Sulfuric Acid

Sulfuric acid is made in greater tonnage than any other chemical on earth, and it is traded in two distinct ways. Most of it is a bulk commodity bought on strength and freight: 93% or 98%, delivered by tanker into steel storage, consumed in fertilizer, leaching, pickling and sulfonation. The rest is bought on impurities, where iron, chloride and heavy metals decide the grade and the assay barely moves. Two properties settle most specifications before anything else. The concentrated acid passivates carbon steel, which is why bulk storage is cheap, and the 98% strength begins to freeze near 3 °C, which is why 93% is the one that survives a cold-climate winter in an outdoor tank. Where sulfur trioxide itself is the reagent, as in sulfonation, the acid is bought as oleum instead, since oleum is the practical way to move and meter trioxide. Ehlinar supplies technical, battery and specified low-metal grades, and oleum by free trioxide content.

Appearance: A clear, colourless to faintly straw oily liquid, odourless when pure and markedly denser than water — around 1.84 g/mL at the 98% strength. It is strongly hygroscopic and miscible with water in all proportions, releasing a great deal of heat as it mixes. Oleum, the fuming form, carries dissolved sulfur trioxide and gives off visible fumes in moist air.

Applications

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Also known as

Sulphuric acid · Oil of vitriol · Battery acid · Hydrogen sulfate · Oleum / fuming sulfuric acid · 66 Baumé acid

Frequently asked questions

What is 66 Baumé sulfuric acid?

A density reading rather than an assay, and an old habit of the trade that has stuck. Sixty-six degrees Baumé corresponds to roughly 93.2% sulfuric acid, which is the standard commercial concentrated strength in much of the world. If a buyer or a tender asks for 66 Bé, they are asking for about 93%, and it is worth confirming the assay in writing rather than trading on the density alone.

Why would I buy 93% instead of 98%?

Freezing point, mostly. The 98% acid starts to crystallise near 3 °C, so an outdoor tank in a cold climate needs heating or it plugs; 93% stays liquid well below freezing, which is why it dominates in cold regions. Against that, 98% carries less water per tonne and is what some processes need. If your storage is outdoors and unheated, specify 93% and say so early — it changes which suppliers can serve you.

Can I really store sulfuric acid in a plain carbon steel tank?

At concentrated strength, yes, and most of the world's bulk storage is exactly that. The acid forms a thin iron sulfate film that protects the steel underneath. The protection has two well-known limits: it fails as the acid is diluted, so any point where water can enter is a corrosion point, and it is scoured away by flow, so velocity in steel pipework is deliberately kept low. Dilute duty, pumps and high-velocity lines use 316L, alloy 20, PTFE-lined steel, PVDF or FRP instead.

What is oleum, and when do I need it rather than 98%?

Oleum is sulfuric acid carrying dissolved sulfur trioxide, sold by its free trioxide content, commonly 20 to 25% or 65%. You need it when the reaction consumes trioxide rather than acid, which in practice means sulfonation and sulfation chemistry — making LABSA from linear alkylbenzene is the everyday example. Using 98% there would dilute the reaction with the water the sulfonation produces. Oleum is also the safe way to move trioxide, which is otherwise a solid-forming hazard in its own right.

Why must acid go into water and never the reverse?

Because the heat of dilution is large and it appears wherever the two first meet. Poured into water, the acid sinks and the whole volume carries the heat away. Water poured onto acid sits on top as a thin layer, boils instantly at the interface, and blows acid out of the vessel. The rule is not caution for its own sake; it is the difference between a warm tank and a spray of concentrated acid at face height.

Is sulfuric acid a controlled chemical?

In most jurisdictions yes, in two separate ways, and both apply to ordinary industrial sales. It is a scheduled drug precursor — category 3 in the EU, List II in the United States — which means transactions are documented and suspicious ones reportable. Separately, in the EU it is a restricted explosives precursor above 15% by weight, so it cannot be supplied to members of the general public and professional buyers state their end use. Neither blocks a normal B2B trade; both mean the paperwork is part of the order rather than an afterthought.