What Is Sulfuric Acid?

by Jessica Clifton

What is sulfuric acid and why it is important? Sulfuric acid, also spelt sulphuric acid, is one of the most useful inorganic mineral acids and plays a crucial role in many industrial processes. It is used as a precursor, reagent, and as a catalyst in manufacturing many types of products such as:

  • Fertilizers
  • Explosives
  • Drugs
  • Detergents
  • Dyes
  • Paints
  • Plastics

Sulfuric acid has many uses. It is the most common type of acid used in batteries, typically in lead-acid batteries for cars and solar power systems. It is also used in purifying metals such as zinc and copper from ores. It is virtually unimaginable for many industries to function without sulfuric acid.

The industrial demand is very high globally, and it is estimated that 260 million tonnes of sulfuric acid are produced annually. This is even to fill roughly 100,000 Olympic-size swimming pools. That’s a lot of acid!

What is the equation for sulfuric acid?

The equation for sulfuric acid is H2SO4.

The chemical equation of sulfuric acid - 3D illustrationTo understand this equation, you first need to know how it is manufactured. Basically, the manufacturing process of this acid involves four stages. These stages are represented by various chemical reactions:

  • 1st stage – extraction of sulfur – 2PbS(s) + 3O2(g)  ? 2PbO(s) + 2SO2(g)
  • 2nd stage – conversion of sulfur to sulfur dioxide – S(s) + O2(g)  ? SO2(g)
  • 3rd stage – conversion of sulfur dioxide to sulfur trioxide – 2SO2(g) + O2(g) ?? 2SO3(g)
  • 4th stage – conversion of sulfur trioxide to sulfuric acid – SO3(g) + H2O(I) ? H2SO4(I)

Typically, sulfur is extracted from fossil fuels. Unrefined petroleum and natural gas contain sulfur compounds that are removed during the refining process. As shown in the first equation above, sulfur can also be extracted from metal ores as sulphides. 

Molten sulfur is then blasted by dry air at 1300 K. At this stage, the sulfur is burnt, converting it to sulfur dioxide.

In the third stage, a catalyst known as vanadium (V) oxide on silica is used to convert sulfur dioxide to sulfur trioxide at 700 K.

Finally, the sulfur trioxide is converted to sulfuric acid by allowing it to react with water at a constant temperature of 400 K.

More than 260 million tons of sulfuric acid are produced globally every year
Sulfur to make sulfuric acid stored in a factory

Sulfuric acid structure

Structurally, a molecule of sulfuric acid, which has a molecular weight of 98.079 g/mol, has a central sulfur atom that is surrounded by four atoms of oxygen. Two of the oxygen atoms are double-bonded with the sulfur. The other two atoms are single-bonded with sulfur but have hydrogen attachments.

Because of valence shell electron pair repulsion (VSEPR), a molecular model that predicts the geometry of the atoms, the oxygen atoms spread out in tetrahedral form. This means that each oxygen is bonded at an angle of 109.5 degrees from the other oxygen atoms.

Is sulfuric acid a strong acid?

Just like other acids, the strength of sulfuric acid is measured by its acid dissociation constant in a solution. This constant, represented by the symbol Ka, does not change regardless of the concentration of the acid. The pH may change based on the concentration but not the Ka.

The acid dissociation constant is also known as the chemical reaction equilibrium of acid with its base conjugate. This is directly associated with the ratio between the formation of hydronium ions and the molecular acid. The larger the Ka, the stronger the acid as its ions dissociate more completely in a solution. Sulfuric acid has a Ka value of 1.0 x 102. This places the acid in higher rank compared to other acids.

In fact, sulfuric acid is one of the seven strong acids. The other strong acids are:

  • HCl – hydrochloric acid
  • HNO3 – nitric acid
  • HBr – hydrobromic acid
  • HI – hydroiodic acid
  • HClO4 – perchloric acid
  • HClO3 – chloric acid

What has sulfuric acid in it?

Although sulfuric acid is essential in many manufacturing processes, it is not commonly found in many products as the active constituent. A few products that contain the acid include:

  • Battery electrolyte – sulphuric acid is the electrolyte for lead-acid batteries of automobiles and solar power systems
  • Household cleaning products – sulfuric acid is good in dissolving protein and grease similar to an alkaline solution
  • Food additive – you probably wouldn’t think that a strong and toxic acid is used as a food additive. Actually, it is an acidifying agent in some dairy products like cheese, but at very low concentration. It is approved in Europe as a food additive under number E513

What is the pH value of sulfuric acid?

Determining the pH value of any acid depends on various factors such as:

  • The type of acid
  • The type of solvent
  • Concentration in a solution
  • Temperature
  • Pressure

The pH or per hydronium value of a substance is measured on a logarithmic scale of 1 to 14. This means that each value is the exponent of base ten.

Any neutral substance like pure water has a pH value of 7. Below 7 this is considered acidic, and above 7 is considered basic or alkaline. The pH scale represents how a substance can donate or receive hydrogen ions in a chemical reaction.

Unlike the dissociation constant, the pH value is directly dependent on the concentration of the acid or base in a solution. Therefore, the pH value of sulfuric acid varies depending on its concentration.

However, under the standard conditions of 1 mMol/L, 25°C, and 1 atmospheric pressure, the pH value of sulfuric acid is 2.75. This makes it highly acidic.

What colour does sulfuric acid turn blue litmus paper?

Litmus paper is available in either red or blue pigmentation. It contains dyes that are extracted from lichens. It does not measure the pH but is sensitive enough to react with an acid or a base.

The litmus paper colour chart measures acids and alkalinesRed litmus paper turns blue if the solution has a pH of 8 or more. Meanwhile, blue litmus paper turns red if the solution has an acid level of between 4 and 5 or lower.

Depending on its concentration, sulfuric acid may not change the colour of blue litmus paper, or it may turn it a shade of red.

Is sulfuric acid an element?

Sulfuric acid is not an element. Rather, it is a compound that is composed of two hydrogen atoms, one sulfur atom, and four oxygen atoms. When dissolved in water, it dissociates into ions of H3O+ and HSO4.

Is sulfuric acid a solution?

Technically, an acid can only react as such if it is in a form of solution. A compound of acid can occur in salt, solid forms but it will almost always have impurities. The purest form of sulfuric acid has a purity grade of 98% with pH value of 0.1. It is most stable for storage in this form.

Other concentrations of the acid are:

  • 10% – dilute sulphuric acid for use in labs (pH 1)
  • 33.5% – battery acid for lead-acid batteries (pH 0.5)
  • 62.18% – chamber or fertilizer acid (pH about 0.4)
  • 77.67% – tower or Glover acid (pH about 0.25)


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