounds, for synthesis of chemical compounds (e.g. hydrazines), and for the production of household cleaning agents.
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