Sulfuric acid

• Sulfuric (VI ) acid
• Oil of vitriol
• Dihydrogen
• Monothionsäure
• E 513

Colorless, odorless, viscous liquid

Liquid

1.84 g · cm -3

10.38 ° C ( 100%)

330 ° C (100 %)

1.3 hPa ( 145.8 ° C)

• -3 (H2SO4 ), -6.62
• 1.9 ( HSO4 - ), 1.99

Completely miscible with water

Risk

0.1 mg · m-3 (measured as the inhalable aerosol fraction)

2140 mg · kg -1 ( LD50, Rat, oral, 25 % solution)

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Sulfuric acid ( according to IUPAC nomenclature dihydrogen, obsolete oil of vitriol ) is a chemical compound of sulfur with molecular formula H2SO4. It is a colorless, oily, highly viscous and hygroscopic liquid. Sulfuric acid is one of the strongest acids and highly corrosive. This mineral acid to form two series of salts, the hydrogen sulphates and sulphates, in which in comparison to the free acid or a two protons have been replaced by cations.

Sulfuric acid is one of the technically most important chemicals at all and is one of the most produced chemical raw materials. 1993 about 135 million tons of sulfuric acid were produced. It is primarily used in the production of fertilizers and to the preparation of other mineral acids, such as the salt or acid. Aqueous solutions of different concentrations are used most often.

The anhydride of sulfuric acid is the sulfur trioxide (SO3). The solution of sulfur trioxide in sulfuric acid over the stoichiometric ratio is also referred to as fuming sulfuric acid or oleum, sulfur trioxide as the included easy escapes as smoke. Related acids include sulphurous acid H2SO3, which is derived from sulfur dioxide and the thiosulfuric H2S2O3 in which an oxygen atom is replaced by sulfur.

History

Sulfuric acid is known under the outdated names oil of vitriol for a long time. First indications are found in the texts of historically controversial alchemist Jabir ibn Hayyān from the 8th century. Followed by possible production methods in the alchemical writings of Albertus Magnus (1200-1280) and Basil Valentine ( 1600) are mentioned. These procedures describe how from naturally occurring sulfates - about Chalkanthit or alum - can be obtained oil of vitriol. The name of oil of vitriol derives from the obsolete name vitriol for these minerals. The first source for large amounts of sulfuric acid was vitriol. From the 16th century sulfuric acid was produced in Bohemia, Saxony, and the resin after Vitriolverfahren. After the first production site Nordhausen Nordhausen, the product was called Vitriol. First scientific studies with sulfuric acid led Johann Rudolph Glauber through. He let the acid to act on salt production and was given the hydrochloric acid and was named after him Glauber's salt sodium sulfate.

The methods in which sulfates used were very complicated and expensive. To obtain larger amounts, a process was developed in the 18th century, were burned, the sulfur and saltpetre in glass vessels. Since the glass vessels were very fragile, in 1746, the reaction was first carried out in lead containers by John Roebuck. Was founded in 1778 in Winterthur with the laboratory the first chemical factory in Switzerland, which produced as the main product oil of vitriol. After 1793 Nicolas Clément- Desormes and Charles Bernard Desormes had discovered that the Salpetermenge could be significantly reduced through the use of air, the lead chamber process could be used industrially. This was particularly important for the 1789 invented by Nicolas Leblanc and 1791 for the first time applied type of Leblanc process for the production of soda ash. The process was repeated several times, for the absorption of nitrous gases by Joseph Louis Gay -Lussac, improved about by the development of methods. It was thus possible to achieve a continuous production management.

The main disadvantage of this method is that only an acid concentration of up to 78 % was reached and concentrated solutions and oleum still had to be made ​​about the consuming distillation of green vitriol. A simple production of highly concentrated sulfuric acid was not until after the development of the contact process from 1870.

