What Is Sulphuric Acid? | The Chemistry Blog - Chemical Suppliers

Basic Properties of Sulphuric Acid

As a substance often encountered in chemistry, you may wonder “what is sulphuric acid?” Well, sulphuric acid, (spelt as sulfuric acid in the US), is a concentrated reactive acid. It either oxidises, dehydrates, or sulfonates many organic materials. It is made by oxidising sulphur dioxide, 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:

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• Fertilisers
• Explosives
• Drugs
• Detergents
• Dyes
• Paints
• Plastics

Not only that, but sulphuric 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 sulphuric acid, which is why so much of the chemical bottling that happens involves solutions that contain this acid.

The global industrial demand is very high globally, and it is with an estimated that 260 million tonnes of sulphuric acid are produced annually, enough even to fill roughly 100,000 Olympic-size swimming pools.

Key Takeaways:

• Sulphuric acid (H₂SO₄)is a highly reactive, versatile acid essential to manufacturing processes across many industries, including fertilisers, metals, and batteries

• It is created through a four-stage process involving sulphur extraction and chemical reactions

• Sulphuric acid has a highly acidic pH of 2.75 under standard conditions, though this varies with concentration and other factors

• It is crucial in industries such as metal processing, chemical manufacturing, battery production, and agriculture due to its reactive properties

Chemical Formula of Sulphuric Acid

The equation for sulphuric acid is H₂SO₄.

This compound is formed in four stages, with each represented by various chemical reactions:

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

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

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

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

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

Sulphuric Acid Structure

Structurally, a molecule of sulphuric acid, which has a molecular weight of 98.079 g/mol, has a central sulphur atom that is surrounded by four atoms of oxygen. Two of the oxygen atoms are double-bonded with the sulphur. The other two atoms are single-bonded with sulphur 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 Sulphuric Acid a Strong Acid?

Just like other acids, the strength of sulphuric 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.

In fact, sulphuric 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

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. Sulphuric acid has a Ka value of 1.0 x 102. This places the acid in a higher rank compared to other acids.

Common Products and Compounds Containing Sulphuric Acid

Although sulphuric 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 – sulphuric 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. At low concentrations, however, it can be used as an acidifying agent in some dairy products like cheese, and is approved in Europe as a food additive under number E513

What is the pH Value of Sulfuric Acid?

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

There are some variables, however: Unlike the dissociation constant, the pH value is directly dependent on the concentration of the acid or base in a solution. This also depends on factors like:

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

What Colour Does Sulphuric Acid Turn Blue Litmus Paper?

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

As litmus paper is available in either red or blue pigmentation, it does not measure the pH but is sensitive enough to react with an acid or a base.

Red 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.

Is Sulphuric Acid an Element?

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

Is Sulphuric Acid a Solution?

Yes, sulphuric acid is a solution because an acid can only react 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 sulphuric 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 include:

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• 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 fertiliser fertiliser acid (pH about 0.4)
• 77.67% – Tower or Glover acid (pH about 0.25)

Industrial and Laboratory Uses of Sulphuric Acid

Industrially, sulphuric acid is key to processes involving chemical synthesis, fertiliser production, petroleum refining, and metals processing. Here’s a closer look at its applications:

Metal Processing

While many reagents and chemicals are used in metal production, sulphuric acid plays a key role in a specific process for steel known as “steel pickling”. While the metal is in a hot working process, its surface oxidises, which can cause issues with the final product.

To counter this before cold rolling, the metal goes through a “pickling liquor” made of sulphuric acid, and sometimes followed by additional washes in hydrochloric and other acids.

Chemical Manufacturing

It may seem hyperbolic, but sulphuric acid is a vital, and arguably one of the most important, compounds made in chemical manufacturing. Many other chemicals – including hydrochloric acid, nitric acid, synthetic detergents, sulphate salts, and more – rely on sulphuric acid as a base component.

