The strength of an acid is determined by a number called the acid-dissociation equilibrium constant. Sulfuric acid is a strong acid, whereas phosphoric acid is a weak acid. In turn, the strength of an acid can determine the way in which a titration occurs. Strong acids can be used to titrate a weak or strong base. A weak acid, on the other hand, will almost always be the analyte, or the thing being titrated.
A polyprotic acid has more than one hydrogen ion that it can donate to a solution. As the hydrogen ion donation increases, a solution's pH lowers; it becomes more acidic. For example, the chemical formula for sulfuric acid is H2SO4. It is diprotic; it has two hydrogen ions it can donate to a solution. The chemical formula for phosphoric acid is H3PO4. It is triprotic; it has three hydrogen ions it can donate to a solution. However, this does not determine whether those hydrogen ions will all dissociate in a solution. Instead, you must consider the acid-dissociation equilibrium constant.
Equilibrium Constants and Dissociation
An acid-dissociation equilibrium constant expresses the ratio of dissociated ions to undissociated compound in the acid at equilibrium. Both sulfuric acid and phosphoric acid have multiple equilibrium constants, corresponding to each of the hydrogen ions that can dissociate. Acids with large equilibrium constants are strong acids. Sulfuric acid has a first equilibrium constant of 1.0 x 10^3, which makes it a strong acid. Acids with small equilibrium constants will not dissociate readily. Phosphoric acid has a first equilibrium constant of 7.1 x 10^-3, which makes it a weak acid.
Sulfuric Acid in Titration
Because sulfuric acid is a strong acid, it can take on many roles in titration. You can use sulfuric acid to titrate either a weak or strong base. Sulfuric acid can also be titrated by a strong base. All titrations involve at least one equivalence point, where the solution of the reaction contains only water and the salt produced by the acid and base. If sulfuric acid is used to titrate a strong base or is titrated by one, the two solutions will completely dissociate and the equivalence point will have a neutral pH of seven. If you use sulfuric acid to titrate a weak base, the equivalence point will contain a weak acid left over from the weak base. Therefore, in such a titration, the pH will be lower than seven.
Phosphoric Acid in Titration
Because phosphoric acid is a weak acid, it is typically used only as an analyte. Phosphoric acid can be the weak acid in a weak acid-strong base titration. When the titration reaches the first equivalence point, the solution will contain the conjugate base H2PO4-. This will give the solution a pH greater than seven at that equivalence point.
Strength, Concentration and pH
The pH of a solution is a measure of the concentration of H3O+ ions in that solution. Therefore, the strength of an acid only partially determines its pH. If a solution of a strong acid has the same molar concentration as a solution of a weak acid, it will have a lower pH. However, if you dilute either solution, the pH will approach seven. This is due to the fact that the addition of water lessens the overall concentration of H3O+ ions.