What Is the Connection between Sulfuric Acid and Sodium Hydroxide?

Sulfuric acid is both a strong acid and a powerful dehydrator. It is made from a derivative of the element sulfur, its trioxide, SO₃. Sodium hydroxide can, likewise, be made from its oxide, Na₂O, from the metal itself, or using a number of other processes, including electrolysis. Metallic sodium is the most common of the alkali metals, and its hydroxide is a strong base. The combination of sulfuric acid and sodium hydroxide can result in either of two compounds, sodium acid sulfate — also called sodium bisulfate, (NaHSO₄) — or sodium sulfate (Na₂SO₄).

Bisulfate preparation from sulfuric acid and sodium hydroxide requires a one-to-one mole ratio of reactants — one mole is the molecular weight, in grams. The sulfate is formed by combining two moles of sodium hydroxide with only the one mole of sulfuric acid. Following a series of preliminary steps leading to its formation, sulfur trioxide, sometimes called sulfuric acid anhydride, can be reacted with water to form sulfuric acid SO₃ + H₂O → H₂SO₄.

Sodium hydroxide is prepared electrolytically, the simplified equation being written 2 NaCl + 2 H₂O → 2 NaOH + Cl₂↑ + H₂↑. The reaction leading to formation of the acid sulfate is NaOH + H₂SO₄ → NaHSO₄ + H₂O. This compound is both a salt and an acid, and can be reacted with additional sodium hydroxide as follows: NaHSO₄ + NaOH → Na₂SO₄ + H₂O.

The reaction of sulfuric acid and sodium hydroxide to form a salt is called as an "acid-base" reaction — achieving a pH of 7.0, similar to pure water, in a range of possibilities from 0 to 14 — is termed "neutralization." Formation of the bisulfate represents an incomplete, or partial, neutralization. Although NaHSO₄ is not as acidic as unreacted sulfuric acid is, a one molar (1.0 m) water solution of the acid-salt has a pH of less than 1.0. For this reason, sodium bisulfate is sometimes chosen for pH-lowering applications. Sodium bisulfate is also useful in metal finishing, in cleaning and as a pickling agent.

Neutral sodium sulfate is sold commercially as the decahydrate (meaning "ten waters") Glauber’s salt (Na₂SO₄·10H₂O). Sodium sulfate can also be purchased — free of water — as the "anhydrous" salt. It is used most notably in powdered laundry detergents and in the manufacture of textiles; it finds declining use in the paper industry, as well. Not considered desirable, sodium sulfate is a byproduct in the manufacture of rayon. Worse, it is known to attack concrete and mortar, posing a risk to new, as well as existing, structures.

Caution must be exercised when handling sulfuric acid and sodium hydroxide. The reactants are dangerous in themselves, and if pure, or even just in concentrated solution, the reaction is violent. Although it results from the combining of the very dangerous chemicals sulfuric acid and sodium hydroxide, sodium sulfate is not considered a hazardous waste.