This becomes all the more clear in laboratory settings, where it is vital as a catalyst for other chemical reactions and in titrations.

Additionally, in the pharmaceutical industry, sulphuric acid helps in the production of drugs by acting as a reactant and catalyst in the synthesis of active pharmaceutical ingredients.

Battery Production

As touched on before, sulphuric acid is vital to the manufacturing of lead-acid batteries, as it is used to activate the lead plates inside the casing, and allow for the transfer of energy.

Much like with metal working, it is one of the last parts of production before the charging cycle begins, with an electrolyte solution made of sulphuric acid and distilled water.

Agriculture

One of the largest users of sulphuric acid is in agriculture, specifically through phosphate fertilisers, ammonium phosphates, and calcium dihydrogenphosphate, though there are other applications. It is used when helping sulphur-deficient land recover, and preventing deficiencies in crops.

Conclusion

Sulfuric acid is a cornerstone of modern industry, holding a place of immense importance in fields ranging from large-scale chemical manufacturing to water treatment, mining, metallurgy, agriculture, automotive applications, and more. It exists in multiple concentrations to accommodate different tasks, with each concentration featuring specific physical and chemical attributes. Understanding these distinctions can be critical for optimizing manufacturing processes, ensuring safety, and selecting the right grade for a given application.

Below, we delve into four common sulfuric acid concentrations—30%, 70%, 93%, and 96%—and discuss their typical uses and essential precautions. We’ll also briefly compare sulfuric acid to other widely used mineral acids, such as hydrochloric acid and nitric acid, and highlight how to handle them safely. For those looking to purchase any of these acid concentrations, or other chemicals, you can explore Alliance Chemical’s extensive listings. Their store offers a variety of acidic products, bases, solvents, oxidizing agents, and more.

1. Sulfuric Acid 30%

30% sulfuric acid is a relatively dilute solution containing 30% by weight of sulfuric acid (H2SO4) and around 70% water. Although it is less aggressive than higher concentrations, it remains corrosive and must be handled correctly. Compared to more potent sulfuric acid grades, its moderated strength can be advantageous in specific scenarios that do not demand highly reactive or dehydrating properties.

Key Applications of 30% Sulfuric Acid

• Battery Electrolyte: Dilute sulfuric acid is commonly used in lead-acid batteries. While many automotive and industrial batteries may contain acids slightly above 30%, this general range can be adjusted or concentrated further. The electrolyte facilitates the electrochemical reactions necessary for power storage and release.
• Water Treatment: Municipal and industrial facilities often dose lower concentrations of sulfuric acid to reduce alkalinity, adjust pH, and support flocculation processes in water and wastewater treatments. Its controlled acidity can help remove dissolved metals and other impurities.
• Cleaning Agents: In cleaning formulations, particularly those aimed at dissolving mineral deposits, scale, or rust, 30% sulfuric acid is occasionally used for a gentler yet effective approach. This might include specialized descalers, rust removers, and surface preps. Even at 30%, it must be handled with caution due to its corrosive nature.

Though milder than highly concentrated acids, 30% sulfuric acid can still cause chemical burns and environmental harm if mishandled. Always follow proper safety measures, use personal protective equipment (PPE), and consult the relevant Safety Data Sheet (SDS).

2. Sulfuric Acid 70%

70% sulfuric acid is a more potent, intermediate-grade solution commonly found in industrial settings. With a higher acid-to-water ratio, it exhibits greater reactivity and a stronger corrosive effect than the 30% variant.

Key Applications of 70% Sulfuric Acid

• Fertilizer Production: One of the largest markets for sulfuric acid lies in manufacturing phosphate fertilizers. When phosphate rock is processed, acids like 70% sulfuric acid help liberate phosphate ions, facilitating the formation of soluble fertilizers such as ammonium phosphate.
• Mineral Processing: Mining operations frequently use sulfuric acid to leach metals from ores. Concentrations around 70% can effectively dissolve or break down metal compounds, assisting in extraction processes for copper, zinc, and other metals.
• Chemical Synthesis: An array of chemicals rely on sulfuric acid at intermediate concentrations for essential reactions. Examples include detergent production, certain dye and pigment manufacturing, and general inorganic salt formation.