Occurrence

Free, not dissociated in oxonium ions and sulfate sulfuric acid occurs in nature very rarely. In the atmosphere it is formed from sulfur dioxide that is produced during the combustion of sulfur-containing substances or volcanic eruptions. The sulfur dioxide is oxidized by hydroxyl radicals and oxygen to form sulfur trioxide. With water eventually forms the free sulfuric acid. Other oxidizing agents that enable the formation of sulfur trioxide, are ozone or hydrogen peroxide. It then passes in the acid rain in the form of diluted acid ( primarily as bisulfate and sulfate) on the ground.

A small amount of free sulfuric acid is also found in some volcanic sources, the so-called solfataras.

In contrast to the free acid, their salts, especially sulfates, in nature much more frequently. There are many different sulfate minerals. Among the best known and most important include gypsum ( CaSO4 · 2 H2O), barite ( BaSO4 ), Chalkanthit ( CuSO4 · 5 H2O) or Glauber's salt (Na2SO4 · 10 H2O).

Outside the earth there is sulfuric acid in the upper atmosphere of Venus. This is caused by photochemical reactions of sulfur dioxide and water. They form droplets containing 80-85 % sulfuric acid. In deeper layers, the acid decomposes due to the high temperatures in sulfur dioxide, oxygen and water that rise up again and can form sulfuric acid.

Production and representation

Raw material for the production of sulfuric acid is often elemental sulfur (2007: 66 million tonnes) in large quantities obtained in the desulfurization of natural gas and crude oil and by the Claus process is worked or is degraded by the Frasch process. This is burned in air to gain sulfur dioxide as a starting material for the actual presentation.

Another source, wherein said sulfur dioxide is produced in large quantities, the smelting sulfur-containing ores. Examples are copper, zinc or lead recovery from the corresponding sulfides. The sulfur dioxide formed during roasting with atmospheric oxygen.

In 1999, only about 3 million tons of pyrite were roasted for sulfuric acid production in Europe. In Asia, however, the proportion of pyrite is higher.

For resource-poor countries that have neither sulfur nor sulfidic ores, the Müller -Kühne process comes into question. This is made from gypsum and coal in the rotary kiln sulfur dioxide. The energy-intensive process can be made more profitable if obtained by the addition of sand and clay as a by- product cement. In the GDR, the procedure was carried out on a large scale.

For more production must be obtained from the sulfur trioxide. The direct reaction of sulfur and oxygen to sulfur trioxide takes place only to a small extent, since the equilibrium at low temperatures only in the reaction of sulfur dioxide to sulfur trioxide on the side of the sulfur trioxide. However, at these temperatures the reaction rate is too low. Therefore, the reaction must be controlled by means of suitable catalysts, a sufficiently rapid response is guaranteed at not too high temperatures.

In today exclusively applied contact process vanadium pentoxide is used as the oxygen -transferring catalyst. It forms a molten salt of vanadium (V ) oxide and the added co- catalysts are alkali metal sulfates. In this, the actual catalyst acting as reactive form complex with the composition [ (VO ) 2O (SO4) 4] 4 -. At this overlap without changing the oxidation state of the vanadium oxygen and sulfur, and respond to sulfur trioxide.

The temperature during the reaction has to 420-620 ° C, because at lower temperatures, the catalyst through the formation of vanadium (IV) compounds are inactive and it decomposes at higher temperatures. The reaction is carried out in so-called Hordenkontaktöfen, in which the catalyst in a total of four layers ( the " trays " ) is placed over each other and the flowing gas is cooled to the appropriate temperature between the trays.

In the so-called double contact process the existing sulfur is washed with concentrated sulfuric acid before the last Horde. This allows an increase in the yield to at least 99.8 % (First General Administrative Regulation on the Federal Pollution Control Act, Technical Instructions on Air Pollution 2002).

After formation of the sulfur trioxide is converted to sulfuric acid this. First you have to remove residual sulfur dioxide with ammonia or sodium thiosulfate. Since the direct reaction of sulfur trioxide with water is too slow, the gas is conducted in concentrated sulfuric acid. It forms rapidly disulphuric H2S2O7. If this is diluted with water, it breaks down into two molecules of sulfuric acid.

