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Context 1: The oxidation of sulphur and acid formation
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Formation of acids Sulphur oxides are primarily formed during the combustion of sulphur or the oxidation of sulphur compounds. In the air, they react to particles of sulphuric acid. Acid rain is an environmental problem mainly based on the release of sulphur oxides (sulphur dioxide and sulphur trioxide) as a result of human activity. Nitrogen oxides also play a role in acid rain formation.
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Sources of sulphur dioxide Suphur dioxide is naturally found in volcanic gases and as a product of the oxidation of dimethyl sulphide from oceanic phytoplankton (algeae) or other sulphur compounds formed by micro-organisms. The latter process is described in ACCENT magazine Nr. 5 (oceanic sulphur).
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Table: Sulphur emissions in the world
> 90% of all biogenous emissions are DMS.
Sulphur dioxide can destroy pigments. Therefore, in industry it is used as bleaching agent. The disinfecting effect of sulphur dioxide is often used for the conservation of food (for example in dried fruit).
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From sulphur dioxide to sulphurous acid If sulphur dioxide is dissolved in water, it forms a weak acid solution, sulphurous acid. H2O + SO2 H2SO3In water, sulphurous acid forms hydrated protons H+ (aq) and two sorts of anions are formed: H2SO3 H+(aq) + HSO3-(aq)Sulphurous acid forms a proton and a hydrogen sulphite anion HSO3-(aq) H+(aq) + SO32-(aq)Hydrogen sulphite gives a proton and a sulphite anion. From each molecule of sulphurous acid two protons can be released. Therefore sulphurous acid is called a bi-protonous acid. Sulphuric acid With the combustion of sulphur and during oxidative reactions in the air not only sulphur dioxide is formed but also sulphur trioxide. The amount formed of each of these gases depends strongly on the temperature, since sulphur trioxide decomposes into sulphur dioxide and oxygen at temperatures above 600°C: 2 SO3 -> 2 SO2 + O2 (at t > 600°C)If sulphur trioxide dissolves in water, sulphuric acid is formed: SO3 + H2O H2SO4Sulphuric acid is also a bi-protonous acid which decomposes in water to yield two protons and an anion. H2SO4 H+(aq) + HSO4-(aq)In the first step sulphuric acid forms a proton and hydrogen sulphate anion. HSO4-(aq) H+(aq) + SO42-(aq)Hydrogen sulphate decomposes to give another proton and a sulphate anion.
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Acid rain The solution of sulphuric acid in the water of rain drops increases the concentration of protons, i.e. the pH value decreases. The rain becomes more acidic. Acid rain does not only dissolve some rocks (carbonate rock), but also attacks metal surfaces and mobilises certain ions in the soil, which are toxic for plants. The consequences are damage to buildings, metal tools, the acidification of lakes and damage to forests. You will find more detailed information on acid rain by choosing the links in the link list.
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