Due to its elevated acidity and potential for violent reactions with water or incompatible materials, 70% sulfuric acid demands strict adherence to safety protocols. Storing it in corrosion-resistant containers and ensuring a well-ventilated workspace are paramount.

3. Sulfuric Acid 93%

93% sulfuric acid is often referred to as a standard “concentrated” sulfuric acid in many industrial practices. It strikes a balance between high reactivity and manageable handling, compared to more extreme fuming acids or oleum (which can exceed 100% by including disulfuric acid).

Key Applications of 93% Sulfuric Acid

• Oil Refining (Alkylation): Petrochemical refineries use concentrated sulfuric acid as a catalyst in alkylation units to produce high-octane gasoline components, enhancing fuel performance.
• Pulp and Paper Industry: In paper manufacturing, sulfuric acid helps remove lignin from cellulose fibers, contributing to improved paper quality. Concentrated acid can efficiently break down unwanted organic matter.
• Metal Pickling: Steel, iron, and other metals are treated with strong acids to remove oxide layers, rust, and scale before further processes like galvanizing or painting. With 93% acid, pickling becomes a rapid and precise stage of metal preparation.

Handle 93% sulfuric acid with rigor. Its dehydrating nature can cause severe damage to skin, organic materials, and certain plastics. Splashes, even minor, can be dangerous, so wearing goggles, gloves, and protective clothing is vital. Proper chemical fume hoods or exhaust systems are recommended in any environment where vapors might accumulate.

4. Sulfuric Acid 96%

96% sulfuric acid stands among the most concentrated commercial forms of this acid, approaching the azeotropic mixture and displaying powerful corrosive and dehydrating properties. Its high purity makes it indispensable in various advanced industrial processes and laboratory work.

Key Applications of 96% Sulfuric Acid

• Chemical Synthesis: Many chemical reactions—such as nitration, sulfonation, or forming certain sulfates—depend on highly concentrated sulfuric acid. The near-anhydrous environment it creates can shift reaction equilibria in ways that weaker acids cannot.
• Laboratory Reagent: At near-absolute concentration, sulfuric acid serves as a standard reagent for titrations, dehydration reactions, and a variety of analytical procedures. Research laboratories often rely on 96% acid for controlled experiments and advanced organic synthesis.
• Catalyst for Esterification and Other Reactions: In certain industrial settings, the acid’s potent proton-donating ability accelerates esterification, nitration, or polymerization. The near-waterless environment fosters conditions that drive reactions to completion.

Extreme care is necessary when working with such a high concentration. Splash-resistant face shields, acid-resistant gloves, and chemical aprons should be standard. Improper handling can lead to rapid exothermic reactions—especially if water is added to the acid rather than the reverse (always add acid slowly to water).

Comparisons with Other Common Acids

Beyond sulfuric acid, chemical operations often involve other strong acids, each with its own set of benefits and limitations. Two frequently encountered options are hydrochloric acid and nitric acid. Understanding how these acids differ from sulfuric acid can inform smarter choices for each application.

1. Hydrochloric Acid

Hydrochloric acid (HCl) is a strong, monoprotic acid widely recognized for its use in cleaning, pH control, and large-scale chemical production. Commonly sold in concentrations ranging from about 20° Baumé (~31.5% by weight) to more specialized strengths, HCl is known for:

• Metal Pickling: Similar to sulfuric acid, hydrochloric acid effectively removes oxides and prepares metals for further processing. It typically generates fewer sulfates but can release chloride ions that pose corrosion concerns for certain equipment.
• pH Adjustment: Widely used in water treatment and industrial processes requiring precise pH control, HCl is easy to handle and efficient at neutralizing bases.
• Production of Inorganic Compounds: Hydrochloric acid underpins the synthesis of numerous chlorides and chlorinated compounds, including disinfectants, food additives, and certain pharmaceutical intermediates.