In this method, there is no pure sulfuric acid, but concentrated acid with 98 % acid. To produce pure sulfuric acid, the amount of sulfur to be introduced into the concentrated acid, which corresponds to the molar amount of the excess water of the concentrated acid.

In recent decades, the sulfuric acid production mainly in China has risen sharply, while in European countries such as Germany, the production has declined. Since early 2000, China has to rely on additional quantities from Europe. The seen in the graph strong upheaval in the years 1990 and 1991 are due to the dissolution of the Soviet Union and a change of statistics in the United States.

It is of considerable economic importance in the large-scale industrial production of sulfuric acid that the three individual steps are exothermic ( for values ​​see on contact method after ). The released amount of heat is used to produce high pressure steam for power generation and industrial heating purposes.

Properties

Physical Properties

Sulfuric acid is a viscous, colorless liquid which solidifies below 10.37 ° C. The frequent light brown staining of technical sulfuric acid relies on organic contaminants. Above the boiling point of 279.6 ° C, sulfuric acid vapors, which also contain excess sulfur trioxide form. At a temperature of 338 ° C, the vapor has an acid content of 98%, which corresponds to a sulfuric acid -water azeotropic mixture. On further heating the sulfuric acid decomposes into water and sulfur trioxide and is almost completely dissociated at 450 ° C.

As solid crystallized sulfuric acid in the monoclinic crystal system, space group C2 / c. The lattice parameters are a = 814 pm, b = 470 pm, c = 854 pm, and β = 111 °. The structure is a corrugated layer structure in which each dihydrogen tetrahedra connected by hydrogen bonding with other four tetrahedrons. In addition to the crystalline pure sulfuric acid and sulfuric acid several hydrates are known. An example is the dihydrate H2SO4 · 2 H2O, which also crystallizes monoclinic with space group C2 / c. Six different hydrates with one, two, three, four, six and eight equivalents of water are known, in which the acid is completely divided into oxonium and sulfate ions. The oxonium ions are associated, depending on the hydrate with a different large number of water molecules. The melting point decreases with increasing number of hydrates, the water molecules. So the monohydrate melts at 8.59 ° C, while the octahydrate already melts at -62 ° C.

Between the individual molecules act strong hydrogen bonds that determine the high viscosity of 24.6 mPa · s at 25 ° C. In comparison, water has 0.89 mPa · s at 25 ° C, a significantly lower viscosity.

Similar to pure water conducts pure sulfuric acid to a small extent electricity. The specific conductivity of 1.044 · 10-2 S / cm. The reason for this is the low dissociation of the acid by autoionization. Other hand, dilute acid is derived due to the oxonium ions contained electric current well.

In the gas phase are single sulfuric acid molecules. They are constructed with tetrahedral bond angles of 101.3 ° between the OH groups and 123.3 ° between the oxygen atoms. The bond lengths of the sulfur -oxygen bonds are different with pm 157.4 ( to OH groups ) and pm 142.2 ( to oxygen atoms ). The molecular structure in the solid state the same as that in the gas phase.

The bonds in the sulfuric acid molecule can be described by several resonance structures. For example, the structure in which the double bonds between sulfur and oxygen are assumed to be present or when only single bonds, and simultaneously a charge separation. In theoretical calculations has shown that the d- orbitals, which are necessary for the description of an OS - double bond contribute very little to the binding. Therefore, the real situation is described in the binding molecule of sulfuric acid most accurately by that structure in which only single bonds are drawn. The shortened S- O bond can be explained by an additional electrostatic interactions between the charged atoms.

Chemical Properties

As a very strong acid is sulfuric acid slightly from protons. With a pKa of -3.0 ( this applies only for dilute solutions ) a H0 value of -11.9 counts sulfuric acid in the first protolysis to strong acids or accurate.

