Sulfuric acid and sodium hydroxide can form sodium hydrogen sulfate or sodium sulfate. Bisulfate requires a one-to-one mole ratio, while sulfate needs two moles of sodium hydroxide. Sodium bisulfate is useful for pH lowering and metal finishing, while sodium sulfate is used in laundry detergents and fabric manufacturing. Handling sulfuric acid and sodium hydroxide requires caution. Sodium sulfate is not considered hazardous waste.
Sulfuric acid is both a strong acid and a powerful dehydrator. It consists of a derivative of the element sulfur, its trioxide, SO3. Sodium hydroxide can, likewise, be obtained 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 give rise to one of two compounds, sodium hydrogen sulfate – also called sodium bisulfate, (NaHSO4) – or sodium sulfate (Na2SO4).
The preparation of bisulfate from sulfuric acid and sodium hydroxide requires a one-to-one mole ratio of the reactants: one mole is the molecular weight, in grams. Sulfate is formed by combining two moles of sodium hydroxide with one mole of sulfuric acid. After 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 is written 2 NaCl + 2 H2O → 2 NaOH + Cl2↑ + H2↑. The reaction that leads to the formation of 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 an “acid-base” reaction – reaching a pH of 7.0, similar to pure water, over a range of possibilities from 0 to 14 – it is called “neutralization”. The formation of the bisulfate represents an incomplete or partial neutralization. Although NaHSO4 is not as acidic as unreacted sulfuric acid, a one molar (1.0 m) aqueous solution of the acidic 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, cleaning and as a pickling agent.
Neutral sodium sulfate is sold commercially as Glauber’s salt (Na2SO4 10H2O) decahydrate (meaning “ten waters”). Sodium sulfate can also be purchased, without water, as an “anhydrous” salt. It is notably used in powdered laundry detergents and fabric manufacturing; it also finds declining employment in the paper industry. Not considered desirable, sodium sulfate is a byproduct in rayon manufacturing. Worse, it has been known to attack concrete and mortar, putting new structures as well as existing ones at risk.
Care must be taken when handling sulfuric acid and sodium hydroxide. The reactants are per se dangerous, and if pure, or even only in concentrated solution, the reaction is violent. Although it derives from the combination of very dangerous chemicals, sulfuric acid and sodium hydroxide, sodium sulfate is not considered a hazardous waste.
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