In acid-base chemistry, a dibasic acid is one that can provide two positively charged hydrogen ions, or protons, when reacting with a base. A more modern term for this type of acid is a diprotic acid. An acid and base will normally react to form a salt and water. The water results from the positively charged hydrogen ion from the acid reacting with the negatively charged hydroxide ion from base: H+ + OH- → H2O. In a dibasic acid molecule, there are two hydrogen atoms available to react, and so two types of salt can be formed, one of which will be an acid salt containing a hydrogen atom.
An acid that can provide just one proton is known as a monoprotic or monobasic acid. Examples are hydrochloric acid (HCl) and nitric acid (HNO3). Polyprotic acids that can provide more than two protons also exist — for example, phosphoric acid H3PO4, which is triprotic. There is no relationship between the number of hydrogen atoms in the acid molecule and the strength of the acid — this depends on the extent to which the hydrogen in the molecule splits off in solution into hydrogen ions. An acid with one hydrogen atom that splits off easily will be stronger than one with two hydrogen atoms that do not; for example, hydrochloric acid (HCl) — a monobasic acid — is a much stronger acid than the dibasic carbonic acid (H2CO3).
The terms monobasic and dibasic are seldom applied to acids nowadays, but older chemistry textbooks may use them. Acids are now usually described as monoprotic, diprotic, triprotic and so on. The term dibasic may still be seen in the context of bases, for example calcium hydroxide (Ca(OH)2) may be described as dibasic, as it has two hydroxide groups that can combine with an H+ ion from an acid to form water.
Sulfuric acid, one of the best-known and most widely used acids, is a good example of a dibasic acid. It can form two types of salts, known as sulfates and hydrogen sulfates, sometimes called bisulfates. Carbonic acid is another common dibasic acid that can, similarly, form carbonates and hydrogen carbonates. The latter are often called bicarbonates; sodium bicarbonate, or baking soda, is the best known of these. These acid salts may be acidic, as in sodium hydrogen sulfate, or basic, as in sodium hydrogen carbonate; the term simply indicates that the salt contains a hydrogen atom that originated from an acid.
Not all of the hydrogen atoms in an acid molecule are necessarily available to form H+ ions and react with bases. It is thus not possible to tell whether an acid is monoprotic, diprotic or polyprotic simply by counting the hydrogen atoms in the molecule. This is especially true of organic acids, which can have relatively complex structures involving hydrogen in other roles. An example is tartaric acid (C4H6O6). There are six hydrogen atoms in the molecule, but it is so structured that only two of them can split off into hydrogen ions in solution; it is therefore a dibasic acid.