It is usually not counted as superacids, but it is chosen as the starting point for the definition of the superacid: All acids, which are stronger than pure sulfuric acid and this can protonate thus are called superacids.

The second protolysis of sulfate to bisulfate has a pKa of 1.9. The hydrogen sulfate ion is therefore only a moderately strong acid.

For this reason is located in a dilute sulfuric acid ( concentration of about 1 mol / l) for the most part before hydrogen sulfate. H2SO4, the molecule is almost completely dissociated during the reaction to sulphate only a small extent ( about 1.3 % at 1 mol / L) takes place. Only at higher dilutions greater levels of sulfate are formed.

Sulfuric acid having a high affinity for water. Be mixed acid and water, produced under excessive heat various hydrates of the form H2SO4 · n H2O ( n = 1-4, 6, 8). The strong water affinity of sulfuric acid manifests itself also in the fact that it is able to split off from organic substances hydroxyl groups and protons. By this removal of carbon is left, the organic material is black and charred. This effect occurs especially for substances that contain many hydroxyl groups. Examples are many carbohydrates such as glucose or polysaccharides. Furthermore, the great affinity for water can be used for condensation reactions. Here, the water is removed without charring of organic compounds. An example of this is the synthesis of 2 -pyrone.

Another indication of the strong hygroscopicity is that the acid is drained to a small degree itself:

Of concentrated sulfuric acid has an oxidizing effect, and is able to dissolve when heated and noble metals such as copper, mercury or silver. The sulfuric acid is thereby reduced to sulfur dioxide. In contrast, even pure, base iron is not attacked by Passivation of concentrated sulfuric acid.

Dilute sulfuric acid, however, acts only to a limited extent by oxidation, because the reaction of sulfur dioxide to be inhibited by the solvent is water. It can be oxidized or dissolved, which can be oxidized as a base element by the reaction of hydrogen protons to only those metals.

Use

Sulfuric acid is used in very large quantities and in many areas. Your production amount applies - in addition to chlorine - as a measure of industrial development and the performance level of a country. Depending on the concentration it is called differently. Between 10 and 20 % is referred to as dilute sulfuric acid or dilute acid, the battery acid has an acid concentration of 33.5 %, up to a content of about 70 % is called chamber acid, up to 80 % of Gloversäure. Concentrated sulfuric acid usually has a content of 98.3 % ( azeotrope ). Dilute acid is obtained in large quantities as a waste product in the titanium oxide or dye production.

Most of it is consumed in the production of fertilizers. With the help of sulfuric acid, especially phosphate and ammonium sulfate fertilizers are obtained. The latter is represented by the reaction of semi- concentrated sulfuric acid with ammonia.

In the production of phosphate fertilizers, sulfuric acid is needed for the digestion of phosphate rock. In the reaction occurs superphosphate Ca ( H2PO4 ) 2/CaSO4

Addition of ammonium sulfate, other sulfates are prepared by reaction of corresponding salts with sulfuric acid. An example is the aluminum sulphate produced from aluminum hydroxide, which is used in large quantities in the paper industry and as a flocculant in water purification.

As many ores are soluble in sulfuric acid, it can be used as disintegrating agents. Examples are the wet process for the production of zinc from zinc oxide and the sulphate process for the recovery of the white pigment titanium dioxide. With the help of sulfuric acid not only oxide ores, but also those can be digested with other anions such as fluoride or phosphate. In the reaction, in doing so the corresponding acids. This method is relevant for the production of several industrially important acids. Examples include hydrofluoric acid from fluorite, apatite phosphoric acid and hydrochloric acid from halite.

As battery acid sulfuric acid is an important part of the lead-acid battery, as it is for example used in automobiles as a starter battery. As well as in the lead storage battery, dilute sulfuric acid is used in electrolytic processes as the electrolyte. The advantages over other electrolytes are the high conductivity and simultaneously low propensity for reduction.