2. Nitric Acid

Nitric acid (HNO3) is another potent acid, typically found at concentrations like 70%. Key uses include:

• Fertilizer Production: Nitric acid is integral in making ammonium nitrate and other nitrogen-rich fertilizers, vital for agriculture worldwide.
• Explosives Manufacturing: The synthesis of nitroglycerin, TNT, and other high-energy materials requires nitric acid for nitration reactions.
• Metal Etching and Cleaning: Nitric acid is famously used in etching processes for electronics, semiconductor devices, and precision instruments, as it dissolves metals in a more controlled manner while producing characteristic nitrates.

While sulfuric, hydrochloric, and nitric acids share common ground as strong mineral acids, they each bring different anions and reactivities. Choosing the right acid often hinges on the specific chemical pathway, the nature of byproducts, and the materials in contact during processing.

Essential Safety Precautions

Regardless of concentration or acid type, strong acids must be handled responsibly. Maintaining a safe environment not only protects staff and equipment but also minimizes the risk of contamination or product loss. Key guidelines include:

• Wear Proper PPE: This typically means acid-resistant gloves, goggles or face shields, lab coats or aprons, and closed-toe shoes. For higher concentrations or large volumes, consider respiratory protection (e.g., a fume hood or appropriate respirator).
• Ensure Adequate Ventilation: Use fume hoods or exhaust systems in closed spaces to disperse any harmful vapors. Always store and use acids in well-ventilated areas, far from incompatible chemicals like bases or oxidizers unless specifically designed processes permit.
• Correct Storage: Keep acids in secure, labeled containers (often polyethylene or specialized alloys). Separate them from alkalis, flammable materials, or reactive metals. Temperature stability is key: extreme heat or cold can affect acid concentration and pressure in storage drums.
• Emergency Measures: Train all personnel to respond to spills or exposures quickly. Chemical showers, eyewash stations, and firefighting equipment (if flammable chemicals are nearby) must be easily accessible.
• Safe Dilution Practices: Always add acid to water slowly, never water to acid. A large exothermic reaction can occur if water is suddenly poured into concentrated acid, potentially causing splashes and generating heat.
• Disposal and Regulatory Compliance: Consult local regulations when disposing of acid waste. Some facilities neutralize spent acid with alkaline solutions before disposal, while others may rely on licensed chemical waste handlers.

Conclusion

Sulfuric acid in concentrations of 30%, 70%, 93%, and 96% performs a vital role across a remarkable spectrum of industrial and commercial processes. From powering lead-acid batteries and refining metals to catalyzing chemical reactions and synthesizing essential fertilizers, each grade brings distinct capabilities and risks. Understanding these nuances can streamline operational efficiency, reduce costs, and maintain a safe environment for workers.

Additionally, contrasting sulfuric acid with other major mineral acids—such as hydrochloric acid and nitric acid—underscores how each acid has its own niche applications and chemical behaviors. Choosing the correct acid depends on the target reaction, regulatory constraints, and the materials or equipment in use.

By following recommended safety measures, employing best practices in storage and handling, and partnering with reputable suppliers like Alliance Chemical, industries can harness the immense utility of sulfuric acid while mitigating risks. For more detailed information on product availability, technical specifications, or bulk orders, visit Alliance Chemical’s main site or contact their support team directly. Proper acid management ensures continued innovation and productivity across sectors ranging from energy to manufacturing, making sulfuric acid an enduring heavyweight in the chemical world.

Disclaimer: The content provided here is for informational purposes only and may not cover all potential applications or hazards. Always review the latest Safety Data Sheets (SDS), consult industry regulations, and seek professional guidance when handling, storing, or disposing of hazardous materials.

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