What Factors Affect Sulfuric Acid Solubility?

Oil of vitriol, better known as sulfuric acid, consists of the three elements hydrogen (H), sulfur (S) and oxygen (O), and possesses the chemical formula H₂SO₄. This is one of the most powerful acids known. It is also a dangerous dehydrating agent, dissolving in water rapidly, and in any proportions. Sulfuric acid solubility in water is so energetically favored, it may even result in damage to life or limb. Factors contributing to its great solubility in water include its high polarity, ease of ionization, stabilization through hydrogen bonding, and the symmetry of its anion.

As a general rule "like dissolves like" indicates water, a highly polar solvent, will dissolve other, polar substances. Polarity bears close relationship to the dielectric constant of a substance, a material’s charge handling capability. The dielectric constant of 100 percent sulfuric acid is approximately 100, which is higher than that of water, at about 80. This constant defines the degree in charge reduction a substance is able to make toward a particle completely encompassed within it. Sulfuric acid solubility is, clearly, energetically favored by this process.

Ionization in sulfuric acid is so strong, it can do it to itself, the process being called auto-protolysis. Mechanistically, the reaction is 2 H₂SO₄ → H₃⁺SO₄ + HSO₄⁻. Since ionization is the breakup of a substance into its charged constituents — part of the dissolution process — sulfuric acid solubility is not only complete in water, it is rapid. The H₃⁺SO₄ portion, itself, ionizes further.

Hydrogen bonding is a variety of "weak" bonding that occurs between electropositive hydrogen atoms and specific electronegative atoms, including oxygen, nitrogen and fluorine. Sulfuric acid’s negative ion, or anion, contains a sulfur atom attached to four oxygen atoms. The structure thus has ample ability to form strong and multiple hydrogen bonds. This form of bonding adds increased stability to the anions, and once formed, affects the recombining of ions. The process is energetically greatly disfavored, providing yet an additional factor guaranteeing sulfuric acid solubility.

The anion of completely ionized sulfuric acid is SO₄⁻². This ion is highly symmetrical, the two charge electrons can easily reside on any of the four oxygen atoms at the perimeter. This resonance stability decreases, further, the tendency of ion recombination, and is part of the sulfuric acid solubility package. The size of this already large anion is greatly increased by surrounding water molecules that hydrogen-bond to it. This spreads the modest negative charge over the much greater outer surface area of the hydrated anion, and may be compared to the process of hydration and hydrolysis of positive, metal cations.