In organic chemistry, by fuming sulfuric acid, the sulfonic acid are added ( sulfonation ). Notably, surfactants for the detergent industry and dyes are prepared. A further functional group which can be introduced by means of sulfuric acid, the nitro group. This is done using the so-called mixed acid, a mixture of sulfuric and nitric acid. This is mainly used for the production of explosives, such as trinitrotoluene, or nitroglycerin.

In chemical laboratories sulfuric acid is one of the most commonly used chemicals. Addition of hydrochloric acid and nitric acid is a strong acid used much. It is used, among others, to adjust the pH - value, as a catalyst, such as for esterification and fuming at outcrops. The highly water -withdrawing effect of the sulfuric acid is used for drying organic substances and gases in desiccators and washing bottles.

Biological Significance

The resulting sulfuric acid from sulfur dioxide in the air is next to the nitric acid formed from nitrogen oxides, a component of acid rain. Due to acid rain may occur, especially in weakly buffered soils and water in a decrease of the pH value. An effect of a low pH value is a change in the solubility of some metal ions. Thus, the harmful plants for aluminum is better at lower pH value in water soluble. As biologically important ions such as potassium or magnesium, can be easily washed out. For these reasons, sulfuric acid is considered as a possible cause of forest dieback in the 1980s. Due to technical measures such as flue gas desulfurization in coal-fired power plants and the introduction of low-sulfur fuels is now released so little sulfur dioxide in Germany, that the rain water contains significantly less sulfuric acid here.

Sulfuric acid acts on fish and other aquatic creatures due to their acidity toxic. Thus, the median lethal concentration in soft water without buffer capacity ( LC50 ) for fish 100-330 mg / l In hard water, in which the acid is converted into sulphates, sulfuric acid is less toxic, the LC50 value is 15 g / l ( when converted to sodium sulfate ) and 2.98 g / l (calcium sulfate). In the slag heaps of ore mines and lignite mines sulfuric acid is formed by the oxidation sulfidhaltiger minerals. In addition, rain water, it is washed out and collects in residual lakes in which living things can be found because of the low pH and high heavy metal contents hardly. This process is known as acid mine drainage.

Safety

Sulfuric acid acts on the skin and mucous membranes highly irritating and corrosive. She is able to destroy living tissue ( burns ). The mechanisms of action of concentrated and dilute sulfuric acid are clearly distinct. In dilute sulfuric acid, the increased proton concentration is corrosive, that is, the effect is similar to that of other dilute acids. The effect of which is in contact with skin, depending on the concentration, mainly in local irritation. It is significantly more dangerous than concentrated sulfuric acid. This acts due to their highly water -withdrawing effect charring and damage already strong in small quantities skin and eyes. It formed only slowly healing, painful wounds. Sulfuric acid can be absorbed through the vapors from the air, the TLV is 0.1 mg/m3, taken by inhalation LC50 in rats over four hours of 510 mg/m3.

Since formed in the reaction of concentrated sulfuric acid with water much heat, they must only be thinned by pouring into water and not by adding water to the acid. When water is added to sulfuric acid, it can inject and so corrosive to bystanders.

Proof

Of concentrated sulfuric acid is demonstrated by the reaction with organic substances. If, for example, a wood chip immersed in concentrated sulfuric acid, it is slowly black. It is possible to distinguish by means of different reactions diluted and concentrated sulfuric acid. Here, the different reactivity of the two acids with base metals such as zinc or iron utilized. While forming with dilute acid at room temperature, hydrogen, reacting the concentrated acid, which contains almost no free oxonium ions, only upon heating to form sulfur dioxide and sulfur.

Since in aqueous solution, the sulfuric acid is dissociated, they can not prove it directly. Instead, one can determine the concentration of protons and thus the acidic pH by means of suitable indicators, or with a pH meter. The sulfate ion can be determined for example by precipitation as sparingly soluble barium sulfate.