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sulfuric acid

Product name sulfuric acid
GOST 2184-77
CAS 7664-93-9

Sulfuric acid H2SO4 is a clear, colorless, oily, odorless liquid. It is a strong dibasic acid; fully miscible in water. Sulfuric acid dissolves in water with liberation of heat. Water being added to concentrated sulfuric acid can boil and spit dangerously. One should always add the acid to the water rather than the water to the acid because water is less dense than sulfuric acid and will tend to float above the acid. Sulfuric acid forms hydrates H2SO4*nH2O, where n=1, 2, 3, 4 and 6.5. Sulfuric acid is completely dissociated in water solutions to form H3O+, HSO4- and SO42-:

H2SO4 + H2O --> H3O+ + HSO4-
HSO4- + H2O --> H3O+ + SO42-

Due to the hydration process of sulfuric acid is thermodynamically favorable (ΔH = -880kJ/mol), it is an effective dehydrating agent. Anhydrous sulfuric acid is a very polar liquid and it has nonzero conductivity. This is because the fact that it can ionized by protonating itself, a phenomenon known as autoprotolysis:

2H2SO4 --> H3SO4+ + HSO4-

Actually the composition of anhydrous acid is more complex than shown above. It containes following species in equilibrium (% wt): H2SO4 (99.5), HSO4- (0.18), H3SO4+ (0.14), H3O+ (0.09), H2S2O7 (0.04), HS2O7- (0.05);

Sulfur trioxide SO3 is fully miscible in sulfuric acid to give compounds H2SO4*SO3 and H2SO4*2SO3. The H2SO4*SO3 or H2S2O7 is called pyrosulfuric acid. The solutions of sulfur trioxide in sulfuric acid are known as oleum.

Like other acids, sulfuric acid reacts with bases to form corresponding salts which called sulfates (neutralization reaction):

H2SO4 + Ba(OH)2 --> BaSO4 + 2H2O

Sulfates also can be obtained in the reaction with basic oxides. If the an excessive amount of sulfuric acid is reacted with base or if acid is interacted with metal sulfate, acid sulfate (also known as bisulfate) forms:

H2SO4 + Na2SO4 --> 2NaHSO4

Diluted sulfuric acid reacts with metals like almost acids, such as hydrochloric or phosphorous. These reactions result in corresponding salts and release hydrogen gas. But concentrated sulfuric acid behaves as oxidizing agent. It can oxidize hydrogen iodide HI and hydrogen bromide into corresponding haloids and in can oxidize such passive metals as copper into Cu2+. Depending on process conditions and reducing agent nature sulfuric acid reduces in these reaction to give sulfur dioxide SO2, sulfur or hydrogen sulfide H2S.

Zn + H2SO4(diluted) --> ZnSO4 + H2
Cu + 2H2SO4(concentrated) --> CuSO4 + SO2 + 2H2O

Sulfuric acid displaces weaker acids from their salts to give corresponding sulfates and to release free acid.

As a strong dehydrating agent, sulfuric acid reacts destructively with some organic compounds. Thus, starch or sugars, being treated with it, become charred. On the other hand, sulfuric acid play an significant role in electrophilic aromatic substitution reactions both as an acid and a dehydrating agent. Most important of these reactions such as nitration, sulfonation and dehydration of alcohols are used in industrial organic synthesis. Thus, nitro-substituted or sulfonated compounds are an intermediates in dyes and pharmaceuticals making. Sulfuric acid can form mono- and di- esters in the reactions with alcohols. Being treated with methanol at ~100°C it gives methylsulfuric acid CH3OSO2OH and then dimethyl sulfate (CH3O)2SO2

Sulfuric acid is produced industrially in several forms used for different purposes. Common concentrations are: Diluted sulfuric acid (10%, pH = 1); battery acid (33.5%, pH = 0.5); fertilizer acid (62.18%, pH ~0.4); tower acid (77.67%, pH ~0.25); concentrated acid (98%, pH ~0.1).

Either diluted or concentrated sulfuric acid is corrosive and can cause irritation or injury to the eyes, mucous membranes and skin.


There are two main methods for industrial making of sulfuric acid: contact process and nitrose (tower) process.

  • Contact route. In the first step molecular sulfur is oxidized with oxygen of air to give sulfur dioxide, which is then oxidized in the presence of vanadium pentoxide V2O5 resulting in sulfur trioxide. Sulfur trioxide is absorbed by 98% sulfuric acid to form oleum, which is then gradually diluted with water to produce 98% acid. Direct dissolving of SO3 is impractical because of this reaction is highly exothermic.

    S + O2 --> SO2 + 297.028 kJ/mol
    SO2 + 1/2O2 --> SO3 + 96.296 kJ/mol (catalysed with V2O5)
    SO3 + H2O --> H2SO4 + 130.56 kJ/mol

  • Nitrose (tower) route. The first step in this route is the same as in contact process. The main difference is in conversion of SO2 into SO3. Sulfur dioxide is oxidized by nitrogen oxides dissolved in sulfuric acid. Tower sulfuric acid is cheaper than contact acid, but it contaminated with nitrogen oxides and iron. Highest possible concentration for tower acid is 78%.


  • The main use of sulfuric acid is in fertilizer production such as superphosphate and ammonium sulfate;
  • in orthophosphoric acid making. Orthophosphoric acid is then used in fertilizers and detergents (trisodium phosphate) preparation:

    Ca5F(PO4)3 + 5H2SO4 + 10H2O --> 5CaSO4*2H2O + HF + 3H3PO4

  • to produce aluminium sulfate Al2(SO4)4 which is used in water treatment to produce aluminium hydroxide (coagulant);
  • as a component of nitrating mixture in making nitro-substituted organic compounds (dyestuff, pharmaceuticals precursors, explosives);
  • as a dehydrating agent in chemical processes, such as diethyl ether preparation, esterification reactions and so on;
  • to produce sulfonated organic compounds used as pharmaceuticals, dyes intermediates and others;
  • as a catalyst for electrophylic reactions in industry and laboratory (i.e. for the reaction of isobutane with isobutylene to give isooctane and to convert cyclohexanoneoxime to caprolactam);
  • in the purification of petroleum to remove impurities out of gasoline and other refinery products;
  • in pickling of metal, electroplating baths, nonferrous metallurgy;
  • as an electrolyte in lead-acid batteries;
  • concentrated acid is used as a general dehydrating agent for gases;

  • Manufacturer(s) JSC Alchevskkoks
    JSC Ammofos Cherepovets
    JSC Balakovo mineral fertilizers
    OJSC Boron
    OJSC Chelyabinsk Zink Plant
    JSC Crimea TITAN
    OJSC Khimvolokno Balakovo
    JSC Konstantinovsky Chemical Plant
    JSC Naftan
    JSC Shchekinoazot
    JSC Sibneft-ONPZ
    JSC Sumykhimprom
    Chemical structure of sulfuric acid

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